Counterions of manganese(III) porphyrin complexes influence diastereoselectivity in cis-stilbene epoxidation and product distribution in cyclohexene epoxidation markedly. In the epoxidation of cis-stilbene by iodosylbenzene carried out in a solvent mixture of CH3CN and CH2Cl2, trans-stilbene oxide is the major product in the reaction of manganese complexes bearing a ligating anion (i.e., Cl-), whereas cis-stilbene oxide is the dominant product in the reactions of manganese complexes bearing a poorly-ligating anion (i.e., CF3SO4-). In cyclohexene epoxidation, the yields of allylic oxidation products such as cyclohexenol and cyclohexenone are higher when the counterion of the manganese catalysts is Cl- than when the counterion is CF3SO4. The product selectivities are also dependent on the nature of iodosylareries and the axial and porphyrin ligands of the manganese porphyrin catalysts. The observation that product selectivities are different depending on the iodosylarenes may indicate the involvement of multiple oxidants in oxygen atom transfer reactions. These results are compared with those observed in manganese salen-catalyzed epoxidation of olefins by iodosylarenes. (C) 2004 Elsevier Inc. All rights reserved.