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Letter
Direct synthesis of unsaturated sugars from methyl glycosides Ji Cao, Masazumi Tamura, Yoshinao Nakagawa, and Keiichi Tomishige ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00589 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019
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ACS Catalysis
Direct synthesis of unsaturated sugars from methyl glycosides Ji Cao, Masazumi Tamura*, Yoshinao Nakagawa and Keiichi Tomishige* Department of Applied Chemistry, School of Engineering, Tohoku University, Aoba 6–6–07, Aramaki, Aoba–ku, Sendai, Miyagi, 980–8579, Japan ABSTRACT: 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 due to the olefin group. Herein, we demonstrated direct transformation of various methyl glycosides, which can be easily obtained from sugar derivatives, to the corresponding unsaturated sugars without protection of the OH groups in high selectivities and yields (up to 90%) using ReOx–Au/CeO2 catalyst at low H2 pressure (≤1 MPa) by deoxydehydration and maintained their original stereo–structures. KEYWORDS: Deoxydehydration, Rhenium, Sugar conversion, Unsaturated sugars, Heterogeneous catalyst.
Conversion of biomass to valuable chemicals is essential for sustainable society because biomass is the only renewable organic carbon resource. Sugars are the main components of lignocellulosic biomass and can be regarded as important intermediates for biomass conversion.1 Conventionally, sugars are transformed to furfurals by dehydration, followed by further transformations of furfurals to various chemicals.2 However, this method loses the original stereo–structure of sugars despite the unique stereo–structure useful for the synthesis of valuable chemicals. Recently, effective transformation methods of sugars with maintaining the original stereo–structure have been developed. Gagné and co–workers reported an effective reaction system of B(C6F5)3 + tertiary silane for selective transformation of silyl–protected polyols to chiral polyol synthons1b, 3 and reductive carbocyclization of unsaturated carbohydrates,4 and very recently reported that B(C6F5)3 + catecholborane was effective for selective C– O bond cleavage of sugar derivatives.5 Huber and co– workers demonstrated a novel transformation route of cellulose via formation of levoglucosenone, providing tetrahydrofurandimethanol and 1,6–hexanediol in high selectivity.6 Unsaturated sugars are one of promising chemicals because they are widely used for the biological active molecules,7 and important intermediates for medicines, biomaterials and agrichemicals and so on8. Traditionally, these unsaturated sugars are synthesized by many reactions9 including Ferrier rearrangement,10 de novo approach,11 Achmatowicz rearrangement,12 or heterocycloaddition,13 leading to large cost and low yield (Scheme 1). Typically, reported methods for the synthesis of dideoxy unsaturated sugars from glucose need multiple steps (5–7 steps) with inadequate yields (18–75%) (Scheme S1). Therefore, direct transformation of sugars without protection of the OH groups to the unsaturated
ones maintaining the original stereo–structure is highly required. Scheme 1. Synthesis method of unsaturated sugars from sugar derivatives Conventional method O O HO HO
OH
OH D-glucose This work
HO Multiple steps
HO
O
O
Problems: Multiple steps (5-7) Many reagants Low total yield
Retention of original stereo-structure Target product HO Medicines O O Agrochemicals -2H2O, H2 HO HO OH Biomaterials Deoxydehydration OH HO Food industries Methyl -D-mannopyranoside Unsaturated sugar etc. (Model) O
O
One promising strategy for that will be application of deoxydehydration (DODH) reaction, where the vicinal OH groups in polyols are simultaneously removed to produce C=C bond. DODH reactions have been actively investigated,14 and effective Re, Mo and V complexes were developed.15 Recently, heterogeneous catalysts such as ReOx/TiO216 and ReOx/Al2O317 were reported to be effective for the reaction, and we also found that isolated ReOx species over CeO2 support was an effective heterogeneous catalyst for DODH reaction using H2 as a reductant. We demonstrated effective catalyst systems of ReOx–Pd/CeO2 for DODH + hydrogenation of 1,4–anhydroerythritol to tetrahydrofuran,18 ReOx–Au/CeO2 for DODH of glycerol to allyl alcohol19 and physical mixture of ReOx–Au/CeO2 and ReOx/C for one–pot multi–step reactions including DODH from 1,4–anhydroerythritol to 1,4–butanediol.20 On the other hand, a few reports have been reported on application of the DODH reaction to transformation of sugars: Toste and co–workers reported a seminal work on the DODH of various sugar derivatives such as sugar alcohols (hexoses and tetroses) and inositols with CH3ReO3 catalyst to afford the corresponding polyenes in moderate to high yields.21 Moreover, the same research group demonstrated
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hydrogenation activity of Au was enhanced by the deposition–precipitation method. The effect of H2 pressure was investigated with ReOx–dpAu0.3/CeO2 (entries 2–8) and ReOx–impAu0.3/CeO2 (Table
conversion of sugar acids to dicarboxylic acids with a combination catalyst system of Pd/C, KReO4 and NH4ReO4.22 CH3ReO3 and MoOx/Carbon black were also applied to the conversion of sugar acids to dicarboxylic acids.23 However, these methods lose all the OH groups at the chiral carbons, and usually use organic reductants such as alcohols and phosphines. Recently, we substantiated direct transformation of methyl glycosides to dideoxy saturated methyl glycosides without protection of the OH groups using ReOx–Pd/CeO2 catalyst, where the cis–vicinal OH groups in methyl glycosides were selectively removed, and the corresponding dideoxy saturated methyl glycosides were obtained in high yields with maintaining the original stereo–structure of the OH groups.24 Methyl glycosides can be easily obtained by methanolysis of lignocellulosic biomass25 or methylation of monosaccharides with methanol. However, unsaturated methyl glycosides were not almost obtained, due to the high reactivity of C=C bond under H2 atmosphere. Herein, we demonstrated direct transformation of methyl glycosides to the corresponding unsaturated methyl glycosides with H2 as a reductant over heterogeneous ReOx– Au/CeO2 catalyst, and high yields of the unsaturated methyl glycosides were obtained (up to 90%). At first, combination of ReOx and metals (M) with H2 activation ability over CeO2 support was investigated in DODH of methyl α–D–mannopyranoside with H2 as a model reaction (Table S1). The catalysts were prepared by impregnation method (labeled as imp on the left side of M) with 1 wt% Re loading amount and M/Re molar ratio of 0.3, which is the best composition of ReOx–Au/CeO2 in our previous work on DODH of glycerol to allyl alcohol.19 The target product is methyl 2,3–dideoxy–α–D–erythro–hex–2– enopyranoside (1), which is produced by removal of the cis– OH groups at C–2 and C–3 positions of methyl α–D– mannopyranoside, and the main byproduct is methyl 2,3– dideoxy–α–D–mannopyranoside (2), which is formed by over–hydrogenation of 1. Au–, Ag– and Cu–loaded ReOx/CeO2 catalysts provided high selectivity to 1 (49– >99%, entries 1–3), which could be attribute to their lower hydrogenation activities than other noble metals. Ir–, Pt–, Rh–, Ru– and Pd–loaded ReOx/CeO2 catalysts showed low selectivity to 1 (99%) to 1 with moderate conversion (10%, entry 1). ReOx/CeO2, CeO2 and impAu/CeO2 catalysts (entries 9–11) showed no or low conversion (≤2%), indicating that all the species of CeO2, ReOx and Au are essential for the high performance. Catalyst preparation method strongly influences formation of metal species, especially for Au catalysts.26 Generally, smaller Au particles have higher hydrogenation activity, and deposition–precipitation method is often used for formation of small Au particles (99
