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

Abstract

The mechanism and origin of asymmetric induction in the Mn III Highly enantioselective Kochi-Jacobsen-Katsuki epoxidation of unfunctionalized olefins with Mn III (salen)-based chiral catalyst provides an efficient route to optically active epoxides (1-4). 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 (5-19). Numerous studies assume the Mn V -oxo species postulated by Kochi and coworkers (20) to be the enantioselective oxidant. Although the Mn V -oxo species has not been detected experimentally, its geometric and electronic structure, as well as the mechanism of the epoxidation, have been the focus of several theoretical investigations (5)(6)(7)(8)(9)(10)(11)(21)(22)(23)(24)(25).The predicted electronic structure of the trans-Mn V -oxo species is controversial due to small energetic differences between various electronic states. Without an axial ligand, the singlet, triplet, and quintet states of the square pyramidal complex are nearly degenerate. In the presence of axial ligands, the singlet state is strongly destabilized, whereas the triplet and quintet states remain nearly degenerate in energy. The mechanism of the epoxidation by the trans-Mn V -oxo species has been predicted to depend on the spin state and presence of an axial ligand. Without an axial ligand, epoxidation is predicted to be concerted for the quintet and singlet states but stepwise (through a radical intermediate) for the triplet state (5, 6). Axial Cl Ϫ ligand alters epoxidation by the quintet state from concerted to stepwise (5, 7-9).The origin of the asymmetric induction is a separate issue from the epoxidation mechanism and is poorly understood (1-4, 11, 26-29). For the trans-Mn V -oxo species, Jacobsen and Cavallo (11) and Houk et al. (28) computationally studied the asymmetric induction, describing only the small core part of the catalyst by quantum mechanics. To describe crucial noncovalent interactions between the olefin and the substituted salen, they used empirical force fields. Both agreed that olefin could approach the reactive MnO moiety in a side-on manner from multiple directions. However, Houk et al. (28) indicated olefin approach along the oxygen side of the salen ligand, whereas Jacobson and Cavallo (11) suggested a perpendicular approach to be the most favorable. Both studies demonstrated that olefin adds to the Mn V -oxo species regioselectively, forming the most stable radical intermediate. 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 (30-34). In particular, in the presence of N-alkyl imidazole or N-oxide axial ligands, high enantioselectivity is realized,...

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

“…More recently, Teixeira et al [31,44,135] used these linear-scaling strategies in their study of the charge and spin distributions in Mn(salen) and related complexes (including their oxo-derivatives). Here, the results obtained by these authors are in good accordance with those obtained previously without the use of such approximations [17,31,37,82].…”

“…Despite this, in the particular case of imidazole-coordinated complexes, a concerted mechanism was observed in the quintuplet surface. In addition to this, the authors confirmed the cis form of oxo-Mn(salen) complexes as being more stable than their transisomers [82]. The behavior of immobilized Mn(salen) complexes was probed by Malek et al [83,84] in two different studies.…”

“…On the other hand Khavrutskii et al [82] pursued the hypothesis of a non-planar, highly bent, conformation of the salen moiety in oxo-Mn(salen) complexes previously proposed by Katsuki in the 1990s [9]. Khavrutskii et al [82] studied the epoxidation of ethylene using a proper Mn(salen) model (cf. Model 2, in Figure 1) and a peroxy acid (CH 3 COOOH) as the terminal oxidant.…”

“…Despite this, the models published in the last 20 years gravitate around a common model for both Mn-and Fe-catalyzed epoxidations; namely, the use of oxo-Mn and oxo-Fe species as the active forms of the catalyst [23,32,68,69,[92][93][94]132], the avoidance of a spin-crossing mechanism [42,48,76] and the possible role of a radical intermediate as a putative explanation for the incomplete stereoselectivity of these reactions [76,82]. It should be noted that many of these insights on the reaction mechanism are subtle enough to elude most of the standard kinetic studies.…”

Synthesis, characterization of palladium hydroxysalen complex and its application in the coupling reaction of arylboronic acids: Mizoroki–Heck type reaction and decarboxylative couplings

“…Since it can be prepared simply in a one step reaction from inexpensive starting materials, transition metal complexes having salen ligands have been widely studied and developed as catalysts for use in many reactions, such as asymmetric synthesis and polymerization for a long time [1][2][3][4][5][6][7][8]. Although several palladium salen complexes have been reported, they have only low activities as catalysts [9][10][11][12][13].…”