Facile and efficient acylation of chitin in deep eutectic solvents

Abstract

This study investigates acylation of chitin in deep eutectic solvents (DESs) as analogous solvents of ionic liquids (ILs), because we have already reported that such reaction smoothly progresses using acyl chlorides in the presence of pyridine/N,N-dimethyl-4-aminopyridine (DMAP) as base/catalyst in the IL, that dissolves chitin. In addition to a DES composed of 1-allyl-3-methylimidazolium chloride (AMIMCl) and thiourea (TUA) as hydrogen bond acceptor and donor (HBA and HBD), respectively, which we previously reported to dissolve chitin, several DESs were prepared from AMIMCl and different HBDs, that is, urea (UA), acetylthiourea (AcTUA), acetylurea (AcUA), and 1,1,3,3-tetramethylguanidine (TMG). These DESs were found to also dissolve chitin. Hexanoylation of chitin in the AMIMCl/TUA-DES under the same conditions, as those previously performed in the IL, i.e., using hexanoyl chloride in the presence of pyridine/DMAP at 100 ​°C for 24 ​h, gave the product with the low degree of substitution (DS). As this was speculated to be due to the high nucleophilicity of TUA, which had a potential to react with hexanoyl chloride, the other HBDs with lower nucleophilicity, mentioned above, were employed to be combined with AMIMCl in the DESs. When hexanoylation of chitin was carried out in the DESs composed of UA, AcTUA, AcUA, and TMG under the same conditions as above, the higher DS products were obtained. In particular, the reaction in the AMIMCl/TMG-DES efficiently occurred in the absence of pyridine/DMAP to produce the high DS product, probably owing to the high basicity and low nucleophilicity of TMG. The structure of the chitin hexanoate produced was evaluated by the IR and ¹H NMR measurements. Accordingly, acylation of chitin using various acyl chlorides was performed in the AMIMCl/TMG-DES under the conditions without the use of pyridine/DMAP to give the corresponding chitin acylates with the high DSs. This study achieves the facile and efficient acylation method of chitin in the DES.