Reaction of an Enantiomerically Pure Phosphaalkene-Oxazoline with MeM Nucleophiles (M = Li and MgBr): Stereoselectivity and Noninnocence of the P-Mesityl Substituent

The addition of alkyl nucleophiles (MeM, M = Li, MgBr) across the P═C bond of an enantiomerically pure phosphaalkene-oxazoline followed by protonation of the C anion affords phosphines with three chirality centers. The formation of palladium(II) complexes of the resultant phosphines permitted structural characterization of the products by X-ray diffraction. The choice of nucleophile has a profound effect on the product distributions. For instance, the Grignard reagent adds in a diastereoselective manner to give one major phosphine product with P- and C-stereocenters. In contrast, addition of methyllithium has proven not only to be less stereoselective but also affords a fascinating cyclic phosphine product. Both the Grignard and RLi reactions involve proton transfer from the o -Me of the P-Mes substituent even though the products are quite different in each case.