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

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)C[triple bond, length as m-dash]CPh and LCuOC(Ar)C[triple bond, length as m-dash]CPh (L = N-heterocyclic carbene (NHC), dppf, -BINAP), to produce monomeric (NHC)CuC[triple bond, length as m-dash]CPh, dimeric [(diphosphine)CuC[triple bond, length as m-dash]CPh], 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)C[triple bond, length as m-dash]CPh is proposed to occur through an organized four-centred transition state a Cu-alkyne π complex based on Eyring analysis of variable-temperature reaction rates by UV-vis kinetic analysis to provide Δ = 24(1) kcal mol, Δ = -8(3) e.u., and Δ (25 °C) = 27 kcal mol 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.