Mechanistic insight for tunable regioselective cyclization of aryl propiolates with radicals: ipso- versus ortho-cyclization

Abstract

A computational study of the reaction mechanism of aryl propiolates with various radicals is examined employing DFT and coupled-cluster methods at the DLPNO-CCSD(T)/cc-pVTZ//M062X-6-311G(d,p) level. Computations demonstrated that the reaction mechanism of aryl propiolates with radicals proceeds via two pathways, electrophilic aromatic substitution (EAS) and homolytic aromatic substitution (HAS), depending on the reaction conditions. Computations reveal that radicals are responsible for ipso-cyclization, while cations are responsible for ortho-cyclization. Our calculations show that HAS proceeds with ester migration following ipso-cyclization, whereas EAS prefers ortho-cyclization and no rearrangement. Our results show that ipso-cyclization generates a rearrangement product by ester migration. Computations show that ortho-cyclization occurs following the oxidation of the alkenyl radical to a cation, contrary to a radical. Our results are significant, shedding light on the controversies in the reaction mechanism of aryl propiolates with radicals. Furthermore, our results will be very useful in determining the reaction conditions for the synthesis of significant coumarin derivatives.