Platinum complexes reductive elimination

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

Platinum-lead complexes undergo a cleavage of Pt—Pb bond with halogens and halogen acids506,507 510. These reactions are believed to occur through an electrophilic attack on Pt(II) leading to oxidative addition with the formation of a hexa-coordinated Pt(IV) complex. Reductive elimination of a plumbane results in the observed products (equation 193). 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

The A-frame hydride [Pt2H2(/i-H)(/i-dppm)2] undergoes reductive elimination of H2 in the presence of tertiary phosphine ligands, L, to give the platinum(I) dimer, [Pt2HL(//-dppm)2]. Hill and Puddephatt have shown that this occurs via the intermediate [Pt2II2(/i-H)L(//-dppm)2] (14).99 Carbon monoxide reacts rapidly and reversibly with [PtH(/r-PP)2Pt(CO)]+, PP = R2P-CH2-PR2, R = Et or Ph, to give [PtH(/i-PP)2Pt(CO)2]+ and [PtH(CO)(/u-PP)2Pt(CO)2]+, the first reported mixed valence, platinum(0)-platinum(ll) complexes.100... 

Monomeric platinum(III) complexes have been observed frequently as transient species in electrochemical or pulse radiolysis studies and they are proposed as an intermediates in reductive elimination and oxidative addition reactions of platinum(IV) and platinum(II) respectively.382-388... 

The most important contributions in this area, however, directly related to bond activation chemistry, and, undoubtedly triggered by theoretical considerations along the lines of Figure 1, were reported by Whitesides and coworkers in 1986 and 1988 [11]. It was shown that the bent, bisphosphine-coordinated platinum chelate complex [(dcpe)Pt(O)] (9) (dcpe = bis(dicyclohexylphosphino)ethane), which could be generated thermally as a "hot" reactive intermediate by reductive elimination of neopentane from its ris-neopentylhydride Pt(II) precursor at around 60-70°C in solution, was able to activate C-H bonds, even of unactivated alkanes. 

A stoichiometric reaction of tetrakis(triphenylphosphine)platinum(0) with bis(pinacolato)diboron gives cis-diborylplatinum(n) complex in high yield (Scheme 3).38 The diborylplatinum complex then reacts with an alkyne, giving m-diboration product.40,41 These results indicate that the diboration proceeds through the general mechanism shown in Scheme 1 (E1 = E2 = Bpin), which involves the formation of diborylplatinum(n), insertion of an alkyne into the B-Pt bond, and reductive elimination. 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

Protonation reactions of the related dimethyl(hydrido)platinum(IV) complex TpMe2PtMe2H (58) leading to rapid methane reductive elimination have also been reported (86). This protonation was shown to occur exclusively at the pyrazole nitrogen, presumably forming a five-coordinate Pt(IV) intermediate. This species should undergo C-H coupling, and while a Pt(II) methane complex is not observed, trapping with... 

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. 

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