Stoichiometric and catalytic reactivity of tert-butylamine-borane with calcium silylamides

The primary amine-borane t-BuNH₂·BH₃ reacts with a β-diketiminate-supported silylamido calcium complex with elimination of HN(SiMe₃)₂ and formation of the corresponding primary amidoborane complex, in which the deprotonated amine-borane is attached to the alkaline-earth center via its nitrogen atom and anagostic interactions with the boron-bound hydrides. Catalytic dehydrocoupling reactions employed with this β-diketiminate precatalyst are found to be slow and complicated by protonation of the supporting ligand and the formation of a number of boron-containing products, all of which have been positively identified. In common with previous studies of group 2 catalyzed secondary amine-borane dehydrogenation, the first formed major product of the catalysis is identified by solution NMR and solid-state single-crystal X-ray studies to be a cyclic diborazane, [H^tBuN-BH₂]₂, the formation of which is accompanied by variable proportions of diamidoborane and aminoborane products. The active calcium species is also observed to be depleted during the catalysis due to the formation of hydrocarbon-insoluble [Ca(BH₄)₂·THF]_∞, which has also been structurally characterized. Continued heating of these reaction mixtures results in the formation of cyclic trimeric 1,3,5-tri-tert-butylborazine, which is proposed to form through the intermediacy of [H^tBuN-BH₂]₂ by an, as yet, undefined sequence of borazane dehydrogenation and ring expansion reactions.