Elucidating Latent Mechanistic Complexity in Competing Acid-Catalyzed Reactions of Salicylaldehyde-Derived Baylis–Hillman Adducts

1H NMR-based kinetic studies have revealed the latent mechanistic complexity of deceptively simple hydrochloric acid-catalyzed reactions of salicylaldehyde-derived Baylis–Hillman adducts. Reactions conducted at 0 °C afforded 2-(chloromethyl)­cinnamic acid derivatives as the major products and the corresponding 3-(chloromethyl)­coumarin derivatives as the minor products. In reactions conducted in refluxing acetic acid, however, the 3-(chloromethyl)­coumarin derivatives are the sole products. Variable-temperature 1H NMR analysis permitted the determination of the rate constants and kinetic parameters involved in the pseudo-first-order formation of (Z)-2-(chloromethyl)-3-(2-hydroxyphenyl)-2-propenoic acid. The kinetic data clearly preclude the operation of classical kinetic versus thermodynamic control and indicate the operation of three independent reaction pathways. Theoretical studies of these pathways undertaken at the B3LYP/6-31G­(d) level permitted rationalization of the experimental data and provided insights into the possible mechanism of the enzymic E–Z isomerization and cyclization of (E)-cinnamic acid analogues to afford coumarins.