PNNL

Abstract

?-Carbon eliminations are key elementary reactions in the proposed mechanisms of homogeneously catalysed cross coupling, group transfer, and annulation of aryl, allylic, and propargylic alcohols by Rh(I), Pd(II), and Cu(I) species. Besides the several known studies of isolable Rh(I)-alkoxides, ?-carbon eliminations of Pd(II)- and Cu(I)-alkoxides are less definitive. Herein, we provide a comprehensive synthetic, structural, and mechanistic study on the ?-alkynyl eliminations of isolable secondary and tertiary propargylic alkoxide Cu(I) complexes, LCuOC(H)(Ph)C=CPh and LCuOC(ArF)2C=CPh (L = N-heterocyclic carbene (NHC), dppf, S-BINAP), to produce monomeric (NHC)CuC=CPh, dimeric [(diphosphine)CuC=CPh]2, and the corresponding carbonyl. Selective ?-alkynyl over ?-hydrogen elimination was observed for NHC- and diphosphine-supported secondary propargylic alkoxide complexes. The mechanism of this transformation proceeds by an organized four-centred transition state via a Cu-alkyne ??complex based on the first-order reaction kinetic of (IPr*Me)CuOC(ArF)2C=CPh by UV-vis kinetic over a temperature range of 60-100 ?C and subsequent Eyring analysis of ?H‡ = 24(1) kcal/mol, ?S‡ = -8(3) e.u., and ?G‡ (25 ?C) = 27 kcal/mol. Quantitative UV-vis kinetic studies conclude that the electronic and steric properties of the NHC ligands engendered marginal effect on the elimination rate, requiring 2-3 h at 100 ?C for completion. Whereas complete ?-alkynyl eliminations of diphosphine-supported propargylic alkoxides were observed in 1-2 h at 25 ?C.