Yamazaki Yasuomi, Ishitani Osamu
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan.
Dalton Trans. 2017 Apr 5;46(14):4816-4823. doi: 10.1039/c7dt00922d.
The addition of a tertiary phosphine and O to reaction solutions strongly affected the reactivity and selectivity of coupling reactions between transition metal complexes. The Mizoroki-Heck reaction between metal complexes with bromo and those with vinyl groups in the diimine ligand did not proceed using Pd(OAc) in the presence of 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (Sphos) under Ar but proceeded selectively after injection of air into the reaction vessel. In the absence of the phosphine ligand, on the other hand, not only the Mizoroki-Heck reaction but also a homo-coupling reaction between the metal complexes with the bromo groups proceeded at the same time. Mechanistic investigation showed that nanoparticles of Pd species were produced in the absence of the phosphine ligand and worked as catalysts for both the Mizoroki-Heck and homo-coupling reactions. On the other hand, larger Pd particles, which were produced in the presence of Sphos but after addition of air for oxidising Sphos, selectively catalysed the Mizoroki-Heck reaction. 'Molecular' Pd species that were stabilised in the presence of non-oxidised Sphos could not catalyse both coupling reactions under the reaction conditions. Based on these results, reaction conditions were established for the selective progress of the Mizoroki-Heck and the homo-coupling reactions.
向反应溶液中添加叔膦和氧气会强烈影响过渡金属配合物之间偶联反应的反应活性和选择性。在氩气氛围下，使用醋酸钯（Pd(OAc)），在2-二环己基膦基-2',6'-二甲氧基联苯（Sphos）存在的情况下，二亚胺配体中含溴金属配合物与含乙烯基金属配合物之间的 Mizoroki-Heck 反应不发生，但向反应容器中注入空气后反应选择性地进行。另一方面，在没有膦配体的情况下，不仅 Mizoroki-Heck 反应，含溴金属配合物之间的均偶联反应也会同时发生。机理研究表明，在没有膦配体的情况下会生成钯物种的纳米颗粒，它们同时作为 Mizoroki-Heck 反应和均偶联反应的催化剂。另一方面，在 Sphos 存在但添加空气氧化 Sphos 后生成的较大钯颗粒选择性地催化 Mizoroki-Heck 反应。在未氧化的 Sphos 存在下稳定的“分子”钯物种在反应条件下不能催化这两种偶联反应。基于这些结果，建立了 Mizoroki-Heck 反应和均偶联反应选择性进行的反应条件。
