The experimental and theoretical determination of combinatorial kinetic isotope effects for mechanistic analysis

Unfortunately, chemists can never experimentally unravel a full reaction pathway. Even our ability to define key aspects of mechanisms, such as short-lived intermediates and the even more ephemeral transition states, is quite limited, requiring subtle experiments and subtle interpretations. Arguably the most important knowledge to be gained about the mechanism of a reaction is the structure and geometry of the transition state at the rate-limiting step, as this is where a reaction?s rate and selectivity are generally decided. The Singleton group has developed a methodology for predicting the combinatorial kinetic isotope effects (KIEs) at every atomic position, typically carbon or hydrogen, at natural abundance. A combination of experimental isotope effects and density functional theory (DFT) calculations has greatly aided our ability to predict and understand a reaction?s pathway and transition state geometries. Precise application of this method has allowed for the mechanistic investigation of a myriad of bioorganic, organic, and organometallic reactions. The technique has been applied in the analysis of the catalytic borylation of arenes via C-H bond activation, dynamic effects in the enyne allene cyclization, palladium catalyzed allylic alkylation, the nature of proton transfer in orotate decarboxylase, and the epoxidation of enones with t-butyl hydroperoxide.