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Chiral Tetraaminophosphonium Salts: A New Catalyst Motif
D. Uraguchi, S. Sakaki, T. Ooi*
Nagoya University, Japan
22 November 2007 (online)
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).