Carbene-platinum complexes

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... 

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. 

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. 

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. 

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