Borane-Catalyzed Water Dehydrogenation at Dioxaphospholes and -Phospholanes.

We report that water undergoes rapid oxidative addition to readily accessible dioxaphospholes and -phospholanes within minutes at room temperature. The resulting phenylphosphinate esters release dihydrogen under mild conditions (room temperature to 60°C) in a borane-catalyzed process. Remarkably, a pinacol-derived dioxaphospholane reacts spontaneously with atmospheric moisture under ambient conditions to form the corresponding phenylphosphinate ester, thereby enabling the direct utilization of water from air for catalytic hydrogen evolution. The mechanism of the borane-catalyzed dehydrogenation was investigated by variable time normalization analysis, targeted synthesis of intermediates, and DFT calculations. These studies support a catalytic pathway initiated by hydrogen elimination from the borane adduct of the phosphinate ester, followed by retro-hydroboration to regenerate the catalyst.