1,2-Diamine-Derived (thio)Phosphoramide Organocatalysts in Asymmetric Michael Additions

Phosphoramides and thiophosphoramides were prepared from optically pure C-2-symmetric 1,2-diamines and were used as chiral organocatalysts in the asymmetric Michael additions of aldehydes and ketones to N-substituted maleimides. The 1,2-diphenylethane-1,2-diamine derived thiophosphoramide, which could be prepared in good yield in a one-step procedure, was found to be more active and selective catalyst in the addition of aldehydes to various maleimide derivatives, when compared to sulfonamides having the same backbone. Products resulted in reactions of ketones with maleimides were also obtained in high yields and enantioselectivities. The thiophosphoramide derivative was also efficient in the asymmetric conjugate addition of carbonyl compounds to beta-nitrostyrene and in the reaction of nitromethane with alpha,beta-unsaturated ketones. Based on results obtained with (thio)phosphoramides in asymmetric additions to maleimides it was suggested that a weaker, more flexible hydrogen-bonding of the rigid electrophile to the catalyst is responsible for the improved performance of these bifunctional organocatalysts, as compared with sulfonamides.