gc 2013 15 1431

Green Chemistry

TUTORIAL REVIEW

Cite this: Green Chem., 2013, 15, 1431

Received 6th March 2013,Accepted 4th April 2013

DOI: 10.1039/c3gc40440d

www.rsc.org/greenchem

Sustainable routes to polyurethane precursors

Oliver Kreye, Hatice Mutlu and Michael A. R. Meier*

Environmentally friendly products and procedures are being developed both in industry and academia,

mainly due to the depletion of fossil resources and the growing global awareness of the need to protect

the environment. Thus, since polyurethanes represent a highly demanded class of polymers, straightfor-

ward, isocyanate and phosgene-free methods are required for the synthesis of their precursors (mono-

mers) in order to achieve a sustainable production. To foster the discussion with the nal goal to meet

such a sustainable production, this review provides an overview of classic as well as modern and more

sustainable routes towards polyurethanes and their precursors.

1. Introduction

Since Otto Bayer and co-workers reported the first polyadditionreaction of diols and diisocyanates to polyurethanes (PUR) in1947, this class of polymers has found various applications inall areas of our daily life.1 In light of the benefits that poly-urethanes oer through their versatile manufacturing possibi-lities and overall performance, numerous applications indierent fields (i.e. medical,2 automotive and industrial) haveshown high growth rates for a long time.3 Common manifes-tations of polyurethane formulations are soft or rigid foams,elastomers, as well as hard solid or flexible plastics.4 In 2011,the share of polyurethanes within all polymers on the Euro-pean market was 7.0% (Fig. 1).5 This demand can be attributedto their light weight, excellent strength to weight ratio, energyabsorbing performance, and comfort features. Their versatilityis instrumental in achieving the mechanical propertiesrequired for specific applications.6

Despite the desirable features of PUR, their industrial syn-thesis involves highly toxic and hazardous reagents.7,8 Thetypical synthesis is carried out by polyaddition reactions of di-isocyanates with diols (polyols) in the presence of tertiaryamines, especially 1,4-diazabicyclo[2.2.2]octane (DABCO), as abasic catalyst (Scheme 1). Most commonly, diisocyanates aresynthesized by reaction of phosgene with the correspondingamines. The most frequently used diisocyanates are methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI),with demands of 61.3% and 34.1%, respectively. Moreover, ali-phatic representatives such as hexamethylene diisocyanate(HDI) and isophorone diisocyanate (IPDI) are often used,although with a smaller demand of 3.4% and 1.2%,

respectively.9 Typical polyols employed for industrial PUR syn-thesis are diols (i.e. polyester or polyether polyols) or multi-functional polyols, such as glycerol.10

Taking into account the increased importance of poly-urethanes for the global market, alternative synthesis routesavoiding the application of extremely toxic phosgene as well astoxic isocyanates are mandatory from a sustainability point ofview. Moreover, considering health and safety reasons, poly-urethanes with the lowest possible free isocyanate content aredesirable. Furthermore, long-term considerations based on theavailability of petrochemicals due to their depletion makerenewable resources very attractive for a sustainable poly-urethane production. Recently, numerous reviews have shownthat the use of renewable resources, which are widely availableand inexpensive, often show reduced environmental impactand can address some of the main concerns of the petroleum-based chemical industry.11

On the basis of a comprehensive survey of the currentlyavailable literature on polyurethanes and possible new precur-sors, this review focuses on environmentally friendly and sus-tainable routes for the production of alternative monomers for

Fig. 1 Share of major polymer materials in the European plastic market in2011.5

Karlsruhe Institute of Technology (KIT), Institute of Organic Chemistry, Fritz-Haber-

Weg-6, 76131 Karlsruhe, Germany. E-mail: [email protected];

Fax: (+49)-721-608-46800; Tel: (+49)-721-608-48326

This journal is The Royal Society of Chemistry 2013 Green Chem., 2013, 15, 14311455 | 1431

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greener polyurethane synthesis avoiding phosgene at any stageof production. Of exceeding importance is the production ofdi- and polyisocyanates, both as commodities and as special-ties, for the production of polyurethanes. Thus, in this context,we summarize classic synthetic routes to isocyanates and car-bamates as well as modern and promising possibly eco-friendly ways to these compounds. Finally, some recent devel-opments in the synthesis of renewable polyurethanes arediscussed.

2. Isocyanates

Isocyanates (RNvCvO) are esters of the unstable isocyanicacid and are known for their high reactivity towards nucleophi-lic additions.12 Isocyanates dimerize in an equilibrium reac-tion to uretidones (Scheme 2). This reaction is catalyzed byphosphanes as well as tertiary amine bases (e.g. pyridine).Better known is their trimerisation to isocyanurates (perhydro-1,3,5-triazine-2,4,6-triones) catalyzed by phosphanes, aminebases, as well as alkali metal salts of formic or acetic acid.Modern protocols report a very eective cyclotrimerization byapplying alkali metal fluorides, N-heterocyclic carbenes(NHCs), or diverse other catalysts and methods.13 Isocyanu-rates play an important role in polymer chemistry for the syn-thesis of polyisocyanurate foams (PIRs). These foams oer ahigh rigidity and are used, for instance, for thermal

insulation.14 The hydrolysis of isocyanates under acidic orbasic conditions gives unstable carbamic acid derivatives,which immediately undergo decarboxylation to primaryamines. Unfortunately, in 1984 the uncontrolled reaction ofmethyl isocyanate with water led to one of the worst chemicalindustry disasters. In the Indian city Bhopal, approximately8000 people died within two weeks due to gas-related diseasescaused by inhalation of methyl isocyanate and thereof derivedmethyl amine.15 The certainly most common reaction of iso-cyanates is their conversion to carbamates (urethanes) withalcohols.16 To increase the reaction rate, tertiary amine baseslike 1,4-diazabicyclo[2.2.2]octane (DABCO) or 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) are used as catalysts. With primary orsecondary amines, isocyanates react readily to urea deriva-tives.17 Furthermore, isocyanates can react to carbodiimides byapplying phospholene 1-oxides as catalysts.18 In modern pro-cedures, the conversion of carboxylic acids or thioacids withisocyanates to amides under base influence is described.19

Besides this, it is worth noting that during the last fewdecades numerous cycloaddition reactions of isocyanates, cata-lyzed by transition metals, were described.20

The application of isocyanates in polymer chemistry isfocused on the production of polyurethanes, but besides this,anionic polymerization strategies aord the synthesis of rigidpolyisocyanates.21 Moreover, isocyanate-grafted polymers aredescribed for additional functionalization and for polymerconjugation.22 Interesting copolymers were synthesized bycycloaddition of isocyanates with multifunctionalized alkynemonomers.23

Scheme 1 Frequently used diisocyanate monomers and their polyadditionreaction with diols to polyurethanes.

Scheme 2 Common reactions of isocyanates.

Tutorial Review Green Chemistry

1432 | Green Chem., 2013, 15, 14311455 This journal is The Royal Society of Chemistry 2013

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In summary, isocyanates are very useful and versatile com-pounds in organic and polymer chemistry, but neverthelessthey are generally toxic and some of them are classified to bevery toxic (e.g. methyl isocyanate and TDI). Moreover, isocya-nates are potentially irritants and cause allergic asthma.24 Ingeneral, it is thus attractive to generate isocyanates in situ andlet them react to the target compounds without isolation.

2.1. Conventional synthesis of isocyanates

Several dierent strategies for the synthesis of isocyanates areknown.12 The most common and also industrially applied pro-cedure is the treatment of primary amines with phosgeneunder release of two equivalents of hydrogen chloride(Scheme 3a).25 To avoid the application of extremely toxic

gaseous phosgene, derivatives with similar properties like tri-chloromethyl chloroformate (diphosgene, liquid) and bis-(tri-chloromethyl) carbonate (triphosgene, solid) are used foreasier handling in laboratory scale synthesis.26 Diphosgeneand triphosgene can be synthesized by chlorination of methylformate and dimethyl carbonate (DMC). Nevertheless, in con-sideration of environmental acceptability and sustainability,alternative methods are desired that avoid the use of thesehighly toxic compounds and the use of chlorine in theirsynthesis.

In 1995, Knlker et al. developed a method using di-tert-butyl dicarbonate (Boc2O) in the presence of stoichiometricamounts of 4-dimethylaminopyridine (DMAP) for the synthesisof isocyanates from amines (Scheme 3b).27 On a laboratoryscale, this method is a very good alternative to the before men-tioned methods, but toxic Boc2O is also obtained by reactingphosgene with carbon dioxide, tert-butanol and potassiumhydroxide. Moreover, the excess of Boc2O and stoichiometricquantity of DMAP lead to the formation of large amounts ofwaste. Thus, this is a practical procedure for the synthesis ofisocyanates in laboratories, but does not meet the require-ments of green chemistry.

Another possibility for the synthesis of isocyanates is thenucleophilic substitution of alkyl halogenides, tosylates, mesy-lates or triflates with metal cyanates (Scheme 3c). For thismethod, diverse side reactions and polymerisations aredescribed and, thus, the yields of isocyanates are normallypoor.28 The recent literature showed that alcohols, thiols andtrimethylsilyl ethers can directly be converted to isocyanates byapplying a mixture of triphenylphosphine, 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetrabutylammoniumcyanate.29 High yields can be obtained by this procedure, butgenerally, this method is certainly not suited for large-scaleisocyanate synthesis.

Another frequently used method, at least for laboratory-scale isocyanate synthesis, is the Curtius rearrangement of acylazides (Scheme 3d).30 Under non-aqueous conditions andinert solvents, isocyanates can be obtained in high yields. TheCurtius rearrangement is certainly a very useful method, but isnot adequate for industrial applications and in terms of greenchemistry due to the high toxicity and explosive properties ofthe needed azides and acyl azides. Alternative procedures suchas the Hofmann rearrangement of amides and the Lossenrearrangement of hydroxamic acids also aord isocyanates asintermediates, but usually these are directly degraded toprimary amines in the presence of water or trapped with alco-hols to the corresponding carbamates. Nevertheless, modifiedprocedures of the Hofmann rearrangement as well as Lossenrearrangement also allow the synthesis of isocyanates.31 Underclassic conditions, these procedures suer from the use of stoi-chiometric amounts of toxic and corrosive reagents (i.e. Br2 inthe case of the Hofmann rearrangement and acetic anhydrideor acetyl chloride with base in the case of the Lossenrearrangement) and produce high amounts of waste.

In 1994, Minisci et al. developed a free radical synthesisprocedure of isocyanates in good yields starting from oxalicScheme 3 Conventional synthesis routes to isocyanates.

Green Chemistry Tutorial Review


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acid monoamides (Scheme 4).32 With an excess of ammoniumpersulfate in the presence of catalytic amounts of silver(I) andcopper(II) salts, carbamoyl radicals are formed, followed bydecarboxylation to isocyanates. Unfortunately, this method isalso limited to small-scale synthesis. The synthesis of oxalicacid monoamides is extensive and the use of excessammonium persulfate makes this process unsustainable.

In 1979, Lesiak and Seyda described a method for the syn-thesis of isocyanates from formamides.33 After optimization,they found that the highest yields were obtained if formamideswere heated with an excess of bromine in benzene in the pres-ence of 1,4-diazabicyclo[2.2.2]-octane (DABCO) to obtain iso-cyanates in moderate yields (Scheme 5). The addition ofDABCO is necessary to bind the released hydrogen bromide.The applied harsh conditions are limited to non-sensitive for-mamides and the used halogens are not suitable for a sustain-able procedure. However, the idea to use formamides in anenvironmentally benign dehydrogenation process for the syn-thesis of isocyanates is promising, because formamides areeasily accessible in quantitative yields by heating primaryamines in recyclable formic acid or formic acid esters.34 Achallenge is certainly to find a green way for the dehydrogena-tion of these formamides.

Other procedures for the generation of isocyanates startfrom carbamates (urethanes). For detailed synthesis pro-cedures of carbamates, see sections 3.1 and 3.2. The decompo-sition of carbamates applying temperatures above 250 C inthe presence of dierent catalysts led directly to isocyanates.35

However, this pyrolysis process is limited to N-aryl carbamates(see Scheme 10 in section 2.2). The thermal fragmentation ofaliphatic carbamates resulted in many side reactions. For thisreason, mild cleavage protocols were developed using silaneand boron compounds in the presence of tertiary amine com-pounds (Scheme 6). Numerous silanes were investigated anddiiodosilane gave the best results under mildest conditions.36

Also boranes can be used for a rapid and ecient cleavage ofcarbamates. First experiments were performed by applyingchlorocatecholborane, but the much cheaper boron trichloridealso eciently cleaves carbamates to isocyanates in nearlyquantitative yields.37 Also these cleavage procedures arelimited to the laboratory scale. Due to the relatively high priceof silanes and toxicity of boron halogenides, their applications

in industrial processes are not reasonable and sustainable.Instead, the challenge to find highly ecient catalysts for thethermal decomposition of aryl- as well as alkylcarbamates toobtain isocyanates quantitatively should be accepted.

2.2. Modern, sustainable and promising pathways toisocyanates

A promising and sustainable pathway to aryl isocyanates is thereductive carbonylation of nitro arenes.16a,38 For the synthesisof TDI and MDI, this is an excellent alternative method com-pared to the phosgenation of the corresponding amines.Already in 1967, Hardy and Benett described the direct conver-sions of aromatic nitro compounds with carbon monoxide andRh/C catalysts at high pressure and temperatures (Scheme 7).39

Several catalysts can be used for this thermodynamically

Scheme 5 Synthesis of isocyanates from formamides.

Scheme 7 Reductive carbonylation of nitro arenes to isocyanates.

Scheme 4 Free radical synthesis of isocyanates from oxalic acid monoamides.

Scheme 6 Synthesis of isocyanates from carbamates.



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favored, highly exothermic process. Normally, non-toxic tran-sition metals of groups 810 are applied, but also sulfur, sel-enium and tellurium catalyze this reaction.38a However,highly-toxic catalyst residues were detected in the final pro-ducts and, thus, this route is until now inapplicable for anindustrial poluyurethane synthesis.40 The generally acceptedmechanism starts with the formation of a metallacycle fromthe nitroarene in the presence of carbon monoxide and thecatalyst.38a,41 This intermediate fragments by decarboxylation,but the nitroso group remains bound to the metal. Sub-sequently, an insertion of carbon monoxide occurs, followedby decarboxylation. The formed nitrene species, as a key inter-mediate, can be carbonylated to give the resulting isocyanate.

An even more exothermic reaction is the more often appliedtwo-step carbonylation to N-phenyl carbamates in the presenceof alcohols, followed by subsequent thermal decomposition toisocyanates (Scheme 8).16a,17a,38a Numerous catalysts andligands are described for this process. Common homogeneouscatalysts are based on ruthenium carbonyl compounds havingchelating ligands,42 palladium(II) compounds with bipyridineand phenanthroline ligands,43 as well as rhodium carbonylcompounds.44 Moreover, other catalysts (e.g. cheaper iron cata-lysts) are described.45 Procedures for the thermal fragmenta-tion of carbamates to isocyanates were already describedabove.35a

Other valuable products of reductive carbonylations of aro-matic nitro compounds are ureas.38a,46 From a few examples, itis also known that aryl azides can be converted with carbonmonoxide to aryl isocyanates.47 However, the entry to arylazides is an additional non-sustainable step.

Another interesting route to isocyanates starts from iso-nitriles (isocyanides). Very recently, Le and Ganem published amild, ecient and eco-friendly oxidation procedure, withwhich isocyanides can be oxidized to isocyanates applyingdimethyl sulfoxide (DMSO) in the presence of catalyticamounts of trifluoroacetic anhydride (TFAA) (Scheme 9).48 Theoxidation of isonitriles to isocyanates is not a new concept, but

the earlier described procedures used toxic or non-sustainableoxidation agents such as mercuric oxide, lead tetraacetate,ozone as well as halogen- and acid-catalyzed oxidations withDMSO and pyridine N-oxide.49

A future goal could be to find other mild and environmen-tally benign oxidation processes to obtain isocyanates fromisonitriles. The drawback is certainly the lack of a sustainableroute to isonitriles. Common procedures to obtain isonitrilesare the dehydration of N-formamides with toxic agents likephosgene, phosphorous oxychloride, thionyl chloride andother highly reactive substances in the presence of aminebases.50 Less applicable methods are nucleophilic substitutionreactions of allyl-, benzyl- and tert-alkylhalogenides with silvercyanide and the reaction of dichlorocarbene with anilines.51

Thus, an eco-friendly synthesis of isonitriles would bedesirable.

3. Carbamates

Since carbamates are key intermediates for the synthesis ofisocyanates as well as for direct conversions with diols to non-isocyanate polyurethanes (NIPUs) via transesterification reac-tions, the following section shall give a closer insight into sus-tainable routes to carbamates. Carbamates (urethanes) areformal esters of the instable carbamic acid. In chemistry, car-bamates found applications in three main categories: inpolymer chemistry as the functional group of polyurethanes,in peptide or related chemistry as protective groups of amines,and in agricultural chemistry as active ingredients of insecti-cides, fungicides and herbizides. Numerous urethane protect-ing groups are well-known for the protection of primary andsecondary amines.52 Typical reagents for the protection ofamines are alkyl or aryl chloroformates as well as organic car-bonates and pyrocarbonates (Scheme 10). The most commonlyused urethane protecting groups are the tert-butyloxycarbonylgroup (Boc), the benzyloxycarbonyl group (Z or Cbz), the fluor-enylmethoxycarbonyl group (Fmoc) and the allyloxycarbonylgroup (Alloc). For each of these groups, dierent conditionsfor their cleavage are required. Besides this, numerous otherurethane protecting groups exist for special applications inpeptide chemistry.52 Moreover, carbamates can be transesteri-fied with alcohols in the presence of catalysts (see section 3.2).Their reaction with primary or secondary amines (aminolysis)to urea derivatives is also well-established.17a,53

3.1. Conventional synthesis of carbamates

Carbamates can be obtained via dierent routes from alcoholsand amines using phosgene or one of its derivatives as a

Scheme 9 Oxidation of isonitriles to isocyanates.

Scheme 8 Reductive carbonylation of nitro arenes to carbamates followed bythermal decomposition to isocyanates.



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reagent (Scheme 11). The simplest synthesis procedure toobtain carbamates is the reaction of primary or secondaryamines with alkyl or aryl chloroformates in the presence of abase to trap the released hydrogen chloride (Scheme 11a).16a,54

Chloroformates are phosgene derivatives, highly toxic andthus this method is far from sustainable and not an eco-friendly entry to carbamates. On the laboratory scale,especially in peptide chemistry, this method is often appliedto introduce amine protecting groups.52 The reaction ofprimary amines with phosgene leads to isocyanates(Scheme 11b). The following addition of alcohols gives carba-mates.16a,55 The conversion of diisocyanates with diols is uti-lized for standard industrial processes for the production of

polyurethanes.1,56 Moreover, some procedures for carbamatesynthesis are described in which carbamoyl chloride, derivedfrom secondary amines and phosgene, is reacted with alcoholsto give N,N-disubstituted carbamates (Scheme 11c).16a,57

Due to the diculty of handling and the high toxicity ofphosgene, the known alternatives di- and triphosgene as wellas pyrocarbonates can also be used in carbamate synthesis(Scheme 12a). A common pyrocarbonate reagent is di-tert-butyldicarbonate (Boc2O), used for the introduction of theBoc-protecting group of amines.52 Due to the importance ofBoc-protection in peptide and related chemistry, severalmodern, mild, and ecient procedures are described regard-ing selectivity and sustainability.58 During the last twodecades, more ecient reagents were developed for the conver-sions of alcohols or amines to carbamates.16a The use of carbo-nyl diimidazole (CDI) as a safer and less toxic reagent is anexcellent alternative method for the synthesis of carbamatesstarting from alcohols or amines (Scheme 12b and c).59,60

However, also CDI is synthesized from phosgene and largeamounts of by-products are obtained during carbamate syn-thesis. With carbamoyl-3-nitro-1,2,4-trizole, another highlyreactive carbamate derivative, useful applications in selectiveprotection of nucleobases are described (Scheme 12d).61 More-over, methyl N-(triethylammoniumsulfonyl)carbamate, betterknown as Burgess reagent, is a versatile reagent in organic syn-thesis. It is easily prepared by the reaction of chlorosulfonylisocyanate with methanol and triethylamine in a two-step pro-cedure.62 It can be used to convert alcohols into olefins andmethyl carbamates, carboxamides into nitriles, formamidesinto isonitriles, nitroalkanes into the corresponding nitrilox-ides, and aldoximes to formamides. Moreover, several appli-cations in the synthesis of heterocycles are known.16a,63 Alsodiols can be converted to carbamates using Burgess reagent inthe presence of nucleophiles (Scheme 12e).64

Furthermore, a direct conversion of primary alcohols toCbz-protected amines applying the benzyl carbamate deriva-tive of Burgess reagent is described.65 The Burgess reagent canalso be used to synthesize carbamates of -amino acids, whichcan be prepared via BaylisHillman reactions.66 Interestingly, asolvent-free synthesis of primary carbamates can be achieveddirectly in a one pot reaction of alcohols with sodium cyanate(Scheme 12f). The reaction is performed in the presence ofperchloric acid on silica gel.67 As shown, many proceduresexist for the synthesis of carbamates from alcohols andamines. However, these methods cannot be considered sus-tainable for several reasons (see discussion above) and most ofthem are limited to lab-scale synthesis.

For the conversions of carboxylic acids to carbamates, threewell-known rearrangement reactions, the Curtius rearrange-ment of acyl azides, the Homann rearrangement of carboxa-mides, and the Lossen rearrangement of hydroxamic acids, areknown (Scheme 13).16a As mentioned in section 2.1, theprimary rearrangement products are isocyanates, but in thepresence of alcohols, carbamates are the final products.

Probably the best known of these reactions is the Curtiusrearrangement, although toxic and explosive acyl azides are

Scheme 10 Common reaction pathways of carbamates.

Scheme 11 Carbamate synthesis by using phosgene and its derivatives.



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the starting materials.30,68 In modern procedures, the acylazide is generated in situ by applying CDI, Boc2O, 1-propane-phosphonic anhydride (T3P) or diphenylphosphoryl azide

(DPPA), followed by rearrangement to carbamates in a one potfashion.69 Similarly, the Hofmann rearrangement is oftenapplied to convert amides to carbamates. Using the traditionalprotocol, amides are treated with bromine or chlorine in alka-line solution to obtain primary amines.70 Modern proceduresallow the conversion of amides with hypervalent iodine com-pounds under mild and neutral conditions to obtain methylcarbamates.71 Furthermore, N-bromosuccinimide (NBS) canalso be used as a reagent to initiate the rearrangement.72

Recently, some other modifications and also electrochemicallyinduced Hofmann rearrangements to carbamates aredescribed.73 The Lossen rearrangement is less often used incomparison with both the aforementioned methods. Onereason can be the slightly extensive access to hydroxamicacids. Nevertheless, to date numerous dierent procedures aredescribed for the synthesis of hydroxamic acids from car-boxylic acid derivatives.74 Hydroxamic acids require an acti-vation to undergo the rearrangement under basicconditions.75 In modern protocols, the activation andrearrangement occur in a one pot reaction sequence.76

Besides the carbamate synthesis from amines, alcohols andcarboxylic acid derivatives, a few examples describe the conver-sion of nitriles, aldehydes and ketones to carbamates.16a

Caddick et al. developed a generic approach for the directcatalytic reduction of nitriles to tert-butyl carbamates applyingcatalytic amounts of nickel(II) chloride hexahydrate and anexcess of sodium borohydride and Boc2O (Scheme 14a).

77

Also ketones and aldehydes can be directly transformed tocarbamates. In 2007, Seijas et al. observed that substitutedacetophenones and other ketones can be converted with carba-moyl chlorides to enol carbamates in the presence of 2,4,6-col-lidine as a strong non-nucleophilic base under solvent-freeconditions supported by microwave heating (Scheme 14b).78

Moreover, Tomkinson et al. described the -carbamoylation ofstructural diverse ketones and aldehydes by reaction withN-methyl-O-carbamoyl hydroxylamine hydrochlorides.79 Aninteresting synthesis procedure of aromatic acyl carbamateswas reported by Nair and co-workers in which aromaticaldehydes react with dialkyl diazocarboxylates and triphenylphosphine.80

Scheme 12 Phosgene alternatives in the synthesis of carbamates.

Scheme 13 Carbamates via Curtius, Hofmann and Lossen rearrangements.



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3.2. Sustainable synthesis procedures of carbamates

3.2.1. Organic carbonates as key compounds for sustain-able synthesis of carbamates. In general, the application oforganic carbonates for the synthesis of carbamates is certainlya very promising process for a sustainable and environmentallybenign synthesis of polyurethane precursors. However, toachieve industrial realization, further developments, both interms of eciency (which generally goes along with sustain-ability) and economic feasibility, of the herein described pro-cedures are certainly necessary. A very promising andsustainable access to carbamates and also ureas is the catalyticconversion of primary and secondary amines utilizing dialkylor diaryl carbonates (Scheme 15).16a The synthesis of dierentcarbamates applying N,N-disuccinimido carbonate as the acti-vated carbonate was achieved under mild conditions at roomtemperature as reported by Ogura et al. in 1983.81 Further reac-tive carbonates are ortho- or para-substituted bis(nitrophenyl)carbonates.82 Pentafluorophenyl carbonate derivatives are alsoapplied for mild conversions to oligopeptidyl carbamates.83

Christensen et al. reported a very useful method to obtain

mono-protected amines in high yields from aliphatic poly-amines by the application of alkyl phenyl carbonates. Theydemonstrated that tert-butyl, benzyl and allyl phenyl carbon-ates used in slight excess to the applied di- or polyamine gavemono-protected Boc-, Cbz- or Alloc-carbamates in yieldsbetween 46 and 98%.84 This oers significant advantages interms of less toxic reagents if compared to the above describedclassic routes employed for the introduction of the respectiveprotecting groups.

However, far more attractive with regard to sustainable pro-cedures, several catalysts were described for the formation ofcarbamates and ureas from non-activated carbonates. In 2007,Porco et al. showed that Zr(Ot-Bu)4 (5.0 mol%), in combinationwith 2-hydroxypyridine (10 mol%), is an ecient catalyticsystem for the reaction of primary and secondary amines withdialkyl carbonates to obtain variable carbamates and asym-metric ureas.85 Later, Vidal-Ferran et al. observed that 1.0 mol%zinc acetate is an excellent catalyst to synthesize bis-isocya-nate precursors (dimethyl carbamates) of TDI and MDI inalmost quantitative yields from the corresponding anilines viareaction with DMC.86 Also sodium acetate (20 mol%) is aneective catalyst for methoxycarbonylation of 1,6-hexanedi-amine by DMC.87 Moreover, Meier et al. observed that thestrong guanidine base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)catalyzes carbamate formation from primary and renewablediamines.88 Similarly, TBD (0.5 to 10 mol%, depending on thealcohol used) can eciently catalyze the formation of sym-metric and unsymmetric carbonates from DMC and primary,secondary, as well as tertiary alcohols.89 Due to the importancein the development of sustainable and environmentally benignsynthesis procedures of carbamates from amines and DMC,several other methods and catalysts have also been reported.90

Very promising is the application of enzymes as biocatalystsfor alkoxycarbonylation reactions. Already in 1993, Gotor et al.showed the synthesis of chiral carbamates by appling CAL(Candida antarctica lipase immobilized on accurrel) in thereaction of racemic amines with n-octyl and n-butyl vinyl car-bonates.91 Later, the same group showed the CAL catalyzedalkoxycarbonylation of 1,25-dihydroxyvitamin D3 A-ring pre-cursors with O-[(vinyloxy)carbonyl]oxime to obtain carbamatederivatives in quite good yields.92 Furthermore, the first regio-selective enzymatic alkoxycarbonylation of primary amines tocarbamates of pyrimidine 3,5-diaminonucleoside derivativesapplying CAL-B was described.93 Diallyl or dibenzyl carbonatesare adequate reagents in this alkoxycarbonylation reaction toobtain allyl and benzyl carbamate derivatives in yields between

Scheme 15 Carbamates and ureas by reaction of amines with dialkyl or diarylcarbonates.

Scheme 14 Carbamates derived from nitriles, ketones and aldehydes.



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63% and 72%.94 Moreover, other enzymatic alkoxycarbonyla-tion reactions have also been reported.95

In 2005, Selva et al. reported a catalyst-free synthesis ofmethyl carbamates from amines and DMC in supercriticalcarbon dioxide.96 At 130 C and a CO2 pressure between 5 and200 bar, primary aliphatic amines react with DMC selectivelyin the presence of anilines and alcohols with conversions upto 90% after four hours reaction time.

The traditional synthesis of organic carbonates involves thereaction of phosgene or phosgene derivatives with alcohols.However, especially for the synthesis of dimethyl and diphenylcarbonate, eco-friendly industrial processes are described.97

Four well-known synthesis procedures of dimethyl carbonate(DMC) without applying phosgene received industrial rele-vance.98 The conversion of carbon dioxide and methanol inthe presence of catalysts is such a well-known method(Scheme 16a). In 1998, first experiments were described toinvestigate the reactivity of carbon dioxide with n-butyl(phenoxy)-, (alkoxy)- and (oxo)-stannanes and other organic tincatalysts for the synthesis of DMC.99 Another method reportedabout the reaction of methanol with carbon dioxide in thepresence of potassium carbonate to form methyl carbonateions as intermediates, which were trapped with methyl iodideto obtain DMC.100 A general problem of a direct conversion ofmethanol with carbon dioxide to DMC is the formed water,which has to be removed from the equilibrium reaction. In2009, Tomishige et al. reported a direct conversion of metha-nol and low pressure carbon dioxide to DMC catalyzed byCeO2 and promoted by acetonitrile hydration.

101 Another syn-thesis of dimethyl carbonate from methanol and carbon

dioxide catalyzed by ZrO2 doped with KCl was investigated byMurzin and co-workers using chemical traps for water to cir-cumvent thermodynamic limitations.102 Moreover, a catalystsystem of cerium and zirconium oxide in dierent ratios(CexZr1xO2) supported by ionic liquid [EMIM]Br seems to be apromising process for the synthesis of DMC.103 Beyond thesementioned methods, diverse other procedures for the directconversion of carbon dioxide with methanol are described.104

Another important industrial process for the synthesis ofDMC is the oxidative carbonylation of methanol (Scheme 16b).In 1998, the Bayer AG described a process for the oxycarbonyla-tion of methanol to DMC using molten salts as catalysts.105

The catalyst system consists of a eutectic mixture of copperchloride and potassium chloride. In 2003, Itoh et al. reportedthe synthesis of DMC by vapor phase oxidative carbonylationof methanol applying CuCl2/NaOH/activated carbon cata-lysts.106 The application of CuY zeolite supported on siliconcarbide as an improved catalytic system for the vapor phaseoxidative carbonylation was reported by Keller and co-workersin 2008.107 Moreover, Li et al. developed an ecient and recycl-able catalyst (Schi base/zeolite) for the oxidative carbonyla-tion.108 In addition to these mentioned methods for theoxidative carbonylation of methanol to DMC, several other pro-cesses have been reported.109

During the last few years, the catalytic conversion of urea toDMC has gained more and more interest (Scheme 16c). In2003, Arai et al. described the conversion of urea and ethyleneglycol to ethylene carbonate catalyzed by ZnO.110 Followed bythe before mentioned transesterification procedures withmethanol, this method is an important step for the urea basedsynthesis of DMC. Later on, Sun et al. reported the direct syn-thesis of DMC from urea and methanol over a ZnO catalyst.111

In 2007, the same group described a high-yielding direct syn-thesis of DMC by urea methanolysis applying a catalytic distil-lation process over a Zn-based catalyst.112 One year later, Sunand co-workers investigated the optimal conditions for the syn-thesis of DMC from methyl carbamate (intermediate from ureamethanolysis) and methanol with zinc compounds as cata-lysts.113 Very recently, Cai et al. reported the selective synthesisof DMC from urea and methanol applying a Fe2O3/HMCM-49catalytic system in a batch reactor.114

Probably the most frequently employed method to preparecarbonates is the two step conversion of ethylene or propyleneoxide with carbon dioxide to ethylene carbonate or propylenecarbonate, followed by transesterification with methanol toDMC (Scheme 16d). In 2003, Arai et al. reported the direct syn-thesis of DMC and glycols from epoxides, methanol andcarbon dioxide using heterogeneous Mg containing smectitecatalysts.115 At the same time, Wang and co-workers describedthe application of KI and K2CO3 as a catalyst mixture for thesynthesis of DMC from ethylene oxide under supercritical con-ditions.116 CaO/carbon composites are also described to cata-lyze the transesterification of ethylene and propylene oxidewith methanol to DMC.117 In 2006, He et al. described theapplication of recyclable inorganic base/phosphonium halide-functionalized polyethylene glycol as a catalyst for theScheme 16 Phosgene-free synthesis procedures of dimethyl carbonate (DMC).



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synthesis of DMC from propylene oxide.118 Furthermore,MgOCeO2 mixed oxide catalysts were established by Yoo andco-workers.119 Garca and co-workers found that gold nanopar-ticles promote the catalytic activity of ceria for the transesterifi-cation of propylene carbonate to DMC.120 Recently, Cai et al.showed an electrochemical synthesis of DMC from methanol,CO2 and propylene oxide in ionic liquids.

121 Besides this,numerous other sustainable and eco-friendly processes aredescribed for the insertion of carbon dioxide to epoxides andfor the transesterification of cyclic carbonates to DMC.122 Assummarized, several sustainable alternatives for the synthesisof DMC have been developed and, thus, DMC appeared to be areadily available and sustainable alternative, being regarded asa non-toxic phosgene substitute for the eco-friendly and safesynthesis of carbamates, ureas and polyurethanes.

Several catalytic systems are able to catalyze the exchangereaction of alcohols in carbamates (transcarbamoylation reac-tions, Scheme 17). Jousseaume et al. reported that bismuth tri-flate in amounts of only 1.0 mol% is a highly ecient catalystfor the transcarbamoylation of N-hexyl O-methyl carbamates toN-hexyl O-octyl carbamates at a temperature of 160 C.123

Lanthanum(III) isopropoxide is also reported to catalyze thetranscarbamoylation of methyl carbamates in an excellentmanner.124 Moreover, Lewis acids such as titanium(IV) isoprop-oxide and tin(II) 2-ethylhexanoate gave similar results.125

Very recently, Meier et al. reported a sustainable way to car-bamates from hydroxamic acids. In the introduced eco-friendlyLossen rearrangement, dialkyl and diaryl carbonates are ableto activate the hydroxamic acids in situ in the presence of cata-lytic amounts of tertiary amine bases (0.10.4 eq.) to initiatethe rearrangement by heating for one day (Scheme 18).126 Firstinvestigations were carried out with DMC, but also severalother dialkyl carbonates as well as diphenyl carbonate couldbe employed in the catalytic Lossen rearrangements of

aliphatic hydroxamic acids to the corresponding alkyl and arylcarbamates in good yields. The application of these conditionsto aromatic hydroxamic acids aorded the direct synthesis ofanilines in yields up to 83%. In consideration of the aspects ofgreen chemistry, the solvent/activation mixture can be recycledseveral times and, thus, the production of chemical waste canbe minimized. For the synthesis of carbamates from carboxylicacids, this method seems to be a promising sustainable andeco-friendly process.3.2.2. Miscellaneous sustainable synthesis procedures of

carbamates. Another access to carbamates is the metal cata-lyzed oxidative carbonylation of amines and anilines(Scheme 19).16a In 1984, Fukuoka et al. reported a novel cataly-tic synthesis of carbamates by oxidative alkoxycarbonylation ofprimary amines and anilines with alcohols in the presence ofpalladium and iodides.127 The obtained yields of methyl andethyl carbamates were in most cases above 90%. Moreover, thesame group mentioned that this reaction was also catalyzed inthe presence of other platinum group metals and alkali metalhalides.128 One year later, Alper and Hartstock showed a mild,catalytic alternative for the conversion of amines into carba-mates.129 Anilines were converted to methyl and ethyl carba-mates applying a catalytic mixture of PdCl2 (10 mol%), CuCl2(20 mol%) and traces of hydrochloric acid at room tempera-ture. The obtained yields ranged from 16% in the case of steri-cally hindered anilines up to 99% for unsubstituted aniline. In1992, Leung and Dombek reported that metallomacrocycliccompounds, such as metalloporphyrins, are excellent catalystsfor the oxidative carbonylation of amines to carbamates.130 Acobalt porphyrin complex (1.0 mol%) promoted by sodiumiodide (10 mol%) showed the highest activity and full conver-sion was observed after three hours at 180 C under highpressure in the case of the synthesis of ethyl carbamates oftert-butyl amine and aniline in the presence of ethanol.Schwartz and co-workers reported that elemental iodine rep-resents a good promoter for palladium catalyzed oxidativecarbonylation in the presence of potassium carbonate.131

Methyl and ethyl carbamates were obtained in yields between24 and 90% from anilines and aliphatic amines performingthe oxidative carbonylation in the presence of methanol orethanol. A polymer supported palladiumcopper catalyst forthe oxidative carbonylation of aniline was developed by Liaoand co-workers.132 Moreover, a PdCl2/ZrO2SO4

2 catalystsystem was shown to be highly ecient for the oxidativecarbonylation of amines.133 Deng et al. developed a highlyecient ionic liquid-mediated palladium complex catalystsystem for the oxidative carbonylation of amines.134 High con-versions and selectivity for the synthesis of methyl N-phenyl-carbamate were achieved with low catalyst loadings. The same

Scheme 17 Transcarbamoylation reactions of carbamates.

Scheme 18 New base catalyzed Lossen rearrangements of hydroxamic acidswith organic carbonates.

Scheme 19 Oxidative carbonylation of primary amines in the presence ofalcohols.



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group also showed that a gold(I)-complex acts as an ecientcatalyst for the oxidative carbonylation.135 Yamanaka et al.demonstrated an electro-organic approach for the synthesis ofN-hexyl carbamate by carbonylation of methanol and hexyl-amine over a gold supported carbon anode.136 Furthermore,other methods, also in the synthesis of cyclic carbamates,applying oxidative carbonylations are described.137

A very simple, eective and environmentally benign accessto carbamates can be achieved by a three component reactionof a primary or secondary amine with carbon dioxide to obtainan instable carbamic acid derivative in an equilibrium reac-tion, which can be trapped by electrophiles to form carba-mates (Scheme 20). Numerous procedures describe thatelectrophiles, such as alkyl halogenides, tosylates, epoxides,alkynes, Michael acceptors and also alcohols, can beapplied.16a

In an early example, Katchalski et al. reported the reactionof ethylene diamine with carbon dioxide, followed by reactionwith diazomethane to obtain the dimethyl carbamate deriva-tive.138 In 1977, Yoshida and Inoue showed that carbamic acidderivatives can react with ethyl vinyl ether to obtain 1-ethoxyethyl carbamates.139 The same group reported also that epox-ides can react with carbamic acids to obtain monocarbamicesters from 1,2-diols.140 Later, Inoue and co-workers describeda conversion of epoxides by the application of aluminum por-phyrin as a catalyst for the fixation and activation of carbondioxide, followed by reaction with secondary amines to obtainN,N-dialkyl carbamates.141 In 1984, Yoshida et al. reported thesynthesis of carbamates by the reaction of amines, carbondioxide and alkyl halides.142 To promote this reaction, Cs2CO3is an ecient additive.143 Moreover, 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) is very useful for the fixation of carbondioxide as the DBUCO2 complex.

144 Instead of applying alkylhalides as electrophiles, tosylates react in the same manner.145

The direct use of alcohols is also possible by activation withdialkyl azodicarboxylates and triphenyl phosphine (Mitsunobureagent).146 Sakakura et al. reported the direct application ofalcohols in the presence of 2.0 mol% dibutyl tin oxide as acatalyst for the synthesis of carbamates.147 Moreover, orthoesters can react as electrophiles with carbamic acids to the cor-responding carbamates.148 Inesi et al. developed electrogener-ated systems for direct synthesis of carbamate esters fromcarbon dioxide, amines and alkyl halides.149

Furthermore, it has been shown that carbamic acidsderived from amines and carbon dioxide can also react withalkyne derivatives mediated by ruthenium catalysts. In 1987,Sasaki and Dixneuf reported the reaction of acetylenic alcohols

with secondary amines and carbon dioxide catalyzed with0.4 mol% Ru3(CO)12 to obtain 2-oxoalkyl N,N-diethylcarba-mates in moderate yields.150 Moreover, the same group men-tioned that numerous ruthenium complexes catalyze thesynthesis of vinyl carbamates from carbon dioxide, terminalalkynes and secondary amines.151 At the same time, Mitsudoet al. found that [Ru(COD)(COT)], in the presence of phos-phines, catalyzes the reaction of terminal alkynes with second-ary amines and carbon dioxide to obtain enol carbamates inyields up to 80%.152 The direct use of carbon dioxide for thesynthesis of carbamates as discussed above seems very promis-ing in view of a sustainable process to polyurethaneprecursors.

Modern procedures with promising aspects regarding sus-tainable and environmentally friendly entries to carbamatesemploy oximes and formamides. Cardona et al. developed thefirst catalytic oxidation procedure to convert aromatic aldox-imes in the presence of alcohols to the corresponding carba-mates (Scheme 21).153 The oxidation is performed withcatalytic quantities of methyl trioxorhenium (MTO) and anexcess of urea hydrogen peroxide as the oxidant. Long reactiontimes (48 days) at room temperature are required to obtainthe carbamates in moderate yields ranging from 35 to 72%.This method seems to be a promising approach for the conver-sion of aromatic aldehydes into carbamates making use ofurea hydrogen peroxide as a sustainable oxidation agent. Fur-thermore, Elghamry showed an unexpected synthesis of N-arylcarbamates by heating 2-oximinoacetoacetates as ketoximederivatives with anilines at 130 C for a few minutes undersolvent-free conditions.154 The N-aryl carbamates wereobtained in good yields between 70 and 75%.

Other interesting starting materials for the synthesis of car-bamates are formamides. In 1993, Kotachi et al. reported aruthenium-catalyzed synthesis of carbamates by dehydrogena-tion of formamides with alcohols (Scheme 22a).155 The reac-tion of substituted formanilides with various alcohols in thepresence of dierent ruthenium catalysts in refluxing mesity-lene aorded N-aryl carbamates in yields ranging from 23% to

Scheme 20 Carbamates by reaction of primary amines with carbon dioxideand electrophilic agents.

Scheme 21 Carbamates derived from oximes.



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82%. The conversion is strongly dependent on the applied for-manilides, alcohols, the kind of ruthenium catalyst as well asthe solvent. Fully substituted formanilides did not react underthese conditions. To convert fully substituted formamides intocarbamates, Reddy et al. recently demonstrated a copper-cata-lyzed oxidative CO coupling by direct CH bond activation offormamides (Scheme 22b).156 The coupling of N,N-dialkyl for-mamides with dierent -ketoesters and 2-carbonyl substi-tuted phenol derivatives aorded enol carbamates andsubstituted phenol carbamates. Formamides can be easily syn-thesized in high yields from the corresponding primary or sec-ondary amine by reaction with sustainable formic acid or itsesters.34 In combination with catalytic oxidation or reductivedehydrogenation processes, this access to carbamates meetsthe requirements of green chemistry.

4. Polyurethane synthesis

As mentioned in the introduction, polyurethanes (PUs) are aclass of polymers that contain urethane linkages and are

obtained by the reaction of diisocyanate with di- or multi-func-tional polyols. A large range of polyurethanes can be preparedwith specific physico-chemical properties, depending on thenature of the polyol and the diisocyanate. Thus, various poly-urethanes have been developed for more than 70 years basedon numerous possible structural variations. The polyol seg-ments, which nowadays are easily derived from diversebiomass feedstocks, are often biocompatible and biodegrad-able. The manifold approaches to obtain such renewablepolyols will not be discussed here, since they are described inmany review articles and are discussed there in detail.157

Despite the use of renewable polyols, toxic polyisocyanatesobtained from the reaction between an amine and phosgene,remain an important issue to be overcome in the chemistry ofpolyurethanes. The design of an ideal commercially applicablemethod, combining sustainability and high mechanical/phys-ical properties, is the main aim of ongoing research, both inacademia and industry. While aiming at sustainable poly-urethane synthesis, alternative and environmentally friendlyroutes that either use non-isocyanate pathways and/or employrenewable resources derived diisocyanates at best synthesizedby the above discussed sustainable methods are certainlyrequired. The most interesting developments on isocyanate-free and non-phosgene routes to polyurethane can be summar-ized as shown in Scheme 23.

An alternative, green and isocyanate-free route is the ringopening of cyclic carbonates with amines yielding urethanelinkages. The step-growth polyaddition of bifunctional five-membered cyclic carbonates and diamines, reported as earlyas 1957158 and recently reviewed by Mller et al.159

(Scheme 23a), is an example of this type of reaction. The result-ing product contains additional hydroxyl groups and appearsas non-porous polyurethane with a pore-free surface sinceneither volatile nor non-volatile by-products are produced bythis reaction. One of the advantages of this method is that thedicyclocarbonates can be obtained in a sustainable manner, asalso discussed above.160 For instance, recent research wasfocused on the synthesis of the dicyclocarbonate reactantsdirectly from renewable resources such as glycerol.161 Theecient and atom economic thiolene coupling of allyl-cyclo-carbonate with a dithiol yielded the corresponding dicarbo-nate precursor for polyurethanes. Additionally, the resultingnon-isocyanate polyurethanes show better thermal stabilitythan conventional polyurethanes due to the absence of ther-mally unstable biurets and allophanates.162 Moreover, thelower toxicity, the biodegradability of the cyclic carbonates,and their high reactivity towards amines make them veryattractive in the field of isocyanate-free PUs, as has beenreviewed by Guan et al.163 Since the polymers obtained possessmolar masses in the range of 1020 kDa, this approach isusually employed in the synthesis of prepolymers163 or ther-mosetting coatings.164 Recently, this method was modified byGuillaume and co-workers in order to be able to synthesizehigher molecular weight polymers.165 Their strategy was basedon the synthesis of ,-bis(cyclic carbonate) telechelic poly-carbonate precursors by ring-opening polymerization of

Scheme 22 Carbamates derived from formamide couplings with alcohols,enols and phenols.



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trimethylene carbonate using a glycerol carbonate as a chaintransfer agent, followed by the ring-opening polyaddition ofthe terminal cyclic carbonate with a diamine. Thus, poly(car-bonate-hydroxyurethanes) with dierent molecular weightscould be achieved.

The related reaction of ethylene carbonate with diaminesand diols can also be considered as a possibility for the prepa-ration of linear polyurethanes in a phosgene-free way(Scheme 23b).166 Thus, a transcarbamoylation of dihydroxyur-ethanes, obtained via the nucleophilic addition of diamines toethylene carbonate, with diols using Bu2SnO as a catalyst wasreported by Rokicki et al.166 Furthermore, ethylene carbonatewas used in the synthesis of urethane containing diols orhydroxy acids using amino alcohols or amino acids, respect-ively, which in turn were enzymatically polymerized.167 Alter-natively, aliphatic polyurethanes were obtained in high yieldsvia the self-polycondensation of dihydroxyurethanes withoutusing diols.168 The addition of a diamine to ethylene carbon-ate gives the respective urethane derivatives with hydroxylethylcarbamates (hydroxyurethanes). A two-step polycondensation,consisting of polycondensation performed under a nitrogenatmosphere followed by that under reduced pressure, waseective in yielding polymers with molecular weights up to10 kDa.

Another method for polyurethane preparation is a chaingrowth polymerisation procedure employing ring-opening

polymerisation (ROP) of aliphatic cyclic urethanes or diur-ethanes (Scheme 23c).169 It is worth mentioning that the five-and six-membered cyclic urethanes can be prepared in a sus-tainable manner by reactions of alkylene diamines or aminoalcohols with reagents, such as dialkyl carbonates,170 or withpressurized carbon dioxide in the absence catalysts.171

However, the drawback of this approach is that not all of thecyclic urethanes are easy to synthesise, and the ones that areeasily formed are thermodynamically stable and thus less reac-tive for polymerization.

The replacement of phosgene by CO2, which is abundant,renewable and environmentally friendly,172 is another emer-ging non-isocyanate route. Aziridines, nitrogen analogues ofepoxides, can react in supercritical CO2 (scCO2) to give cyclicurethanes, and polymers consisting of urethane and amineunits (Scheme 23d).173 Thus, substituted aziridines were co-polymerized with carbon dioxide to give random copolymers ofpolyurethane in the presence or absence of catalysts. This doesnot only avoid phosgene and isocyanates, but also highlightsthe potential applications of CO2 in the synthesis of indust-rially useful chemicals as the alternative carbon source. Ihataet al. reported the reaction of 2-methylaziridine and scCO2 togive a polymer with a high content of urethane units, whichexhibited a thermoresponsive behaviour (i.e. lower criticalsolution temperature, LCST) in water, undergoing a sharpphase change over a broad range of 4185 C.174 It was

Scheme 23 Polyurethane synthesis via isocyanate- and phosgene-free routes: (a) the polyaddition of bifunctional cyclic carbonate and diamine; (b) the polycon-densation of ethylene carbonate, diamines and diols; (c) the cationic ring-opening polymerization of cyclic urethane; (d) the copolymerization of substituted aziri-dines with carbon dioxide.



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observed that an increased CO2 pressure can be a reason for asignificant decrease in the lower critical solution temperatureof the polymers, possibly due to an increase in their urethanecontents.175

Scheme 24 Synthesis of saturated (a) and unsaturated (b) diisocyanate fromoleic acid.180 Scheme 25 Thiol-ene additions onto double bonds as an approach for the

synthesis of fatty acid based diisocyanates.181

Scheme 26 New AB-type condensable monomers from various fatty acid derivatives for bio-based polyurethanes synthesis.182



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The cycloaddition of CO2 to epoxides, as discussed insection 3.2, produces cyclic carbonates and is one of the fewindustrial processes that utilize CO2 as a raw material. Withinthis concept, the reaction of epoxidized soybean oil withcarbon dioxide has been studied intensively and someresearch groups have synthesized PU networks by reacting thiscarbonated soybean oil with dierent diamines adopting themethod (a) in Scheme 23.176 Moreover, a wide variety of cross-linked terpene-based renewable polyurethanes were syn-thesized by curing novel limonene dicarbonates with polyfunc-tional amines, such as citric aminoamides.177 The limonenedicarbonates were obtained from limonene dioxide with achemical fixation of 34 wt% CO2. Additionally, recently a newbio-based non-isocyanate urethane was obtained by the reac-tion of a cyclic carbonate synthesized from a modified linseedoil and an alkylated phenolic polyamine from cashew nut shellliquid.178 On the other hand, only a few reports are focused onthe carbonation of vegetable-based precursors with the objec-tive to prepare linear non-isocyanate polyurethanes. Forinstance, Cramail and co-workers have synthesized linear non-isocyanate polyurethanes with moderate molar masses by poly-addition of various diamines with linear bis-carbonates,obtained by the carbonation of bis-epoxidized fatty acid di-esters from methyl oleate in scCO2.

179

As a current trend, fully bio-based thermoplastic poly-urethanes with comparable properties to those derived fromcommercially available diisocyanates have been synthesizedemploying diisocyanates obtained from azelaic and oleic acidvia the Curtius rearrangement (see also section 2.1), thusavoiding the direct exposure to isocyanates.180 The self-meta-thesis of oleic acid yielded 1,18-octadec-9-enedioic acid, whichwas converted into the necessary diisocyanate derivative bythe aforementioned Curtius rearrangement (Scheme 24b).

Scheme 27 Biobased polyureas synthesis via the metal-free catalysed isocya-nate-free route.184

Scheme 28 Synthesis of (R)-(+)- and (S)-()-limonene based polyurethanes with various renewable diols in a sustainable manner.88

Scheme 29 (A) Nucleophilic amine-thiol-ene conjugation: aminolysis of the thiolactone ring (a), followed by thia-Michael addition (b). EWG = electron-withdraw-ing group. (B) Representative AB-monomer, containing a thiolactone and an acrylate group as reactive entities and its subsequent in situ polymerization to yield iso-cyanate-free functionalized polyurethanes.186



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A phosgene-free method based on a diacyl hydrazide inter-mediate was employed in the transformation of fatty acidderivatives into diisocyanates (Scheme 25), which subsequentlywere reacted with commercially available and fatty acid baseddiols to obtain partially or fully bio-based polyurethanes withthermo-mechanical properties close to polyethylene.181

Cramail and co-workers have recently discussed the syn-thesis of isocyanate-free, fully-biobased polyurethanes via aone-pot AB-type polyaddition method, either by self-

condensation or transcarbamoylation.182 Fatty acid derivatives,namely, ricinoleic acid, methyl oleate and methyl 10-un-decenoate, were transformed to AB-type monomers containinghydroxyl-acyl azide or hydroxyl-methyl urethane functionalities(Scheme 26).

A thiolene addition onto the double bonds of eithermethyl oleate or methyl 10-undecenoate was the key step toobtain more reactive AB-type monomers (Scheme 26B). Thepolyaddition of these AB-type monomers, with or without a

Scheme 30 Overview of sustainable synthesis procedures to polyurethane precursors reported in this review.



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catalyst, was carried out either in bulk or in solution to yieldthe desired polyurethanes. However, rather low molar masspolyurethanes were obtained for AB-monomers presented inScheme 26A, especially for the carbamate derivative, due to thepresence of secondary hydroxyl functions and the formation ofcyclic structures. Polyurethanes based on methyl oleate andricinoleic acid were of amorphous nature due to the pendentalkyl chains, but displayed well-defined glass transition temp-eratures, while polyurethanes from AB-type monomers basedon methyl 10-undecenoate displayed semi-crystalline behav-iour with well-defined melting transitions.

As a sustainable starting material for polyurethanes syn-thesis, dicarbamates can also be employed. Dicarbamates canbe synthesized from diamines by reacting them with dicarbo-nates, such as dimethyl carbonate (see also section 3.2) in aneco-friendly manner. For instance, Deepa et al. reported thatpolyurethanes can be obtained by melt polycondensation ofdicarbamates with diols (or diamines) in the presence of ametal based catalyst (titanium tetrabutoxide).183 It wasobserved that only in the presence of the catalyst high conver-sions of 97% were achieved. With the aim to propose a moresustainable method for isocyanate-free polyurethanes/poly-ureas, Koning and co-workers adopted the aforementionedprocedure (Scheme 27).184 In the corresponding work, TBDwas applied for the first time to prepare polyureas, avoidingthe use of metal catalysts (see also section 3.2).

Along the same idea, Meier and co-workers synthesizedterpene based dicarbamates and studied their behavior inpolycondensation with various renewable diols for the syn-thesis of renewable polyurethanes with molecular weights upto 12.6 kDa in a non-isocyanate and phosgene-free manner(Scheme 28).88 The addition of cysteamine hydrochloride to(R)-(+)- and (S)-()-limonene was described as an eective wayto obtain the amine functionalized renewable monomers,which were easily transformed into dicarbamates via a phos-gene-free route employing dimethyl carbonate and TBD as acatalyst.

Other renewable polyurethanes containing carbohydrate-derived units have also been prepared. For instance, relativelyhigh molecular weight, linear, stereoregular and opticallyactive polyurethanes were synthesized by polymerization of aconveniently substituted 1-amino-1-deoxyalditol prepared fromD-galactono-1,4-lactone.185

Very recently, nucleophilic aminethiolene conjugation(Scheme 29) was developed to enable the one-pot, additive-freesynthesis of isocyanate-free polyurethanes.186 According to thisprocedure, AB-type monomers containing both an acrylateand a thiolactone unit are undergoing aminolysis and theresulting intermediate thiolacrylate reacts in situ via Michaeladdition to yield polyurethanes with a large structural variety.

5. Conclusions

As mentioned in the introduction, a major concern in poly-urethane chemistry is the high toxicity of the precursors used,

namely phosgene and isocyanate. Thus, research both inindustry and academia is facing increasing demand for envir-onmentally benign and safe processes for the synthesis ofthese compounds. Scheme 30 summarized the herein dis-cussed sustainable strategies for phosgene-free polyurethanesynthesis. Obviously, carbon dioxide and monoxide as well asorganic carbonates play an important role in these processes.In principle, the production of these sustainably preparedpolyurethane precursors is technologically feasible and theseroutes are capable of substituting hazardous phosgene.However, more research and development is certainly necess-ary before the transition to a greener production of isocya-nates, carbamates and thereof derived polyurethanes becomesreality. With this review we thus aimed to provide a basis forthe discussion and further development of sustainable andgreen synthesis routes to carbamates, isocyanates and poly-urethanes and of course illustrate the very recent progress inthis field.

Notes and references

1 O. Bayer, Angew. Chem., 1947, A59, 257272.2 R. J. Zdrahala and I. J. Zdrahala, J. Biomater. Appl., 1999,14, 6790.

3 T. Thomson, in Polyurethanes as Specialty Chemicals:Principles and Applications, CRC Press, Boca Raton, FL,2004.

4 (a) G. Woods, The ICI Polyurethanes Book, Wiley, New-York,2nd edn, 1990; (b) V. Sharma and P. P. Kundu, Prog.Polym. Sci., 2008, 33, 11991215; (c) Z. S. Petrovic, Polym.Rev., 2008, 48, 109155.

5 Plastics the Facts 2012, Plastics Europe, http://www.plas-ticseurope.org/Document/plastics-the-facts-2012.aspx?FolID=2 (accessed on 25.11.2012).

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15 The Bhopal disaster: (a) I. Labunska, A. Stephenson,K. Brigden, R. Stringer, D. Santillo and P. A. Johnston, TheBhopal Legacy: Toxic contaminants at the former UnionCarbide factory site, Bhopal, India: 15 years after the Bhopalaccident, Technical Note 04/99, Greenpeace Research Labo-ratories, Department of Biological Sciences, Universityof Exeter, Exeter UK, November 1999; (b) I. Eckerman,Chemical Industry and Public Health: Bhopal as an Example,Essay in Master of Public Health, Nordic School of PublicHealth, Gteborg, Sweden, 2001; (c) S. Sriramachari, Curr.Sci., 2004, 86, 905920; (d) I. Eckerman, The Bhopal Dis-aster 1984 working conditions and the role of the tradeunions, Asian-Pacific Newslett. Occup. Health Safety, 2006,13, 4850.

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