Search

DFT calculations on the photoisomerization reactions of pyridazine N -oxide derivatives are presented. The irradiation of 3,6-diphenylpyridazine N -oxide allowed to obtain the first excited singlet state that underwent to a ring opening reaction induced to the oxygen migration to the adjacent carbon atom, with the formation of a diazo derivative. This intermediate can give a ring closure reaction to give a pyrazole derivative, or, with a higher transition energy, it can lose nitrogen allowing the formation of 2,5-diphenylfuran. The irradiation of 3-phenylpyridazine N -oxide allowed the formation only of 2-phenylfuran. In this case, calculations allowed to explain this behavior. The first excited singlet state gave a photoisomerization reaction with the formation of the corresponding diazo intermediate. This intermediate could not be converted into the corresponding pyrazole derivative because it was converted into a N -formyl pyrazole never reported in literature. The only possible reaction is the loss of nitrogen and the formation of 2-phenylfuran. [Display omitted] • The first photochemical reactions of 3,6-diphenylpyridazine N -oxide is a ring opening reactions. • In the photoisomerization of 3,6-diphenylpyridazine N-oxide oxaziridine intermediates were not obtained. • The formation of furan derivatives occurs without any intermediate. • The selective formation of furan in 2-phenylpyridazine N-oxide derive from an unusual isomerization reaction. [ABSTRACT FROM AUTHOR]

DFT calculations on the photoisomerization of 2,6-dimethylpyrazine allowed us to confirm the role of benzvalene isomers in the isomerization of hexatomic heterocyclic compounds. 2,6-Dimethylpyrazine in the excited singlet states can be converted into the corresponding Dewar isomers. If the S2 state is populated, two Dewar isomers can be obtained, while the S1 state allows the formation of only one of the possible Dewar isomers. Both Dewar isomers can be converted into the benzvalene isomer, that is, the precursor of 4,5-dimethylpyrimidine, the reaction product. In fact, the benzvalene isomer can be obtained from the Dewar isomers in processes that occur without an activation energy, and it is the more stable benzvalene isomers that can be obtained from the Dewar isomers. CASSCF study indicates the presence of a conical intersection allowing the direct formation of the benzvalene isomer. [ABSTRACT FROM AUTHOR]

DFT calculations on the photoisomerization of hydrazones of 1,2,4-oxadiazole derivatives to 1,2,5-triazoles have been performed showing that the reaction occurred through the first excited singlet state. The Z isomer gave the reaction through a hydrogen atom transfer of the hydrazonic nitrogen atom to the nitrogen atom in four position on the oxadiazole ring. In this case, the isomerization was a concerted reaction. The E isomer could undergo the same reaction. However, it could not be a concerted reaction but required the presence of a ring opening intermediate. [ABSTRACT FROM AUTHOR]

The photochemical version of the Boulton–Katritzky reaction has been studied, examining the behaviour of the arylhydrazones of 3-benzoyl-5-X-1,2,4-oxadiazoles. The effect of several modifications of the substrates structure (the E and/or Z structures of arylhydrazones, the possible presence of substituents in the arylhydrazono moiety, and the nature of substituents at C-5 of the 1,2,4-oxadiazole ring) on the course of the photochemical rearrangement has been examined. [ABSTRACT FROM AUTHOR]

Abstract: The reasons why colchicine undergoes photochemical cyclization to give lumicolchicine, whereas thiocolchicine does not, are investigated by theoretical methods. The relative stability of the reagent and of the products is computed at DFT level of theory and indicates that such a photochemical reaction is energetically possible. The photochemical reaction path of thiocolchicine is studied within the relaxed energy scan, at MC-SCF level of theory. The results are again rather similar to those of colchicine. Finally, the spin–orbit coupling between S1 and T1 has been computed and are two order of magnitude greater than those of colchicine, that makes plausible that spin–orbit is the main reason of the observed reaction quenching. [Copyright &y& Elsevier]

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 π,π* triplet with LSOMO at −9.47 eV and HSOMO at −6.84 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 π,π* species. It shows the LSOMO at −10.70 eV and the HSOMO at −8.14 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. [Copyright &y& Elsevier]

The photochemical isomerization reactions of thiophene, thiophene-2-carbonitrile, and 2-phenylthiophene were studied using ab initio methods. The results are in agreement with the previously reported data obtained through semiempirical methods. Triplet excited thiophene is a π,σ* triplet with LSOMO at −9.94 eV and HSOMO at −9.51 eV and the biradical intermediate is a π,π* species with LSOMO at −10.12 eV and HSOMO at −4.82 eV. In this case, the singlet excited state can evolve giving the Dewar thiophene, while the corresponding excited triplet state cannot be obtained. Furthermore, the triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. Triplet excited thiophene-2-carbonitrile is a π,π* species. It shows the LSOMO at −11.38 eV and the HSOMO at −7.36 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; however, its interconversion into the corresponding biradicals cannot be efficient considering that the biradicals show a higher energy, even if for a little amount, than that of the triplet state. Triplet excited 2-phenylthiophene is a π,π* species. It shows the LSOMO at −9.32 eV and the HSOMO at −6.24 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 energy of the excited singlet state was obtained from the UV absorption of the substrate. The intersystem crossing to the triplet state cannot occur; because it shows higher energy than the corresponding singlet state. Furthermore, its interconversion into the corresponding biradicals cannot be efficient considering that the biradicals show the same energy of the triplet state. The high efficiency of this reaction can be explained on the basis of low energy of the Dewar isomer. [Copyright &y& Elsevier]

Studies the photochemical isomerization reactions of furan, 2-methylfuran, 2-trimethylsilylfuran, and furan-2-carbonitrile using ab initio methods. Sensitized irradiation of furan derivatives in the first excited triplet state of the furan; Relationship of the efficiency of these processes with the presence or the absence of isomerized furan derivatives.

The isomerization occurs only from trans first excited singlet state • Furylidentetralone derivative undergoes photochemical isomerization through S 1 state. • The photochemical isomerization occurs only from E -isomer. • Triplet state of furylidenetetralone gives the transposed product via a high energy transition state. • The triplet state of furylidenetetralone cannot give dimerization products (endothermic process). A DFT study on the photochemical isomerization of furylidenetetralone derivatives is reported. The excitation of the trans isomer of the starting material can give the cis one, converts the S1 into the corresponding triplet state, or can be converted directly into the transposed product. The excitation of the cis isomer of the starting material can give the same triplet state observed with the trans isomer; however it cannot be converted into the transposed product because the energy of the transition state is too high. Finally, compounds with the structure Het CH CH COPh can give a sensitized dimerization reaction, never observed in this case. The calculations showed that, while with compounds such HetCH CH COAr the biradical intermediate of the dimerization reaction was obtained in very fast exothermic reaction, in the case of the furylidenetetralone derivative, the formation of the dimeric biradical intermediate is highly endothermic and, than, less probable. [ABSTRACT FROM AUTHOR]