Efficient and chemoselective hydroboration of organic nitriles promoted by TiIV catalyst supported by unsymmetrical acenaphthenequinonediimine ligand

Abstract

We report the synthesis, characterization, and utilization of a titanium (IV) complex [(η5-C5H5){L}TiCl2] (1) supported by a monoanionic ligand (L), N-(2, 6-diisopropyl)acenaphthenequinonediimido, as a molecular pre-catalyst for the hydroboration of nitriles. The unsymmetrical N-silylated N-(2, 6-diisopropyl)-N-(trimethylsilyl)-acenaphthenequinonediimine ligand (LSiMe3) was obtained upon the completion of a one-pot reaction between N-(2, 6-diisopropyl)iminoacenaphthenone and lithium hexamethyldisilazide in the presence of trimethylsilyl chloride in 1:1:1 M ratio at 90 °C. The reaction of LSiMe3 with {η5-(C5H5)TiCl3) in equal proportion (1:1) at 60 °C afforded the titanium complex [(η5-C5H5){L}TiCl2] (1) in good yield. The molecular structures of the N-silyl ligand (LSiMe3) and Ti(IV) complex 1 were established by single-crystal X-ray analysis. Complex 1 was tested as a pre-catalyst for hydroboration of nitriles with pinacolborane (HBpin) and catecholborane (HBcat) to afford diboryl amines at ambient temperature. Titanium complex 1 exhibited high conversion, superior selectivity, and broad functional group tolerance during hydroboration of nitriles with both HBpin and HBcat under mild conditions.