作者:Michael S. Hill、David J. Liptrot、Dugald J. MacDougall、Mary F. Mahon、Thomas P. Robinson
DOI:10.1039/c3sc51797g
日期:——
The homoleptic alkaline earth hexamethyldisilazides, [MN(SiMe3)2}2]2 (1: M = Mg; 2: M = Ca; 3: M = Sr), have been demonstrated as active pre-catalysts for the cross-dehydrocoupling of Si–H and N–H bonds under mild (25–60 °C) conditions. The reactions are applicable to the coupling of a wide variety of amine and silane substrates and are proposed to occur via a sequence of discrete Si–H/M–N and N–H/M–H metathesis steps. Whereas reactions of dialkyl group 2 species with 2,6-di-iso-propylaniline and phenylsilane delivered a series of well-defined compounds consistent with this rationale, kinetic analysis of the cross-coupling of diethylamine with diphenylsilane provided evidence for a more complex and subtly variable mechanistic landscape. Although reactions performed with all three pre-catalysts presented a number of common features, in every case the calcium species, 2, was found to provide notably superior catalytic activity, an order of magnitude higher than both 1 and 3 and in excess of many previously described benchmark transition metal- or f-element-mediated processes. Variations in overall reaction order, mode of pre-catalyst activation and the nature of the rate determining process are postulated to arise as a consequence of the marked change in M2+ radius and resultant charge density as group 2 is descended.
均配位碱土金属六甲基二硅氨基化合物 [MN(SiMe3)2}2]2 (1: M = Mg; 2: M = Ca; 3: M = Sr) 已被证明是在温和(25–60 °C)条件下进行 Si–H 和 N–H 键交叉脱氢耦合的活性前催化剂。这些反应适用于各种胺和硅烷底物的耦合,推测通过一系列离散的 Si–H/M–N 和 N–H/M–H 置换步骤进行。尽管与 2,6-二异丙基苯胺和苯基硅烷反应的二烷基物种 2 给予了一系列与此推断一致的定义良好的化合物,但在二乙胺与二苯硅烷交叉耦合的动力学分析提供了更为复杂且微妙变化的反应机制证据。虽然使用三种前催化剂进行的反应表现出一些共同特征,但在每种情况下,钙物种 2 显示出明显优越的催化活性,显著高于 1 和 3,并且超出了许多先前描述的基准过渡金属或 f 元素介导的过程。整体反应级数、前催化剂活化方式及速率决定过程的性质变化被推测为由于 M2+ 半径的显著变化及随族下降而导致的电荷密度变化。