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(PPh3)(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)(SiR3)(Cl)(PPh3)(2) (8), the formation of which is observed in a solution of CDCl3 by H-1 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 degreesC. Rh-4(CO)(12) (3b) or a cationic rhodium complex [Rh(cod)(dppb)]PF6 (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.