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(C ₆F ₅) ₃ gave the RCH=C(C ₆F ₅)B(C ₆F ₅) ₂ systems 4a(R = n-C ₃H ₇), 4b (R = n-C ₄H ₉), 4c (R = Ph), and 4d (R = t-C ₄H ₉), respectively. The alkenylborane/tBu ₃P FLPs derived from compounds 4a-d reacted rapidly with dihydrogen (2.5 bar) at ambient temperature. The bulky system 4d left the C=C double bond of the alkenylborane unsaturated and gave the dihydrogen cleavage product [tBu ₃PH][tBuCH= C(C ₆F ₅)BH(C ₆F ₅) ₂] (10d). In contrast, the less bulky systems 4a/tBu ₃P and 4b/tBu ₃P split dihydrogen under these conditions and had their C=C double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [tBu ₃PH][RCH ₂CH(C ₆F ₅)BH(C ₆F ₅) ₂] 9a (R = n-C ₃H ₇) and 9b (R = n-C ₄H ₉), respectively. The 4c/tBu ₃P FLP gave a mixture of both product types (9c/10c). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R ₂C=C(C ₆F ₅)B(C ₆F ₅) ₂4e (R = C ₂H ₅), 4f (R = n-C ₃H ₇), 4g (R = Ph), and 4h (R = p-MeC ₆H ₄), respectively. The 4e-h/tBu ₃P FLPs cleaved dihydrogen under mild conditions but retained their C=C double bonds to give the respective [tBu ₃PH][R ₂C=C(C ₆F ₅)BH(C ₆F ₅) ₂] products (10e-h). 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/H ₂ 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.