Alkenylborane-Derived Frustrated Lewis Pairs: Metal-Free Catalytic Hydrogenation Reactions of Electron-Deficient Alkenes

A series of alkenylboranes were prepared by 1,1-carboboration routes and used as Lewis acid components for the generation of frustrated Lewis pairs (FLPs). The reactions of 1-alkynes with B­(C₆F₅)₃ gave the RCHC­(C₆F₅)­B­(C₆F₅)₂ systems <b>4a</b> (R = <i>n</i>-C₃H₇), <b>4b</b> (R = <i>n</i>-C₄H₉), <b>4c</b> (R = Ph), and <b>4d</b> (R = <i>t</i>-C₄H₉), respectively. The alkenylborane/<i>t</i>Bu₃P FLPs derived from compounds <b>4a</b>–<b>d</b> reacted rapidly with dihydrogen (2.5 bar) at ambient temperature. The bulky system <b>4d</b> left the CC double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product [<i>t</i>Bu₃PH]­[<i>t</i>BuCHC­(C₆F₅)­BH­(C₆F₅)₂] (<b>10d</b>). In contrast, the less bulky systems <b>4a</b>/<i>t</i>Bu₃P and <b>4b</b>/<i>t</i>Bu₃P split dihydrogen under these conditions and had their CC double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [<i>t</i>Bu₃PH]­[RCH₂CH­(C₆F₅)­BH­(C₆F₅)₂] <b>9a</b> (R = <i>n</i>-C₃H₇) and <b>9b</b> (R = <i>n</i>-C₄H₉), respectively. The <b>4c</b>/<i>t</i>Bu₃P FLP gave a mixture of both product types (<b>9c</b>/<b>10c</b>). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R₂CC­(C₆F₅)­B­(C₆F₅)₂ <b>4e</b> (R = C₂H₅), <b>4f</b> (R = <i>n</i>-C₃H₇), <b>4g</b> (R = Ph), and <b>4h</b> (R = <i>p</i>-MeC₆H₄), respectively. The <b>4e</b>–<b>h</b>/<i>t</i>Bu₃P FLPs cleaved dihydrogen under mild conditions but retained their CC double bonds to give the respective [<i>t</i>Bu₃PH]­[R₂CC­(C₆F₅)­BH­(C₆F₅)₂] products (<b>10e</b>–<b>h</b>). Selected examples of these alkenylboranes undergo FLP reactions acting as catalysts for the hydrogenation of imines. Perhaps most remarkably, some of these alkenylboranes retain the CC double bonds under FLP/H₂ reaction conditions and heterolytically split dihydrogen in the presence of the Lewis base DABCO and catalyze the hydrogenation of the electron-poor CC double bonds of diaryl-substituted enones