Homogeneous Catalytic Dehydrocoupling/Dehydrogenation of Amine−Borane Adducts by Early Transition Metal, Group 4 Metallocene Complexes

Abstract

The efficient catalytic dehydrocoupling of a range of amine−borane adducts, R′RNH·BH₃ (R′ = R = Me <b>1a</b>; R′ = R = ^<i>i</i>Pr <b>1b</b>; R′ = Me, R = CH₂Ph <b>1c</b>) by a series of group 4 metallocene type precatalysts has been demonstrated. A reduction in catalytic activity was detected upon descending the group and also on substitution of the cyclopentadienyl (Cp) ligands with sterically bulky or electron-donating substituents. Precatalysts Cp₂TiCl₂/2^<i>n</i>BuLi and Cp₂Ti(PMe₃)₂, which are believed to act as precursors to [Cp₂Ti], were found to promote the transformation of <b>1a</b> to [Me₂N-BH₂]₂ (<b>3a</b>) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me₂NH-BH₂−NMe₂-BH₃ (<b>2a</b>) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer <b>3a</b>. Synthesis of the ²H-isotopologues of <b>1a</b> allowed the extraction of phenomenological kinetic isotope effects for <b>1a</b> → <b>2a</b> and <b>2a</b> → <b>3a</b> from initial rate data, which permitted the proposal of a catalytic cycle with plausible intermediates. Support for the presence of an active Ti(II) catalyst was provided by the lack of reactivity of Ti(III) complexes TiCl₃ and Cp₂TiCl or Ti(0) in the form of THF soluble colloids or bulk Ti powder toward <b>1a</b> or <b>1b</b>. Modeling of the rates of consumption of <b>1a</b> and formation of <b>3a</b> during catalysis by Cp₂Ti(PMe₃)₂ supported this conclusion and allowed the proposal of a two cycle, four step reaction mechanism. The proposed first cycle generates <b>2a</b> in a two step process. In the second cycle, interaction of <b>2a</b> with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form <b>3a</b>, also in a two step process.