Development, synthetic scope, and mechanistic studies of the palladium-catalyzed cycloisomerization of functionalized 1,6-dienes in the presence of silane

A 1:1 mixture of the pi-allyl palladium complex (eta (3)-C3H5)Pd(CI)PCy3 Cia) and NaB[3,5-C6H3(CF3)(2)](4) in the presence of HSiEt3 catalyzed the cycloisomerization of diethyl diallylmalonate (2b) to form 4,4-dicarbomethoxy 1,2-dimethylcyclopentane (3b) in 98% yield with 98% isomeric purity. The procedure tolerated a range of functionality including esters, ketones, sulfones, protected alcohols, and substitution at the allylic and terminal olefinic carbon atoms. Cycloisomerization of 2b obeyed zero-order kinetics to >3 half-lives with initial formation of 1,1-dicarboethoxy-4-methyl-3-methylenecyclopentane (4b), followed by secondary isomerization to 3b. Deuterium labeling studies revealed that the conversion of 2b to 4b was accompanied by significant H/D exchange, consistent with an addition/elimination pathway coupled with facile H/D exchange of the Pd-H(D) intermediates with free silane.