Origin of Enantioselectivity in the Jacobsen Epoxidation of Olefins

Abstract

It is proposed that facial selectivity in the Jacobsen epoxidation is determined by electrostatic and steric factors with a two-step pathway involving a carbocationic intermediate.

Mechanism of the Enantioselective Oxidation of Racemic Secondary Alcohols Catalyzed by Chiral Mn(III)−Salen Complexes

“…The above pathway involves the same canted (i.e., nonplanar) geometry for the Mn(V)–salen complex III as proposed recently for the Jacobsen epoxidation 7,8. That canted conformation places one of the ortho tert -butyl groups above and one below the N 2 O 2 ligand plane, with the further assumption that the hydrogen is transferred to that phenoxy oxygen with the proper orientation of the one electron pair acceptor 20.…”

“…At the outset of this study, perhaps the most obvious pathway for enantioselective oxidation of secondary alcohols by the reagents Mn(III)–salen, KBr, and PhI(OAc) 2 under the conditions of Xia et al1 that would follow from our earlier mechanistic proposal for the Jacobsen epoxidation (Figure 1)8 seemed to be the one summarized in Figure 2. Such a pathway involving abstraction of the carbinol C–H by the oxygen of a salen-Mn(V)–oxo complex with H bonding of the hydroxyl proton and one of the oxygens of the salen ligand can definitely be ruled out, as this process would lead to the wrong enantiopreference.…”

“…This line of analysis based on our earlier work8 together with the experiments described herein have led to a new and rational explanation for the Mn–salen-catalyzed enantioselective oxidation of secondary alcohols. We have followed a heuristic that has been extremely useful in many of our previous research efforts on the design and understanding of enantioselective chemical reactions 9.…”

“…We surmise further that the Mn(V)–alkoxide complex III is converted to the Mn(III) species V by intramolecular hydrogen transfer from the carbinol C–H to the neighboring phenolic oxygen along a five-membered cyclic pathway via IV . This cyclic pathway is reasonable because the electron lone pair density on one of the metal-bound phenoxy oxygens is properly oriented to facilitate the carbonyl-forming elimination process in a fashion analogous to the electrostatic effect of that oxygen in organizing the transition state and stabilizing the positive charge during the Jacobsen epoxidation of olefins, as pictured in Figure 1 8. There is also an obvious analogy with the Westheimer chromate ester pathway for oxidation of secondary alcohols by Cr(VI) reagents 17.…”

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…Kürti et al [73] rationalized the experimental data available at the time in order to propose an alternative model for this type of epoxidation reaction, mostly based on steric considerations. According to Kürti, electrostatic, van der Waals and π − π interactions govern the stereoselectivity of Mn(salen) catalysts.…”