(Salen)Mn-Catalyzed Epoxidation of Alkenes:  A Two-Zone Process with Different Spin-State Channels as Suggested by DFT Study

Abstract

[structure: see text] A novel (two-zone process with different spin-state channels) mechanistic picture for the Jacobsen-Katsuki reaction is presented that provides insight into the still elusive understanding of the epoxidation mechanism. For the first time, we show that the salen moiety of the catalyst can be explicitly involved in the epoxidation process.

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

“…These include the formation of a metallaoxethane intermediate (Figure 4a), a concerted mechanism (Figure 4b), the formation of a five-membered ring structure including a nitrogen or oxygen atom of a spectator ligand, such as salen, or porphyrin (Figure 4c,d, respectively), or the formation of a carbon radical intermediate (Figure 4e) [34]. Each of these mechanisms allows the explanation of at least some of the experimental data available for these reactions, in particular the high enantioselectivity of Mn(salen)-and Fe(porphyrin)-catalyzed epoxidation of cis-olefins [23,50,69,70].…”

“…In the triplet surface, the oxygen transfer step occurs via a radical intermediate. Thus, the stereoselectivity of the reaction would depend on the likelihood of this spin crossing process, which in turn would be affected by the nature of the substrate and changes in the catalyst [69].…”

“…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.…”

Epoxides and aziridines - a mini review

“…In one outcome on a rather complex reaction surface, a covalent bond is formed beween the carbon atom of the substrate and the oxygen atom of the salen framework. 2 A novel immobilized (salen)Mn catalyst has been constructed by incorporating a chiral sulfonato-salen complex into a zinc-aluminum layered double hydroxide (LDH) host. The active catalyst is proposed to exist as an intercalated species spanning the interlayer space (i.e., 1).…”

“…The active catalyst is proposed to exist as an intercalated species spanning the interlayer space (i.e., 1). The complex was shown to promote the selective epoxidation of limonene (2) with molecular oxygen at room temperature in the presence of pivaldehyde and N-methylimidazole (N-MeI) under Mukaiyama conditions. The activity of the catalyst is stable even after multiple cycles, and no leaching of manganese has been observed.…”