Time-Resolved 3D Hammett Correlation to Monitor Catalyst Behavior with No Differential Data in Hand

Hammett correlation is one of the widely applied tools in the mechanistic studies of catalytic processes. From the beginning, it was applied for measuring reaction rates with various substituted substrates, particularly benzoic acid derivatives (under competitive or noncompetitive modes), to construct a two-dimensional plot of the relative reactivity vs the σ-constant of the substituents. In this study, we use time-resolved fundamental Hammett correlation to obtain a three-dimensional plot for a catalytic reaction using experimental data of cross-coupling reactions. Using this approach, possible variations of the Hammett correlation parameter ρ during the catalytic reaction can be easily monitored. Importantly, the suggested approach does not require differentiation of primary experimental kinetic data on the concentrations of substrates and products. The study was conducted for the Suzuki–Miyaura reaction, and demonstrative data were obtained for different behaviors of the Pd species activating aryl iodides, aryl bromides, or aryl chlorides under “ligand-free” catalytic conditions. The proposed approach was tested for other reactions of aryl halides and related substrates to confirm its validity for the monitoring of possible changes in active catalyst species during the reactions.