Division of Chemical Sciences, Lawrence Berkeley National Laboratory, and Department of Chemistry, University of California , Berkeley, California 94720, United States.
J Am Chem Soc. 2017 Sep 13;139(36):12758-12772. doi: 10.1021/jacs.7b07124. Epub 2017 Sep 1.
Mechanistic studies of the copper-catalyzed asymmetric hydroboration of vinylarenes and internal alkenes are reported. Catalytic systems with both DTBM-SEGPHOS and SEGPHOS as the ligands have been investigated. With DTBM-SEGPHOS as the ligand, the resting state of the catalyst, which is also a catalytic intermediate, for hydroboration of 4-fluorostyrene is a phenethylcopper(I) complex ligated by the bisphosphine. This complex was fully characterized by NMR spectroscopy and X-ray crystallography. The turnover-limiting step in the catalytic cycle for the reaction of vinylarenes is the borylation of this phenethylcopper complex with pinacolborane (HBpin) to form the boronate ester product and a copper hydride. Experiments showed that the borylation occurs with retention of configuration at the benzylic position. β-Hydrogen elimination and insertion of the alkene to reform this phenethylcopper complex is reversible in the absence of HBpin but is irreversible during the catalytic process because reaction with HBpin is faster than β-hydrogen elimination of the phenethylcopper complex. Studies on the hydroboration of a representative internal alkene, trans-3-hexenyl 2,4,6-trichlorobenzoate, which undergoes enantio- and regioselective addition of HBpin catalyzed by DTBM-SEGPHOS, KOtBu, and CuCl, also was conducted, and these studies revealed that a DTBM-SEGPHOS-ligated copper(I) dihydridoborate complex is the resting state of the catalyst in this case. The turnover-limiting step in the catalytic cycle for hydroboration of the internal alkene is insertion of the alkene into a copper(I) hydride formed by reversible dissociation of HBpin from the copper dihydridoborate species. With SEGPHOS as the ligand, a dimeric copper hydride was observed as the dominant species during the hydroboration of 4-fluorostyrene, and this complex is not catalytically competent. DFT calculations provide a view into the origins of regio- and enantioselectivity of the catalytic process and indicate that the charge on the copper-bound carbon and delocalization of charge onto the aryl ring control the rate of the alkene insertion and the regioselectivity of the catalytic reactions of vinylarenes.
报道了铜催化的乙烯基芳烃和内烯烃不对称硼氢化反应的机理研究。研究了以 DTBM-SEGPHOS 和 SEGPHOS 为配体的催化体系。以 DTBM-SEGPHOS 为配体时，硼氢化反应中催化剂的休眠状态（也是催化中间体）是一个被双膦配体配位的苯乙基铜（I）配合物。该配合物通过 NMR 光谱和 X 射线晶体学得到了充分的表征。在乙烯基芳烃反应的催化循环中，限制步骤是该苯乙基铜配合物与频哪醇硼烷（HBpin）的硼化反应，生成硼酸酯产物和铜氢化物。实验表明，在苄位的构型保持下发生硼化反应。在没有 HBpin 的情况下，β-氢消除和烯烃的插入反应可逆，但在催化过程中不可逆，因为 HBpin 的反应速度快于苯乙基铜配合物的β-氢消除。还对具有代表性的内烯烃反-3-己烯基 2,4,6-三氯苯甲酸酯的硼氢化反应进行了研究，该反应在 DTBM-SEGPHOS、KOtBu 和 CuCl 的催化下，经历 HBpin 的对映选择性和区域选择性加成，这些研究表明，在这种情况下，催化剂的休眠状态是 DTBM-SEGPHOS 配位的铜（I）二氢硼酸酯配合物。内烯烃硼氢化反应的催化循环中的限速步骤是烯烃插入由 HBpin 可逆解离形成的铜（I）氢化物。以 SEGPHOS 为配体时，在 4-氟苯乙烯的硼氢化过程中观察到二聚铜氢化物作为主要物种，而这种配合物没有催化活性。DFT 计算为催化过程的区域和对映选择性的起源提供了一个视角，并表明铜结合碳上的电荷和电荷在芳环上的离域控制了烯烃插入的速度和乙烯基芳烃催化反应的区域选择性。
