Platinum carbene

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

The most common reaction of platinum(II) isocyanides is their reaction with nucleophiles. This reaction is a useful one for the formation of platinum carbene complexes, and this will be discussed in the next section. 

A logical route to the synthesis of platinum carbene complexes is by using a carbene precursor. This method has been successfully used with electron-rich alkenes. Reacting (30) with ci -PtCl2(PPh3)2 and similar complexes leads to the formation of carbene complexes (equation 125).378-380 Heterocyclic chloro compounds have also been used to prepare carbene complexes (equation 126).381... 

Platinum-cadmium clusters, luminescent properties, 12, 773 Platinum carbenes... 

There are numerous Pd and Pt carbene complexes. A silicon analogue of a Fischer-type platinum carbene is also known, [H(PR3)2Pt=Si(SEt)2]+.42... 

As can be seen in Equation (99), a platinum-catalyzed intramolecular cyclization was also utilized in the formation of benzo[, ]furan-based tricycles. A platinum carbene was proposed as an intermediate in this reaction <2003JA5757>. 

Although the platinum-catalyzed reactions are not as practical as palladium used for the same purpose, several types of unique discoveries still are noteworthy. Besides electrophihc C-H activation, the platinum complexes are efficient catalysts for hydrosilation see Hydrosilation Catalysis) that is important to silicon polymer industry. Platinum carbene catalysts are prepared from Karstedt catalyst (9) and imidazolium salts. The trials of model hydrosilylation reactions show very high yields of regioselective products (<1% impurity) with remarkable TON (see Turnover) (<30ppm catalyst load) (Scheme 81). ... 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cycloisomerization might undergo a metallacyclic intermediate that proceeds to eliminate /3-H. The formation of cyclopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted aUcene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). 

This lends some credence to the notion that nucleophilic attack of the imine is at the platinum carbene intermediate 8 formed from the migration of the silicon atom to the nitrogen atom that leads to compound 5.12... 

A cationic hydrido-methyl complex, HPtCHs ", is formed upon the almost barrierless insertion of a Pt ion into a C-H bond of methane and then 1,2-migration of a hydrogen atom and subsequent elimination of H2 from the transition-metal center gives the platinum carbene cation PtCHi"" with an apparent rate constant of k - 8.2 x 10" cm molecule" s. The reaction of ground-state platinum atoms produced by the photodissociation of [PtMe3-(CsHiMe)] follows a termolecular mechanism [4c] ... 

This chapter describes how the use of JV-heterocychc carbenes (NHCs) has enabled the discovery of unique platinum(O) catalysts for the hydrosilylation of unsaturated carbon-carbon bonds. Independently, a similar strategy has been explored by Elsevier and coworkers [9]. NHC platinum(0) complexes have also been used as hydroboration and diboration catalysts as well as precursors of platinum(II) carbene complexes [10]. Supported platinum carbene complexes on mesoporous cross-linked polystyrene particles, precipitated siUca and porous glass beads have been reported as well as their application in hydrosilylation [11]. Water-soluble analogs have also been prepared [12]. These will not be discussed here. 

Scheme S.16 Synthesis of bis-silyl platinum carbene species 55.

Platinum compounds yield various kinds of carbene complexes [118-137]. For example, if isocyanate platinum compounds react with alcohols or amines, the carbon of the isocyanate bonds to the hetero atom to afford the reactive CXY (X and Y are hetero atoms) group, and the carbene complex yields as shown in eqs. (21.44) [119,127] and (21.45) [120]. On the other hand, as shown in eq. (21.46) [121], the platinum carbene complexes are also obtained by the reaction of cyclic XYC = CXY with platinum compounds on heating for a long time. In these carbene complexes, the cw-isomer is more stable than the /ra/75-isomer as shown in eq. (21.47), the rra/j5-isomers being isomerized to the m-isomer at around 200 C [125]. The Pt-C bond length of these carbene eomplexes is about 2.00 A slightly shorter than 2.16 A, the sum of the covalent radii of the Pt-C bond (Pt= 1.39 A, C = 0.77A). 

Properties and Reactivity of Platinum Carbene Complexes References... 

The vast majority of platinum carbene complexes are stabilized by one or more adjacent heteroatoms, which allow TT-delocalization. However, there have been a few reports of compounds where there is no such stabilization, and we consider these first, before moving on to look at those with heteroatoms. 

Highly reactive platinum carbene species have been generated in the gas phase via the fast atom bombardment ionization of ethyl diazoethanoate complex 136 (Scheme 33). " Elimination of dinitrogen leads to carbene species 137, which are implicated in the reactivity of the precursor complexes, but had not previously been detected in solution. 

In an analogous fashion, oxidative addition to a platinum(O) species gives platinum(ii) species 157 (Equation (27)). Complex 157 is a rare example of an 18-electron platinum(ii) complex, and the platinum-carbene distance is reported as 2.006(6) A. 

Previously, Hashmi et al. found the gold-catalyzed cyclization of furans bearing a terminal alkyne moiety 75 into the phenol derivatives 76 (Scheme 21.32) [42]. Martin-Matute et al. reported a similar platinum-catalyzed reaction in which platinum-carbene complexes were proposed as reactive intermediates [43]. However, some results of experimental and DFT studies by Hashmi, RabaS, and co-workers indicated... 

The stereodefined silyl-substituted diene was prepared by hydrosilylation of the corresponding conjugated enyne in the presence of platinum carbene complex (for details see Sect. 4). 

More recently, Fiirstner has described the hydrosilylation of internal enyne compound 227 to be catalyzed by platinum carbene complex 228 (Scheme 89) [24]. The corresponding silylated diene 229 is thus efficiently obtained in a regio-and stereocontrolled way, en route to lactimidomycin. 

Scheme 89 Platinum carbene catalyzed hydrosilylation of enyne...

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