On-the-fly H2 degassing: Towards selective borohydride reduction of α, β-unsaturated esters to allylic alcohols in continuous microflow

Herein, we demonstrate a hybrid microflow system consisting of a coaxial double-tube microreactor and a capillary microreactor for performing borohydride reduction of α , β -unsaturated esters to allylic alcohols in a highly controlled and safe manner. Using methyl cinnamate as the model substrate and LiBH 4 as reductant, complete conversion was achieved in 75 s residence time at 80 ℃ with up to 85% yield of the target product cinnamyl alcohol under optimized conditions in the hybrid microflow system, enabling selective borohydride reduction of α , β -unsaturated esters to allylic alcohols without the need of using metallic salts as additives or co-reagents. The kinetics of this reduction reaction has been examined and modelled, providing an understanding of the intensified reaction behaviour and the reaction performance of the hybrid microflow process. The results presented can shed new light on the development of continuous microflow processes for the hydridic type of reductions.