ring-opening of epoxides in water - stuba.skszolcsanyi/education/files/chemia... · 2012-10-23 ·...

MICROREVIEW

DOI: 10.1002/ejoc.201001693

Ring-Opening of Epoxides in Water

Simona Bonollo,[a] Daniela Lanari,[a] and Luigi Vaccaro*[a]

Keywords: Synthetic methods / Green chemistry / Water / Epoxides / Nucleophilic addition

Epoxides and water mix well together. The use of water rep-resents an intriguing alternative to organic solvents, not onlyfor developing green processes but also for exploring a tot-ally new and peculiar reactivity. Epoxides react very ef-ficiently in water with several nucleophiles, and several ex-

1. IntroductionThe synthetic utility of epoxides resides mainly in their

versatile reactivity[1] and in the availability of establishedand simple methods for their preparation as racemates orin optically enriched form.[2] Therefore it is not surprisingthe extent of literature covering the chemistry of this classof compounds.

[a] Laboratory of Green Synthetic Organic Chemistry,Dipartimento di Chimica Università di Perugia,8, Via Elce di Sotto, 06123 Perugia, ItalyFax: +39-075-5855560E-mail: [email protected]

Simona Bonollo studied Chemistry at the University of Firenze and graduated in 2003. She then moved to the Universityof Perugia where obtained her Ph.D. in 2008, working on ring opening of epoxides in water. She is currently carrying outpostdoctoral research on stereoselective processes in water and novel solid catalysts for the C-H activation processes.

Daniela Lanari obtained her MS degree in Organic Chemistry at the University of Perugia in 2001. She received herPhD degree from the same institution in 2005 working on the synthesis and reactivity of [2.2] paracyclophane derivatives.In the same year she joined the research group of James Fraser Stoddart at University of California Los Angeles (UCLA)where she worked as postdoctoral fellow on the template synthesis of mechanically-interlocked molecules. In 2010 she wasappointed Lecturer at University of Perugia and her current research interests include the design and synthesis of solidcatalysts for the development of synthetic procedures in unconventional media.

Luigi Vaccaro obtained his Laurea in Chemistry at the University of Naples – Federico II, then he moved to the Universityof Perugia where he earned a PhD in Chemical Sciences studying catalytic methods for organic transformations in water.Presently, he is an Associate Professor at the University of Perugia leading Green S.O.C. group. His research is currentlyfocused on the use of novel catalytic systems in ecofriendly reaction media and on the definition of automated continuous-flow reactors with the aim of minimizing waste in the preparation of target molecules and materials.

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amples in the literature report that the use of water as reac-tion medium is essential for realizing processes that cannotbe performed alternatively in other reaction media. This con-tribution is devoted to highlight the unique role of water inthe ring-opening reactions of epoxides.

Nucleophilic addition to epoxides plays a pivotal role inthe stereoselective preparation of 1,2-disubstituted productsand it has been certainly the most thoroughly studied reac-tion of these compounds and for which different promotersand conditions have been proposed.[1]

It is generally accepted that the epoxide ring-opening re-action proceeds under neutral or basic conditions via SN2mechanism giving inversion at the attacked C atom (gen-erally the less substituted one, when no additional electroniceffects are operating, i.e. normal attack) and furnishing 1,2-disubstituted products with a trans or anti relationship ofthe nucleophile to the oxygen leaving group. Under acidic

S. Bonollo, D. Lanari, L. VaccaroMICROREVIEWconditions, a borderline SN2 mechanism has been evokedto justify the electronic pull on the oxygen by an acid. Fi-nally, SNi (ion pair), carbocationic SN1 and double inver-sion mechanisms have been proposed to take account forthose reactions that proceed with retention of configurationwith formation of a carbocation species.[1g,1j]

Among all the possible reaction media, water has provedto be a chemically efficient option for various organic trans-formations and sometimes has allowed to efficiently realizeprocesses that cannot be performed in organic media.[3] Forthis reason water can be also considered a valid alternativeto classic organic medium for realizing green innovativeprocesses. In several cases, by exploiting the unique proper-ties of water it has been possible to realize more selectiveand efficient processes than those performed in organic me-dia. In addition, reactions of water-insoluble organic com-pounds that take place in aqueous suspensions (“on water”)have recently received a great deal of attention because oftheir high efficiency and straightforward synthetic proto-cols.[3e–3f] In several cases, that will be discussed herein, pro-tocols for the ring opening of epoxides using water as reac-tion medium, have resulted to be the most efficient availableand promising results have also been obtained in asymmet-ric transformations.

Based on literature examples water, thanks to its peculiarphysical properties and structure (e.g. pH control, H-bond-ing capabilities, excellent activation of ionic nucleophiles),[4]

may represent the ideal reaction medium for achieving thebest efficiency in synthetic processes based on epoxides.

This review will deal with all the reactions of epoxideswith nucleophiles in water highlighting those cases wherethe use of water has played a crucial role for realizing highlyefficient processes, mainly focusing on catalytic protocols.The material selected will be presented according to thetype of nucleophile involved in the process and is not in-tended to be exhaustive of the field. The endo-regioselectiveepoxide-opening cyclizations in water have been recentlycovered in an excellent tutorial review by Jamison et al., andtherefore this topic will not be treated herein.[1d] Enzyme-catalyzed processes have not been discussed herein.[1e,2a,2f]

N-Nucleophiles

Nitrogen-containing nucleophiles have been largely em-ployed in the ring-opening reactions of epoxides to directlyaccess 1,2-amino alcohol moiety[1,5] or its precursors, mostcommonly 1,2-azido alcohols.[1,6]

The efficiency of the available procedures for the reactionof epoxides with amines strongly depends on the type ofamine used (aliphatic or aromatic). Typical limitations arerelated to the use of high temperatures, large amounts ofcatalysts, hazardous solvents, formation of side products(mainly bis adducts).

Surprisingly, water has been scarcely used as reaction me-dium for this transformation, although a kinetic study andan investigation on the distribution of possible products,carried out in water and in the absence of any catalyst, al-ready appeared more than thirty years ago.[7a,7b]

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The beneficial effect of water on this reaction is well rep-resented by the data reported in 2003 by Hou et al. in theirstudy on the use of tributylphosphane as catalyst for thering-opening of epoxides in water.[8] For example in the re-action of cyclohexene oxide (1) with aniline (2a) or benzyl-amine (2b) besides the catalytic effect of tributylphosphane(10 mol-%) that allowed to obtain most satisfactory yieldsof 3, the presence of water is essential for the success of theprocess, in fact the same reactions performed in MeCN gaveonly traces of products 3 (Scheme 1).[8] This catalytic sys-tem revealed to be efficient in water also when phenol wasused as nucleophile, while in the case of thiols the yieldswere lower respect to those obtained in organic solvent.[8]

Scheme 1. Beneficial effect of water in the nBu3P-catalyzed ami-nolysis of cyclohexene oxide (1) in water.[8]

Later, in 2005 aminolysis of epoxides with arylamines hasbeen efficiently performed in water in the presence of 1 mol-% of the base 1,4-diazabicyclo[2.2.2]octane (DABCO) ortriethylamine (Scheme 2).[9] With this protocol also benzyl-amine gave satisfactory results but isopropylamine gave noreaction at all. Also in this case other nucleophiles wereused and very good results were obtained with aromaticthiols, while aliphatic thiols were scarcely reactive.[9]

Scheme 2. Base-catalyzed (1 mol-%) aminolysis of epoxides inwater.[9]

In apparent contrast, the same year Azizi and Saidi re-ported excellent results on the aminolysis of aliphatic epox-ides with aliphatic amines in water without any additionalcatalyst.[10] In this paper it has also been reported that ani-line and p-isopropylaniline reacted only with styrene oxide,while other arylamines and epoxides gave only discouragingresults (Scheme 3).

Scheme 3. Uncatalyzed aminolysis of epoxides in water.[10]


Accordingly, Rao et al. reported that β-cyclodextrin (β-CD) is an efficient catalyst for the aminolysis of aromaticamines with styrene oxide derivatives 4 and highly reactiveglycidyl ethers 5 in diluted water (15 mL per 1.0 mmol ofreactants) at room temperature. The Authors highlightedthat the same reactions do not proceed in the absence of β-CD or in the presence of α-cyclodextrin (Scheme 4).[11]

Scheme 4. Aminolysis of glycidol derivatives in water promoted byβ-cyclodextrin (β-CD).[11]

Our group tried to rationalize these data according toour previous experiences where the pH value of the aqueousmedium played a crucial role in determining the efficiencyof the nucleophilic ring-opening process of epoxides inwater.[12] Therefore, it was undertaken a study devoted toevaluate the dependence on the pH of a variety of reactionsof aliphatic and aromatic amines with several epoxides.[13]

As representative example it was found that the reactionof cyclohexene oxide (1) with almost equimolar amount ofaniline (2a) was very slow at pH lower than 7.0 with thecompetitive formation of the corresponding trans-1,2-cyclo-hexandiol by-product (coming from the attack of water tothe epoxide). As expected, under basic conditions (pH 8–10) the reaction was faster and interestingly the pH re-sulting from the simple mixing of the reactants was 8.30 at30 °C, therefore sufficiently basic to warrant a 90% conver-sion to 3a. The best conversion (95 %) was achieved by rais-ing the temperature at 60 °C (resulting pH was 8.0) or at30 °C but raising the pH to 10. By extending the study toother amines, it was generally concluded that aliphaticamines are sufficiently basic to define an adequate pH con-dition for a successful uncatalyzed aminolysis. In the caseof substituted poorly nucleophilic anilines the pH resultingfrom the mixing of the reactants was lower than 7.0 andtherefore reactions were slow and significant amount of thecorresponding diols were formed. By raising the pH of thereaction mixture to 10 (aq. NaOH addition) satisfactoryyields were obtained also in these cases (Scheme 5).

The Authors remarked in some cases the unavoidableand spontaneous formation of a bis-product coming fromthe attack of the ring-opened product to another moleculeof epoxide, which can be reduced only by running the reac-tions with an excess of amine.

As expected the regioselectivity of the ring openings fav-oured the product at the less hindered carbon (β-attack)except in the case of styrene oxides where α-attack was pre-ferred and in the case of β-CD-promoted protocol where acomplete α-regioselectivity was observed.

Several other protocols have been recently pro-posed,[14,15] where for example the addition of variousamines to epoxides has been promoted by ultrasounds[14a]

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Scheme 5. pH-influence on the aminolysis of representative epoxide1 in water.[13]

or monodispersed silica nanoparticles,[14b] or catalyzed byerbium(III) triflate,[15a] zirconium dodecyl sulfate[15b] oraluminum dodecyl sulfate trihydrate.[15c]

Recently, water has proved to be an efficient reaction me-dium for the enantioselective ring opening of epoxides byamines, allowing to reach better results than those obtainedin organic medium. Kobayashi et al. employed Lewis acid-surfactant-combined catalysts (LASCs) to promote severalorganic transformations.[16] These catalytic systems furnishboth the Lewis acidity needed for the activation of a basiccentre and the hydrophobic environment that favours theapproaching of the reactants.[16,17]

In 2005 Kobayashi et al. used LASCs based on ScIII [17]

and BiIII [18] in combination with a chiral bipyridine 9 asligand, in the desymmetrization of highly hydrophobic ep-oxides 8 by aromatic amines 2 in water.[17,18]

The best results were obtained by using Scandium(III)dodecylsulfate [Sc(DS)3] (1 mol-%), (S,S)-6,6�-bis(1-hy-droxy-2,2-dimethylpropyl)-2,2�-bipyridine (9) (1.2 mol-%)and equimolar amounts of reactants (Scheme 6). Althoughthere are no mechanistic insights on the precise role ofwater in this process, this reaction medium allowed to reachbetter results than those obtained by using an organic sol-vent[19] in terms of either enantioselectivity and of isolatedyields of products.

Scheme 6. ScIII LASCs for the catalytic enantioselective aminolysisof epoxides in water.[17]

S. Bonollo, D. Lanari, L. VaccaroMICROREVIEWThe influence of the type of LASC catalyst was tested in

the case of BiIII, where different catalysts were examined byvarying the surfactant. The best results were obtained byusing Bi(OTf)3 (5 mol-%), sodium dodecylbenzene sulfate(SDBS) (20 mol-%) and bipyridine 9 (6 mol-%) (Scheme 7).In this case the LASC system was generated in situ. Otheranionic surfactants like sodium dodecylsulfate (SDS) andsodium dioctyl sulfosuccinate (AOT) gave the desired prod-ucts 10 in low yields. In this study only cis-stilbene oxideswere considered.[18]

Scheme 7. BiIII LASCs for the catalytic enantioselective aminolysisof epoxides in water.[18]

Other LASCs have been used for the same process. Inparticular, in 2008, we have reported that transition metalcatalysts ZnII, CuII, NiII, and CoII, combined with SDS, arealso able to efficiently promote the desymmetrization of cis-stilbene oxide (8a) with aniline (2a) (Scheme 8, Ar = Ph) inwater by using (R,R)-bipyridine 11 (Scheme 8).[20] Satisfac-tory results were obtained in the case of ZnII and CuII cata-lysts. It is noteworthy that the use of (R,R)-bipyridine 11with these transition metals allowed to obtain the sameproduct (+)-(S,S)-10 coming from the reaction of Sc(DS)3/(S,S)-bipyridine 9 (Schemes 6 and 8).

Scheme 8. Zn(OTf)2/SDS/11 catalytic system for the enantioselec-tive aminolysis of epoxides in water.[20]

This enantioselective outcome was confirmed by Kobay-ashi et al. who reported the use of ZnII and CuII undecansu-lfonates in combination with (S,S)-bipyridine 9 in the de-symmetrization of epoxides by amines in water.[21] The dif-ferent enantioselection was also justified by accountingsome significant differences between the crystal structuresof the two complexes of (S,S)-9 with CuBr2 and with ScBr3.Kobayashi et al. also confirmed the importance to haveboth hydroxy groups in the structure of 9 for achieving thebest enantioselectivity, in fact by protecting one or bothhydroxys as a methyl ether the enantiomeric excessdropped.[19,21]


In our contribution,[20] other parameters were consideredand by varying temperature and concentration it was foundthat the catalytic system Zn(OTf)2/SDS/11, was most effec-tive at 4 °C and at 0.5 m. This result was extended to a vari-ety of epoxides 8 and anilines 2 including very small epox-ides such as cyclohexene oxide (1), cyclopentene oxide and2-butene oxide, for which rare and generally very poor re-sults have been reported, and none of them obtained withwater as reaction medium. The enantiomeric excess of 85%achieved in the case of cyclohexene oxide (1) with aniline(2a) is a very good and representative result (Scheme 8).

Kobayashi et al. also reported that Sc(DS)3 is a goodcatalyst for the enantioselective ring-opening of cis-stilbeneoxide (8a) by N-nucleophile benzotriazole (12)(Scheme 9).[22]

Scheme 9. Sc(DS)3/(S,S)-9 catalyzed enantioselective addition ofbenzotriazole (12) to cis-stilbene (8a) in water.[22]

The use of catalytic systems formed by the combinationof a metal salt with SDS has been applied also in otherreactions of epoxides with N- and other nucleophiles. Inparticular, the system made by Ce(OTf)4 (10 mol-%) andSDS (30 mol-%) has been used[23] for promoting the reac-tions of a variety of epoxides with nitrite, nitrate, thiocyan-ate, azido ions besides cyanide, chloride and bromide ions(Scheme 10). Other surfactants were also considered butSDS gave the best results.[23]

Scheme 10. Ce(OTf)4/SDS system in the ring-opening of epoxidesin water.[23]

Azidolysis of epoxides has been deeply studied as a validalternative for the preparation of 1,2-azido alcohols that areprimarily 1,2-amino alcohol precursors.[1,6]

In the past decades, the generally reported classical pro-tocol for preparing 1,2-azido alcohols used NaN3 as reagentand NH4Cl as a coordinating salt in alcohol-water at 65–80 °C. Under these conditions, azidolysis is generally car-ried out over a long reaction time (12–48 h) and the azido-hydrin is often accompanied by isomerization, epimeri-zation and rearrangement products.[1,24] Azidolysis of un-symmetrical epoxides occurs with the attack of the azide


ion at the less substituted C atom, except for the aryl-substi-tuted epoxides. Attempts to reverse the regioselectivity havehad little success[6a–6d] with the exception of the protocolthat uses Et3Al/HN3 in dry toluene at –70 °C.[6e]

In 1999,[12o] we have reported the first use of water asreaction medium for the reactions of epoxides and sodiumazide. The possibility of adjusting the pH of the aqueousmedium, which is a peculiar property of water, has allowedto significantly influence both the rate and regioselectivityof the process.

Generally under basic condition (pH 9.5) at 30 °C theazido ion slowly attacks the less substituted β-carbonthrough an expected SN2 mechanism. At acidic pH (4.2)the reaction is much faster likely due to the protonation ofthe oxygen of the epoxide ring. Under these conditions theazido ion showed an increase preference for attacking themore substituted α-carbon trough an SN2 borderline mecha-nism (Scheme 11).[12o]

Scheme 11. pH-controlled regioselectivity of the azidolysis of repre-sentative epoxide 13 in water.[12o]

Recently, polyethylene glycol supported on silica gel[25a]

or Dowex resin,[25b] have been used as solid recoverable cat-alysts for the reaction of epoxides with sodium azide inwater under reflux.

The stereoselectivity of epoxide ring openings is stronglydependant on the substituents to the oxirane moiety andthe presence of nucleophilic groups directly attached to thering can promote the formation of retention products lead-ing to non-stereoselective processes. Representative exam-ples are given by styrene oxides and carbonyl-substitutedepoxides.[1j] Azidolysis of α,β-epoxycarboxylic acids 14 andesters is a well-studied process that if regio- and stereoselec-tive, opens a direct access route to α-hydroxy β-amino acids,a key moiety of several pharmaceutical target com-pounds.[26] The classical azidolysis protocol that uses NaN3

in alcohol or alcohol/water (8:1) generally furnishes a mix-ture of products including the formation of some retentionproducts.[27a,27b] To overcome this problem, the use of largeamounts (150–500 mol-%) of various Lewis acids in an or-ganic reaction medium has been adopted.[27]

The use of water as reaction medium and a careful ad-justment of the pH has allowed to define very efficient pro-tocols for the completely β-regio- and anti-stereoselectiveazidolysis of a variety of α,β-epoxycarboxylic acids 14 byemploying for the first time catalytic amounts (1 or 10 mol-%) of a metal salt such as Cu(NO3)2, InCl3, AlCl3 at pH 4or 7 according to the catalyst used.[12e,12j–12n]


By controlling the pH, the complete stereoselectivity ofthe ring-opening process was achieved and several β-azido-α-hydroxycarboxylic acids 15 were prepared in high yields(Scheme 12).

Scheme 12. Metal-catalyzed regio- and stereoselective azidolysis ofα,β-epoxycarboxylic acids in water.[12e,12j–12n]

These data are the result of a study where the efficiencyof the metal salts employed as catalysts has been comparedin water and in organic media proving that the role of waterand the regulation of its pH, are crucial for realizing thisprocess satisfactorily, in fact only in water a Lewis acid canbe used in a catalytic amount while in organic media largeexcess is needed. Both the catalyst used and water used asreaction medium have been recovered and reused in furtherruns without observing any decrease in the efficiency of theprocess, contributing also to the environmental efficiency ofthese procedures.

By coupling these novel protocols with an azido groupreduction catalyzed by the same metal salts,[28] it has beenpossible to define the first one-pot synthesis of α-hydroxyβ-amino acids (norstatines) starting from the correspondingα,β-epoxycarboxylic acids avoiding at all the use of organicsolvent (Scheme 13).[12e] Among all the metal catalyststested (CuII, CoII, AlIII, and InIII), CuII salts proved to bethe most efficient for this one-pot protocol. Also in this casethe catalyst used could be completely recovered and reusedefficiently.

Scheme 13. CuII-catalyzed one-pot synthesis of norstatines inwater.

A similar approach has been used to realize very efficientprotocols for the Lewis acid-catalyzed ring-opening of ep-oxides by thiols and halides.[12a,12i,12l] In all cases the carefulcontrol of the pH has allowed to reach the highest efficiencyand realize processes that in organic media cannot be suc-cessfully performed.

S. Bonollo, D. Lanari, L. VaccaroMICROREVIEWKinetic resolution of epoxides in water has also been re-

ported using β-CD as promoter.[29] More interesting resultswere reported by Rao et al. in the case of the reactions ofglycidols 5 with trimethylsilyl azide (TMSN3) and isoprop-ylamine (Scheme 14). The amount of β-CD influences theenatioselectivity outcome. Best results have been obtainedby using 1.33 or 2.0 equiv. of β-CD while the use of substoi-chiometric amounts led to very low ee values.[29a]

Scheme 14. β-CD-promoted kinetic resolution of glycidols 5 inwater.[29a]

The reaction of sodium azide with epoxides at room tem-perature has been recently proposed by using Zr(DS)4 ascatalyst.[30]

O-Nucleophiles

Among the different nucleophiles, water, alcohols andphenols react very poorly with epoxides.[31] Anyway this re-action is very important because is the most direct accessroute for the preparation of 1,2-diols, β-alkyloxy and -aryl-oxy alcohols.

Jafarpour et al. reported the use of Zr(DS)4 (5 mol-%) asa recoverable and reusable LASC for the ring opening ofepoxides with water under reflux. The same catalyst waseffective in the reactions with alcohols but the transforma-tions were conducted in the same alcohol as reaction me-dium.[30]

As a unique example Sc(DS)3 was also employed asLASC for the desymmetrization of cis-stilbene oxide (8a)with benzyl alcohol in water proceeding with 34% yield and86% ee.[22]

With the idea of creating a localized area where hydro-phobic substrates are concentrated in order to make reac-tions proceed more efficiently (micellar catalysis), We-berskirch et al. have reported an alternative approach toLASC for the hydrolytic kinetic resolution (HKR) of epox-ides in water.[32]

The authors prepared core-shell type nanoreactors (par-ticle radius in the range of 10–12 nm) where a hydrophobiccore furnishes the favorable environment for the catalyticcentre, that is a CoIII-(salen) complex [H2salen = N,N�-bis(salicylidene)ethylenediamine], and the substrate epox-


ide, while a hydrophilic shell warrants the solubility in waterof the whole nanoreactor. CoIII(salen) unit was covalentlyattached to an amphiphilic copolymer able to create micel-lar aggregates in water to form complex 18 (Scheme 15).The formation of micellar aggregrates of 18 in water at a0.18–0.39 mmol/L dilution was studied with TEM analysisand dynamic light scattering (DLS).

Scheme 15. Hydrolytic kinetic resolution (HKR) of epoxides inwater catalyzed by 18.[32]

The efficiency of 18 was studied in the case of aromaticterminal epoxides that usually need large amounts ofJacobsen catalyst and long reaction times under homogen-eous conditions.[2b] The results obtained were comparableto those reached under homogeneous conditions. Theauthors successfully generated a nanoreactor with high lo-cal concentration of the catalyst in the hydrophobic core,while the amount of water that could penetrate into themicelle was very little, which is crucial for achieving highyields and stereoselectivity. The catalyst was recovered andreused in four consecutive runs without decrease of its effi-ciency.

The use of a polymeric CoIII-(salen) complex was alsoreported by Zheng et al. in organic solvent or under sol-vent-free conditions. These Authors also showed that betterees were achieved for the preparation of diols when the re-covered catalyst was used with water as reaction mediumand as reactant.[33]

Feringa et al. reported for the first time the use of DNAas a chiral scaffold to develop asymmetric catalysis.[34] Thisresearch group developed a protocol for the HKR of aseries of 2-pyridyloxiranes 19 in water, buffered at pH 6.5.By using a DNA-bound CuII complex 20 where CuII isbounded to DNA via an achiral ligand (21–23)(Scheme 16).[35]

The efforts of this research group show that DNA canbe used as a viable source of chirality for the HKR reaching63 % ee of the recovered epoxide [selectivity (s) = 2.7]. It ishighly interesting that DNA-based catalysts can be used insuch HKR in water but obvious limitations are evident forsynthetic application.

An example of resolution of the diastereoisomeric mix-ture of limonene 1,2-epoxides (cis and trans) based on aHg2+-promoted addition of water to the carbon–carbondouble bond was reported by Franssen et al.[36] in pH 7.0buffer.


Scheme 16. DNA-bound CuII complex 20 as catalyst for the HKRof epoxides in water.[35]

Rao et al. reported the oxidation of terminal epoxidesin water by N-bromosuccinimide (NBS) or 2-iodoxybenzoicacid (IBX) in the presence of β-CD.[37] The role of β-CD isessential for realizing the process and mechanistic studieshave proved that the cyclodextrin not only activates the ep-oxide, but also forms a complex with the oxidizing agentthrough H-bonding, which first oxidize the epoxide to 1,2-diol and then further oxidation of the secondary carbonfurnishes the corresponding β-hydroxy ketone. Evidences ofthe complexation of β-CD with the epoxide and the oxidiz-ing agent were deduced from 1H NMR and IR spec-troscopy.[37]

S-Nucleophiles

Thiolysis of epoxides is a direct access route to β-hydroxysulfides. Organic chemists have usually performed thiolysisin organic solvents (THF, CH2Cl2, MeOH, MeCN) generat-ing reactive thiolate under anhydrous conditions.[38] The re-actions occur with good yields in short times but reactionconditions are often harsh and only functional groupswhich tolerate basic conditions can be used.[38a] When thiolsare used as nucleophiles, milder reaction conditions are usu-ally required but an activating agent is necessary (generallya Lewis acid).

Considering that both arene- (pKa 6–8)[39] and alkane-thiols (pKa 10–11)[39] can be deprotonated in water by hy-droxide ions, to form the corresponding highly nucleophilicthiolates, the use of aqueous basic conditions represents anideal approach to realize efficiently this process.

We have reported that at pH 9.0[12g,12i] thiolysis of a vari-ety of substituted epoxides by different arenethiols were fastand in 0.08–4.0 h a complete conversion was reached at30 °C with the prevalent formation of the β-products(� 95 %) coming from the totally anti nucleophilic attack atthe less substituted carbon of the oxirane ring. A little α-addition (3–5%) was sometimes observed (Scheme 17). Thehigher nucleophilicity of ArS–, with respect to N3

–,[39]

makes the thiolysis of alkyl oxiranes in basic aqueous me-


dium much more β-regioselective than the azidolysisone.[12o] Formation of 1,2-diol products due to the competi-tion of nucleophilic oxygen species (OH–, H2O) with ArS–

was rarely observed[40] and this by-product was never anobstacle to the purification of sulfide because the latter ispoorly soluble in aqueous medium and, since it is a solidcrystalline, it can be separated easily from the former byfiltration. Also, in the case of highly sterically hinderedthiols or epoxides such as ortho-methyl-benzenethiol or 2-methyl-2,3-heptene oxide, the reactions were complete aftera reasonable time (0.04–22 h). In all cases the yields of theisolated β-hydroxy sulfides are very satisfactory(� 80 %).[12g,12i]

Scheme 17. Thiolysis of epoxides under aqueous basic condi-tions.[12g,12i]

In water and by varying the pH from basic to acidic,it has been possible to realize a one-pot synthesis of 1,4-benzoxathiepinone exploiting the thiolysis of epoxides bythiosalicylic acid.

Thiolysis of α,β-epoxy ketones in water has been used asa key step for the one-pot multisteps synthesis of α-carbonylvinyl sulfoxides starting from the corresponding α,β-unsatu-rated ketones.[12b] Generally the thiolysis of such a class of1,2-epoxides is considered to be neither regio- nor stereose-lective at the C-α position, especially in the case of acyclicsubstrates.[41]

The basicity of the reaction medium plays a crucial rolein the reaction of the representative 3,4-epoxyheptan-2-one(25) (Scheme 18) depending on the nature of the thiol 26employed in the process. β-carbonyl-β-hydroxy sulfides arevery base-sensitive and easily give epimerization reaction atC-3, retroaldol and dehydration reactions to give complexreaction mixtures.

Scheme 18. Thiolysis of representative α,β-epoxy ketone 25 inwater.[12b]

S. Bonollo, D. Lanari, L. VaccaroMICROREVIEWAfter an accurate study on the influence of pH on this

transformation, we found that a catalytic amount of NaOH(0.02–0.3 molar equiv.) was sufficient to complete the thi-olysis of 25 in water at 30 °C with thiols 26a–d. The processis completely α-regio- and anti-stereoselective with forma-tion of only anti-β-hydroxy sulfides 27a–d with excellentyields (97–98 %). The one-pot synthesis of the correspond-ing vinyl sulfides 28 was accomplished by coupling the thi-olysis process with a stereoselective dehydration achievedby treating compounds 27a–d with HCl at 70 °C for 18 h(Scheme 18).[12b]

A variety of α,β-epoxy ketones were also tested and inthe case of cyclic substrates 29, the corresponding vinylsulfides 30 were obtained directly in very good yields(Scheme 19).

Scheme 19. Thiolysis of cyclic α,β-epoxy ketone 29 in water.

By combining the completely stereoselective protocol forthe thiolysis of α,β-epoxy ketones in water with the epoxid-ation of α,β-enones previously developed,[12c] it was also re-ported that the preparation of α-carbonyl sulfoxides 32 andtriazole 33 starting from cyclohex-2-en-1-one (31) resultedin very good yields (Scheme 20). This is a very good exam-ple that confirms the efficiency of water as reaction mediumfor achieving high selectivity and realizing one-pot pro-cesses, results that in this case cannot be obtained by usingan organic reaction medium.

Scheme 20. One-pot protocols including thiolysis of α,β-epoxyketones.[12b]


Thiolysis of α,β-epoxycarboxylic acids is a key syntheticstep in the preparation of calcium channel blocker Diltiaz-em.[38a] We investigated the thiolysis by benzenethiol of aseries of α,β-epoxycarboxylic acids in sole water.[12a] Thereactions were very slow under acidic conditions (pH 4.0)and sometimes occurred with very low conversions, whilethey became very fast at pH 9.0 and occurred quantita-tively. Under basic conditions, benzenethiolate predomi-nantly attacked the more electrophilic C-α carbon, exceptin the case of β-phenyl-substituted α,β-epoxypropanoic acidand when an alkyl substituent was present at C-α position.

While in the case of azidolysis of simple alkyl- and aryl-substituted epoxides metal salts did not show any catalyticeffect, in the reaction of thiols with epoxides a strong cata-lytic effect was found by using several metal catalysts andin particular ZnII and InIII salts.[12a,12d,12f,12i] We have foundpreviously that the catalytic efficiency of metal ion (Lewisacid) catalyst in water in the reaction of epoxides, is ex-pected to be maximum at a pH value lower than its pK1,1

hydrolysis constant, at which the maximum concentrationof the aqua ion is present.[12k] According to this, the bestcatalyst under acidic pH was InCl3[12a,12i] (pK1,1 ca. = 4)while ZnCl2 (pK1,1 = 8.96) proved to be more versatile andshowed a high catalytic efficiency also at pH 7.0 (biomime-tic conditions).[12d]

ZnCl2 under neutral conditions was effective in the thi-olysis of a variety of epoxides. In all cases, excellent yields(94–97%) and generally short reaction times were obtained(5–300 min). The thiolysis by a variety of substituted ben-zenethiols was also investigated. An example is illustratedin Scheme 21. No catalytic effect was observed in the caseof o- and p-NH2, and o- and p-CO2H-substituted benz-enethiols, supposedly due to the formation of a stable com-plex with Zn2+ and its consequent deactivation as oxiranering-opening catalyst.[42] The efficiency of ZnCl2 as catalystwas regained in the case of o-Me-, p-NHAc, and o-CO2Mebenzenethiols, that is, when the thiol carries functionalitieswith reduced binding properties.

Scheme 21. ZnII-catalyzed thiolysis of epoxides under biomimeticconditions.[12d]

The use of ZnCl2 at pH 4.0 in the thiolysis of epoxideshas also allowed the definition of a one-pot protocol forthe selective preparation of sulfoxide or sulfone based on


the thiolysis of epoxide and pH controlled oxidation byH2O2.[12f]

Kobayashi et al. applied the use of Sc(DS)3 with chiralligand 9 also to the reactions of cis-stilbene oxide (8a) withbenzenethiols, achieving also in this case very good results(Scheme 22).[22]

Scheme 22. Sc(DS)3/(S,S)-9 catalyzed enantioselective thiolysis ofcis-stilbene oxide (8a) in water.[12d]

Water as proved to be an efficient reaction medium alsofor the addition of sulfinates to epoxides for the direct syn-thesis of β-hydroxy sulfones.[43] By combining the NaOHcatalyzed thiolysis of epoxide and the oxidation by tert-but-yl hydroperoxide, β-hydroxy sulfoxides have been preparedby coupling the use of water and microwave in a two stepone-pot procedure.[44]

Kiasat et al. reported the addition of ammonium thiocy-anate to epoxides in water catakyzed by the multi-sitephase-transfer catalyst α,α�,α��-N-hexakis(triethylammoni-umdichloromethane)melamine[45a] or the polymeric catalystPEG-SO3H.[45b] In both reports the corresponding β-hy-droxy thiocyanates have been obtained in short times andgood yields (7 examples, 0.25–1 h, 70–96 %).[45]

By exploiting its basic properties, borax (Na2B4O7) wasused as an alternative to NaOH to catalyze (10 mol-%) thethiolysis of allyl- and arenethiols to alkyl epoxides (14 ex-amples, room temp., 2–12 h, 43–98% yield).[46]

C-, Se-, and H-Nucleophiles

Kobayashi et al. extended the use of Sc(DS)3 (5–10 mol-%) as LASC and (S,S)-9 (6–12 mol-%) in water, they re-ported the enantioselective desymmetrization of cis-stilbeneoxides 8 by indoles 39 (Scheme 23).[22,47]

Scheme 23. Sc(DS)3/(S,S)-9 catalyzed desymmetrization of epox-ides 8 by indoles 39 in water.[22,47]

The reactions are sensitive to concentration achieving thebest results at 1.0 m. In the case of cis-stilbene oxide (8a)and indole (39a) (Scheme 23, Ar = Ph, R1 = R2 = H), going


from 0.5 m to 1.0 m the reaction proceeded in 5 and 6 h,respectively, giving 50 and 85% yield with 96 and 93 % ee,respectively. Details on the role of concentration in theseprocesses are not available but comparison with the samereaction performed in dichloromethane, gave lower yieldsand ee.[21]

Generally, the reactions proceeded at room temp. for 4–6 h with good yields (56–85 %) and high enantioselectivities(85–93% ee) (Scheme 23).

β-Cyclodextrin has been used to promote the addition ofsodium cyanide to several epoxides in water/acetone(7.5:1).[48] The reaction proceeded satisfactorily at roomtemp. and gave very good yields (Scheme 24). The Authorsalso reported that β-CD was able in two cases to promotethe process with a 15–17% ee. In Scheme 24 the case ofchlorophenyl glycidol 41, has been representatively re-ported.

Scheme 24. β-CD catalyzed addition of sodium cynide to chlo-rophenyl glycidol 41.[48]

Similarly, Rao et al. reported also the use of β-CD forthe promotion of the reaction of epoxides with benzenesele-nol in water. A variety of epoxides were considered (10 ex-amples) and always in short times (25–40 min) the corre-sponding β-hydroxy selenides were obtained in good yields(75–86 %).[49]

Concellón et al. reported the ring-opening of 3-aryl-2,3-epoxyamides in water or deuterium oxide by samarium iod-ide. The reaction proceeded with a complete β-regioselectiv-ity (12 examples, 50–79% yields) and by starting from en-antioenriched epoxides 3-aryl-2-hydroxyamines were pre-pared with complete retention of configuration.[50]

The use of α-, β-, and γ-cyclodextrins (α-, β-, and γ-CDs)was investigated for the kinetic resolution of epoxides inwater by sodium, lithium or potassium borohydrides.[51]

Takahashi et al.,[51c,51d] found that in the reaction of styreneoxide (Scheme 25) with NaBH4, as it happened in the caseof azidolysis of epoxide (Scheme 14),[29a] the efficiency ofthe process strongly depends on the amount of CD used.The best results were obtained by using 2 equiv. of β-CDand after 72 h at room temp. the (S)-1-phenylethanol (44b)was the main product (94%) with a 46 % ee and the (S)-epoxide 43 was recovered in 49% yield and 31% ee. Theuse of α-CD and γ-CD led to an almost 1:1 mixture ofproducts 44a/44b (Scheme 25).[51d]

The reduction of ortho- and para-substituted styrene ox-ides 45 and 47 respectively, was investigated in the presenceof β-CD. As reducing agent sodium borohydryde gave bet-ter results than the corresponding lithium and potassiumreagents.[51a,51b]

S. Bonollo, D. Lanari, L. VaccaroMICROREVIEW

Scheme 25. β-CD promoted enantioselective reduction of styreneoxide 43 in water.[51d]

The Authors found that by using 1.0 or 2.0 equiv. of β-CD and 4.0 equiv. of hydride, the β-regioselectivity for theformation of chiral alcohol 46b was generally high (92–100%) except in the case of o-methoxystyrene oxide(Scheme 26, 45 R = OMe) where 14% of the corresponding46b was formed when NaBH4 was used. More interestingly,it was found that para-substituted styrene oxides 47 prefer-entially gave the corresponding chiral (S)-alcohol 48b, whilethe ortho-substituted gave the (R)-enantiomer (Scheme 26).

Scheme 26. β-CD promoted enantioselective reduction of ortho-and para-styrene oxides in water.[51a,51b]

Conclusions

There is much chemistry between water and epoxide.This review presents the intriguing role of water in severalprocesses based on the epoxide ring opening. We have high-lighted those cases where water has been used not as a sim-ple substitution of the organic medium or as an exotic op-tion to claim the greenness of a process, but where waterplays a crucial role for reaching the highest chemical effi-ciency. The accurate use of the peculiar properties of waterand epoxides has allowed to realize processes that some-times cannot even be performed in other reaction media.

Acknowledgments

The authors gratefully acknowledge financial support by the Minis-tero dell’Istruzione, dell’Università e della Ricerca (MIUR) and theUniversità degli Studi di Perugia within the projects “Firb-Futuro


in Ricerca” (grant numbers RBFR08TTWW and RBFR08J78Q)and PRIN 2008.

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Received: December 16, 2010Published Online: March 30, 2011

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