Singlet state formation

For several compounds probabilities of excited singlet state formation as a result of TTA processes have been determined in our previous paper ( ) with the help of a method based on measurement of the ratio of the integrated intensities of ADF and prompt fluorescence (PF) under the conditions of powerful photoexcitation... 

The calculated relative energies of all the possible intermediates involved in the photochemical isomerization are collected in Fig. 4 (OOOJOC2494). In this case the irradiation can involve the excited singlet state, and then the formation of Dewar isomer is possible. As in 2-methylfuran, the fission of a O—Cq, bond in the triplet state of the molecule is not so favored as in furan. The corresponding biradicals... 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

Computational results are reported for the isomerization of 1,4,5-trimethyl-imidazole (99MI233). They show that the isomerization occurs through the Dewar isomer arising from the excited singlet state. The formation of the triplet state is energetically favored however, the biradical intermediate cannot be produced because it has higher energy than the excited triplet state. 

Calculations account for the experimental results (Fig. 20) (99MI233). The first excited singlet state (which accounts for the absorption at 269 nm calculated value 267 nm) was converted into the corresponding triplet state (69 kcal moF experimental 68 kcal moF ). The triplet state gives the cleavage of the S—N bond with the formation of the biradical intermediate. This intermediate leads to the product. 

Oxadiazole was obtained through the first excited singlet state. When the reaction was carried out in the presence of a triplet sensitizer, 99 was not detected but the quinazolinone 100 was obtained (Scheme 41) [91JCS(P2)187]. Compound 99 cannot be obtained via the Dewar isomer. The author supposed the formation... 

Irradiation of Z-but-2-ene 8 initiates a cyclodimerization reaction, even without a photosensitizer." This cycloaddition proceeds from a singlet state and is likely to be a concerted, one-step reaction. Bond formation occurs suprafacial with respect to both reactants, whereupon only the tetramethylcyclobutanes 9 and 10 can be formed ... 

N-Aminobenzoxazolin-2-one (4), which was readily prepared by animation of benzoxazolin-2-one with hydroxylamine-O-sulfonic acid, is also a useful nitrene precursor (Scheme 2.2). Oxidation of 4 with lead(iv) acetate in the presence of a conjugated diene resulted in exclusive 1,2-addition of nitrene 5, to yield vinylazir-idine (6) in 71 % yield [6]. The formation of vinylaziridines through 1,2-additions of methoxycarbonylnitrene (2) or amino nitrene 5 contrasts with the claimed 1,4-ad-dition of nitrene itself to butadiene [7]. Since the reaction proceeded stereospecif-ically even at high dilution, the nitrene 5 appears to be generated in a resonance-stabilized singlet state, which is probably the ground state [8]. 

In the singlet state of Jt-type 1,3-diradical (e.g., TM, 2), there may also exist the through-space interaction between radical centers, i.e., p...q interaction (Fig. 9), in addition to the previously addressed cyclic -p-o -q-o- orbital interactions (Fig. 6). The through-space interaction is indispensable for the bond formation between the radical centers. The corresponding delocalization of the a-spin electron is shown in Fig. 9a. Clearly, the involvement of the through-space p... q interaction gives rise to two cyclic orbital interactions, -p-o -q- and -p-o-q-. From Fig. 9, one can find that the cyclic -p-o -q- orbital interaction can satisfy the phase continuity requirements for the a-spin electron the electron-donating radical orbital, p (D) can... 

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