Selective cross-metathesis of cellobiose derivatives with amido-functionalized olefinic structures: A model study for synthesis of cellulosic diblock copolymers

Abstract

This work describes a model study for synthesis of cellulose-based block copolymers, investigating selective coupling of peracetyl β-d-cellobiose and perethyl β-d-cellobiose at their reducing-ends by olefin cross-metathesis (CM). Herein we explore suitable pairs of ω-alkenamides that permit selective, quantitative coupling by CM. Condensation reactions of hepta-O-acetyl-β-d-cellobiosylamine or hepta-O-ethyl-β-d-cellobiosylamine with acyl chlorides afforded the corresponding N-(β-d-cellobiosyl)-ω-alkenamide derivatives with an aromatic olefin or linear olefinic structures. Among the introduced olefinic structures, CM of the undec-10-enamide (Type I olefin) and the acrylamide (Type II olefin) gave the hetero-block tetramers, N-(hepta-O-ethyl-β-d-cellobiosyl)-N′-(hepta-O-acetyl-β-d-cellobiosyl)-alkene-α,ω-diamides, with >98 % selectivity. Moreover, selectivity was not influenced by the cellobiose substituents when a Type I olefin with a long alkyl tether was used. Although the amide carbonyl group could chelate the ruthenium atom and reduce CM selectivity, the results indicated that such chelation is suppressed by sterically hindered pyranose rings or the long alkyl chain between the amido group and the double bond. Based on this model study, selective end-to-end coupling of tri-O-ethyl cellulose and acetylated cellobiose was accomplished, proving the concept that this model study with cellobiose derivatives is a useful signpost for selective synthesis of polysaccharide-based block copolymers.