Oxorhenium Catalysts Research Articles

Highly Stereoselective Glycosylation Reactions of Furanoside Derivatives via Rhenium (V) Catalysis.

A novel approach for the formation of anomeric carbon-functionalized furanoside systems was accomplished through the employment of an oxo-rhenium catalyst. The transformation boasts a broad range of nucleophiles including allylsilanes, enol ethers, and aromatics in addition to sulfur, nitrogen, and hydride donors, able to react with an oxocarbenium ion intermediate derived from furanosidic structures. The excellent stereoselectivities observed followed the Woerpel model, ultimately providing 1,3-cis-1,4-trans systems. In the case of electron-rich aromatic nucleophiles, an equilibration occurs at the anomeric center with the selective formation of 1,3-trans-1,4-cis systems. This anomalous result was rationalized through density functional theory calculations. Different oxocarbenium ions such as those derived from dihydroisobenzofuran, pyrrolidine, and oxazolidine heterocycles can also be used as a substrate for the oxo-Re-mediated allylation reaction.

Deoxydehydration of Biomass-Derived Polyols with a Reusable Unsupported Rhenium Nanoparticles Catalyst

Oxorhenium catalysts effectively transform biomass-derived polyols into corresponding alkenes via deoxydehydration (DODH) with a secondary alcohol as a hydrogen donor. This study describes the preparation of unsupported rhenium oxide nanoparticles (ReOx NPs), their use as DODH heterogeneous catalyst, and changes in the speciation and structure of Re during catalysis leading to a possible mechanism for DODH by a heterogeneous catalyst. The unsupported nanoparticles catalyze various polyols transformation to produce alkenes with high efficiency and recyclability. X-ray spectroscopy analysis shows that the structure of as-prepared ReOx NPs is similar to perrhenate with oxidation state between Re(VI) and Re(VII). Under reaction conditions and postcatalysis, the ReOx NPs are reduced by approximately two oxidation units. Kinetic isotope effect demonstrates that dissociation of a C–H bond during reduction of ReOx NPs by 3-octanol is part of the rate-determining step. Based on all experimental observations, the m...

Diarylmethane synthesis through Re2O7-catalyzed bimolecular dehydrative Friedel-Crafts reactions.

This manuscript describes the application of Re2O7 to the syntheses of diarylmethanes from benzylic alcohols through solvolysis followed by Friedel-Crafts alkylation. The reactions are characterized by broad substrate scope, low catalyst loadings, high chemical yields, and minimal waste generation. The intermediate perrhenate esters are superior leaving groups to chlorides and bromides in these reactions. The polarity and water sequestering capacity of hexafluoroisopropyl alcohol are critical to the success of these processes. Re2O7 is a precatalyst for HOReO3, which serves as a less costly and easily handled promoter for these reactions. Oxorhenium catalysts selectively activate alcohols in the presence of similarly substituted acetates, indicating a unique chemoselectivity and mechanism in comparison to Brønsted acid catalysis.

New solid oxo-rhenium and oxo-molybdenum catalysts for the deoxydehydration of glycols to olefins

Several new solid oxo-rhenium and oxo-molybdenum catalysts were tested for the deoxydehydration (DODH) of 1,2-diols to terminal olefins employing aromatic solvents and reaction temperatures of 150–200°C. A catecholato oxo-rhenium catalyst tethered to silica was prepared and found to have comparably high DODH activity to its soluble analog with triphenylphosphine as the reductant. Catalyst re-use and leach-test experiments show significant leaching into the liquid phase. Silica, ceria and, for the first time, alumina and iron oxide were tested as supports for perrhenate, with ceria producing the least active material in 1,2-decane diol DODH. Decene yields obtained with ReOx/SiO2, ReOx/Fe2O3, ReOx/Al2O3 were highest when triphenylphosphine was the reductant and reached 70−78% (at 90–100% conversion) at 150°C. These three catalysts were also effective with gaseous reductants H2 or CO. Leaching of rhenium species occurred for all supports and was most pronounced for SiO2. New solid oxo-molybdenum catalysts were synthesized. At 200°C, decene yields observed using MoOx/Fe2O3, MoOx/SiO2, MoOx/Al2O3, MoOx/TiO2 and MoOx/ZrO2 were comparable and did not exceed 20%. There were homogeneous contributions through leached molybdenum species.

Deoxydehydration of polyols.

The development of sustainable chemical processes for the conversion of highly oxygenated biomass feedstocks to chemical products requires efficient and selective processes for partial oxygen removal and refunctionalization. Here we review the development of the deoxydehydration (DODH) reaction, which converts vicinal diols (glycols) to olefins. Uncatalyzed deoxygenative eliminations were first established. The catalyzed DODH reactions have largely employed oxo-rhenium catalysts and a variety of reductants, including PR₃, dihydrogen, sulfite, and alcohols. A variety of glycol and biomass-derived polyol substrates undergo the DODH reaction in moderate to good efficiency, regioselectively, and stereoselectively. Observations regarding selectivity, mechanistic probes, and computational studies support the general operation of a catalytic process involving three basic stages: glycol condensation to an M-glycolate, reduction of the oxo-metal, glycol condensation to produce a metal-glycolate, and alkene extrusion from the reduced metal-glycolate. Recent practical developments include the discovery of non-precious V- and Mo-oxo DODH catalysis.

Role of Low-Valent Rhenium Species in Catalytic Hydrosilylation Reactions with Oxorhenium Catalysts

The catalytic competency of a Re(III) complex has been demonstrated. In the presence of silane, oxorhenium(V) catalysts are deoxygenated to produce species that are significantly more active than t...

Silica-Immobilized Highly Dispersed Oxo-Rhenium and its Catalytic Activity for the Direct Synthesis of Nitrones

A novel covalently attached silica-immobilized oxo-rhenium catalyst was prepared by the reaction of perrhenic acid and 3-chloropropyl functionalized mesoporous SBA-15, in which the oxo-rhenium species is chemically bonded to the support, preventing leaching. The prepared silica-immobilized oxo-rhenium was found to be an efficient and recyclable catalyst for the one-pot' condensation/oxidation of aldehydes and primary amines to nitrones in high yields using urea-hydrogen peroxide as a terminal oxidant under mild reaction conditions.

Hydrogen Production from Hydrolytic Oxidation of Organosilanes Using a Cationic Oxorhenium Catalyst

We describe herein the novel application of a transition metal oxo complex, a cationic oxorhenium(V) oxazoline, in the production of molecular hydrogen (H2) from the catalytic hydrolytic oxidation of organosilanes. The main highlights of the reaction are quantitative hydrogen yields, low catalyst loading, ambient conditions, high selectivity for silanols, water as the only co-reagent, and no solvent requirement. The amount of hydrogen produced is proportional to the water stoichiometry. Thus, reaction mixtures of polysilyl organics such as HC(SiH3)3 and water contain potentially >6 wt % hydrogen. Kinetic and isotope labeling experiments have revealed a new mechanistic paradigm for the activation of Si-H bonds by oxometalates.

Olefin epoxidation with bis(trimethylsilyl) peroxide catalyzed by inorganic oxorhenium derivatives. Controlled release of hydrogen peroxide.

The replacement of organometallic rhenium species (e.g., CH(3)ReO(3)) by less expensive and more readily available inorganic rhenium oxides (e.g., Re(2)O(7), ReO(3)(OH), and ReO(3)) can be accomplished using bis(trimethylsilyl) peroxide (BTSP) as oxidant in place of aqueous H(2)O(2). Using a catalytic amount of a proton source, controlled release of hydrogen peroxide helps preserve sensitive peroxorhenium species and enables catalytic turnover to take place. Systematic investigation of the oxorhenium catalyst precursors, substrate scope, and effects of various additives on olefin epoxidation with BTSP are reported in this contribution.