Electronic Effects on the Stereoselectivity of Epoxidation Reactions Catalysed by Manganese Porphyrins

Abstract

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

Manganese‐Catalyzed Oxidation with Hydrogen Peroxide

“…Improvement in the stereoselectivity of the oxidation of cis-stilbene was observed by increasing the number of substituents on the aryl groups of the porphyrin ligand, pointing to an enhanced preference for a concerted pathway. In general, it should be noted, however, that trans-alkenes are poor substrates for these catalysts [57,62].…”

Parallel mechanistic studies on the counterion effect of manganese salen and porphyrin complexes on olefin epoxidation by iodosylarenes

“…Further, the manganese complexes bearing electron-deficient porphyrin ligands showed the same product selectivities irrespective of iodosylarenes, in which cis-stilbene oxide was the dominant product in cis-stilbene epoxidation and cyclohexene oxide was the major product with the formation of small amounts of allylic oxidation products in cyclohexene epoxidation (data now shown). The electronic effect of manganese porphyrin complexes on the diastereoselectivity in cis-stilbene epoxidation was interpreted with the competition of two reaction pathways previously (i.e., the epoxide ring closure (Scheme 2(b), pathway C) vs the C-C bond rotation (Scheme 2(b), pathway D)) [60,61].…”

Partially and fully β-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

“…However, among the Mn-porphyrins used in this study, the partially b-brominated one, Mn(III)(Br 6 TCMPP)Cl, surpassed the reactivity of the fully b-brominated analogue and was consistently the most efficient catalyst for both epoxidation (with either PhIO or ÔPhIO + O 2 Õ) and hydroxylation reactions. Such a catalytic reactivity pattern, where partially b-brominated Mn-porphyrins are more active then their fully brominated counterparts, parallels with that on H 2 O 2 -and PhIO-based oxidation systems described by Baciocchi et al [7,23].…”