Transition metal catalysis reductive elimination

The possibilities for the formation of carbon-carbon bonds involving arenes have been dramatically increased in recent years by the use of transition metal catalysis. Copper-mediated reactions to couple aryl halides in Ulknann-type reactions [12, 13] have been known for many years, and copper still remains an important catalyst [14, 15]. However, the use of metals such as palladium [16,17] to effect substitution has led to such an explosion of research that in 2011 transition metal-catalyzed processes comprised more than half of the reactions classified as aromatic substitutions in Organic Reaction Mechanisms [18]. The reactions often involve a sequence outlined in Scheme 6.6 where Ln represents ligand(s) for the palladium. Oxidative addition of the aryl halide to the paiiadium catalyst is followed by transmetalation with an aryl or alkyl derivative and by reductive elimination to give the coupled product and legeuCTate the catalyst. Part 6 of this book elaborates these and related processes. 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

An alternative mechanism for catalysis of Si-Si bond formation by later transition metal complexes was originally proposed by Curtis and Epstein.33 This mechanism, illustrated in Scheme 3, also invokes a sequence of oxidative addition/reductive elimination steps but does not resort to a silylene intermediate. A serious weakness in this scheme is that, as far as we know, no example of the spontaneous reductive elimination of a disilane from a bis(silyl) complex has yet been observed. 

In this Scheme, pC stands for pro-catalyst, C for catalyst, CS for a complex between catalyst and substrate, CP for a complex between catalyst and product, I for an initiator. S for a structural variation of the substrate, R for an added reagent. In cases 1.1 and 1.2 the catalysis is based on a coordinative interaction between catalyst and substrate in case 1.1 the product is released to regenerate C (for example by reductive elimination) whereas in case 1.2 the regeneration of CS results from a substitution of the complexed product by S. It should be clear that cases 1.1 and 1.2 do not exhaust the formal possibilities offered to photogenerated catalysis. One may actually imagine a photogeneration of catalyst from a selected pro-catalyst for any of the multiple catalytic cycles identified in homogeneous catalysis centered on transition metal complexes [12]. 

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