Reaction Pathway Discrimination in Alkene Oxidation Reactions by Designed Ti‐Siloxy‐Polyoxometalates

Abstract

Titanium complexes of silanol functionalized polyoxometalates (THA)3[SbW9O33(RSiO)3Ti(OiPr)] (Ti‐SiloxPOMs) catalyze alkene oxidation with tert‐butyl hydrogen peroxide (TBHP). However catalytic activity and product distribution in the oxidation of allylic alcohols are shown to depend on the steric surrounding generated by the SiloxPOM (R=tBu, iPr, nPr). Combined experimental and computational studies clarify how steric repulsions between the oxidant (tBu group) and the surrounding SiloxPOM govern the reaction pathways leading either to oxidation of the alcohol function (R=tBu) or to alkene epoxidation (R=nPr). Moreover, another consequence of this steric repulsive interactions is that outer‐sphere mechanisms become competitive with the inner‐sphere ones (coordination of allylic alcohol), whether for the oxidative dehydrogenation reaction or for the epoxidation reaction. In the case of unfunctionalized olefins (linear and cyclic), we show that reducing the bulkiness surrounding the active site leads to higher conversion to epoxide, emphasizing that these Ti‐SiloxPOMs may behave as structural and functional models for metal single‐site in Ti‐Silicates.

Unveiling the mechanism of the photocatalytic reduction of CO2to formate promoted by porphyrinic Zr-based metal–organic frameworks

“…Such a pattern facilitates the hydride-transfer by delocalizing the positive charge generated in the process, as recently observed for structurally similar allylic alcohols capable of transferring a hydride to an electrophilic Ti IV -hydroperoxo group. 77 Finally, the release of the formic acid product from species 4 to regenerate 1 0 is endergonic by 9 kcal mol À1 , in agreement with the experimental observations reporting the strong adsorption of the product in MOF-545 (vide supra). It is worth noting that upon releasing an aqua ligand, species 4 may also evolve to the slightly more stable 4 0 where the formate anion coordinates two neighbouring Zr ions in a bridged fashion (see Fig.…”

“…Such a pattern facilitates the hydride-transfer by delocalizing the positive charge generated in the process, as recently observed for structurally similar allylic alcohols capable of transferring a hydride to an electrophilic Ti IV -hydroperoxo group. 77…”

Titanosilicate Epoxidation Catalysts: A Review of Challenges and Opportunities

“…As regard to the epoxidation of geraniol, Guillemot et al . reported in 2020 that outer‐sphere O‐transfer may become competitive with the inner‐sphere mechanism (coordination of the allylic alcohol to the Ti center, see Scheme 3b) when using TBHP as oxidant and Ti‐siloxy‐polyoxometalates as molecular models for titanosilicates [268] . In the outer‐sphere mechanism, a preference for the more nucleophilic 6,7 bond rather than for the 2,3 one is expected and was indeed experimentally observed, thereby accounting for the larger formation of the 6,7‐epoxy geraniol over the 2,3‐epoxy geraniol (see Scheme 1).…”

Porous Membrane Reactors for Liquid-Phase Heterogeneous Catalysis

“…The use of homogeneous catalysts is advantageous in the high catalytic activity and elucidation of detailed catalytic mechanisms, but it has suffered from some disadvantages, such as high cost, difficult separation, and low recycling performance, limiting their further applications in the catalytic processes . In contrast, heterogeneous catalysts, which are present in the distinct solid phase, are highly favored over the homogeneous counterparts for large-scale industrial production processes, because of their easy separation and recyclability. , Numerous types of catalysts have been synthesized, for example, metal oxides, − transition-metal coordination complexes, − polyoxometalates (POMs), − and metal nanoparticles (NPs) − supported on several solid materials, such as metal–organic frameworks (MOFs), , zeolites, − carbon materials, , silicas, and metal oxides, , which are in very high demand for achieving the goals of green chemistry and sustainable development. The activity and selectivity of catalysts can be enhanced by decreasing the size of the catalyst particles .…”