View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Springer - Publisher Connector

Reactivity and Mechanism in Organogold Chemistry

A FIRM FOUNDATION FOR FUTURE RESEARCH

R. J. Puddephatt Donnan Laboratories, University of Liverpool, England

The reactivity of alkylgold compounds towards thermal decomposition, reactions with olefins and acetylenes, oxidation and reduction, and reactions involving cleavage of gold-carbon bonds can be rationalised once the mechanisms of the reactions are known. Synthesis of new compounds and some catalytic reactions involving organogold species have been developed as a result.

The factors influencing the reactivity of organic This compound was very unstable, but Coates was molecules are well understood through detailed later able to prepare stable derivatives by addition of studies of organic reaction mechanisms. The tertiary phosphine ligands (4). mechanisms of very few organometallic reactions are known in the same detail and, as a result, our under- "AuMe," + PEt, -- [Me,AuPEt,] standing of the factors influencing the reactivity of organometallic compounds leaves much to be desired. These tertiary phosphines were thought to stabilise This is particularly unfortunate since organometallic the organogold compounds by formation of a dn d7z compounds are often used as catalysts and are fre- bond between gold and phosphorus. Coates was also quently proposed as intermediates in catalytic pro- able to prepare the first alkylgold(I) complexes by cesses. Logical design of catalysts cannot be achieved using the same stabilising ligands (4). until it is possible to predict organometallic reactivity. Recently, the mechanisms of several important [C1AuPEt,] + MeLi -^ [MeAuPEt,] + LiC1 reactions of organogold complexes have been investi- gated. It is hoped to point out in this article how such For several years it was thought that such n-bond- mechanistic investigations have already led to a better ing ligands were necessary if thermally stable alkyl- understanding of the factors influencing the stability gold(I) and trialkylgold(III) complexes were to be and reactivity of alkylgold complexes, and how the prepared. However, recently a considerable number mechanisms of some catalytic reactions have been deduced.

Stability of Alkylgold Complexes The first organogold compounds were prepared early this century and the chemistry of dialkylgold (III) complexes was thoroughly developed by C. S. Gibson and his co-workers (1, 2). The preparations typically involved the use of Grignard reagents. Et Br Et

[Au,Br e] + 4EtMgBr —> Au Au +4MgBr2 /.\/ \ Et Br Et Gilman later showed that, using the more reactive CH,-Au-CH2 organolithium reagents, trimethylgold could form (3). (Ref.8)

[Au,Br,] + 6MeLi -+2 "AuMe s" + 6LiBr

108 of complexes which do not conform to this rule have Once the factors which influence the thermal been prepared and have been shown to be thermally stability of organogold complexes were understood, stable. The simplest such species are the ions (5, 6) it was relatively straightforward to synthesise the new

[AuMe 2] - and [AuMe 4] - ; some other examples are types of compounds shown in the table on page 108. given in the table. The problem is resolved by a study of the mechanism Oxidative Addition Reactions of decomposition of alkylgold complexés. For ex- Organogold(I) complexes are easily oxidised, even ample, the decomposition of [Me3Au(PPh 3)] is shown by such mild reagents as alkyl halides. The simplest to proceed by reductive elimination of ethane after kind of behaviour is illustrated by the following dissociation of a phosphine ligand (9). reaction (11): Me -PPh, 1 [Me,Au(PPh,) =± [Me,Au]->C,H 6 + [MeAu] [MeAuPMe 3] 4- CF 3I CF, Au—PMe, ] PPh, [MeAu(PPh,)] I

The methylgold(I) complexes decompose in a However, such reactions may often be more com- similar way (10). plex, as shown in the following example (13, 14).

[MeAu(PPh s)] + MeI -- cis-[Me,AuI(PPh,)]

-PPh, McAuPh, cis-[Me 2Aul(PPh,)] + [MeAu(PPh,)] -> [MeAu(PPh,)] — [MeAu] . ' [Me,AuAu(PPh,)] [Me,Au(PPh s)] + [IAu(PPh,)]

3 C,H,+2Au+PPh [Me,Au(PPhg)] > C 2H 6 + [MeAu(PPh,)] Thus the decomposition involves the three co- Here the initial product of oxidative addition under- ordinate gold(III) species [Me,Au] or the one co- goes further methylation to give [Me,Au(PPh,)] ordinate gold(I) species [MeAu], and the thermal which can then undergo reductive elimination of stability of a complex [Me,AuL] or [MeAuL] will ethane, as discussed earlier. Overall the reaction is: depend on the Base with which the ligand L can [MeAu(PPh,)] + MeI -; [IAu(PPh,)] + C,H6 dissociate to give the reactive intermediate. The thermal stability of the complexes when L is a The reactivity of alkylgold(I) complexes to oxida- tertiary phosphine ligand is then due simply to the tion increases as the electron density at the gold centre gold-phosphorus bond being strong and not readily increases. Thus the anionic species [AuMe 2]- is very cleaved. Similarly the ions [AuMe,] - and [AuMe 4] - reactive in this sense, and the coupling of alkyl groups are thermally stable because the gold-carbon bond is between alkyl-lithium reagents and alkyl halides can strong. be catalysed by gold complexes as illustrated (5): The three co-ordinate gold(III) species such as [Me,Au] are thought to have T-shaped stereo- [MeAu(PPh,)] + MeLi±Li[Me 2Au] -1- PPh 3 chemistry (a trigonal planar species would be para- Li[Me,Au] -{- PPh 3 + MeI -> [Me,Au(PPh,)] ± Lir magnetic with two unpaired electrons), and they are [Me,Au(PPh,)] --> C,H R + [MeAu(PPh,)] thought to be involved as intermediates in other [MeAu(PPh,)] reactions as illustrated below (9). MeLi-i-MeI -+ C,H 6 +LiI No useful synthetic applications of Me -PPh, Me Me PPh 3 Me this type of catalysis have yet been Me—Au—PPh, Me—Alu±Et—Au Et—Au—PPh, made, but the reactions are interest- 1 I 1 1 ing as models for similar reactions Et Et Me Me of organocopper complexes which are widely used in organic synthesis. Me -PPh, Me Me 1 1 1 CMe, Finally in this section, it is interest- Me,C—Au—PPh,± Me,C—Aug Me—Au-11 ing to note that the gold(II) com- CH plexes can be prepared by oxidative Me H 2 Me I addition to dimeric organogold(I) Me PPh, 1I Me complexes (8), as shown at the top of the following page. Me 2 CHCH Au—PPh 3 ^_ Me,CHCH2—Au 1 1 Me Me

109

Me 1 CH R Au—CH 2 CH,-Au-CH2 / \ / Me 2P PMe2 + MeI Me 2P PMe2 / \ / CH 2—Au—CH 2 CH2-Au-CH2 I

Reactions of Alkylgold Complexes with the methylgold bonds of both gold(I) and gold(III) Unsaturated Reagents derivatives (14). Methylgold(I) complexes react with unsaturated O reagents in a number of ways. For example, tetra- II cyanoethylene forms a simple olefin complex while [MeAu(PMe,)] + SO, -+ Me — S — Au(PMe3) tetrafluoroethylene inserts into the methylgold bond (14, 15).

Me C(CN)z Me [MeAu(PMe,)] + (CN) 2C = C(CN) 2 --> Au—ll O

Me,P/ C(CN) 2'9 [Me,Au(PMe,)] -f- SO Z -^ Me — S — Au — Me 0 [MeAu(PPh,)] + CF 3 = CF 2 --. [MeCF 2CF 2Au(PPh,)] PMe,

Hexafluorobut-2-yne reacts with methylgold(I) Cleavage of Alkylgold Bonds by complexes in a complex way as shown in the scheme Electrophilic Reagents below (16): When an electrophilic reagent E8+-XS- (such as In all these reactions it is likely that an olefin or HC1) attacks an alkylmetal bond, R-M, the electro- acetylene complex of gold is formed initially and then phile ES+ has generally been considered to attack the rearranges to the final products. Under similar con- alkyl group. If the nucleophile X S- simultaneously ditions trialkylgold(III) complexes are unreactive, attacks the metal centre the mechanism is termed

Me CF3 ^, C=C

[MeAu(PMe 2Ph)] + CF,C= CCF, - / Au Au Ph2Me,P Me PMe,Ph CF, CF, CF, jF,

[MeAu(PMe 2Ph)] + C =C C,H0 + \C=C Me AuPMe2Ph Au Au / PhMe.P PMe2Ph

perhaps because in this higher oxidation state the gold SE2 (cyclic) and if it does not the mechanism is S E2 centre is unable to form a strong bond to the olefin. (open). These are illustrated by the equations given In contrast, sulphur dioxide inserts rapidly into at the top of the next page.

110 E---X that found in the gold(III) derivatives discussed Barlier. Í -.RE-I-MX R—M+E—X-+ Í With other electrophiles, the S $2 (cyclic) mechan- ism probably operates, as in the cleavage of the R---M methyl-gold(I) bond by cis-[Me 2AuI(PPh,)] (22).

X Me i E i i Me — Au —1 + [CD,Au(PPh,)] -> R—M+E—X-->R----M->RE+M++X - -* Me --'-RE + MX PPh, I 1 /\ --> Me —Au Au(PPh) The S E2 (open) mechanism is thought to operate in cleavage of alkyl groups from trialkylgold(III) com- CD/ plexes by acids (17). Thus the reaction rates are PPh, determined by the strength of the attacking acid Me (nitric > trifluoroacetic > acetic acid) and not by the Me — Au — PPh, + [IAu(PPhe)] nucleophilic power of the conjugate base X. D,

Me,Au(PPh,) + HX -. cis-[Me 2AuX(PPh,)] + CH 4 From this brief survey it can be seen that the relative rates of cleavage of different alkyl groups from a given gold centre, as well as the relative react- In these reactions it is always one of the mutually ivity of alkylgold(I) and alkylgold(III) complexes trans alkyl groups which is cleaved by the acid, and towards Au-C bond cleavage, can only be rationalised the ease of cleavage of an alkyl group is determined after the detailed mechanisms of reaction are under- mostly by steric factors, with smaller alkyl groups stood. being lost preferentially. The general S„2 mechanism is also thought to Free Radical Reactions in Organogold operate in the cleavage of alkylgold(III) complexes with electrophiles such as mercury(II), palladium(II) Chemistry and platinum(II) halides and probably with many It has been shown quite recently that alkylgold(I) other reagents (18, 19). complexes readily undergo free radical substitution With gold(I) complexes another mechanism is reactions with loss of an alkyl radical. The attacking possible in which the electrophile first oxidises gold(I) radical may be oxygen, sulphur or carbon centred as to gold(III), followed by reductive elimination. This illustrated by the following equations (23, 24) : mechanism is favoured in reactions with acids, though the proposed hydridogold(III) intermediates shown [PhCO 2Au(PPh,)] ;- Me' in the equations at the foot of the page are too PhCO j PhS' short-lived to be characterised directly (5, 20, 21). [AuMe(PPh,)] -> [PhSAu.(PPh,)] +Me - -- As a result of this different mechanism, the selec- Me' tivity towards cleavage of different alkyl groups R [*MeAu(PPh,)] +Me and R' from [AuRR'] - by acids is quite different from

Me PPh, 1 [Me 2Au] - + HE H —Au — PPh, -> CH4 + [MeAu(PPh9)] 1 Me

H 1 [MeAu(PMe 9)] + PhSH -> Me — Au — PMe, -> CH4 + [Au(SPh)(PMe,)] SPh

111

These reactions probably occur via short-lived Cationic bis(carbene) complexes of gold(I) can be gold(II) complexes formed by addition of the attack- prepared similarly. ing radical to the gold centre. Such gold(II) com- plexes may then expel an alkyl radical as in the above examples or be further oxidised to gold(III). An [(MeNC),Au]+ + 2Me 2NH , example where this latter reaction probably occurs is in the oxidative addition of CF 3I to [MeAu(PMe 3)] McNH which is initiated by formation of a CF 3 • radical and C Au then proceeds as shown below (11): / a

Me 2N

CF, + [MeAu(PMe 3)] -^ [Me(CF 3)Au(PMe3)] Strong ligands for gold(I) are able to displace the carbene from gold, and the carbene rearranges to give [Me(CF 3)Au(PMe,)] + CF3I -> a derivative of formamidine. Alkyl isocyanides are able to effect this displacement. [Me(CF,)AuI(PMe 3)] + CFg

McNH + This is an example of a free-radical chain mechanism. \ A similar mechanism probably operates in reactions C Au + 2MeNC -; of thiols and selenols with methylgold(I) complexes ( Me,N (20, 21). MeN

PhS + [MeAu(PMe,)] -. [MeAu(SPh)(PMe 3)] -> 2 CH + [(MeNC) 2Au] -l-

Me 2N -M PhSH -CH, [Au(SPh) (PMe,)] +-- [Me(H)Au(SPh) (PMe,)] Now combination of the last two equations leads to the prediction that gold(I) isocyanide complexes will catalyse the addition of amines to isocyanides to give formamidines. Alkylgold(III) complexes do not appear to undergo attack by free radicals nearly as readily as alkylgóld(I) complexes and, as a result, they are much less reactive [(MeNC) 2Au]+ Me 2NH + MeNC -> MeN = CHNMe 2 towards thiols and selenols. In fact it has been known for some time that H[AuC1 4] will catalyse such reactions. It is now Carbene Complexes of Gold established that the H[AuC1 4] reacts with isocyanide Carbene complexes of gold can be prepared by and is reduced to give the isocyanidegold(I) complex. reaction of an isocyanide complex with an alcohol or The catalytic mecbanism outlined above then al es amine as shown below (25-27). over (25-27).

RNH C—Au—Cl R'O MOM

[RN =_ C — Au — Cl] R'NH, RNH C—Au—Cl R'NH

112 References 1 V. J. Pope and C. S. Gibson, J. Chem. Soc., 1907, 91, 15 C. M. Mitchell and F. G. A. Stone, 3. Chem. Soc., 2061 Dalton Trans., 1972, (1), 102 2 B. Armer and H. Schmidbaur, Angew. Chem., Internat. 16 J. A. J. Jarvis, A. Johnson and R. J. Puddephatt, Y. Ed. En., 1970, 9, (2), 101 Chem. Soc., Chem. Commun., 1973, (11), 373 3 H. Gilman and L. A. Woods,,7. Am. Chem. Soc., 1948, 17 S. Komiya and J. K. Kochi,,. Am. Chem. Soc., 1976, 70, (2), 550 98, (24), 7599 4 G. E. Coates and C. Parkin, J. Chem. Soc., 1963, 421 18 B. J. Gregory and C. K. Ingold,,7. Chem. Soc. B, 1969, 5 A. Tamaki and J. K. Kochi, Y. Chem. Soc., Dalton (3), 276 Trans., 1973, (23), 2620 19 R. J. Puddephatt and P. J. Thompson, ,J. Chem. Soc., 6 G. W. Rice and R. S. Tobias, Inorg. Chem., 1976, 15, Dalton Trans., 1975, (18), 1810 (2), 489 20 A. Johnson and R. J. Puddephatt, ,7. Chem. Soc., 7 R. Usón, A. Laguna and J. Vicente, Y. Chem. Soc., Dalton Trans., 1975, (2), 115 Chem. Commun., 1976, (10), 353 21 R. J. Puddephatt and P. J. Thompson,.7. Organometal. 8 H. Schmidbaur, Accounts Chem. Res., 1975, 8, (2), 62 Chem., 1976, 117, (4), 395 9 S. Komiya, T. A. Albright, R. Hoffman and J. K. Kochi, 22 G. W. Rice and R. S. Tobias, ,7. Organometal. Chem., .7. Am. Chem. Soc., 1976, 98, (23), 7255 1975, 86, (2), C37 10 A. Tamaki and J. K. Kochi, J. Organometal. Chem., 1973, 23 R. Kaptein, P. W. N. M. van Leeuwen and R. Huis, 61, 441 .7. Chem. Soc., Chem. Commun., 1975, (14), 568 11 A. Johnson and R. J. Puddephatt, J. Chem. Soc., 24 P. W. N. M. van Leeuwen, R. Kaptein, R. Huis and Dalton Trans., 1976, (14), 1360 C. F. Roobeek, J. Organometal. Chem., 1976,104, (3), C44 12 A. Tamaki and J. K. Kochi,,. Organometal. Chem., 1974, 25 G. Minghetti, L. Baratto and F. Bomti,, . Organometal. 64,(3),411 Chem., 1975, 102, (3), 397 13 A. Johnson and R. J. P_udd'ephatt, J. Organometal. Chem., 26 J. E. Parks and A. L. Balch, J. Organometal. Chem., =4=1975, 85, (1), 115 1974, 71, (3), 453 14 A. Johnson, R. J. Puddephatt and J. L. Quirk, J. Chem. 27 J. A. McCleverty and M. M. M. da Mota,J. Chem. Soc., Soc., Chem. Commun, 1972, (16), 938 Dalton Trans., 1973, (23), 2571

Gold and Its Alloys in Dentistry A REVIEW OF THEIR PROPERTIES AND APPLICATIONS In a recent International Metallurgical Review- platinum metals. The type of casting alloy selected published by the Metals Society, London, as Review depends largely on the stresses the casting is likely 125-a comprehensive survey is given of the metals to be subjected to, the relatively soft Type I golds and alloys used as restorative materials in dentistry. being used where minimum stresses occur while at The authors, Dr J. F. Bates of the Welsh National the other end of the range the extra hard Type IV School of Medicine, Dental School, Cardiff, and Dr are used for thin sections which must withstand high A. G. Knapton of the Johnson Matthey Research masticatory forces. The properties of these Type IV Centre, Reading, consider the metallic materials used golds also enable theet to be used for certain re- both for restorations fixed in the mouth and for movable appliances. The review ably describes the removable prostheses, and a considerable part of the functions of the various components of the gold review relates to gold and its alloys, which find most alloys; their composition and mechanical properties use in the first application. are tabulated and appropriate phase diagrams given. An-tong the essential properties required by all One approach to the search for better materials for materials to be used for any dental restorative work dentistry has been the development of composites are high resistance to corrosion and compatibility consisting of an accurate gold casting which is enam- with both hard and soft tissues. Gold, which does not elled with porcelain. This can produce a restoration collect plaque on polished surfaces as readily as other which is indistinguishable from the natural tooth and materials, atnply satisfies these criteria and has been strong enough to withstand the forces involved. The used in dentistry for over 2,500 years. The other success of this process, which is basically one of necessary properties depend to a considerable extent vitreous enamelling, depends on a number of require- on the application the material is to be used for, ments which are described and discussed. and many of the requirements are unique to dentistry. Because many dental appliances are too large to Dental fillings are the most common type of fixed be made as a single casting it is frequently necessary restorative and the metallic filling employed to fill to join smaller pieces together and gold solders are cavities include pure gold which, when well made, is generally used for this purpose. Once again these are regarded as the most serviceable type of restoration. based on the gold-silver-copper system with ap- An alternative to packing filling material into a propriate additions to lower the melting point. cavity is to cement a dental casting into the tooth. The high cost of gold has undoubtedly stimulated Such castings are made to a high degree of accuracy considerable effort to find cheaper alternative materials by the lost wax process and a variety of gold alloys, but nevertheless it is still very widely used. With the which are classified into four types according to technical properties of gold so eminently suitable for mechanical strength or hardness, are used for this applications in restorative dentistry, and its appearance purpose. The composition of these materials is based so aesthetically pleasing, any further significant largely on the gold-silver-copper system with varying replacement by base metals seems unlikely in the additions of other elements including zint and the immediate future. I. E. C.

113