E-H bond activation

Our group has explored in detail the reactivity of 82 and the dicyclohexylphos-phino analog 84 (Scheme 6.19). These Pt chloride complexes were utiHzed as precursors for the synthesis of Pt alkyl complexes and cations of the type [(PSiP)Pt]", with the goal of accessing highly reactive metal complexes that would engage in E-H bond activation chemistry [77]. Indeed, square planar Pt alkyl and aryl complexes of the type (R -PSiP)PtR (85, R = Ph, R = CH2Ph 86,... 

The reaction between Pt(0) or Pt(II) hydride precursors and silane or germanes with E-H bonds (E = Si, Ge) can provide a variety of E-H bond activated products, for example, a series of silyl or germyl bridging diplatinum(I) compounds (Eigure 10.19) [130]. Further conversion of these compounds was possible including E-E bond coupling as well as the formation of Pt(II) silyl/germyl hydride. 

Metallocycles as intermediates in synthesis of heterocycles by transition metal-catalyzed coupling reactions under C—H bond activation 99AG(E)1698. 

However, recently, a theoretical paper has been published, which provides interesting arguments for a conventional silylmetal hydride rather than for a 3c2e M(H)Si bond [132,133]. For the great implications of these compounds for Si —H bond activation reactions consult, e.g., on the work of Crabtree [129]. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

A free carbene B base-containing complex C transmetallation D oxidative addition E C=C activation F C-H bond activation... 

Synthetic Reactions via C-H Bond Activation C-C and C-E Bond Formation... 

---