Ab initio study on the photochemical isomerization of thiazole derivatives Maurizio D’Auria * Dipartimento di Chimica, Universita ` della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy Received 18 June 2002; revised 24 June 2002; accepted 15 August 2002 Abstract—The photochemical isomerization reactions of 2-phenyl and 2-acetylthiazole were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. Triplet excited 2-phenylthiazole is a p,p p triplet with LSOMO at 29.47 eV and HSOMO at 26.84 eV. In this case, the singlet excited state can evolve giving the Dewar thiazole while the corresponding excited triplet state cannot be obtained. Furthermore, the triplet state cannot be converted into the biradical intermediates because these intermediates show a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. Triplet excited 2-acetylthiazole is a p,p p species. It shows the LSOMO at 210.70 eV and the HSOMO at 28.14 eV. In this case, the direct irradiation involves the population of the excited singlet state, and then the formation of the Dewar isomer is possible. The intersystem crossing to the triplet state can occur; the intersystem crossing quantum yield for this conversion is nearly quantitative. The triplet state cannot convert into Dewar thiazole but it can give the corresponding biradicals. These biradicals are not responsible for the isomerization reactions but they are able to give only decomposition products. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Five mechanisms can be invoked in order to justify the photochemical isomerization of pentaatomic aromatic heterocycles: (1) the ring contraction – ring expansion route (RCRE) (Scheme 1A); (2) the internal cyclization– isomerization route (ICI) (Scheme 1B); (3) the van Tamelen–Whitesides general mechanism (VTW) (Scheme 1C); (4) the zwitterion–tricycle route (ZT) (Scheme 1D); (5) the fragmentation –readdition route (FR) (Scheme 1E). Recently we reported that the photochemical isomerization of pentaatomic aromatic heterocycles 1 can be described using a unifying hypothesis. 2–6 In this hypothesis, if the first excited singlet state of a molecule is populated, the molecule can convert into the corresponding triplet state or into the corresponding Dewar isomer. The efficiency of these processes will depend on energetic factors. If the Dewar isomer is formed, the isomeric product is obtained. If the triplet state is formed, cleavage of the X–C a bond can occur to give ring opening products, decomposition products or ring contraction products. However, if the radical formed after the X–C a cleavage shows a higher energy than the triplet state, the triplet state will not be able to give the biradical with high efficiency, and, then, it will be quenched in radiative and not radiative processes. In this case, the Dewar isomer could be responsible for the isomerization reaction, but the isomerized product will probably be produced in very low quantum yields. 0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(02)01003-7 Tetrahedron 58 (2002) 8037–8042 Scheme 1. Proposed mechanisms for photochemical isomerization of pentaatomic aromatic heterocycles. * Tel.: þ39-971-202-240; fax: þ39-971-202-223; e-mail: dauria@unibas.it Keywords: thiazole photoisomerization; ab initio study; 2-acetylthiazole; decomposition products.