Stoichiometric and Catalytic Reactivity of tert-Butylamine–Borane with Calcium Silylamides

Abstract

The primary amine borane t-BuNH 2 .BH 3 reacts with a β-diketiminate-supported silylamido calcium complex with elimination of HN(SiMe 3) 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 pre-catalyst 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 t BuN-BH 2 ] 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(BH 4) 2 .THF] ∞ , which has also been structurally characterized. Continued heating of these reactions results in the formation of cyclic trimeric 1,3,5-tri(tert-butyl)borazine which is proposed to form through the intermediacy of [H t BuN-BH 2 ] 2 by an, as yet, undefined sequence of borazane dehydrogenation and ring expansion reactions.

The Catalytic Dehydrocoupling of Amine–Boranes and Phosphine–Boranes

“…The mechanism of formation of these species is proposed to proceed via the formation of an amidoborane complex which undergoes β-hydride elimination to give an aminoborane and a metal hydride. The free amine then reacts with the The first example of a main-group catalyst which formed a product with an equal B:N ratio was also from the group of Hill [132] BuH] 2 (45%) although 14% of the starting substrate remained unreacted [133].…”

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

“…Our own interest in this topic is motivated by the use of alkaline-earth (Ae) elements as non-toxic and inexpensive catalysts for the synthesis of inorganic polymers. 15,16 Following previous investigations into Ae-mediated catalytic and stoichiometric amine-borane dehydrogenation and dehydrocoupling, [17][18][19][20][21][22][23][24][25][26] we recently reported a series of β-diketiminate (BDI) supported alkaline-earth (Ae) phosphidoborane complexes, which are prepared by exposing diphenylphosphine-borane to readily accessible BDI-Ae hydride, alkyl, or amide precursors (Scheme 2a). 27 Attempts to achieve complete phosphine-borane dehydrogenation were unsuccessful, however, and the use of super-stoichiometric quantities of phosphine-borane resulted in BH 3 transfer to provide the phosphinodiboronate complex 2 and uncomplexed diphenylphosphine (Scheme 2a).…”

Ionic Liquid Facilitated Dehydrogenation of tert-Butylamine Borane

“…However, it takes less time when compared to several metal catalyst-facilitated dehydrogenations. 5,6 However, for diammoniate diborane (DADB), which is formed during the dehydrogenation of AB, Stowe et al 25 identifies the −BH 2 resonance at δ = −13 ppm. Similarly, Neiner et al 26 observed the −BH 2 resonance at δ = −10 ppm that originates from the bimolecular EDAB resulting from the thermal dehydrogenation of pure EDAB.…”

“…Before assigning the 11 B resonances of the moieties of TBAB/IL systems during dehydrogenation, we take a closer look of assigned 11 B NMR characterization from the literature. The prominent route for dehydrogenation of TBAB remains transition metal catalyst-facilitated dehydrogenation. − The dehydrogenation, as described by Bellham et al, follows the β-hydride elimination from TBAB to form aminoborane [ t -BuNHBH 2 ] having a chemical shift at δ = 38.2 ppm. Additionally, several new born intermediates are reported, such as diamidoborane [HB­(NH- t -Bu) 2 ] at δ = 29.1 ppm (doublet) and diborazane [( t -BuHN–BH 2 ) 2 ] at δ = −6.1 ppm (triplet).…”

“…Metal-catalyzed dehydrogenation is the most prominent route for dehydrogenation of TBAB. Bellham et al . reported the catalytic dehydrogenation of TBAB with β-diketiminate-supported silylamido calcium complex.…”