Related Experiment Video
Updated: May 18, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
Published on: November 29, 2018
Photochemical electrocyclic ring closure and leaving group expulsion from N-(9-oxothioxanthenyl)benzothiophene
Majher I Sarker1, Tasnuva Shahrin, Mark G Steinmetz
1Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881, USA.
Abstract:
N-(9-Oxothioxanthenyl)benzothiophene carboxamides bearing leaving groups (LG(-) = Cl(-), PhS(-), HS(-), PhCH(2)S(-)) at the C-3 position of the benzothiophene ring system photochemically cyclize with nearly quantitative release of the leaving group, LG(-). The LG(-) photoexpulsions can be conducted with 390 nm light or with a sunlamp. Solubility in 75% aqueous CH(3)CN is achieved by introducing a carboxylate group at the C-6 position of the benzothiophene ring. The carboxylate and methyl ester derivatives regiospecifically cyclize at the more hindered C-1 position of the thioxanthone ring. Otherwise, the photocyclization favors the C-3 position of the thioxanthone. Quantum yields for reaction are 0.01-0.04, depending on LG(-) basicity. Electronic structure calculations for the triplet excited state show that excitation transfer occurs from the thioxanthone to the benzothiophene ring. Subsequent cyclization in the triplet excited state is energetically favourable and initially generates the triplet excited state of the zwitterionic species. Expulsion of LG(-) is thought to occur once this species converts to the closed shell ground state.
Related Concept Videos
Thermal and Photochemical Electrocyclic Reactions: Overview
Photochemical Electrocyclic Reactions: Stereochemistry
Selection Rules: Photochemical Activation
Thermal Electrocyclic Reactions: Stereochemistry
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Acid-Catalyzed Ring-Opening of Epoxides
meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H
Base-Catalyzed Ring-Opening of Epoxides

