Lewis acidic catalysts for olefin epoxidation by iodosylbenzene

Article abstract of DOI:10.1021/ja00019a016

A ferric complex, (Et₃HN) Fe^III(bpb)Cl₂, has been synthesized, and its structure has been determined by X-ray crystallography. This complex and its inflate derivative, (Et₃HN)Fe^III(bpb)(OTf)₂, are found to catalyze the epoxidation of a variety of olefins by iodosylbenzene, OIPh. These reactions give little allylic oxidation of cyclohexene and stereochemical retention with cis-stilbene. Al(OTf)₃, a nonredox metal salt, has also been found to catalyze the epoxidation of cyclohexene by iodosylbenzene, and the reactivity is quite similar to that of Fe(OTf)₃, which, we studied previously. In addition to epoxides, other products were observed. For the reactions containing Fe(OTf)₃, Al(OTf)₃, or (Et₃HN) Fe^III(bpb)(OTf)₂, cis-1,2-cyclohexanediol ditrifilate and 3-acetamidocyclohexene were found. The amide oxygen in 3-acetamidocyclohexene was derived from iodosylbenzene as verified by isotopic labeling using ¹⁸OIPh. For the reactions containing (Et₃HN) Fe^III(bpb)Cl₂ FeCl₃, or AlCl₃, trans-1,2-dichlorocyclohexane and 3-chlorocyclohexene were observed. 1,4-Diiodobenzene was found in all of the reactions. The presence of these products suggests strongly that the mechanisms of these reactions are related to those occurring between soluble iodine(III)-containing compounds and olefins in the absence of any metal catalysts. A new mechanisms that accounts for all of the products is proposed which involves electrophilic attack on the olefin by the iodine(III) center in a metal-iodosylbenzene complex. The reactions of PhI(OAc)₂ with norbornenecarboxylic acid or nortornene in different solvents were also investigated. The products isolated were shown to be 5-(acetyloxy)-3,3a,4,5,6,6a-hexahydro-[3β,3aα,5α,6β, 6aα]-3,6-methano-2H-cyclopenta [b] furan-2-one (1), 5-acetamido-3,3a,4,5,6,6a-hexahydro-[3β,3aα,5α,6β, 6aα]-3,6-methano-2H-cyclopenta [b] furan-2-one (2), and exo-2-acetoxy-syn-7-acetamidonorbornane (3). The structures of 1 and 3 were determined by X-ray crystallography. The formation of these products provides additional evidence for the electrophilic character of iodine(III) compounds.