Olefin Epoxidation with Transition Metal η2-Peroxo Complexes: The Control of Reactivity

Abstract

The activation energies for olefin epoxidation with Mimountype η 2 -peroxo complexes have been calculated using density functional methods. Six degrees of freedom of the complex [MOL(O 2 )(OER 3 )] and the olefin CH 2 CHRЈ have been systematically modified. The calculations were based on the assumptions that the reaction follows a concerted oxygentransfer mechanism suggested by Sharpless and that a peroxo oxygen atom trans to the phosphane oxide ligand is transferred. This was recently proved for the epoxidation of ethylene with the parent complex [MoO(O 2 ) 2 {OP(CH 3 ) 3 }]. It has been found that the diperoxotungsten complexes (M = W; L = O 2 ) are more reactive than the diperoxomolybdenum complexes (M = Mo; L = O 2 ). The activation barriers for the monoperoxomolybdenum complexes (M = Mo; L = O) are

Olefin Epoxidation by H2O2/MeCN Catalysed by Cyclopentadienyloxidotungsten(VI) and Molybdenum(VI) Complexes: Experiments and Computations

“…This is in accordance with previously reported DFT calculations on Mo and W peroxo and diperoxo complexes. [34][35][36][37][38][39][40] In addition, the comparison of the and rationalising the earlier transition state for W than for Mo, as suggested by the bond parameters (vide supra). On the basis of previous works [37,38] and our NBO analysis, the transition state can be described as two fused three-mem- (Figure 13 and Table 3).…”

“…[34][35][36][37][38][39][40] In addition, the comparison of the and rationalising the earlier transition state for W than for Mo, as suggested by the bond parameters (vide supra). On the basis of previous works [37,38] and our NBO analysis, the transition state can be described as two fused three-mem- (Figure 13 and Table 3). In agreement with the participation of the metal to the olefin epoxidation reac-tion, the electrophilic character of the [M-O a -O b ] fragment has to be related not just to the O a charge (almost identical for both compounds), but rather to the sum of the partial charges at the metalladioxirane moiety, q(M) + q(O a ) + q(O b ).…”

“…In agreement with the participation of the metal to the olefin epoxidation reac-tion, the electrophilic character of the [M-O a -O b ] fragment has to be related not just to the O a charge (almost identical for both compounds), but rather to the sum of the partial charges at the metalladioxirane moiety, q(M) + q(O a ) + q(O b ). [37] On going from Mo to W, although the charge at the metal centre increases, the charges at the hydroperoxo oxygen atoms are almost identical. The comparison between the Mo and W systems, therefore, clearly indicates that the [W-O a -O b ] fragment is more positive by + 0.2 electrons than the Mo analogue.…”

Assessment of DFT Methods for Computing Activation Energies of Mo/W-Mediated Reactions

“…12,13 Molybdenum and tungsten are also two of the most widely used early transition metals in organic and organometallic chemistry, [14][15][16][17][18][19][20][21][22] and many important organic reactions are epoxidation, and C-H bond activation. Their mechanisms have also been widely studied both experimentally and theoretically, 34,37,50, which enables us to focus only on crucial steps in reactions, such as rate limiting step or step with relatively high activation energy, as depicted in Scheme 1.…”

Reaction of a monomeric titanium hydride with dioxygen does not produce a stable titanium hydroperoxide

“…However, η 2 -peroxo Ti 4+ complexes were shown to be incapable of epoxidizing alkenes, and instead it is believed that Ti 4+ -OOH complexes are the active species in Ti-catalyzed epoxidations [16]. Most closely related to the work reported here are heterogeneous catalysts, such as the commercial Enichem TS-1 catalyst (Ti-OOH on silicalite), that are utilized for epoxidations with H 2 O 2 as the oxidant.…”