N‑Heterocycle-Ligated Borocations as Highly Tunable Carbon Lewis Acids

The relative (to
BEt3) hydride ion affinity (HIA) of
a series of acridine borenium salts has been calculated, with some
HIAs found to be similar to that for [Ph3C]+. The HIA at the acridine C9 position is controlled by both acridine
and the boron substituents, the latter presumably affecting the strength
of the BN bond in the acridane-BY2 products from
hydride transfer. Through a range of hydride abstraction benchmarking
reactions against organic hydride donors the experimental HIA of [F5acr-BCat]+ (cat = catechol, F5acr =
1,2,3,4,7-pentafluoroacridine) has been confirmed to be extremely
high and closely comparable to that of [Ph3C]+. The high HIA of [F5acr-BCat]+ enables H2 and alkene activation in a FLP with 2,6-di-tert-butylpyridine. Finally, the HIA of pyridine and quinoline borenium
cations has been determined, with the HIA at boron in [PinB­(amine)]+ (pin = pinacol, amine = pyridine or quinoline) found to be
relatively low. This enabled the hydroboration of pyridine and quinoline
by HBPin to be achieved through the addition of 5–10 mol %
of bench-stable cationic carbon Lewis acids such as 2-phenyl-N,N-dimethylimidazolium salts.