Department of Chemistry, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Department of Food Science and Technology, The Ohio State University , Columbus, Ohio 43210, United States.
J Am Chem Soc. 2017 Apr 26;139(16):6008-6016. doi: 10.1021/jacs.7b02512. Epub 2017 Apr 18.
Studies of the stoichiometric and catalytic reactivity of a geometrically constrained phosphorous triamide 1 with pinacolborane (HBpin) are reported. The addition of HBpin to phosphorous triamide 1 results in cleavage of the B-H bond of pinacolborane through addition across the electrophilic phosphorus and nucleophilic N-methylanilide sites in a cooperative fashion. The kinetics of this process of were investigated by NMR spectroscopy, with the determined overall second-order empirical rate law given by ν = -k[1][HBpin], where k = 4.76 × 10 M s at 25 °C. The B-H bond activation process produces P-hydrido-1,3,2-diazaphospholene intermediate 2, which exhibits hydridic reactivity capable of reacting with imines to give phosphorous triamide intermediates, as confirmed by independent synthesis. These phosphorous triamide intermediates are typically short lived, evolving with elimination of the N-borylamine product of imine hydroboration with regeneration of the deformed phosphorous triamide 1. The kinetics of this latter process are shown to be first-order, indicative of a unimolecular mechanism. Consequently, catalytic hydroboration of a variety of imine substrates can be realized with 1 as the catalyst and HBpin as the terminal reagent. A mechanistic proposal implicating a P-N cooperative mechanism for catalysis that incorporates the various independently verified stoichiometric steps is presented, and a comparison to related phosphorus-based systems is offered.
研究了一种几何约束的膦三酰胺 1 与频哪醇硼烷（HBpin）的计量比和催化反应性。HBpin 加至膦三酰胺 1 中，通过在亲电磷和亲核 N-甲基苯胺位点上的加成协同断裂频哪醇硼烷的 B-H 键。通过 NMR 光谱研究了这个过程的动力学，确定的总二阶经验速率定律为 ν = -k[1][HBpin]，其中 k 在 25°C 时为 4.76×10 M s。B-H 键活化过程生成 P-氢代-1,3,2-二氮杂膦烯中间体 2，其具有氢化物反应性，能够与亚胺反应生成磷三酰胺中间体，这一点通过独立合成得到了证实。这些磷三酰胺中间体通常寿命较短，通过消除亚胺氢硼化的 N-硼胺产物并再生变形的磷三酰胺 1 而演变。后一过程的动力学呈一级，表明是单分子机制。因此，各种亚胺底物的催化硼氢化作用可以用 1 作为催化剂和 HBpin 作为末端试剂来实现。提出了一种涉及 P-N 协同催化机制的机理建议，该机制包含了各种独立验证的计量步骤，并与相关的磷基体系进行了比较。
