Synthesis of Iron P-N-P′ and P-NH-P′ Asymmetric Hydrogenation Catalysts

Complexes of the type mer,trans-[Fe(P-N-P')(CO)(2)Br]BF4 are known to be precatalysts for the asymmetric direct hydrogenation of ketones and imines. Employing related ligand scaffolds, we successfully generated and tested the series of three new precatalysts [Fe(PCy2CH2CH-NCH(R)CH2PPh2)(CO)(2)Br]BF4 with chirality derived from (S)-amino alcohols with phenyl, benzyl, and isopropyl substituents (R), yielding fairly active and selective systems. For the reduction of acetophenone to (S)-1-phenylethanol turnover frequencies up to 920 h1 and up to 74% enantiomeric excess at 50 degrees C and 525 atm of H-2 were obtained. We found, however, that placing these large groups R next to nitrogen was found to be deleterious to catalytic activity. Extending the scope of the ligand structure, we then developed a series of six P-N-P and five P-NH-P' systems starting with o-diphenylphosphinobenzaldehyde and the phosphine-amines PPh2CHR(1)CHR2NH(2) (R-1 = H, Ph, CH2Ph, iPr with R-2 = H or R1 = Me, Ph with R-2 = Ph) as well as their corresponding [Fe(P-N-P')(NCMe)(3)][BF4](2) and [Fe(P-NH-P')(NCMe)(3)][BF4](2) complexes, which were not catalytically active. Finally, we made the new achiral iron complex mer,cis-Fe(PPh2(o-C6H4)CHNCH2CH2PPh2)(CO)Br-2, which was active for the direct hydrogenation of acetophenone, achieving turnover frequencies of 800 h(-1) at 50 degrees C and 25 atm of H-2.