Rhodium-Catalyzed Silylative Cyclization of 1,6-Heptadiyne Derivatives: A Versatile Route for the Synthesis of a 1,2-Dialkylidenecyclopentane Skeleton

The reaction of 1,6-heptadiyne derivatives with a hydrosilane in the presence of a catalytic amount of RhCl(PPh 3 ) 3 ( 3a ) affords 1,2-dialkylidenecyclopentane derivatives ( 4 ) in moderate yields. This skeleton is constructed by the insertion of two alkyne moieties involved in the same molecule into the Rh−Si bond and the Rh−vinyl bond, consecutively. The key intermediate in this process is Rh(H)(SiR 3 )(Cl)(PPh 3 ) 2 ( 8 ), the formation of which is observed in a solution of CDCl 3 by 1 H NMR analysis. 1,6-Heptadiynes react smoothly with hydrosilanes to give 4 at room temperature in the presence of an equimolar amount of 8 . The quantity of 8 can be reduced to 3 mol % of the starting diynes to complete the identical reaction at 80 °C. Rh 4 (CO) 12 ( 3b ) or a cationic rhodium complex [Rh(cod)(dppb)]PF 6 ( 3c ) is also effective as a catalyst precursor for the present transformation, although the selectivity of 4 is rather low. The stereochemistry of the silylvinylidene moiety of 4 has the E -configuration in the reaction catalyzed by 3a or 8, while 4 with the Z -configuration is obtained from the reaction catalyzed by 3b . Two exo-methylene groups on the contiguous carbons of the products 4 behave as a diene unit for the Diels−Alder reaction with some dienophiles. Hydrogenation of 4 proceeds smoothly in the 1,4-addition mode to give the allylsilane 25 in high yield under 1 atm of H 2 .