Tran Ba L, Fuller Jack T, Erickson Jeremy D, Ginovska Bojana, Raugei Simone
Institute for Integrated Catalysis, Pacific Northwest National Laboratory Richland WA 99352 USA
Chem Sci. 2024 Sep 23;15(42):17481-9. doi: 10.1039/d4sc02982h.
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.
铑（I）、钯（II）和铜（I）的芳基、烯丙基和炔丙基醇盐的β-碳消除反应是均相催化交叉偶联、基团转移和环化反应机理中的关键基元反应。除了对可分离的铑（I）-醇盐进行的少数研究外，钯（II）-和铜（I）-醇盐的β-碳消除反应还不太明确。在此，我们对可分离的仲和叔炔丙基醇盐铜（I）配合物LCuOC（H）（Ph）C≡CPh和LCuOC（Ar）C≡CPh（L = N-杂环卡宾（NHC）、dppf、-BINAP）的β-炔基消除反应进行了全面的合成、结构和机理研究，以生成单体（NHC）CuC≡CPh、二聚体[（二膦）CuC≡CPh]和相应的羰基化合物。对于NHC和二膦支持的仲炔丙基醇盐配合物，观察到选择性的β-炔基消除而非β-氢消除。通过紫外可见动力学分析对变温反应速率进行艾林分析，提出（IPr*Me）CuOC（Ar）C≡CPh的β-碳消除一级反应机理通过一个有序的四中心过渡态——基于铜-炔π配合物发生，在60-100°C的温度范围内提供ΔH‡ = 24（1）kcal mol，ΔS‡ = -8（3）e.u.，以及ΔG‡（25°C） = 27 kcal mol。额外的定量紫外可见动力学研究得出结论，NHC配体的电子和空间性质对消除速率产生了边际效应，在100°C下需要2-3小时才能完成，而二膦支持的炔丙基醇盐在25°C下1-2小时内即可观察到完全的β-炔基消除。
