A series of Mn III porphyrins progressively halogenated in the electrophilic pathway of addition and a stereospecific pathway of oxygen insertion, the former being disfavoured ฮฒ-pyrrolic positions was employed to catalyse the epoxidation of cis-stilbene by iodosylbenzene, and to study by electron-withdrawing substituents. Alternatively, the formation of an open intermediate between the Mn V oxene the role of the electronic effects on the stereoselectivity of this process. A gradual improvement in the stereoselectivity and the substrate could be suggested, where the stereoselectivity ought to be determined by the competition on increasing the number of ฮฒ-halogen atoms was observed. The role of steric effects upon the epoxidation was also between closure of the epoxide ring and rotation around the C-C bond. In this case, the enhanced stereoselectivity given investigated by placing ortho-substituents in the mesophenyl rings, and it was found that steric effects are more by our polyhalogenated porphyrins might be attributed to an acceleration of the epoxide ring closure caused by the important than electronic effects toward the stereoselectivity of this process. These results can be rationalised by electron-withdrawing effect of the halogen substituents. proposing a competition between a nonstereoselective depends upon the structure of the reactants and also upon