Hydroboration-oxidation: A chemoselective route to cellulose ω-hydroxyalkanoate esters

We describe the first synthesis of hydroxy-functionalized polysaccharide esters via chemoselective olefin hydroboration-oxidation in the presence of ester groups. Cellulose esters with terminally olefinic side chains were first synthesized by esterification of commercially available cellulose esters (e.g., cellulose acetate) with undec-10-enoyl chloride or pent-4-enoyl chloride. Subsequent two-step, one-pot hydroboration-oxidation reactions of the cellulose esters were performed, using 9-borabicyclo[3.3.1]nonane as hydroboration agent, followed by oxidizing the intermediate borane to a hydroxyl group using mildly alkaline H2O2. Sodium acetate was used as a weak base to catalyze the oxidation, thereby minimizing undesired ester hydrolysis. Characterization methods including FTIR, 1H, and 13C NMR proved the selectivity of the hydroboration-oxidation pathway, providing a family of novel cellulose ω-hydroxyalkanoyl esters that were previously difficult to access.