Photo-Initiated Radical Hydrophosphination at Titanium Compounds Capable of Ti–P Insertion
Dennis M. Seth, Rory Waterman
Abstract
Titanium compounds supported by the triamidoamine ligand (N 3 N, N(CH 2 CH 2 NSiMe 3 3– ) 3 ) have been investigated for hydrophosphination catalysis. The simple titanium alkyl compound (N 3 N)TiMe ( 2 ) demonstrates modest activity as a precatalyst for the photocatalytic hydrophosphination of styrene, but the terminal phosphido compound, (N 3 N)TiPHPh ( 4 ), is inactive. Analysis of these reactions by EPR spectroscopy indicates that (N 3 N)TiMe undergoes homolytic Ti–C bond cleavage to achieve radical hydrophosphination. This pathway was further supported in a radical trapping experiment. The phosphido derivative 4 does not produce radicals under similar conditions, despite undergoing facile migratory insertion reactions with polar substrates featuring C–N and C–O multiple bonds. Both nitrile and isonitrile substrates insert with ultimate formation of phosphaalkene products, a change in reactivity as compared to the zirconium congener. Molecular structures of (N 3 N)TiPHPh ( 4 ), (N 3 N)TiNBnC═PPh ( 5 ), and (N 3 N)TiN(H)C(Ph)═PPh ( 6 ) are reported.