Zirconium-Catalyzed Amine Borane Dehydrocoupling and
Transfer Hydrogenation
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Abstract
κ<sup>5</sup>-(Me<sub>3</sub>SiNCH<sub>2</sub>CH<sub>2</sub>)<sub>2</sub>N(CH<sub>2</sub>CH<sub>2</sub>NSiMe<sub>2</sub>CH<sub>2</sub>)Zr (<b>1</b>) has been found to dehydrocouple amine borane
substrates, RR′NHBH<sub>3</sub> (R = R′ = Me; R = <sup><i>t</i></sup>Bu, R′ = H; R = R′ = H), at
low to moderate catalyst loadings (0.5–5 mol %) and good to
excellent conversions, forming mainly borazine and borazane products.
Other zirconium catalysts, (N<sub>3</sub>N)ZrX [(N<sub>3</sub>N) =
N(CH<sub>2</sub>CH<sub>2</sub>NSiMe<sub>2</sub>CH<sub>2</sub>)<sub>3</sub>, X = NMe<sub>2</sub> (<b>2</b>), Cl (<b>3</b>), and O<sup><i>t</i></sup>Bu (<b>4</b>)], were found
to exhibit comparable activities to that of <b>1</b>. Compound <b>1</b> reacts with Me<sub>2</sub>NHBH<sub>3</sub> to give (N<sub>3</sub>N)Zr(NMe<sub>2</sub>BH<sub>3</sub>) (<b>5</b>), which
was structurally characterized and features an η<sup>2</sup> B–H σ-bond amido borane ligand. Because <b>5</b> is unstable with respect to borane loss to form <b>2</b>,
rather than β-hydrogen elimination, and <b>2</b>–<b>4</b> do not exhibit X ligand loss during catalysis, dehydrogenation
is hypothesized to proceed <i>via</i> an outer-sphere-type
mechanism. This proposal is supported by the catalytic hydrogenation
of alkenes by <b>2</b> using amine boranes as the sacrificial
source of hydrogen
