progress in inorganic chemistry · 2013-07-23 · f. silver-mediated oxidative ring enlargement /...

PROGRESS IN

INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY

JOHNS HOPKINS UNIVERSITY

BALTIMORE, MARYLAND

VOLUME 56

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Progress inInorganic Chemistry

Volume 56

Advisory Board

JACQUELINE K. BARTON

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

JAMES P. COLLMAN

STANFORD UNIVERSITY, STANFORD, CALIFORNIA

ALAN H. COWLEY

UNIVERSITY OF TEXAS, AUSTIN, TEXAS

RICHARD H. HOLM

HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS

EIICHI KIMURA

SHIZUOKA UNIVERSITY, SHIZUOKA, JAPAN

NATHAN S. LEWIS

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

STEPHEN J. LIPPARD

MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE,

MASSACHUSETTS

TOBIN J. MARKS

NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS

KARLWIEGHARDT

MAX-PLANCK-INSTITUT, MÜLHEIM, GERMANY

PROGRESS IN

INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY

JOHNS HOPKINS UNIVERSITY

BALTIMORE, MARYLAND

VOLUME 56

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Contents

Chapter 1 Silver-Mediated Oxidation Reactions: Recent Advances

and New Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

ZIGANG LI, DAVID A. CAPRETTO, and CHUAN HE

Chapter 2 Roles of Metal Ions in Controlling Bioinspired

Electron-Transfer Systems. Metal

Ion-Coupled Electron Transfer . . . . . . . . . . . . . . . . . . . . . . 49

SHUNICHI FUKUZUMI

Chapter 3 Cyanide-Bridged Complexes of Transition Metals:

A Molecular Magnetism Perspective . . . . . . . . . . . . . . . . . 155

MICHAEL SHATRUK, CAROLINA AVENDANO,

and KIM R. DUNBAR

Chapter 4 The Use of Metalloligands in Metal-Organic

Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

SERGIO J. GARIBAY, JAY R. STORK,

and SETH M. COHEN

Chapter 5 Exploring the Supramolecular Coordination

Chemistry-Based Approach for Nanotechnology . . . . . . . . . 379

HENRIQUE E. TOMA and KOITI ARAKI

Chapter 6 Synthetic Models for the Urease Active Site . . . . . . . . . . . . 487

FRANC MEYER

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543

Cumulative Index, Volumes 1–56 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

v

Silver-Mediated Oxidation Reactions: Recent Advances

and New Prospects

ZIGANG LI, DAVID A. CAPRETTO, AND CHUAN HE

University of Chicago, Department of Chemistry, Chicago, IL 60637

CONTENTS

I. INTRODUCTION 2

II. SILVER-MEDIATED OXIDATION OF ALKANES 6

III. SILVER-MEDIATED OXIDATION OF ALKENES 7

A. Introduction to Heterogeneous Systems / 7

B. The Oxametallacycle As an Intermediate / 8

C. Styrene Oxide: An Interesting Case / 11

1. Geometry of Double-Bond Interactions / 12

2. Identifying Additional Side Products: Klust and Madix’s Studies of Styrene Oxide / 13

D. Promoters / 15

E. Assorted Topics / 16

IV. SILVER-MEDIATED OXIDATION OF ALKYNES 17

V. SILVER-MEDIATED OXIDATION OF ALCOHOLS, ALDEHYDES, IMINES,

AND DECARBOXYLATION 17

A. Silver-Mediated Selective Benzylic and Allylic Alcohol Oxidation / 17

B. Lattice Silver(110)-Mediated tert-Butyl Alcohol Oxidation / 18

C. Silver-Mediated Amine, Imine, and Aldehyde Oxidation / 19

D. Silver-Mediated Oxidative Decarboxylation / 19

VI. SILVER-MEDIATED OXIDATION WITH THE FORMATION OF C��N BONDS 21

A. Aziridination of Olefins with Chloramine-T / 22

B. Aziridination of Olefins with PhI¼NTs / 23

Progress in Inorganic Chemistry, Volume 56 Edited by Kenneth D. KarlinCopyright � 2009 John Wiley & Sons, Inc.

1

C. Intramolecular Amidation / 24

D. Intermolecular Amidation / 24

VII. SILVER-MEDIATED CARBENE AND SILYLENE TRANSFERS 27

A. Wolff Rearrangement / 27

B. Aziridination and Cyclopropanation / 27

C. C��X Bond Activation / 29D. C��C Bond Formation / 30E. C��Si Bond Formation / 31

VIII. MISCELLANEOUS SILVER-MEDIATED OXIDATION REACTIONS 33

A. Silver I2 Oxidation / 33

B. Silver-Mediated Hydroxymethylation / 33

C. Silver-Mediated C��X Bond Formation / 34D. Silver-Mediated Oxidation Reactions with Sulfoxides and Sulfides / 34

E. Silver-Mediated Coupling Reactions with Grignard, and Alkyl Halides / 35

F. Silver-Mediated Oxidative Ring Enlargement / 37

IX. CONCLUSION 39

ACKNOWLEDGMENT 39

ABBREVIATIONS 39

REFERENCES 40

I. INTRODUCTION

In its metallic form, silver is a white, lustrous metal that was known by

humans for at least 5000 years. Most of its early use was as a coinage metal,

although its tarnishing in the presence of arsenic and sulfur made it useful as a

poison detector in utensils. Silver is also a bacteriocide, resulting in the usage of

electrocolloidal silver to treat infections, burns, and as an antibiotic from 1902 to

1947 (1–4). The photography industry is responsible for the majority of silver

used today, accounting for roughly 30% of the use of all silver produced in the

United States (5).

The stability and excellent conductivity of silver helped foster the early devel-

opment of silver plating in the electrochemical industry (6). Although silver is

perceived to be and designated a “precious’’ metal, it is actually more abundant

and less costly than most other late transition metals (5). Silver metal and silver-

based compounds are utilized as catalysts, additives, oxidants, and ligand-transfer

reagents in various chemical transformations (7–11).

It would be a formidable task to comprehensively introduce every aspect of

the chemical and biochemical applications of silver. This chapter focuses on

recent silver-mediated organic reactions, more specifically silver-mediated

2 ZIGANG LI, DAVID A. CAPRETTO, AND CHUAN HE

oxidation reactions. Generally, literature published after the year 2000 will be

discussed.

The promise of silver and silver complexes in oxidation chemistry lies in the

high redox potentials of silver. The oxidation states of silver are typically the 0,þ1,þ2, and þ3 states, with the þ1 state being the most common and stable. The þ4state was achieved and is described below; however, it is rare and too unstable to

be of any use in chemical reactions. The intense interest in the use of silver

inoxidation reactions lies in thehigh reductionpotential of theAg2þ ion.TheAg2þ/Agþ couple(4MHClO4 E� ¼ 1.980V) is strong enough to oxidizewater to oxygenat appreciable rates, making the formation of active Ag2þ compounds valuable forreactivitypurposes (12).Adi-silver(I) systemcapableof performinga two-electron

transfer is particularly attractive in homogeneous catalysis, especially with the

prevalence of dirhodium systems in catalysis today. The Latimer diagram for silver

is shown in Fig. 1. These values are reported in an aqueous acidic solution, the

values in basic solutions are markedly lower in most cases (13).

Silver(II) complexes are not technically difficult tomake, but they are unstable in

aqueoussolutionsunlessstabilizedbyligands(typicallynitrogenbased).Anumberof

different oxidants includingS2O82�, PbO2, andozone canbeused tooxidize silver(I)

to silver(II).Electrochemicalmethodsalsocanoxidize silver(I) compounds tohigher

oxidation states. Silver(III) species also can be accessed chemically or electrochemi-

cally, but are often unstable in the presence of water or many organic molecules and

show irreversible electrochemical reductions (12,13).

Due to these high redox potentials, the crystalline oxides of silver arevery useful

in batteries due to their high energy and power densities, finding most of their use

in silver oxide-zinc alkaline and silver–aluminum systems. Research efforts were

made in>30years to understand these systems,with themain focus on understand-ing the formation of Ag2O fromAgO (14–19). Decomposition of AgO leads to the

formationofAg2O,whichhasahigherohmicresistanceandshortensbatterylifedue

to voltage regulation problems. The formation of Ag2O is a complex process,

involvingat least threestages,allofwhichareunderdebate.Cyclicvoltammograms

(CVs) of AgO can vary with tuning of experimental procedures, but the general

Figure 1. Latimer diagram of silver in acidic solution.

SILVER-MEDIATED OXIDATION REACTIONS 3

structure is shown in Fig. 2. Detailed descriptions and discussions of the species

believed to formduring theprocess canbe found in theabovementioned references.

In terms of its coordination chemistry, the silver(I) ion is typically characterized

as “soft’’. Although originally believed to only bind ligands in a linear arrange-

ment, it was soon shown that it can adopt a variety of coordination environments,

themost common one being a four-coordinate tetrahedral geometry. Square-planar

complexes are not rare, and various silver(I) cluster complexes also contain three-

and five-coordinate silver(I) ions.

Silver(II) and (III) are characterized as hard ions, and this is evidenced by their

affinity for nitrogen- and oxygen-donor ligands. Silver(II) and (III) pyridine

derivatives and macrocycles were known for almost a century. Silver(II) nicoti-

nate–isonicotinate, various silver pyridine complexes, and silver(II) picolinate

were known as some early examples of ligand stabilized silver(II) complexes.

Generally, the silver(I) complexes were prepared first and then oxidized to their

silver(II) counterparts. Due to their d 9 configuration, silver(II) complexes tend

to have dark colors, varying from deep orange to black. The most common

geometry for a d 9 silver(II) complex is square planar; however, some distorted

six-coordinate octahedral structures were reported (20,21).

Porphyrin-based ligands are excellent generators of silver(II) complexes.

Silver(I)–porphyrin complexes disproportionate to silver(II)–porphyrin and

silver(0), partially because fitting a silver(I) ion with an ionic radii of 1.16A�

into a rigid porphyrin ring significantly lowers its oxidation potential. The silver(II)

ion, with an ionic radii of 0.93A�, is more size suitable for porphyrin frameworks,

while the strongly donating character of the porphyrin also helps to stabilize

formation of the silver(II) complex. The strong-field ligand environment increases

the highest occupied molecular orbital (HOMO) energy of the d 9 silver(II) ion

10

5

0

-5

-10

-15

-20

j (m

A c

m-2

)

-25

-30-0.2 -0.1 0.0 0.1

C

A1A2

A3

0.2 0.3

E (V)

0.4 0.5 0.6 0.7

Figure 2. Cyclic voltammogram of Ag2O in basic solution. [Reproduced with permission of ECS –

The Electrochemical Society from (18).]


and, consequently, some porphyrin–silver(II) complexes [and other silver(II)–

macrocyclicamine complexes] can be easily oxidized into corresponding silver

(III) complexes. Silver(IV) can be generated from [AgIIIF4]� under the presence of

high pressure fluorine and CsF in the form of Cs2[AgIVF6] (12,13,20,21).

The first air-stable silver(III) complexwas reported by Furuta et al. (22) in 1999,

who synthesized and characterized a silver(III) complex with 5,10,15,20-

tetraphenyl-2-aza-carbaporphyrin (nctpp) as the supporting ligand. The crystal

structure shows an inequivalence in the Ag��N bonds and the Ag��C bond, whichreflects the asymmetric nature of the porphyrin. Later, the same group reported

another silver(III) complex with ethoxy-5,10,15,20-tetrapentylflorophenyl-3,7-

diaza-21,22-dicarbaporphyrin [(n2cp)2] as the supporting ligand (Fig. 3) (23).

In 2002, Lash and co-workers (24) reported the first silver(III) complex

supported by a non-nitrogen fused carbaporphyrin system by reacting silver(I)

acetate with diphenylcarbaporphyrin (dpcp). Two years later they reported that

reacting silver(I) acetatewith semiquinone, cycloheptatriene, or indene undermild

conditions yields stable silver(III) oganometallic complexes (Fig. 4) (25).

Figure 3. X-ray structures of Ag(III)nctpp and Ag(III)(n2cp)2. [Adapted from (22 and 23).]

Figure 4. X-ray crystal structure of silver(III)–dpcp. [Adapted from (24).]


Br€uckner (26) first reported a series of silver(III)–meso-tetra-p-tolylcorrolato(ttc) complexes by reacting the silver(I) saltswith the corresponding corrol ligands.

In these cases, the silver(III) center is not in perfect square-planar geometry as a

result of the silver ion (0.81A�) being larger than the size of the corrole cavity

(Fig. 5).

As stated previously, the silver(III) state is stabilized by the electron-rich nature

of the ligand in addition to the cavity size contribution. The silver(III) center may

easily obtain an electron from the ligand to forma ligand radical,which satisfies the

metal ion’s high electrophilicity. This finding may explain why in some silver(III)

complexes the silver(II) and (III) states can switch reversibly.Whether this property

of silver can be utilized in oxidation catalysis the way iron and manganese

porphyrin systems were used still has to be seen (27–30).

II. SILVER-MEDIATED OXIDATION OF ALKANES

When dealing with the oxidation of alkanes in general, the main challenge for

the chemical community lies in the oxidation ofmethane into higher hydrocarbons,

either saturated or unsaturated. Methane is an ubiquitous fuel source; however, its

transport is difficult due to the high temperatures and pressures needed to liquefy it.

Therefore, a low-energy and high-yieldingmethod of forming liquid hydrocarbons

is urgently needed.

Also, methane is considered a viable alternative to currently used long-chain

hydrocarbons as a source of combustible fuel. However, it is also a more harmful

greenhouse gas than CO2. Therefore, if methane is to be used as a fuel source, it

must either be completely oxidized on its first pass through the system or recycled

back to ensure that no unreacted methane is released.

Figure 5. X-ray crystal structure of silver(III)–ttc (Tol��tolyl). [Adapted from (26).]


Lack of understanding of the above mentioned issues has led to intense study of

not only what is happening on the atomic level, but also the design of new systems

that have both higher selectivity and rates of conversion. Three main systems were

studied thus far: silver–alumina type catalysts, silver-modifiedmanganese species,

and silver-modified ceria (CeO2) systems.

Silver–alumina type catalysts are by far the most widely used, especially since

theyare themain catalytic source in the epoxidationof ethylene.Therefore, they are

readily available and already have undergone extensive studies. Many systems

have sought to utilize the presence ofNOx (another harmful environmental species)

ingas feeds. In this case, theNOx specieswouldbe reduced toN2, causingoxidation

of the hydrocarbonwith the support of the catalyst. Studies have helped to elucidate

the active species on the catalyst surface at varying temperatures and species

leading to the desired products (31). Results from a recent study point to the active

silver species being a [Ag��O��Al] bound intermediate that leads to N2 formation(32). If the silver is present in nanoparticle form, it is simply believed to be a

spectator. Other work showed mixed results on the benefit of silver-based alumina

systems for the oxidation of methane and higher hydrocarbons. The effect is

dependent on the type of reactor system prepared (33,34).

In the case of silver-modified manganese systems, recent studies agree that the

addition of silver increases the activity of methane oxidation, both in the case of

Ag–Mn composite catalysts and Ag modified MnO2 catalysts (35,36). Silver–

manganese–lanthanide oxide catalyst systems alsowere shown to be highly active,

and recent studies suggested the reasons for this high activity (37).

Ceria-based systems showed mixed effects for methane oxidation. Composite

catalysts of Ag/CeO2 fall apart, forming large silver metal aggregates and deac-

tivating the catalyst system (38). The only system in which silver-modified ceria

found any promise is in solid oxide fuel cells utilizing yttira-stabilized zirconia;

however, the silver-based system was not the optimum one in this case (39).

III. SILVER-MEDIATED OXIDATION OF ALKENES

A. Introduction to Heterogeneous Systems

The oxidation of olefins by silver is best known in the industrial heterogeneous

productionofethyleneoxide.Thisprocess isofmajor importance,withproductionof

almost 7 million metric tons of ethylene oxide (EO) reported in 2005 based on the

reaction shown in Fig. 6 (40). Epoxides are extremely useful starting materials for

a variety of organic functionalities, and hence production of them is sought after.

In addition to ethylene, epoxidation reactions of essentially any type of olefin were

patented, includingolefinswith andwithout vinyl hydrogens.However, the epoxida-

tion of other olefins containing allylic C��H groups with heterogeneous silver


catalysts proved difficult to scale up due to overoxidation. Without allylic C��Hbonds, the process becomes more amenable, with the epoxidation of butadiene to

make1-epoxy-3-butene (EpB)havingbeendevelopedalready intoan industrial scale

reaction (41) (Fig. 6).

Understanding exactly which species are involved in ethylene epoxidation was

the subject of four decades of study. Especially controversial was the active oxygen

species that forms the epoxide. Although extensively debated in the 1980s and

1990s, it is now generally accepted that atomic oxygen (as opposed to molecular

oxygen) forms EO (42).

The majority of studies aimed at understanding the mechanisms of ethylene

epoxidation rely on either experimental physical chemistry studies of single

crystal-silver surfaces or computational studies looking at enthalpies of different

reaction states. One unfortunate caveat of the experimental studies lies in the fact

that the binding of ethylene to the silver surface is too weak for ultrahigh vacuum

(UHV) studies to be of any use. Therefore, the majority of silver surface epoxida-

tion reactions use olefins that “stick’’ to the surface better both before and after

epoxidation (e.g., butadiene or styrene).

B. The Oxametallacycle As an Intermediate

One species thatwas proposed bymanyas an intermediate on theway to epoxide

formation is the surface oxametallacycle. It is believed that this species is attached

to the metal in a ��C��C��O�� type linkage. Previous density functional theory(DFT) calculations showed that the oxametallacycle is slightly energetically

favorable, and the experimental evidence for this species is based only on the

reaction of 2-iodoethanol with a silver surface as opposed to an epoxide (43–45).

Stronger experimental evidence for the oxametallacycle intermediate in epoxide

formation was reported in 2000 with thework of Barteau and co-workers (46). This

report studied the interaction of EpB, the product of the epoxidation of butadiene,

with the single-crystal Ag(110) surface. In this study, temperature programmed

desorption (TPD) studies were used to show that the EpB must be dosed onto the

silver surfaces at higher temperatures (300K) in order to make any sort of strong

interaction with the surface. Dosing the silver at 120K followed by TPD led to

major desorptions of EpB at 165 and 215K, with a small amount of desoprtion at

490K.However, thepeak at 490Kwas too small to be able to be identified asEpB. If

the silver was dosed with EpB at 300K, the TPD led to desportion of EpB at 490K

and 2,5-dihyrofuran at 510K. This higher temperature of desorption indicates the

stronger interaction with the silver surface as well as the requirement that an

Figure 6. Simple description of the epoxidation of ethylene by silver.


appreciable amount of energy be put into the EpB in order for it to interact with the

silver surface (maybe enough to break a bond). By comparing peak intensities for

EpB and 2,5-dihydrofuran as various dosing temperatures, they were able to

calculate activation energy of 8.4 kcalmol�1 for the formation of the “stronglyinteracting’’ EpB species. In addition to this data, the fact that EpB is the major

product seen helps show that most likely a C��O bond is being broken uponinteracting with the surface. Had various C��C or C��H bonds been broken,a variety of different desorption products would were observed as opposed to

only two.

In order to show that the strongly bound species was actually an EpB

molecule, high-resolution electron energy loss spectroscopy (HREELS) was

used to study the species present at the various dosing temperatures. When dosed

at lower temperatures, most of the observed peaks in the HREELSmatched those

of the vibrational spectrum of liquid EpB, suggesting that intact EpB is inter-

acting with the silver surface at lower temperatures. However, the silver surface

dosed with EpB at 300K showed noticeable differences in the HREELS

spectrum. In addition, DFT calculated vibrational frequencies of the surface

bound oxametallacylce matched well with those determined experimentally.

Computational studies were used also to determine the binding mode of the

oxametallacycle. In the previous studies with 2-iodoethanol, the oxametallacycle

intermediate was characterized as the five-membered ring containing two metal

atoms. However, the casewith a singlemetal center forming a four-membered ring

intermediate is also possible, and was considered in these studies. To determine

this, both the calculated enthalpies of formation and calculated vibrational spectra

of the different oxametallacycles were compared with the HREELS data of the

300K dosed silver surface. Comparisons showed the optimum structure to be the

Figure 7. TheDFT calculated interaction of 1-epoxy-3-butene (EpB)with a seven-atom silver cluster.

[Adapted from (46)].


four-membered type, as shown in Fig. 7. In this arrangement, note that the C3–C4

double bond is also in a favorable position to interact with the silver surface.

As stated above, the lack of this interaction in ethylene is what makes its binding to

the silver surface weaker and harder to study. Later work by Barteau and

co-workers, (47) using near-edge X-ray absorption fine structure (NEXAFS) in

combination with DFT techniques, supported the interaction of the C3–C4 double

bond in EpB with the silver surface (Fig. 7).

Barteau and co-worker (48) performed more extensive DFT calculations in

order to fully understand the actual structure of the oxametallacycle. There are two

energetically favorable structures, shown in Fig. 8, for the oxametallacycle:

The first involves formation of a four-membered ring system with a single silver

site interacting with the substrate, commonly known as the OME structures

(oxygen–metal–ethyl), while the other involves two silver centers and a five-

membered ring known as the OMME (oxygen–metal–metal–ethyl) structure.

Barteau’s studies compared the enthalpies of reaction for varying sizes of basis

sets. Their fear was that smaller basis sets might not take into account factors

that may be important in the interaction of the substrate with the silver surface.

While the different basis sets were in close agreement with regards to the heat

of reaction of epoxidation of ethylene, they were surprisingly different for the

formation of the surface oxametallacycle. For this case, the simplest basis set gave

values closest to those determined experimentally. However, it was noted that all

basis sets gave good agreement for the general structure of the oxametallacycle

(based on calculation of vibrational spectra). No basis sets actually showed the

oxametallacycle to not be a reasonable surface-bound structure (Fig. 8).

The size of the silver cluster on calculated energies also was examined.

By looking at the optimized geometry of EpB on a seven-atom silver cluster in

Fig. 7, it might be argued that due to the size of the EpB, interactions with silver

atoms beyond the seven included in the calculation may be significant. Increasing

the size of the silver cluster to 16 atoms and performing the calculations again

actually showed a slight destabilization of 5 kcal mol�1 of the oxametallacycle.They propose that this may point to increased stability of EpB on surface defects or

smaller silver particles as opposed to flat single-crystal surfaces. This argument is

sensible; especially considering the industrial process itself takes place on silver

particles and not on a single-crystal face.

Figure 8. Two proposed oxametallacycle intermediates. (a) This structure depicts theOMEgeometry,

while (b) depicts the OMME geometry.


In order to fully understand the role of the second double bond in butadiene

epoxidation, TPD and DFT studies were undertaken with butylene oxide (BO) on

the Ag(110) surface. Barteau hypothesized that without a double bond binding to

the silver surface, BOs interaction with the silver surface would be greatly

decreased. With these studies, Bareteau and co-workers (48) could only observe

chemisorbed BO species; no ring-opened BO bound to the silver surface was

observed. No adsorption of BO was seen at temperatures>200K, where the EpBmolecule was opened forming the oxametallacycle. The DFT studies showed a

destabilization of 15 kcal mol�1 for the BO molecule bound to the silver surface.In addition, when the ring opened the BOwas bound as an OMME instead of OME

and in a different confirmation (the C1��O bond breaks as opposed to the C2��Obond). Also, the ethyl group is not interacting with the silver surface. The

comparison between the BO and EpB calculated geometries can be seen in

Figs. 7 and 9. Other geometries were tested in order to assure that a more stable

intermediate was not possible, including various OME geometries. However, the

lowest energy conformer was the highly destabilized one shown in Fig. 9. It is

unfortunate that some surface-bound intermediate could not be observed in these

studies with BO: they would have provided a more accurate representation of

EO interactions than any previous substrates studied (Fig. 9).

C. Styrene Oxide: An Interesting Case

The interest in using styrene to gain an understanding of ethylene epoxidation

comes from its functionality as a terminal alkenewith no competing double bonds

for epoxidation. However, in recent years there were reports showing that the

interaction of styrene with single crystals is a very complex process, raising many

questions about its usefulness as a good model for ethylene.

Figure 9. TheDFTcalculated interaction ofBOwith a silver surface. [Reprintedwith permission from

(48). Copyright � 2001 American Chemical Society.]


1. Geometry of Double-Bond Interactions

One concern that was raised by some scientists is the orientation of the

interacting double bond in styrene with the silver surface. It was already shown

that in ethylene, the double bond attaches parallel to the silver surface (49). More

complicated substrates showed different interaction geometries and it is possible

that the phenyl substitution may cause the C¼C double bond to twist in a way thatmakes the interaction a bad model for ethylene (50,51).

In 2004, Lambert and co-workers (52a) studied the interaction of styrene with

Ag(100) single crystals in order to elucidate the binding mode of the double bond.

By using TPD, they first showed that at 100K, the styrene molecule adsorbed and

desorbed from the silver surface without any degradation. In addition, as long as

they did not add too much styrene to the system, the system showed normal

behavior. At coverage above one monolayer of styrene (one molecule styrene for

each silver on the first layer of the silver surface), multilayer behavior began to take

over, leading to differently desorbed species. Knowing that the styrene adsorbed

and desorbed without falling apart, they were able to do both temperature-

programmed X-ray photoelectron spectroscopy (TPXPS) and near-edge X-ray

absorptions fine structure (NEXAFS) in order to determine the orientation of the

binding. The TPXPS studies helped to show that the different states observed at

different coverage densities correlated with distinct binding geometries. In addi-

tion, studies at very high coverage densities suggested that intermolecular styrene–

styrene repulsion energies are present and cannot be ignored. Studies showed that if

surface coverage is>0.19 monolayers, interactions besides those resulting from aflat-lying molecule begin to occur, especially a tilted styrene molecule interacting

with the surface. Also, at lower coverage, the ramping of temperature in order to

cause desorption of a styrene molecule shows no detection of degraded carbon

species. This helps to back up the TPD data and strengthen the argument that the

styrene absorbs and desorbs in a facile manner.

The NEXAFS studies were performed in two ways in order to show that the

styrene binds flat both qualitatively and quantitatively. In the first (qualitative)

study, the photon incidence angle to silver was varied from 10� to 90� and theintensity of the electronic transitions (C 1s ! p* and C 1s ! s*) weremonitored.The changes in the intensities of the transitions correlatewith the styrene molecule

laying parallel to the silver surface. The quantitative studies showed a tilt angle of

5� � 5� by looking at the polarization dependence of theC1s ! p* intensitieswithphoton incidence angle. This procedure and data are described both in the work

(52a) and in other published data (52b). The fit of the data can be seen in Fig. 10,

withA andA0corresponding to thep* resonances of the phenyl andvinyl portions of

styrene, respectively (Fig. 10).

In later studies, Lambert and co-workers (53) also used TPD and X-ray

photoelectron spectroscopy (XPS) to show that only atomic oxygen is responsible


for epoxidation of styrene and as he states “Hopefully lay the issue to rest once and

for all’’. This work also shows that increasing surface coverage of molecular

oxygen actually decreases selectivity.

In studies similar to those done with EpB, Barteau and co-workers (54) in 2005

reported the results of TPD, HREELS, and DFT studies of styrene oxide. The TPD

and HREELS results showed the formation of a surface oxametallacycle and,

at 485K, the re-formation of styrene oxide with small amounts of phenylaceta-

laldehyde.Thehigher temperatureneeded to re-form theepoxide (EpBneededonly

300K) suggests the additional stability imparted by thephenyl group interaction. In

addition, theTPDandHREELSdatacorrelatedwellwithDFTcalculations, placing

thephenyl ringnearlyparallel to thesilver surface.Althoughstudiedwithadifferent

singlecrystal, this data correlateswellwith that describedabovebyLambert andco-

workers (52a). Based on their experimental data in addition to the lack of acet-

ophenone product, they hypothesize that the C��O bond that breaks to form theoxametallacycle is the carbon closest to the benzene, in the same manner as EpB.

The DFT calculations also support this configuration (Fig. 11).

2. Identifying Additional Side Products: Klust and Madix’s Studies

of Styrene Oxide

Then, in early 2006, Klust and Madix (55,56) reported, in two papers, results

showing that the case of styrene epoxidation was not simple as once thought.

Since adsorbed atomic oxygen is generally agreed to be the active species in the

Figure 10. The NEXAFS data showing styrene lying nearly parallel to the silver surface. [Reprinted

with permission from (52). Copyright � 2003 American Chemical Society.]


epoxidation procedure, Klust andMadix first adsorbed molecular oxygen onto the

Ag(111) surface, then introduced styrene into the system, and monitored the

products evolved with temperature using temperature programmed reaction spec-

troscopy (TPRS).Although previous reports only reported the formation of styrene

oxide, they were able to observe large amounts of benzoic acid, benzene, and

combustion products CO2 and H2O. In addition, they failed to see any acetophe-

none or phenylacetaldehyde. Their XPS studies do show the formation of an

oxametallacycle, an important point that is discussed in a comment on the work

written by Barteau (57).

Based on their results, Klust andMadix (56) suggest the reaction scheme shown

in Fig. 12. In this model, the oxametallacycle that forms is in the “branched’’

Figure 12. Reaction scheme supporting product distributions observed by Klust and Madix’s studies

of styrene oxide on Ag(111).

Figure 11. The DFT calculated interaction of styrene oxide with a silver surface. [Reprinted with

permission from (54). Copyright � 2005 American Chemical Society.]


structure,with the benzylic carbon bonded to an oxygen atomon the surface. In this

configuration, the benzylic carbon is highly susceptible to attack from a nucleo-

phile, in this case another adsorbed oxygen atom. When attacked, the C��C bondmaking the styrene breaks, forming surface bound CH2 and benzoate. Further

reaction of these species would lead to the formation of benzoic acid and

carbon dioxide in a 1:1 ratio, which is observed experimentally. The other case,

where the oxametallacycle forms in the linear conformation and a nucleophilic

oxygen atom attacks the other carbon, would form phenylacetic acid, which is not

observed in their studies (Fig. 12).

The surface coverage of oxygen atoms is shown to be of some importance in

product distribution.Theoxygencoverage systemused toproduce theabove results

[Ag(111)–p(4� 4)–O] is not the optimum case (58). Detailed studies of differentmonolayer coverage are discussed in their reports (56).

These works are very important in the questions that they raise. All previous

studies with styrene only pointed to the liner oxametallacycle being able to make

styrene oxide. However, the side products observed show that the branched

structure cannot be ignored. If the branched structure does not occur, then more

complex mechanisms are taking place in order to make benzoic acid and benzene,

and those must be elucidated as well. There is no doubt that the reports from Klust

andMadix (56) raisedmore questions thatmust be answered. The authors postulate

two different pathways after the formation of the oxametallacycle to account for

the products produced: a C��H bond activation pathway and a nucleophilic attackby adsorbed oxygen atoms.

In another recently reported study, the oxidation of styrene was studied on an

oxygen covered Ag(110) surface (59). In contrast to Ag(100) and Ag(111), this

surface showed a very different product distribution, yielding phenylacetylalde-

hyde, phenylketene, and benzene, along with smaller amounts of benzoic acid,

phenylacetic acid, and biphenyl. The same productswere observed if styrene oxide

was reacted with a clean- or oxygen- covered Ag(110) surface. Even styrene

oxide reactedwith a clean silver surface led to the production of styrene, suggesting

that the oxygen in styrene oxide may be reacting with other surface-bound

oxametallacycles.

D. Promoters

In silver-based epoxidations, the role of promoters is to enhance selectivity of

epoxide formation. The “classic’’ promoters used in industry are cesium and

chloride, typically added as the CsCl salt. In some cases, only chloride is added in

the form of an organic halide (e.g., 1,2-dichloroethane). This subject was recently

reviewed, so only a short introduction will be given (60).

Studies of promoter effects typically revolve around studying the production

of epoxide as a function of the coverage of promoter(s). Numerous different


promoters were studied, includingmost of the alkali metals and all of the halogens.

In addition, NOx species were shown to achieve selectivities of up to 90% for

epoxidation of ethylene, warranting increased studies as well.

When studying coverage, there aremany different aspects thatmust be taken into

account. The first involves the understanding of the type and density of surface

coverageatvarying temperatures.Forexample, inchloridepromotion, the formation

ofAgCl isofgreat importance, since its formation is irreversibleandcatastrophically

shuts down epoxidation (61). This issue is also relevant for the case of the alkali

metals, as cesium lowers the efficiency of butadiene epoxidationwhen it is left in air.

The storage in air exposes theCs tomoisture, causing the formationofCs aggregates

on the surface (62).Diffusionof chloride atomsbelow the top layer of silver alsowas

studied and shown to have some relevance to epoxidation selectivity. However, the

effects are different depending on the surface coverage of chloride (63–67).

Similar to the basic surface studies discussed above, promoters often show

markedly different behaviors depending on the alkene species used. Lambert and

co-workers (68) reported a study of ethene and propene epoxidation with different

promoters that showedno real correlation based on the promoter used. In the case of

NOx species as promoters, therewas no effect for the formation of propylene oxide,

which is interesting considering the high activity of NOx in formation of ethylene

oxide. Also, addition of potassium ions into the NOx promoter feed decreased both

activity and selectivity for propylene oxide formation, again completely opposite

to the behavior seen for EO. As in the other surface studies, the authors postulate

a chemical effect from the presence of allylic hydrogens.

E. Assorted Topics

Someother systemswere explored in the literature as alternatives to the standard

silver systems. One notable set of studies looks at bimetallic Ag��Cu catalysts forincreased epoxide selectivity. Studies were performed looking at the selectivity of

EO formation both with and without promoters present (69–71). Selectivity is

increased with these systems; however, conversion is either worse or only slightly

better.

A2002 study reports the use of amicroreactor system to increase rate of production

of EO (72). The use of microreactors eliminates the formation of “hot spots’’ that are

common in large reactor systems. Hot spots commonly affect selectivities by increas-

ing the amount of combustion products formed. Microreactor systems also make

harmful waste products easier to handle and limit the size of explosions if they occur.

Finally, metal nanostructures were examined as new materials for ethylene

epoxidation, which is in the form of silver nanoparticles, nanotubes, nanowires,

nanocubes, and a silver-containing polyoxometallate (73,74). With the current

popularity of nanotechnology in newmaterials, more of these systems are bound to

be seen in the future.


IV. SILVER-MEDIATED OXIDATION OF ALKYNES

Since 2000, there are limited reports of catalytic alkyne oxidation by silver

complexes. In 2003, Liao et al. (75) reported acetylinic homocoupling to form

symmetric 1,3-diynes utilizing an AgOTs��CuCl2��N,N,N0,N0-tetramethylethyl-enediamine (tmeda) system. The importance in this coupling is that it takes place in

the presence of polymeric solid supports (see Fig. 13). The formation of symmetri-

cal domains on solid supports has some interest in biological chemistry, having

been shown to modulate cellular processes and even provide binding sites for

proteins. The coupling is able to take place with a variety of substituted benzofur-

ans. The reaction is thought to go through an intermediatewith silver(I) interacting

with the triple bond of the acetylene (Fig. 13).

V. SILVER-MEDIATED OXIDATION OF ALCOHOLS, ALDEHYDES,

IMINES, AND DECARBOXYLATION

A. Silver-Mediated Selective Benzylic and Allylic Alcohol Oxidation

Silver iswell known in its ability to oxidize alcohols,with tertiary and secondary

alcohols being oxidized into ketones or tetrahydrofuran derivatives, respectively,

Figure 13. Acetylinic homocoupling catalyzed by a simple silver salt (rt¼ room temperature,DBU¼1,8–diazabicyclounclec dec-7-ene).


with bromine and silver salts (76–80). Alcohols also can be oxidized by silver(II)

generated by reacting silver(I) with persulfates, although persulfates themselves

are able to oxidize alcohols as well. Mechanistic studies show that the reaction

can go through either an alkoxyl radical pathway when silver is present or an H-

abstraction pathwaywith only persulfate present. Silver catalysts alsowere used to

selectively oxidize benzylic and allylic alcohols (81,82).

Tsuruya and co-workers (83,84) recently reported that addition of alkaline earth

metals (e.g., Ca, Sr, and Ba) to an Ag/SiO2 catalyst by a coimpregnation method

enhanced the catalytic activity of the partial oxidation of benzyl alcohols

into benzaldehydes, with production of only small amounts of byproducts

(carbon dioxide, toluene, and benzene). The formation of carbonaceous material

was thought to be inhibitedby the alkaline earthmetals,which alsohelps to disperse

the metallic silver and facilitate oxygen adsorption. This effect causes the forma-

tion of an oxygenated silver surface that is generally believed to be responsible for

the partial oxidation of benzyl alcohol.

B. Lattice Silver(110)-Mediated tert-Butyl Alcohol Oxidation

Lattice silver also can perform a dehydrogenative oxidation of alcohols

with O2. For example, tert-butyl alcohol can be oxidized to isobutylene oxide

on an O2 covered Ag(110) surface at elevated temperatures (85). However, other

oxidation products also were produced. Experiments using 18O2 labeling re-

vealed that the oxygen in the product is from the original alcohol and they believe

the hydrogen atom from the methyl C��H bond is directly transferred to either O2or another molecule of tert-butyl alcohol. Lattice silver is still widely used in

industry and further studies hold promise for other industrially suitable methods

(Fig. 14).

Figure 14. tert-Butanol oxidation on silver(110).


To date, most industrial silver catalyst systems that were developed are hetero-

geneous (e.g., Ag–zeolite, lattice silver, nano-silver, and/or foamed silvers) (86).

Homogeneous silver-mediated oxidation remains underdeveloped despite recent

advances.

C. Silver-Mediated Amine, Imine, and Aldehyde Oxidation

Silver is well known in its ability to oxidize amines into imines or hydroxyl-

amine compounds. Normal conditions use persulfate as an oxidant, with the

mechanism considered to be a radical chain reaction (87–93). In some cases,

oxidative condensations may occur between two substrate molecules to yield

heterocycles. Imines can be oxidatively converted into aldehydes or ketones with

aldehydes eventually converting into carboxylic acids. In 1999, Enders and

co-workers (94) reported the mild oxidation of a-amino nitriles into a-hydro-xyenones. The reaction was regioselective with good reported yields.

Silver also was utilized to make pyrrole rings via intramolecular hydroamina-

tions (95). In 2004, Kn€olker and Aggarwal (96) reported an interesting oxidativecyclization of homopropargylamines at room temperature to yield pyrroles. In this

reaction, the silver salt acts as a single electron-transfer reagent to deprotect a

trimethylsilyl (tms) moiety and the substrate cyclizes in a tandem reaction (96).

In 2007, an impressive silver-catalyzed CO2 fixation process was reported by

Yamada et al. (97a) with similar propargyl alcohol substrates. Only 1 atm of CO2was required and the reaction was run at room temperature. Although Yamada’s

systemdoes not actually performanoxidation, it is highly possible that both his and

Kn€olker’s systems employ a similar neighboring group (N or O) stabilized silveralkyne to activate the substrates (97a). A similar phenomenon alsowas discovered

by Pale and co-workers in 1999 (97b). Silver alkyne interactions recently attracted

more attention from the chemical community and additional novel reactivity may

be discovered (Fig. 15).

The oxidation of aldehydes by silver oxides was reported early and eventually

developed into a potentiometric method for the determination of aliphatic alde-

hydes (13, 98a). Later, zeolite-supported silver catalysts were found to catalyze the

rearrangement of 1,3-dioxanes into aldehydes, which were subsequently oxidized

into their corresponding carboxylic acids under an oxygen atmosphere (98b).

D. Silver-Mediated Oxidative Decarboxylation

In 1970, Anderson and Kochi (99) reported a silver-mediated oxidative decar-

boxylation reaction with peroxydisulfate as the oxidant. Kinetic studies showed

that the reaction is first order in both silver and peroxydisulfate and zero order in

carboxylic acid. Silver(II) species and alkyl radicals are considered intermediates.


(Fig. 16) Ifweak allylic or benzylicC��Hgroups are present, a lactone productmayform through an acyloxyl radical, while amino acids may go on to form imines or

aldehydes (99,100).

The alkyl radicals generated from the process are nucleophilic and were able to

readily attack nitrogen-containing heterocycles (e.g. quinolines, pyridines, pyr-

idazine, etc.) with high selectivity. Electron-withdrawing groups on the hetero-

cycles were shown to enhance this reactivity. A tert-Butyl alkyl radical was found

to be more reactive than the corresponding n-butyl radical, which shows that polar

effects are more important than steric and enthalpic effects in determining the

reaction rates (101–103).

In 2001, Jain and co-workers (104) used Anderson and Kochi’s discovery to

introduce substitution on a histidine ring. Although yields are moderate and the

scope is limited, this selective introduction of an alkyl group to a heterocyclic

system has potential to be optimized for synthetic applications. Later, Jain et al.

(105) used this method to build various quinoline derivatives that may have anti-

Tuberculosis activity. In 2001, Frost and co-workers (106) used a silver-mediated

Figure 16. Formation of silver(II) species and silver-mediated oxidative decarboxylation.

Figure 15. Oxidation of amides and influence of neighboring propargyl groups.


oxidative decarboxylation in the synthesis of hydroquinone from glucose instead

of benzene, which has promise to be developed into an industrially useful method

(Fig. 17).

Although silver-mediated oxidative decarboxylation was known for years, its

application in synthetic chemistry was very limited (107–110). Systematic studies

of this chemistry and other silver-mediated oxidation chemistry in homogeneous

solution is rare. This result may be due to the inherent difficulties in working with

silver catalysts, which include sensitivity to ligand environment and relative

inertness toward oxidation. However, these drawbacks may be overcome with

carefully tuned reaction conditions and/or supporting ligand systems. Some of the

recent successeswith silvernitreneandcarbene-transfer reactionswill bediscussed

in detail in Sections VI and VII.

VI. SILVER-MEDIATED OXIDATION WITH THE FORMATION

OF C��N BONDS

Due to the prevalence of nitrogen-based functionalities in organic chemsitry,

strategies for the direct introduction of newC��Nbonds from both C��C andC��Hbonds are still needed due to only minor advances. Silver has helped contribute to

Figure 17. Applications of the silver-mediated oxidative decarboxylation.


significant advances in this field by utilizing pyridine-based ligand systems

(111–113). With the design of more efficient and/or chiral ligand systems,

this chemistry has potential to be extended to yield both a broader substrate

scope and enantioselectively, even in the case of saturated C��H bonds assubstrates.

A. Aziridination of Olefins with Chloramine-T

In 2001, Rai and co-workers (114) reported a silver-mediated aziridination of

olefins in THF with Chloramine-T. In their case, aprotic solvents gave better

yields versus protic solvents. Then, in 2003, Komatsu and co-workers (115) used

similar conditions and found no reaction in THF (solvent)while they detected 70%

conversion in CH2Cl2. Silver nitrate (AgNO3) was required stoichiometrically in

this transformation. Komatsu proposed a nitrene-radical mechanism based on the

fact that the reaction shut down in the presence of oxygen. They designed a model

reaction using 1,6-dienes, and as they expected, bicyclic pyrrolidineswere isolated

as products instead of aziridines. The role of silver in this reaction is not clear and

most likely a free nitrene radical is released with the precipitation of silver(I)

chloride (Fig. 18).

Figure 18. Silver-mediated C��N bond formation with chloramine-T.


progress in inorganic chemistry · 2013-07-23 · f. silver-mediated oxidative ring enlargement /...

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