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 6 F 5 ) 3 gave the RCH═C(C 6 F 5 )B(C 6 F 5 ) 2 systems 4a (R = n -C 3 H 7 ), 4b (R = n -C 4 H 9 ), 4c (R = Ph), and 4d (R = t -C 4 H 9 ), respectively. The alkenylborane/ t Bu 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 [ t Bu 3 PH][ t BuCH═C(C 6 F 5 )BH(C 6 F 5 ) 2 ] ( 10d ). In contrast, the less bulky systems 4a / t Bu 3 P and 4b / t Bu 3 P split dihydrogen under these conditions and had their C═C double bonds cleanly reduced to yield the corresponding 1-pentafluorophenylalkyl hydridoborate salts [ t Bu 3 PH][RCH 2 CH(C 6 F 5 )BH(C 6 F 5 ) 2 ] 9a (R = n -C 3 H 7 ) and 9b (R = n -C 4 H 9 ), respectively. The 4c / t Bu 3 P FLP gave a mixture of both product types ( 9c / 10c ). 1,1-Carboboration of symmetrical internal alkynes gave the alkenylboranes R 2 C═C(C 6 F 5 )B(C 6 F 5 ) 2 4e (R = C 2 H 5 ), 4f (R = n -C 3 H 7 ), 4g (R = Ph), and 4h (R = p -MeC 6 H 4 ), respectively. The 4e – h / t Bu 3 P FLPs cleaved dihydrogen under mild conditions but retained their C═C double bonds to give the respective [ t Bu 3 PH][R 2 C═C(C 6 F 5 )BH(C 6 F 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.