Chiral Tetraaminophosphonium Salts: A New Catalyst Motif

tetraaminophosphonium salts - Henry reaction - ion pairing - hydrogen bonding

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).

Comment

The authors could show that tetra­aminophosphonium salts besides serving as precursors of P-1 phosphazenes exhibit truly cata­lytic potential. Access to phosphonium chlorides 3 is given by means of a concise ex-chiral-pool synthesis starting from l-valine. In the future, chiral P-spirocyclic salts of type 3 and related structures might also compete with the well-established N-spirocyclic or cinchona alkaloid derived ammonium salts in phase-transfer catalysis. Ensuing reports on the use of chiral phosphonium salts in further reactions proceeding via anionic intermediates are expected.