Organostannanes conjugate additions

The application of organostannanes in rhodium-catalyzed 1,4-addition reactions was first studied by Oi and co-workers.137,137a The treatment of enones or enolates with a slight excess of aryltrimethylstannane and catalytic amounts of [Rh(COD)(MeCN)2]BF4 generates the conjugate addition products in good yields. The use of protic additives enhanced the yield of the reaction (Scheme 48).138... 

In 1998, the rhodium-catalyzed conjugate addition of organostannanes to a,/9-unsatu-rated ketones and esters was reported by Oi [29]. In the reaction, which was carried out in the presence of [Rh(COD)(MeCN)2]BF4 in THF at 60°C, a variety of a,/9-unsatu-rated ketones and esters were transformed into the corresponding conjugate addition products. It is noted that the yield was highly dependent on the substitution pattern of the substrates. For example, /9-substituted enone 40 afforded an 86% yield of 41, whereas methyl vinyl ketone 42 gave only an 18% yield of 43 (Scheme 3.14). The addition of water to the reaction mixture improved the reactivity for the formation of 41 (98%) and 43 (80%) [30]. 

Scheme 3.15 Catalytic cycle for rhodium-catalyzed conjugate addition of organostannanes [29],...

The required organostannanes are accessible by conjugate addition of trialkyltinlithium reagents to cyclic enones, followed by treatment with the appropriate RLi or RMgX reagent, as shown in Scheme 16. 

Coupling of organometallics. The demetallative dimerization of RLi by CuCl2 is different from the conjugate addition catalyzed by Cul. Organostannanes RSnBuj give R—R when R is an alkynyl, alkenyl, or aryl group."... 

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