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

The efficient catalytic dehydrocoupling of a range of amine-borane adducts, R'RNH center dot BH3 (R' = R = Me 1a; R' = R = 'Pr 1b; R' = Me, R = CH2Ph 1c) 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(n)BuLi and CP2Ti(PMe3)(2), which are believed to act as precursors to [CP2Ti], were found to promote the transformation of 1a to [Me2N-BH2](2) (3a) in a homogeneous catalytic process. Mechanistic studies identified the linear dimer Me2NH-BH2-NMe2-BH3 (2a) as a reaction intermediate, which subsequently undergoes further catalytic dehydrogenation to form cyclic dimer 3a. Synthesis of the H-2-isotopologues of 1a allowed the extraction of phenomenological kinetic isotope effects for 1a -> 2a and 2a -> 3a 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 1a or 1b. Modeling of the rates of consumption of la and formation of 3a 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 2a in a two step process. In the second cycle, interaction of 2a with the same catalyst then results in a catalytic dehydrogenative ring closing reaction to form 3a, also in a two step process.