Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

Abstract

The epoxidation reaction of cyclohexene is investigated for the system vanadyl acetylacetonate (VO(acac) 2 ) as catalyst with tert-butyl hydroperoxide (TBHP) as oxidant with the aim to identify the most active species for epoxidation and to retrieve insight into the most plausible epoxidation mechanism. The reaction mixture is composed of various inactive and active complexes in which vanadium may either have oxidation state +IV or +V. Inactive species are activated with TBHP to form active complexes. After reaction with cyclohexene each active species transforms back into an inactive complex which may be reactivated again. The reaction mixture is quite complex containing hydroxyl, acetyl acetonate, acetate or a tert-butoxide anion as ligands and thus various ligand exchange reactions may occur among active and inactive complexes. Also radical decomposition reaction allow transforming V +IV to V +V species. To obtain insight into the most abundant active complexes, each of previous transformation steps has been modeled through thermodynamic equilibrium steps. To unravel the nature of the most plausible epoxidation mechanism, first principle chemical kinetics calculations have been performed on all proposed epoxidation pathways. Our results allow to conclude that the concerted Sharpless mechanism is the preferred reaction mechanism and that alkylperoxo species V +IV O(L)(OOtBu) and V +V O(L 1 )(L 2 )(OOtBu) species are most abundant. At the onset of the catalytic cycle vanadium +IV species may play an active role but as the reaction proceeds, reaction mechanisms that involve vanadium +V species are preferred as the acetyl acetonate is readily oxidized. Additionally an experimental IR and kinetic study has been performed to give a qualitative composition of the reaction mixture and to obtain experimental kinetic data for comparison with our theoretical values. The agreement between theory and experiment is satisfactory.

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

“…Contrary to what is generally accepted for the Jacobsen-Katsuki epoxidation, the active epoxidizing agents in the Sharpless process are peroxo, hydro-peroxo or alkyl-peroxo-complexes [104]. Titanium and vanadium appear to bear similar chemical behavior, and both have attracted considerable attention in the last decade [105][106][107][108][109][110].…”

“…Titanium and vanadium appear to bear similar chemical behavior, and both have attracted considerable attention in the last decade [105][106][107][108][109][110]. In general, the coordination sphere in both cases can be quite labile (especially in the case of vanadyl(IV) acetylacetonate, VO(acac) 2 ) [104], which requires additional study of the effect of the different species available during the epoxidation process. In the case of vanadium, this was first systematically studied by Vandichel et al [104], whereas the case of titanium was tackled earlier by Sever and Root [111].…”

“…In general, the coordination sphere in both cases can be quite labile (especially in the case of vanadyl(IV) acetylacetonate, VO(acac) 2 ) [104], which requires additional study of the effect of the different species available during the epoxidation process. In the case of vanadium, this was first systematically studied by Vandichel et al [104], whereas the case of titanium was tackled earlier by Sever and Root [111]. Some theoretical studies on titanium-catalyzed epoxidations also focused on this metal's ability to efficiently catalyze the epoxidation of olefins when immobilized into a heterogeneous matrix, such as silicates [112] or polyoxometalates [113,114].…”

“…These theoretical studies follow the same trend observed in the case of manganese-and iron-catalyzed epoxidations, with a predominance of DFT, with B3LYP as the preferred density function. In this regard, Vandichel's work stands out due to the inclusion of Grimme's semi-empirical corrections for long-distance interactions [104].…”

Insight in the activity and diastereoselectivity of various Lewis acid catalysts for the citronellal cyclization

“…TAMkin has already proved its efficiency in the computation of reaction kinetics [44,[70][71][72]. This program is used to investigate the kinetics and the activation barriers of the citronellal cyclization.…”