‘Carbene Radicals’ in Co^II(por)-Catalyzed Olefin Cyclopropanation

Abstract

The mechanism of cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters was studied. The first step--reaction of cobalt(II)-porphyrin with ethyl diazoacetate (EDA)--was examined using EPR and ESI-MS techniques. EDA reacts with cobalt(II)-porphyrin to form a 1:1 Co(por)(CHCOOEt) adduct that exists as two isomers: the 'bridging carbene' C' in which the 'carbene' is bound to the metal and the pyrrolic nitrogen of the porphyrin that has a d(7) configuration on the metal, and the 'terminal carbene' C in which the 'carbene' behaves as a redox noninnocent ligand having a d(6) cobalt center and the unpaired electron residing on the 'carbene' carbon atom. The subsequent reactivities of the thus formed 'cobalt carbene radical' with propene, styrene, and methyl acrylate were studied using DFT calculations. The calculations suggest that the formation of the carbene is the rate-limiting step for the unfunctionalized Co(II)(por) and that the cyclopropane ring formation proceeds via a stepwise radical process: Radical addition of the 'carbene radical' C to the C=C double bonds of the olefins results in formation of the gamma-alkyl radical intermediates D. Species D then easily collapse in almost barrierless ring-closure reactions (TS3) to form the cyclopropanes. This radical mechanism readily explains the high activity of Co(II)(por) species in the cyclopropanation of electron-deficient olefins such as methyl acrylate.

A Mechanistic Study of Direct Activation of Allylic Alcohols in Palladium Catalyzed Amination Reactions

“…The minimal correction term is a correction for the condensed phase (CP) reference volume (1 L mol −1 ) compared to the gas phase (GP) reference volume (24.5 L mol −1 ). This leads to an entropy correction term (SCP = SGP + Rln{1/24.5} for all species, affecting relative free energies (298 K) of all associative steps of −2.5 kcal mol −1 [39]. Larger correction terms of −6.0 kcal mol −1 have been suggested based on solid arguments [40,41].…”

Asymmetric Cyclopropanation of Olefins Catalyzed by a Chiral Cobalt(II) Porphyrin

“…[16][17][18][19][20][21][22][23] The EPR, ESI-MS as well as DFT studies revealed that the cobalt(II)-porphyrin-mediated cyclopropanation of olefins with diazoesters proceeds via the formation of the Fischer-type radical carbene II followed by a stepwise radical addition of an olefin (1) and cyclization cascade toward the corresponding cyclopropanes 3 and 4 (Scheme 1). 17,18 The "terminal radical carbene species" II and the "bridging carbene species" III were proposed to exist as redox isomers and in dynamic equilibrium with each other, although the latter was determined to be thermodynamically somewhat more stable according to DFT calculations. 18 While the dynamic equilibrium between the species II and III is expected to prevent the formation of dimerization byproducts (e.g.…”

“…radical reactivity that is unusual for carbenes bound to other metal porphyrin complexes. 17,10 The nucleophilic character of the carbene radicals II enables cobalt(II) porphyrins to catalyze also the cyclopropanation of electron-deficient olefins such as methyl acrylate.…”

Hydroformylation of 1-Octene Mediated by the Cobalt Complex [CoH(dchpf)(CO)2]

“…Cobalt coordination compounds in general have become popular for H 2 conversion research [4][5][6][7][8], alkene hydrogenation [9][10][11][12][13], controlled radical polymerisation [14][15][16][17][18][19] and carbene-and nitrene-transfer reactions [20][21][22][23][24]. Cobaltmediated hydroformylation is well-known and has been continuously reviewed [25][26][27].…”