Base catalysed rearrangements involving ylide intermediates. Part 15. The mechanism of the Stevens [1,2] rearrangement

Abstract

The Stevens [1,2] rearrangement of acyl-stabilised ammonium ylides has been investigated with regard to stereoselectivity, intramolecularity and the formation of products in addition to the [1,2] rearrangement product. A detailed study of the effects of reaction conditions upon the rearrangement of the ylide derived from the salt (13) has shown that the stereoselectivity (retention of the configuration of the chiral migrating group) and intramolecularity decrease as solvent viscosity decreases. The rearrangement of the salt (13) in water at 0 °C is essentially intramolecular with virtually complete retention of the configuration of the migrating group. These results, together with the isolation of products that can be rationalised on the basis of random free-radical coupling, indicate that the [1,2] rearrangement of acyl-stabilised ammonium ylides normally involves a radical pair mechanism.