Rhodium-catalyzed silylcarbocyclization (SiCaC) and carbonylative silylcarbocyclization (CO-SiCaC) reactions of enynes

The reaction of a 1,6-enyne with a hydrosilane catalyzed by Rh(acac)(CO)₂, Rh₄(CO)₁₂, or Rh₂-CO₂(CO)₁₂ under ambient CO atmosphere or N₂ gives 2-methyl-1-silylmethylidene-2-cyclopentane or its heteroatom congener in excellent yield through silylcarbocycization (SiCaC) process. The same reaction, but in the presence of a phosphite such as P(OEt)₃ and P(OPh)₃ under 20 atm of CO, affords the corresponding 2-formylmethyl-1-silylmethylidene-2-cyclopentane or its heteroatom congener with excellent selectivity through carbonylative silylcarbocycization (CO-SiCaC) process. The SiCaC reaction has also been applied to a 1,6-enyne bearing a cyclohexenyl group as the alkene moiety and a 1,7-enyne system. The functionalized five- and six-membered ring systems obtained by these novel cyclization reactions serve as useful and versatile intermediates for the syntheses of natural and unnatural heterocyclic and carbocyclic compounds. Possible mechanisms for the SiCaC and CO-SiCaC reactions as well as unique features of these processes are discussed.