Hidden Boron Catalysis: A Simple Colourimetric Indicator

‘Hidden’ catalysis plagues the development and understanding of all catalytic processes. Hidden acid catalysis and catalysis by trace metal contamination being two widely recognised examples. Since 2010, over 600 new catalysed hydroboration protocols have been reported despite the prevalence of hidden borane catalysis across hydroboration reactions using HBcat and HBpin. Nucleophilic species, present as either activators, additives or inherent to the catalyst structure, readily mediate the decomposition of HBpin and HBcat to boranes, including (ligated) BH3. These boranes are themselves catalysts for alkene and alkyne hydroboration, so often serve as the active, ‘hidden’, catalyst rather than the intended metal/metalloid species reported as a the ‘catalyst’. Following our introduction of the TMEDA test for hidden borane catalysis 2020, the proportion of catalysed hydroboration publications testing for hidden borane catalysis has increased from 5% to 23%, but this is still well short of routine. We now report, a simple, rapid and colourimetric method for the determination of hidden borane catalysis. This method uses nothing more than a colour change visible to the naked eye, akin to litmus paper acid/base indicators. The colourimetric test uses a bench-stable, widely commercially-available reagent, crystal violet, at low concentration to identify hidden borane catalysis in seconds. in situ BH3 formation from the decomposition of HBpin by species from across the periodic table has been positively identifed using this new test. The colourimetric indicator does inhibit the hydroboration reaction and shows no reactivity with substrates, HBpin, HBcat, nucleophilic species or any of the ‘catalysts’ tested. This test is easily applied to all past and future catalysed hydroboration reactions and represents the first example of a colour indicator for hidden catalysis.