cis-Stilbene and (1α,2β,3α)-(2-Ethenyl-3-methoxycyclopropyl)benzene as Mechanistic Probes in the Mn^III(salen)-Catalyzed Epoxidation:  Influence of the Oxygen Source and the Counterion on the Diastereoselectivity of the Competitive Concerted and Radical-Type Oxygen Transfer

Abstract

cis-Stilbene (1) has been epoxidized by a set of diverse oxygen donors [OxD], catalyzed by the Mn(III)(salen)X complexes 3 (X = Cl, PF(6)), to afford a mixture of cis- and trans-epoxides 2. The cis/trans ratios range from 29:71 (extensive isomerization) to 92:8, which depends both on the oxygen source [OxD] and on the counterion X of the catalyst. When (1 alpha,2 beta,3 alpha)-(2-ethenyl-3-methoxycyclopropyl)-benzene (4) is used as substrate, a mechanistic probe which differentiates between radical and cationic intermediates, no cationic ring-opening products are found in this epoxidation reaction; thus, isomerized epoxide product arises from intermediary radicals. The dependence of the diastereoselectivity on the oxygen source is rationalized in terms of a bifurcation step in the catalytic cycle, in which concerted Lewis-acid-activated oxygen transfer competes with stepwise epoxidation by the established Mn(V)(oxo) species. The experimental counterion effect is attributed to the computationally assessed ligand-dependent reaction profiles and stereoselectivities of the singlet, triplet, and quintet spin states available to the manganese species.

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: A theoretical study

“…To verify this prediction, experimental kinetics data on the epoxidation rates would be indispensable. Unfortunately, recent experimental mechanistic studies of the Mn III (salen)-catalyzed epoxidation by peracids lack solid kinetics data due to very fast (within 1 min) epoxidation rates even at Ϫ78°C (14,(16)(17)(18)(19)35). Nevertheless, for closely related Mn III (porphyrin) catalysts the rates of the Mn V oxo species formation and͞or subsequent epoxidation have been estimated in water and acetonitrile at room temperature and agree with our predictions (51,52).…”

“…Distortions of both types have been shown to increase the chirality content of the trans-Mn V -oxo species (29). Although the trans-Mn V -oxo hypothesis can successfully explain most of the experimental data, in certain cases enantioselectivity depends on the oxygen source and reaction conditions, suggesting alternative oxygenating species (17,18,(30)(31)(32)(33)(34). The most dramatic enantioselectivity variations are observed for a synthetically important class of oxidants, organic peracids (30)(31)(32)(33)(34).…”

“…The latter is tentatively attributed to the direct epoxidation by acylperoxo complexes of Mn III (salen) (30)(31)(32). Despite recent experimental mechanistic studies of the Mn III (salen)-catalyzed epoxidation by peracids, the nature of oxygenating species remains uncertain (14,(16)(17)(18)(19)35).…”

“…Understanding the mechanism of the epoxidation and the origin of asymmetric induction can lead to the development of new efficient catalysts and therefore is actively pursued (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Numerous studies assume the Mn V -oxo species postulated by Kochi and coworkers (20) to be the enantioselective oxidant.…”

Study of Catalytic Activity of Silica Supported Schiff Base Complexes

“…Among these reported Schiff base cobalt(II) complexes catalysts were known to bind dioxygen more or less reversibly and therefore frequently studied as model compounds for natural oxygen carrier and for their use in O2 storage, as well as in organic syntheses due to their catalytic properties under mild conditions [13,14]. Copper(II) complex with a tridentate imine, was revealed an efficient catalyst for oxidation of phenol, cyclohexene, styrene and benzyl alcohol using hydrogen peroxide and tert-butyl hydroperoxide as oxidant in homogeneous reaction [15][16][17].…”