Phosphonium asymmetric Henry reaction

In 2007, Ooi and coworkers introduced chiral tetraaminophosphonium salts as a new class of Bronsted acids [166]. Similar to the guanidine/guanidinium case, these tetraaminophosphonium salts act as Bronsted bases in their neutral/ deprotonated (triaminoiminophosphorane) form, while they can also be used as mono-functional Bronsted acids in their protonated, phosphonium form. Phos-phonium salt 67, when neutralized in situ with KO Bu, was shown to be a highly effective catalyst in the enantioselective Henry reaction of nitroalkanes with various aromatic and aliphatic aldehydes (Scheme 10.65). The same strategy was further applied to the catalytic asymmetric Henry reaction of ynals [167] and hydrophosphonylation of ynones (Scheme 10.66) [168]. Brfunctional catalysis using this scaffold were also obtained using the carboxylate salts of tetraaminophosphoniums in the direct Mannich reaction of sulfonyl imines with azlactones (Scheme 10.67) [169]. 

Scheme 29.5 Proposed mechanism for the phosphonium salt mediated asymmetric Henry reaction.

QM/MM calculations and experimental kinetic study have explored the effects of solvation on the transition states for reaction between nitromethane and formaldehyde and between nitropropane and benzaldehyde. Asymmetric reactions of nitromethane with various aldehydes have been promoted by Cu(II) coordinated with amino alcohols, 0 imidazolium/pyrrolidinium-tagged Indabox, and imidazolium-taggedbis(oxazoline)-based chiral ligands. The Henry reaction has also been promoted by Mn(OAc)2/Schiff bases bearing a triazole structure, with up to 99% yield, and by phosphonium ionic ligands MeP+(octyl)3 R0C02 without solvent. ... 

---