Competition-Derived Relative Reactivity and 3D Electronic-State Analysis of Site- and Facial Selectivity in NaBH4/MeOH Ketone Reductions

Abstract Yield-based measures of reactivity can be distorted by side reactions and reaction conditions and therefore may not accurately reflect intrinsic reaction rates. In this study, competition experiments, in which two substrates react simultaneously with the same reagent, were used to build a unified kinetic scale for the reduction of ketones by sodium borohydride (NaBH4) in methanol, expressed as activation free energy differences (ΔΔG‡). Kinetic data for 89 structurally diverse ketone reaction faces tracked substituent electronic effects and steric congestion, and transition-state calculations identified methanolysis as the step that determines the identity of the active reductant. A Lasso model trained on 83 of these reaction faces with three-dimensional electron-density, electrostatic-potential, and carbonyl-centered projected C═O π* descriptors achieved R2 = 0.804 under strict nested outer leave-one-out cross-validation. The monoketone-derived model recovers the dominant site and face for every diketone examined, with 85.5% of the cross-validation models reproducing all assignments; however, the predicted product distributions are indicative rather than quantitative. The workflow integrates a yield-independent kinetic scale with experimental measurements, theoretical calculations, and data-driven analyses within the evaluated NaBH4/MeOH domain and provides a basis for testing broader applicability through reaction-specific data, retraining, and validation.