Palladium-Catalyzed Cyclopropanation of Alkenyl Silanes by Diazoalkanes: Evidence for a Pd0 Mechanism

Alkenyl silanes are efficiently converted to the corresponding I silyl cyclopropanes in the presence of a slight excess of diazomethane (24 equiv) and a low loading of Pd(OAc)(2) (< 0.5 mol %). Diazoethane and diazobutane can also be employed and yield silyl cyclopropanes with diastereoselectivities of up to 10:1 for the trans isomer. When conducted oil a 4 g scale. the reaction only required a catalyst loading of 5 x 10(3) mol%. which corresponds to a turnover frequency of 40000 h(-1). Competition experiments revealed that vinyl silanes can he selectively cyclopropanated in the presence of an aliphatic terminal alkene and styrene. The complex [Pd-2(0)(DVTMS)(3)] (38, DVTMS=divinyltetramethyldisiloxane) proved to be an exceptionally active catalyst for the cyclopropanation reaction, giving complete conversion at -35 degrees C in 1 min. Intermolecular and intramolecular competition experiments with DVTMS (36), both with Pd(OAc)(2) and 38, provided strong evidence for a Pd-0(alkenyl silane)(3) resting state. Detailed density functional calculations on the reaction pathways for the cyclopropatiation of trimethylvinylsilane and DVTMS by diazomethane with Pd-0 corroborated the experimental observations.