Palladium-catalyzed cyclopropanation of alkenyl silanes by diazoalkanes: evidence for a Pd(0) mechanism.
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 5x10(-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 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 cyclopropanation of trimethylvinylsilane and DVTMS by diazomethane with Pd(0) corroborated the experimental observations.
Berthon-Gelloz
Guillaume
G
Département de Chimie, Université catholique de Louvain, Place Louis Pasteur, 1, 1348 Louvain la Neuve, Belgium.
Marchant
Mélanie
M
Straub
Bernd F
BF
Marko
Istvan E
IE
D000475:Alkenes / Q000737:chemistry*; D001391:Azo Compounds / Q000737:chemistry*; D002384:Catalysis; D003500:Cyclization; D003521:Cyclopropanes / Q000138:chemical synthesis* / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D010165:Palladium / Q000737:chemistry*; D012821:Silanes / Q000737:chemistry*; D012833:Siloxanes / Q000737:chemistry*; D013237:Stereoisomerism; D020058:Styrene / Q000737:chemistry; D013816:Thermodynamics
D000475:Alkenes; D001391:Azo Compounds; D003521:Cyclopropanes; D012821:Silanes; D012833:Siloxanes; C528333:divinyltetramethyldisiloxane; D020058:Styrene; D010165:Palladium