Electronic effects on the stereoselectivity of epoxidation reactions catalysed by manganese porphyrins

A series of Mn-III porphyrins progressively halogenated in the beta-pyrroli c positions was employed to catalyse the epoxidation of cis-stilbene by iod osylbenzene, and to study the role of the electronic effects on the stereos electivity of this process. A gradual improvement in the stereoselectivity on increasing the number of beta-halogen atoms was observed. The role of st eric effects upon the epoxidation was also investigated by placing ortho-su bstituents in the meso-phenyl rings, and it was found that steric effects a re more important than electronic effects toward the stereoselectivity of t his process. These results can be rationalised by proposing a competition b etween a nonstereoselective electrophilic pathway of addition and a stereos pecific pathway of oxygen insertion, the former being disfavoured by electr on-withdrawing substituents. Alternatively, the formation of an open interm ediate between the Mn-V oxene and the substrate could be suggested, where t he stereoselectivity ought to be determined by the competition between clos ure of the epoxide ring and rotation around the C-C bond. In this case, the enhanced stereoselectivity given by our polyhalogenated porphyrins might b e attributed to an acceleration of the epoxide ring closure caused by the e lectron-withdrawing effect of the halogen substituents.