On the Viability of Carbonyl Hydroboration Catalysed by Aluminium Hydrides

The hydroboration of aldehydes and ketones by pinacolborane (HBpin) catalysed by (Nacnac)^DippAl(OTf)H ((Nacnac)^Dipp = HC(MeCDippN)₂; Dipp = C₆H₃ ⁱPr₂-2,6; Tf = SO₂CF₃) was first reported in 2015. This study represented a landmark in main group catalysis, and advanced a widely-accepted and oft-cited mechanistic paradigm. However, in contradiction of that study, we show here that: i) the mechanism proposed, involving turnover via Al─O/B─H metathesis at the intermediate (Nacnac)^DippAl(OTf)(OCH₂Ph), does not occur; ii) when using pre-purified HBpin, the hydroboration reaction with acetophenone 'catalysed' by (Nacnac)^DippAl(OTf)H (reportedly giving 51% conversion over 6 h at 2 mol% loading), actually shows no conversion; and iii) the active species in catalysis is a BH₃ adduct derived either from the use of impure HBpin, or from the degradation of HBpin by the action of aluminium species present in the reaction mixture. More broadly, our study i) calls into question the nature of the true catalyst species in reports of carbonyl hydroboration by aluminium complexes (since Al─O/B─H metathesis proceeds spontaneously in the reverse direction to that necessitated catalytically); and ii) presents further evidence that the hydroboration of benzaldehyde by HBpin is not a good catalytic probe, given the significant rate of the uncatalysed background reaction.