Direct observation of β-alkynyl eliminations from unstrained propargylic alkoxide Cu(i) complexes by C–C bond cleavage

Abstract

β-Carbon eliminations of aryl, allylic, and propargylic alkoxides of Rh(I), Pd(II), and Cu(I) are key elementary reactions in the proposed mechanisms of homogeneously catalysed cross-coupling, group transfer, and annulation. Besides the handful of studies with isolable Rh(I)-alkoxides, β-carbon eliminations of Pd(II)- and Cu(I)-alkoxides are less definitive. Herein, we provide a comprehensive synthetic, structural, and mechanistic study on the β-alkynyl eliminations of isolable secondary and tertiary propargylic alkoxide Cu(I) complexes, LCuOC(H)(Ph)CCPh and LCuOC(Ar^F)₂CCPh (L = N-heterocyclic carbene (NHC), dppf, S-BINAP), to produce monomeric (NHC)CuCCPh, dimeric [(diphosphine)CuCCPh]₂, and the corresponding carbonyl. Selective β-alkynyl over β-hydrogen elimination was observed for NHC- and diphosphine-supported secondary propargylic alkoxide complexes. The mechanism for the first-order reaction of β-carbon elimination of (IPr*Me)CuOC(Ar^F)₂CCPh is proposed to occur through an organized four-centred transition state via a Cu-alkyne π complex based on Eyring analysis of variable-temperature reaction rates by UV-vis kinetic analysis to provide ΔH^‡ = 24(1) kcal mol⁻¹, ΔS^‡ = −8(3) e.u., and ΔG^‡ (25 °C) = 27 kcal mol⁻¹ over a temperature range of 60–100 °C. Additional quantitative UV-vis kinetic studies conclude that the electronic and steric properties of the NHC ligands engendered a marginal effect on the elimination rate, requiring 2–3 h at 100 °C for completion, whereas complete β-alkynyl eliminations of diphosphine-supported propargylic alkoxides were observed in 1–2 h at 25 °C.