The Elusive Nature of Aromatic Carbocation Intermediates in Confined Catalytic Environments

Wheland intermediates play a pivotal role in the mechanistic understanding of electrophilic aromatic substitution, yet their existence under realistic conditions remains debated.Here, we address this question for aromatic alkylation in acidic zeolites, where charged intermediates are strongly influenced by confinement, electrostatics, hydrogen bonding, and finite-temperature dynamics.By combining enhanced free-energy sampling based on machine-learning interatomic potentials with free energy perturbation and embedded-cluster CCSD(T) calculations, we construct chemically accurate free-energy surfaces for the alkylation of benzene and phenol by cyclohexene in H-BEA.We find that the classical stepwise picture breaks down when the free-energy surface is described at gold-standard coupled-cluster accuracy.For benzene alkylation, both the carbenium ion and the Wheland intermediate are transient configurations rather than genuine intermediates.For phenol alkylation, the OH substituent provides additional stabilization, yielding metastable cationic species that nevertheless remain short-lived.These results show that the existence and lifetime of reactive intermediates are not inherent to the reaction class but emerge from the interplay of substituent effects, confinement, and finite-temperature dynamics.More broadly, we demonstrate that free-energy surfaces beyond DFT quality are required to obtain chemically accurate insights necessary for truly predictive materials design.