Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine–Borane Complexes

We report a series of titanium and
zirconium complexes supported
by dianionic amidophosphine–borane ligands, synthesized by
amine elimination and salt metathesis reactions. The TiIV complex [{Ph2P­(BH3)­N}2C6H4Ti­(NMe2)2] (1) was
obtained by the reaction between tetrakis-(dimethylamido)­titanium­(IV)
and the protic aminophosphine–borane ligand [{Ph2P­(BH3)­NH}2C6H4] (LH2) at ambient temperature. Both the heteroleptic zirconium
complexes[η5-(C5H5)2Zr­{Ph2P­(BH3)­N}2C6H4] (2) and [[{Ph2P­(BH3)­N}2C6H4]­ZrCl2] (3)and the homoleptic zirconium complex [[{Ph2P­(BH3)­N}2C6H4]2Zr] (4) were obtained in good yield by the salt metathesis
reaction of either zirconocene dichloride [η5-(C5H5)2ZrCl2] or zirconium tetrachloride
with the dilithium salt of the ligand [{Ph2P­(BH3)­NLi}2C6H4] (LLi2),
which was prepared in situ. The molecular structures of the complexes 1, 2, and 4 in their solid states
were confirmed by single-crystal X-ray diffraction analysis. Of these
complexes, only titanium complex 1 acts as an effective
catalyst for the facile hydroboration of terminal alkynes, yielding
exclusive E-isomers. The hydroboration of organic
nitriles yielded diborylamines with a broad substrate scope, including
broad functional group compatibility. The mechanism of hydroboration
occurs through the formation of titanium hydride as an active species.