Methods to Predict Potential Reagents in Iridium-Based Photoredox Catalysis Calibrated with Stern–Volmer Quenching Rate Constants
Antoine Juneau, Taylor O. Hope, Jason Malenfant, Mihai Mesko, Jacob McNeill, Mathieu Frenette
Abstract
Visible light photoredox catalysts with strongly oxidizing excited states have been broadly applied in organic synthesis. Following photon absorption by the photocatalyst, electron transfer from an organic reagent is the most common mechanistic outcome for this class of reaction. Reduction potentials for organic reagents are therefore useful to predict reactivity, and density functional theory (DFT) proved to be useful as a predictive tool in this regard. Due to the complex mechanisms that follow electron transfer, kinetics play a crucial role in the success of photoredox reactions. We extend the predictive tools of DFT to estimate the electron-transfer rates between an excited photocatalyst and various organic substrates. To calibrate our model, 49 electron-transfer rate constants were experimentally measured in acetonitrile for the catalyst Ir[dF(CF3)ppy]2(dtbpy)+. The quenching rate constants, kq, gave a trend when compared to calculated ionization energies, which was a better predictor than standard reduction potentials in our case. The calculated kq gave an average error of 10% for log(kq) values between 4.6 and 10.2. This simple method can rapidly predict the reactivity of reagents in silico. Notably, the calculations offered insight that we could translate into success for the C–H activation of 2,4-pentanedione as a proof of concept.