Alkene Epoxidation Catalyzed by Ti-Containing Polyoxometalates: Unprecedented β-Oxygen Transfer Mechanism

Abstract

A DFT study revealed that the mechanism of alkene epoxidation with hydrogen peroxide catalyzed by Ti-containing polyoxometalates (POMs) depends on the Ti coordination environment: For rigid and hindered Ti centers, the unprecedented β-oxygen transfer from the titanium hydroperoxo species becomes favored over the α-oxygen one. Improving the model for catalyst description, the calculations were able to reproduce the Arrhenius activation energy values determined in kinetic studies. Unlike protonation, the possible ion-pairing between POMs and countercations has a minor effect on the electrophlicity of the catalyst and, consequently, on the activity of epoxidation.

Mechanistic Investigation into Olefin Epoxidation with H2O2 Catalyzed by Aqua‐Coordinated Sandwich‐Type Polyoxometalates: Role of the Noble Metal and Active Oxygen Position

“…It can be seen that step 2 is typical beta‐oxygen transfer from the bridging hydroperoxo moiety to the olefin. Recently, Poblet and co‐workers have proposed that the mechanism of Ti−OOH through beta‐oxygen has a lower energy barrier for dititanium‐substituted sandwich‐type POMs . As could be expected, this process will proceed with a slightly lower energy barrier; the calculated energy barrier was 16.6 (Δ E ) or 29.7 kcal mol −1 (Δ G ) at the located transition state, TS3Rh (Figure ).…”

“…According to previous reports on the epoxidation by POMs, the catalytic reaction proceeded through a two‐step mechanism: 1) activation of H 2 O 2 and 2) oxygen transfer; in addition, three distinct H 2 O 2 activation mechanism have been reported . Musaev and co‐workers reported a divanadium‐substituted POM, [γ‐1,2‐H 2 SiV 2 W 10 O 40 ] 4− , which contained a core of [OV−(η‐OH) 2 −VO] .…”

“…For the second type, several computational studies have reported the lacunary‐substituted POM, [γ‐H 2 SiW 10 O 36 ] 4− , as well as mono‐ and dititanium‐substituted POMs [PTi(OH)W 11 O 39 ] 4− and [Ti 2 (OH) 2 As 2 W 19 O 67 (H 2 O)] 8− . A two‐step mechanism has been proposed.…”

“… Epoxidation mechanism based on the mono‐ and ditransition‐metal‐substituted polyoxometlates, [PTM(OH)W 11 O 39 ] 4− and [TM 2 (OH) 2 As 2 W 19 O 67 (H 2 O)] 8− , TM=Ti, V, Zr, Nb, Mo, W and Re …”

Room Temperature Epoxidation of Alkenes With Hydrogen Peroxide Catalyzed by Heteropolyoxometalate

“…Therefore, the development of efficient routes for catalytic epoxidation under mild conditions using inexpensive terminal oxidants remains as an important challenge in synthetic chemistry. Recently, catalytic epoxidation reaction in presence of hydrogen peroxide (H2O2) as terminal oxidant has attracted much attention [6][7][8][9][10][11][12][13] because: (i) It generates only water as a side product. (ii) It contains the largest amount of active oxygen species among the known oxidants, and (iii)…”