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 Ti IV complex [{Ph 2 P(BH 3 )N} 2 C 6 H 4 Ti(NMe 2 ) 2 ] ( 1 ) was obtained by the reaction between tetrakis-(dimethylamido)titanium(IV) and the protic aminophosphine-borane ligand [{Ph 2 P(BH 3 )NH} 2 C 6 H 4 ] ( LH2 ) at ambient temperature. Both the heteroleptic zirconium complexes-[η 5 -(C 5 H 5 ) 2 Zr{Ph 2 P(BH 3 )N} 2 C 6 H 4 ] ( 2 ) and [[{Ph 2 P(BH 3 )N} 2 C 6 H 4 ]ZrCl 2 ] ( 3 )-and the homoleptic zirconium complex [[{Ph 2 P(BH 3 )N} 2 C 6 H 4 ] 2 Zr] ( 4 ) were obtained in good yield by the salt metathesis reaction of either zirconocene dichloride [η 5 -(C 5 H 5 ) 2 ZrCl 2 ] or zirconium tetrachloride with the dilithium salt of the ligand [{Ph 2 P(BH 3 )NLi} 2 C 6 H 4 ] ( 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.