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

Abstract

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.

Recent Developments in Pd‐Catalyzed Reactions of Diazo Compounds

“…Experimental evidence suggesting that the olefin-complexed Pd 0 species is a highly active catalyst has been obtained, mostly likely to be involved in a resting state in the catalytic cycle (Scheme 3). [16] Scheme 3. Cyclopropanation catalyzed by a Pd 0 complex.…”

Stable N‐Heterocyclic Carbene (NHC)–Palladium(0) Complexes as Active Catalysts for Olefin Cyclopropanation Reactions with Ethyl Diazoacetate

“…The already‐described Pd 0 ‐based catalysts for these transformations contain phosphanes or olefins as ancillary ligands 9d. 11c On the other hand, Pérez and co‐workers have reported that group 11 metal complexes containing N‐heterocyclic carbene (NHC) ligands are active catalysts towards carbene transfer from ethyl diazoacetate (N 2 CHCO 2 Et; EDA) to several saturated or unsaturated substrates 4q.…”

Cyclopropanation of poly(isoprene) using NHC‐Cu(I) catalysts: Introducing carboxylates

“…[7] Cyclopropanation of unsaturated moieties by generating carbenoids using diazo compounds in the presence of transition metal-catalysts is a particularly versatile method reported in the literature, even if usually applied only for low molecular weight olefins. [10] Frequently used metal-catalysts based on palladium and rhodium, [11][12][13][14] but also Cu(I/II)-based catalysts were considered for such cyclopropanations. [15][16][17][18] Cyclopropanation of poly(diene)s, like poly(isoprene) (PI) or poly(butandiene), are only rarely known examples in the literature.…”