Department of Chemistry, University of California, Berkeley, California 94720, United States.
Division of Chemical Sciences, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
J Am Chem Soc. 2020 Oct 21;142(42):18213-18222. doi: 10.1021/jacs.0c08746. Epub 2020 Oct 12.
We report the incorporation of large substituents based on heavy main-group elements that are atypical in ligand architectures to enhance dispersion interactions and, thereby, enhance enantioselectivity. Specifically, we prepared the chiral biaryl bisphosphine ligand (TMG-SYNPHOS) containing 3,5-bis(trimethylgermanyl)phenyl groups on phosphorus and applied this ligand to the challenging problem of enantioselective hydrofunctionalization reactions of 1,1-disubtituted alkenes. Indeed, TMG-SYNPHOS forms a copper complex that catalyzes hydroboration of 1,1-disubtituted alkenes with high levels of enantioselectivity, even when the two substituents are both primary alkyl groups. In addition, copper catalysts bearing ligands possessing germanyl groups were much more active for hydroboration than one derived from DTBM-SEGPHOS, a ligand containing 3,5-di--butyl groups and widely used for copper-catalyzed hydrofunctionalization. This observation led to the identification of DTMGM-SEGPHOS, a bisphosphine ligand bearing 3,5-bis(trimethylgermanyl)-4-methoxyphenyl groups as the substituents on the phosphorus, as a new ligand that forms a highly active catalyst for hydroboration of unactivated 1,2-disubstituted alkenes, a class of substrates that has not readily undergone copper-catalyzed hydroboration previously. Computational studies revealed that the enantioselectivity and catalytic efficiency of the germanyl-substituted ligands is higher than that of the silyl and -butyl-substituted analogues because of attractive dispersion interactions between the bulky trimethylgermanyl groups on the ancillary ligand and the alkene substrate and that Pauli repulsive interactions tended to decrease enantioselectivity.
我们报告了基于重主族元素的大取代基的引入，这些取代基在配体结构中不典型，以增强分散相互作用，从而提高对映选择性。具体来说，我们制备了含有磷上 3,5-双（三甲基锗基）苯基基团的手性联芳基双膦配体（TMG-SYNPHOS），并将该配体应用于 1,1-二取代烯烃的对映选择性氢官能化反应这一极具挑战性的问题。事实上，TMG-SYNPHOS 形成铜配合物，可催化 1,1-二取代烯烃的硼氢化反应，具有高对映选择性，即使两个取代基都是伯烷基。此外，含有锗基的配体所负载的铜催化剂比广泛用于铜催化氢官能化的 DTBM-SEGPHOS 配体所负载的铜催化剂对硼氢化反应的活性高得多。这一观察结果导致了 DTMGM-SEGPHOS 的鉴定，它是一种含有 3,5-双（三甲基锗基）-4-甲氧基苯基基团的双膦配体，作为磷上的取代基，是一种形成高活性催化剂的新配体，用于未活化的 1,2-二取代烯烃的硼氢化反应，这一类底物以前不易发生铜催化的硼氢化反应。计算研究表明，由于大体积的三甲基锗基辅助配体和烯烃底物之间存在吸引力的分散相互作用，以及 Pauli 排斥相互作用趋于降低对映选择性，取代基的手性锗取代配体的对映选择性和催化效率高于硅和 -丁基取代的类似物。
