Radical stereoselectivity transition-metal catalysts

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Three major classes of reaction mechanisms can be identified (1) Radical processes for cobalt and manganese carbonyl complexes that give the expected products with little stereoselectivity [11, 12, 49, 50]. Although some claims have appeared of the intervention of radicals in HDS-related hydrogenation, most of the evidence points to other types of surface mechanisms which can be better related to coordinative mechanisms therefore no further mention will be made of radical reactions in this Chapter. (2) Reactions involving Ziegler-type catalysts (a transition metal complex mixed with an alkyl aluminum co-catalyst) these poorly defined systems have proved to be difficult to study in detail [20, 22-25], and they appear rather unrelated to HDS-active... 

The carbostannylation of carbon-carbon unsaturated bonds is a useful synthetic method for synthesis of more functionalized organostannanes. Various additives or catalysts (Lewis acid, radical initiator, and transition metals) are effective for this transformation. The carbostannylation usually proceeds with high regioselectivity and stereoselectivity. 

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