.Figure 1. 1H NMR (DMSO-d6) of AcO[TM]Cl.
.
Synthesis of Ionic Liquid AcO[TM]Cl
5-(2-Acetoxyethyl)-4-methylthiazole (1.85 g, 10.0 mmol) was mixed with an excess of benzyl chloride (6.33 g, 50.0 mmol) in a round-bottom flask. The mixture was heated at 80 °C for 24 h, during which time precipitates formed. The solid was filtered off and washed with ethyl acetate. After drying under vacuum, AcO[TM]Cl was obtained as a white solid; yield: 2.51 g (81%).1H NMR (DMSO-d6): δ 10.26 (s, 1H, thiazole ring proton), 7.25–7.45 (m, 5H, Ph), 5.77 (s, 2H, CH2Ph), 4.12 (t, J = 4.5 Hz, 2H, OCH2), 3.22 (t, J = 4.5 Hz, 2H, OCH2CH2), 2.31 (s, 3H, CH3C=O), 1.92 (s, 3H, C=C–CH3). 13C NMR (DMSO-d6): δ 169.9 (C=O), 158.6 (SCN), 142.4 (NC=C), 133.8 (SC=C), 131.1, 129.3, 129.1, 128.1 (Ph), 62.4 (OCH2), 57.0 (CH2Ph), 26.2 (OCH2CH2), 20.6 (CH3C=O), 12.4 (C=C–CH3). HRMS calculated for C15H18ClNO2S [M-H]−: 310.0674; found: 310.0670.
3.3. Synthesis of Ionic Liquid Ac[TM]I
4-Methylthiazole-5-carboxylate (1.0 g, 32 mmol) was mixed with an excess of methyl iodide (2.0 mL) in a round-bottom flask, and the mixture was heated at 40 °C for 24 h. After removal of the excess methyl iodide, the partly crystalline residue was dissolved in absolute alcohol. On careful addition of diethyl ether, the product crystallized in large light yellow needles (0.85 g, 45% yield on the first crop).1H NMR (DMSO-d6): δ 10.02 (s, 1H, thiazole ring-H), 4.09 (s, 1H, OCH3), 3.89 (s, 3H, NCH3), 2.75 (s, 3H, C=C–CH3). 13C NMR (DMSO-d6): δ 162.3 (C=O), 159.9 (SCN), 152.9 (NC=C), 125.6 (SC=C), 54.2 (O–CH3), 41.3 (NCH3), 13.1 (C=C–CH3). HRMS calculated for C7H10INO2S [M + H]+: 299.9477; found: 299.8741.
Int. J. Mol. Sci.
2015,
16(4),
7143-7158;
doi:10.3390/ijms16047143
Article
Organocatalytic Upgrading of Furfural and 5-Hydroxymethyl Furfural to C10 and C12 Furoins with Quantitative Yield and Atom-Efficiency
Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872, USA; E-Mail: zanghongjun@tjpu.edu.cn
†Current address: Department of Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China.
*Author to whom correspondence should be addressed; E-Mail: eugene.chen@colostate.edu; Tel.: +1-970-491-5609; Fax: +1-970-491-1801.
Academic Editor:
James H. Clark
Received: 13 February 2015 / Accepted: 26 March 2015 / Published: 30 March 2015
Abstract
: There is increasing interest in the upgrading of C5 furfural (FF) and C6 5-hydroxymethyl furfural (HMF) into C10 and C12 furoins as higher energy-density intermediates for renewable chemicals, materials, and biofuels. This work utilizes the organocatalytic approach, using the in situ generated N,S-heterocyclic carbene catalyst derived from thiazolium ionic liquids (ILs), to achieve highly efficient self-coupling reactions of FF and HMF. Specifically, variations of the thiazolium IL structure have led to the most active and efficient catalyst system of the current series, which is derived from a new thiazolium IL carrying the electron-donating acetate group at the 5-ring position. For FF coupling by this IL (0.1 mol %, 60 °C, 1 h), when combined with Et3N, furoin was obtained in >99% yield. A 97% yield of the C12 furoin was also achieved from the HMF coupling by this catalyst system (10 mol % loading, 120 °C, 3 h). On the other hand, the thiazolium IL bearing the electron-withdrawing group at the 5-ring position is the least active and efficient catalyst. The mechanistic aspects of the coupling reaction by the thiazolium catalyst system have also been examined and a mechanism has been proposed.
Keywords:
furfural; 5-hydroxymethyl furfural (HMF); umpolung coupling; organocatalysis; N-heterocyclic carbene; ionic liquid; biomass; biorefining
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