Tetrahydroquinolines are an important structure often seen in natural products and bioactive compounds. The asymmetric hydrogenation of quinolones is seemingly the easiest method for obtaining enantioenriched tetrahydroquinolines. Existing methods for such hydrogenations are primarily based on iridium and ruthenium catalysis. Although some of these reported catalysts show extraordinary enantioselectivity, the prices of the metal and the rather low turnover number still narrows their applications. More importantly, due to the higher reactivity of alkenes, alkynes and electron rich aromatics towards transition metal catalyzed hydrogenations, quinolines containing these functional groups cannot be selectively reduced. In fact, most currently reported methods focus on the hydrogenation of unfunctionalized quinolones.

    In order to overcome the challenges and difficulties in quinoline hydrogenation, Prof. Xiaochen Wang and co-workers developed a class of spiro-bicyclic bisboranes based on their previous studies on these intriguing reductants (Angew. Chem. Int. Ed. 2018, 57, 15096–15100). These bisboranes are formed in situ from spiro[4.4]diene and HB(C₆F₅)₂ (or HB(p-C₆F₄H)₂)，and show extremely high reactivity in the hydrogenation of 2-subsituted quinolones (TON up to 460) and high enantioselectivity (up to 99% ee). The reactions shows great substrate scope, other unsaturated functional groups such as alkenes and alkynes are preserved under the reaction conditions. A TON of 460 is currently the highest recorded TON for asymmetric hydroborations.

    This work is published on Angew. Chem. Int. Ed.(DOI: 10.1002/anie.201900907). Xiang Li and Junjie Tian contributed equally.

    This work is funded by the National Natural Science Foundation of China, the Natural Science Foundation of Tianjin the Fundamental Research Funds for Central Universities.