The 'hidden' BH3 colourimetric test was applied to reagents with motifs typically used in catalysed hydroboration systems, including organolithium, organomagnesium, amide, alkoxide, borohydrides, and alkyl borane reagents (Table , Entries 1 -12). Importantly, control reactions where the 'catalyst', nucleophilic reagent, (10 μM) was added to a chloroform solution of crystal violet (1 μM) showed in all cases. The test for 'hidden' BH3 was carried out by reaction of the nucleophilic reagent and HBpin and addition of a crystal violet solution (1 mL, 1 mM in CHCl3). When BH3 was generated a colour change from purple to colourless should be observed. In all cases upon addition of the indicator solution, a colour change from purple to colourless was observed, successfully indicating the presence of BH3. As organomagnesium reagents react with chloroform to generate various alkanes, toluene was used as the solvent. For all examined nucleophiles, the indicator test was in accordance with the TMEDA test results, validating the accuracy of the colourimetric method. All positive indications of BH3 formation, and therefore hidden catalysis, were confirmed by independent testing (see Supporting Information, S3.10). Importantly, positive controls were also successful; NaOTf, which does not react with HBpin to form BH3, gave no observable colour change; the purple solution remining (Table Entry 13).