Silylformylation of 1-Hexyne Catalyzed by Rhodium-Cobalt Mixed-Metal Carbonyl Clusters

Reactions of hydrosilanes with 1-hexyne catalyzed by Co₂Rh₂(CO)₁₂ and Rh₄(CO)₁₂ at 25 °C and atmospheric pressure-150 psi of carbon monoxide give (Z)-1-silyl-2-formyl-1-hexenes (1), which are the products of “silylformylation”, and/or (E)-1-silyl-1-hexenes (2). The ratio of silylformylation vs hydrosilylation products depends on the electronic nature of hydrosilane used. Depending on the conditions, the reaction gives 2,4-di-n-butyl-3-silylcyclopent-2-en-1-one (3) as a major side product, which is a unique carbometalation-cyclization product. The novel Rh-Co mixed-metal complexes (PhMe₂Si)₂Rh-(CO)_nCo(CO)₄ (n = 2, 3); 6a and RhCo(n-BuC=CH)(CO)₅ (7) are found to be important catalyst species for silylformylation catalyzed by Co₂Rh₂(CO)₁₂. Possible catalytic cycles of silylformylation that can accommodate the observed results are proposed.