(Salen)Mn(III)-Catalyzed Epoxidation Reaction as a Multichannel Process with Different Spin States. Electronic Tuning of Asymmetric Catalysis:  A Theoretical Study

Abstract

The (salen)Mn(III)-catalyzed epoxidation reaction mechanism has been investigated using density functional theory (DFT). There is considerable interest in and controversy over the mechanism of this reaction. The results of experimental studies have offered some support for three different reaction mechanisms: concerted, stepwise radical, and metallooxetane mediated. In this paper, a theoretical examination of the reaction suggests a novel mechanism that describes the reaction as a multichannel process combining both concerted and stepwise radical pathways. The competing channels have different spin states: the singlet, the triplet, and the quintet. The singlet reaction pathway corresponds to a concerted mechanism and leads exclusively to a cis epoxide product. In contrast, the triplet and quintet reactions follow a stepwise mechanism and lead to a product mixture of cis and trans epoxides. We show that the experimentally observed dependence of isomer product ratios on electronic effects connected with the substitution of the catalyst ligands is due to changing the relative position and, hence, the relative activities of the channels with different cis-trans yields. Because the results and conclusions of the present work dramatically differ from the results and conclusion of the recent DFT theoretical investigation (Linde, C.; Akermark, B; Norrby, P.-O.; Svensson, M. J. Am. Chem. Soc. 1999, 121, 5083.), we studied possible sources for the deep contradictions between the two works. The choice of the DFT functional and a model has been shown to be crucial for accurate results. Using high level ab initio calculations (coupled cluster-CCSD(T)), we show that the computational procedure employed in this study generates significantly more reliable numerical results. It is also shown that the smaller cationic model without a chlorine ligand that was used by Linde et al. is too oversimplified with respect to our larger neutral model. For this reason, using the cationic model led to a qualitatively wrong quintet reaction profile that played a key role in theoretical postulates in the earlier work.

The ab-initio density matrix renormalization group in practice

“…In this active space, we find that the singlet is lowest in energy, with a very small singlet-triplet energy gap of 0.6 mE h . This is reminiscent of the nearly degenerate ground state picture found in previous theoretical works, 51,97 although a quantitative ordering requires augmentation by dynamic correlation.…”

“…95,96 The ordering of the lowest spin states is considered important as different reaction paths have been posited depending on the spin state. 97 In our DMRG calculations, we used the 6-31G(d) basis set 81,98,99 and computed the restricted open-shell HartreeFock (ROHF) orbitals for the triplet state in the C 1 point group symmetry. Further, the active space was defined by ROHF molecular orbitals with the following dominant atomic orbital characters: (1) five Mn 3d orbitals, (2) 2p z of the equatorial C, N and O atoms, giving 10 π orbitals, (3) 2p x and 2p y of the equatorial O and N atoms for the Mn-N and Mn-O σ bonds, (4) 2p x , 2p y and 2p z of the axial Cl atoms, (5) an axial O 2p z , and combination of the axial O 2p x , 2p y orbitals, responsible for the Mn-O σ and π bonds.…”

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

“…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.…”

“…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 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)(8)(9).…”