Trans-silylation vs cross-metathesis of styrene with 2,2,4,4,6,6,8,8-octamethyl-1,5-dioxo-2,4,6,8-tetrasila-3,7-exo-dimethylenecyclooctane catalyzed by ruthenium complexes

2,2,4,4,6,6,8,8-Octamethyl-1,5-dioxo-2,4,6,8-tetrasila-3,7-exo-dimethylenec yclooctane (1) (whose X-ray structure has been determined) is a good model to study the reactivity of an exocyclic vinyl bond at silicon with styrene occurring in the presence of RuHCl(CO)(PPh3)(3) (I). Two products are forme d initially: the linear 2 by ring opening of 1 and the cyclic 3 by condensa tion with evolution of ethylene. Ln the presence of an excess of styrene, 2 undergoes consecutive reactions giving subsequent styryl derivatives. Effe ctive catalysis by I and the lack of activity of Ru-carbenes (II) for the r eaction were examined. An MS study of the products (2(D) and 3(D)) of the r eaction of deuterated styrene with 1 provides convincing evidence for a mec hanism that does not involve a metallacarbene. A sequential insertion-elimi nation mechanism is proposed for the catalytic process.