Reversible aryl migrations in metallated ureas: controlled inversion of configuration at a quaternary carbon atom

Deprotonation with strong bases of N-vinyl ureas carrying an N'-aryl substituent leads to migration of the N'-aryl group from N to C via an allyllithium; with weaker bases and electron-deficient aryl rings the direction of the migration reverses, and aryl substituents alpha to the urea N atom may migrate from C to N.