No abstract

Transformation of sugars, while maintaining the intrinsic stereochemical structure, is desirable. However, such a transformation requires multistep synthesis with protection and deprotection of the OH groups. Herein, a new method for selective transformation of sugar derivatives into chiral building blocks and a diol synthon, with retention of the intrinsic configuration (stereo- and regioselectively), is demonstrated. The method is based on the selective recognition of cis-vicinal OH groups in sugars and leads to the one-pot removal of the cis-vicinal OH groups, without protection of OH groups (except the OH group of the hemiacetal group), over a heterogeneous CeO -supported ReO and Pd (ReO -Pd/CeO ) catalyst by using H as a reducing agent.

Olefin production from polyols via deoxydehydration (DODH) was carried out over Ag-modified CeO 2 -supported heterogeneous Re catalysts with H 2 as a reducing agent. Both high DODH activity and low hydrogenation ability for C=C bonds were observed in the reaction of erythritol, giving a 1,3butadiene yield of up to 90 % under "solvent-free" conditions. This catalyst is applicable to other substrates such as methyl glycosides (methyl α-fucopyranoside: 91 % yield of DODH product; methyl β-ribofuranoside: 88 % yield), which were difficult to be converted to the DODH products over the DODH catalysts reported previously. ReO x -Ag/CeO 2 was reused 3 times without a decrease of activity or selectivity after calcination as regeneration. Although the transmission electron microscopy energy-dispersive X-ray spectroscopy and X-ray absorption fine structure analyses showed that Re species were highly dispersed and Ag was present as metal particles with various sizes from well-dispersed species (< 1 nm) to around 5 nm particles, the catalysts prepared from size-controlled Ag nanoparticles showed similar performance, indicating that the catalytic performance is insensitive to the Ag particle size.

We found that nonprotected methyl glycosides with cis-vicinal OH groups could be converted to the corresponding methyl dideoxy glycosides by deoxydehydration and consecutive hydrogenation (DODH + HG) over a ReO x –Pd/CeO2 catalyst with gaseous H2. In the study, the reactivity of the methyl glycosides in DODH was clearly lower than that of simple cyclic vicinal diols, such as cis-1,2-cyclohexanediol and cis-1,2-cyclopentanediol, and the reactivity of the methyl glycosides was also different. Herein, we investigated the reactivity difference based on kinetic studies and density-functional theory (DFT) calculations. The kinetic studies suggest that the reactivity difference between the methyl glycosides and the simple diols is derived from the OH group of methyl glycosides except the cis-vicinal diols, and that the reactivity difference among the methyl glycosides will be associated with the configuration of the substituents adjacent to the cis-vicinal diols, while the reaction mechanism of DODH is suggested to be basically similar judging from almost the same reaction orders with respect to the substrate concentration and H2 pressure in all substrates. The adsorption and transition states of methyl α -l- rhamnopyranoside and methyl α-l-fucopyranoside, which have a large reactivity difference (methyl α-l-rhamnopyranoside≫ methyl α-l-fucopyranoside), were estimated by DFT calculations with ReO x /CeO2 as the active site of the ReO x –Pd/CeO2 catalyst, showing that the main difference is the activation energy in DODH of these substrates (65 kJ mol–1 for methyl α-l-rhamnopyranoside and 77 kJ mol–1 for methyl α-l-fucopyranoside), which was also supported by the results of Arrhenius plots (63 and 73 kJ mol–1 for methyl α-l-rhamnopyranoside and methyl α-l-fucopyranoside, respectively). The activation energy was influenced by the torsional angle of the substituents adjacent to the cis-vicinal OH groups, which is derived from the interaction of the OH group adjacent to the cis-vicinal OH groups and the surface hydroxy groups on CeO2.

Transformation of sugars without protection of the OH groups is an ideal method and a powerful tool for biomass utilization, and particularly, unsaturated sugars are a promising target because they can be transformed to versatile chemicals because of the olefin group. Herein, we demonstrated direct transformation of various methyl glycosides, which can be easily obtained from sugar derivatives, without protection of the OH groups to the corresponding unsaturated sugars with maintaining their original stereostructures in high selectivities and yields (up to 90%) using ReO x -Au/CeO2 catalyst at low H2 pressure (≤1.2 MPa) by deoxydehydration.

A heterogenous catalyst for deoxydehydration (DODH) reaction was developed using less expensive Mo than Re as the active center. Combination of Mo with anatase-rich TiO2 and Au as the support...

Biomass valorization has emerged as a promising and sustainable means of accommodating value-added chemicals. To produce such chemicals, various transformation methods of the top biomass-derived platforms nominated by the U.S....

The detailed structures of monomeric ReO x catalysts supported on the CeO 2 surface and the reaction mechanism of the deoxydehydration (DODH) reaction were investigated by density functional theory calculations. After examining various ReO x H y structures over CeO 2 without substrate adsorption, the stable structure under the experimental condition at 400 K was determined to be the Re VII O 2 species. The reaction mechanism of DODH was then investigated for the conversion of 1,4-anhydroerythritol to 2,5-dihydrofuran as a model reaction. Through the investigations of several reaction pathways, an oxygen vacancy-assisted mechanism, in which the starting structure is the Re IV O species and the oxidation state of the Re atom changes between +IV and +VI during the reaction, was postulated to be the most plausible pathway, considering the energies of the intermediates and the barrier height for the cleavage of the two C−O bonds.