Improving Reproducibility of Catecholase-like Catalysis by Investigating Reaction Conditions and Autoxidation

The oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) is easy to run (using O₂ as an oxidant), easy to monitor by UV-vis spectroscopy (the growth of a peak at 400 nm indicates formation of the product, 3,5-DTBQ), low hazard, low cost, and biomimetic. It is therefore unsurprising that hundreds of papers report this reaction as a way to demonstrate the capacity of new complexes for oxidation catalysis. Yet between these many reports, there are few commonalities in terms of the reaction conditionswith respect to solvent, reaction time, catalyst and substrate concentrations, the use of base as a cocatalyst, or buffering of the solutionand the reports even disagree on the molar absorptivity of 3,5-DTBQ, so that a direct comparison between complexes has many caveats. Indeed, our observation of significant autoxidation and base-enhanced autoxidation further motivated this study, which includes the determination of the molar absorptivity of 3,5-DTBQ in several solvents and a thorough examination of the contributions of autoxidation to 3,5-DTBC oxidation by comparison with a mono-Cu catalyst both with and without an added base (KOH). With effective mixing, reproducible values for the molar absorptivity of 3,5-DTBQ at 400 nm were determined to be 1670 ± 32 M^-1 cm^-1 in methanol, 1870 ± 15 M^-1 cm^-1 in acetonitrile, and 1960 ± 25 M^-1 cm^-1 in tetrahydrofuran. In catalytic trials, the initial substrate concentration, solution pH, and time between the dissolution of 3,5-DTBC and the start of measurements were all found to impact the outcome of the reaction. We report protocols with recommendations for our fellow catalyst developers to help design catalytic trials that will avoid conflating these effects with real differences in catalytic activity.