Tandem Photoarylation-Photoisomerization of Halothiazoles: Synthesis, Photophysical and Singlet Oxygen Activation Properties of Ethyl 2-Arylthiazole-5-carboxylates

• Published in: European Journal of Organic Chemistry 2010-06, Vol.2010 (18), p.3416-3427
• Main Authors: Amati, Mario, Belviso, Sandra, D'Auria, Maurizio, Lelj, Francesco, Racioppi, Rocco, Viggiani, Licia
• Format: Article
• Language: English
• Subjects: Arylation; Chemistry; Density functional calculations; Exact sciences and technology; General and physical chemistry; Heterocycles; Heterocyclic compounds; Heterocyclic compounds with n hetero atom and also o and/or s, se, te hetero atoms; Heterocyclic compounds with o, s, se, te hetero atom and condensed derivatives; Isomerization; Noncondensed benzenic compounds; Organic chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Preparations and properties
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201000096

The photochemical reaction between ethyl 2‐chlorothiazole‐5‐carboxylate and benzene gave no product. In contrast, the reaction with ethyl 2‐iodothiazole‐5‐carboxylate gave ethyl 3‐phenylisothiazole‐4‐carboxylate. The same behaviour was observed if the reaction was performed in the presence of furan, thiophene and 2‐bromothiophene. The products thus obtained were studied for their photophysical properties and it was found that the observed absorptions [λmax=257 nm (ϵ = 7000 M–1 cm–1) for ethyl 3‐phenylisothiazole‐4‐carboxylate, 278 nm (ϵ = 7000 M–1 cm–1) for ethyl 3‐(2‐furyl)isothiazole‐4‐carboxylate, 269 and 285 nm (ϵ = 7500 M–1 cm–1) for ethyl 3‐(2‐thienyl)isothiazole‐4‐carboxylate] are mainly due to π → π* transitions from the HOMO to the LUMO+1 orbital. The tested compounds showed fluorescence [λem = 350–400 (λexc = 300 nm), 360 and 410 nm (λexc = 320 nm), 412 nm (λexc = 340 nm) for ethyl 3‐phenylisothiazole‐4‐carboxylate, 397 and 460 nm (λexc = 300 nm), 381, 394 and 460 nm (λexc = 320 nm), 381, 398 and 466 nm (λexc = 340 nm) for ethyl 3‐(2‐furyl)isothiazole‐4‐carboxylate, 372, 377 and 414 nm (λexc = 300, 320 and 340 nm respectively) for ethyl 3‐(2‐thienyl)isothiazole‐4‐carboxylate], possibly due to dual emission from different excited states. The use of the compound obtained as a sensitizer in the photo‐oxidation of trans‐α,α′‐dimethylstilbene showed that all the new compounds are singlet‐oxygen sensitizers. An unusual photoarylation–photoisomerization of ethyl 2‐iodothiazole‐5‐carboxylate is described. Photophysical and singlet oxygen activation properties are also reported.