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 4a (R = n-C3H7), 4b (R = n-C4H9), 4c (R = Ph), and 4d (R = t-C4H9), respectively. The alkenylborane/tBu(3)P 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(3)PH][tBuCH=C(C6F5)BH(C6F5)(2)] (10d). In contrast, the less bulky systems 4a/tBu(3)P and 4b/tBu(3)P split dihydrogen under these conditions and had their C=C double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [tBu(3)PH][RCH2CH(C6F5)BH(C6F5)(2)] 9a (R = n-C3H7) and 9b (R = n-C4H9), respectively. The 4c/tBu(3)P FLP gave a mixture of both product types (9c/10c). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R2C= C(C6F5)B(C6F5)(2) 4e (R = C2H5), 4f (R = n-C3H7), 4g (R = Ph), and 4h (R = p-MeC6H4), respectively. The 4e-h/tBu(3)P FLPs cleaved dihydrogen under mild conditions but retained their C=C double bonds to give the respective [tBu(3)PH][R2C=C(C6F5)BH(C6F5)(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.