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/tBu3P 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
[tBu3PH]­[tBuCHC­(C6F5)­BH­(C6F5)2]
(10d). In contrast, the less bulky systems 4a/tBu3P and 4b/tBu3P split dihydrogen under these conditions and had their
CC double bonds cleanly reduced to yield the corresponding
1-pentafluorophenylalkyl hydridoborate salts [tBu3PH]­[RCH2CH­(C6F5)­BH­(C6F5)2] 9a (R = n-C3H7) and 9b (R = n-C4H9), respectively. The 4c/tBu3P 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/tBu3P FLPs cleaved dihydrogen under mild conditions
but retained their CC double bonds to give the respective
[tBu3PH]­[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/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.