Hetero-dehydrocoupling of silanes and amines by heavier alkaline earth catalysis

Abstract

The homoleptic alkaline earth hexamethyldisilazides, [M{N(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.