Reductive elimination variation with

The mechanism for the Pd-catalyzed Csp2—P bond formation proposed by Xu et al. is virtually the same as Hirao s with a slight variation. Oxidative addition of 2-bromothiophene to Pd(0) results in Pd(II) intermediate 84, which then undergoes a ligand exchange to give intermediate 85 with the aid of triethylamine. Triethylamine here serves as a base to neutralize HBr so that the reaction is driven forward. Finally, reductive elimination of 85 furnishes unsymmetrical alkyl arylphosphinate 83, regenerating Pd(0). 

This reaction is a variation of the hydroformylation reaction. Transmetallation of Rh(I)(acac) with the alkylmercury(I) compound gives ClHg(acac) and an alkylrhodium(I) compound. Oxidative addition of H2 gives a Rh(III) compound, and coordination and insertion of CO gives the acylrhodium(IH) compound. Reductive elimination then gives the product and regenerates Rh(I) — but as a Rh-H, not as Rh(acac). 

Another variation of the Pauson-Khand is the cyclization of w-alkenyl-aldehydes 249 in the presence of HSi(OEt)3 resulting in siloxycyclopentadienes (Scheme 40)187-189 The observed products, 250, arise from functionalization of the intermediate titanacycle with silane followed by reductive elimination of product. 

The mechanism of homogeneous catalysis invoives the same steps as heterogeneous catalysis. An initial tt complex is formed with the reactant. Metal-hydride bonds then react with the complexed alkene to form a C-H bond and a bond between the metal and alkyl group. There can be variation in the timing of formation of the M—H bonds. The metal carbon bond can be broken by either reductive elimination or protonolysis. Note that reductive elimination changes the metal oxidation state, whereas protonolysis does not. The catalytic cycle proceeds by addition of alkene and hydrogen. 

In the insertion pathway, carbene G transforms into the seven-membered complex H, which collapses to its triplet ground state before reductively eliminating to give E. This pathway was found to be the most favored one with hydrogen cyanide (R = H) and trifluoroacetonitrile (R = CF3). Thus, it was concluded that electron-rich nitriles rather follow the [4 + 2] mechanism, whereas electron-poor nitriles follow the insertion mechanism [56]. The case of benzonitrile was studied recently and led to the [4 + 2] pathway as the best option [57]. Thus, in the case of pyridines, the two options are possible and the switch between them seems to depend on subtle electronic variations. 

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