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.