Chiral Tetraaminophosphonium Salts: A New Catalyst Motif

Significance

Ooi and co-workers have introduced chiral tetraaminophosphonium chlorides of type 3 as catalysts of the asymmetric Henry reaction of a broad range of aldehydes 1 and nitroalkanes 2. The authors’ working hypo­thesis includes (1) the in situ generation of tri­aminoiminophosphorane 5 upon deprotonation of 3 with KOt-Bu. (2) Phosphorane 5 will subsequently deprotonate nitroalkanes 2. The resulting chiral phosphonium nitronate ion pair 6 benefits from hydrogen bonding and enables (3) a highly stereo­selective addition of aldehydes 1 in the final step. The ability of phosphonium salts 3 acting as hydrogen bonding donors via the N-H protons could be demonstrated by an X-ray crystal structure of one representative (Ar = Ph).