Palladium‐Catalyzed Cyclopropanation of Alkenyl Silanes by Diazoalkanes: Evidence for a Pd 0 Mechanism

Pd 0 does the trick! Alkenyl silanes are efficiently cyclopropanated by diazoalkanes at low Pd loadings (see scheme). Clear evidence for the involvement of a Pd 0 resting state for this reaction is given.Alkenyl silanes are efficiently converted to the corresponding silyl cyclopropanes in the presence of a slight excess of diazomethane (2–4 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 on a 4 g scale, the reaction only required a catalyst loading of 5×10 −3  mol %, which corresponds to a turnover frequency of 40 000 h −1 . Competition experiments revealed that vinyl silanes can be selectively cyclopropanated in the presence of an aliphatic terminal alkene and styrene. The complex [Pd 0 2 (DVTMS) 3 ] ( 38 , DVTMS=divinyltetramethyldisiloxane) proved to be an exceptionally active catalyst for the cyclopropanation reaction, giving complete conversion at −35 °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 cyclopropanation of trimethylvinylsilane and DVTMS by diazomethane with Pd 0 corroborated the experimental observations.