The phototransposition reactions in acetonitrile of p-, m-, and o-methylbenzonitrile have been studied. Any one of the three is converted to the other two by either a 1,2-or 1,3-isomerization in a primary photochemical step. However, the reactivities are quite different with the relative values for para:meta:ortho ) 32:4:1. For both the para and meta isomers, extended irradiations approach a calculated steady-state composition of para:meta:ortho ) 3:20:77. Quenching of the excited triplet state of the para and meta isomers with 2,4-dimethyl-1,3-butadiene indicates that these reactions are occurring from the excited singlet state. Irradiation of selectively labeled 2,6-dideuterio-4-methylbenzonitrile demonstrates that only the cyano-substituted carbon undergoes migration.

The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH2-CH=CH2) has been examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisen rearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from the well-established radical pair mechanism. The excited singlet state properties (absorption spectra, fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallel set of unreactive 4-X- and 3-X-anisoles 4. The excited-state properties of three substituted 4-X-aryl 4-(1-butenyl) ethers 14 (ArOCH2CH2-CH=CH2) were also examined. The model compounds 4 and the reactive allyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of, the rate constant for homolytic cleavage from S(1) of the allyl ethers to give the radical pair. The difference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtain values of for all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2 orders of magnitude with electron-donating groups (CH3O, CH3) accelerating the reaction and electron-withdrawing ones (CN, CF3) slowing it down. The parallel range of rate constants observed in both methanol and cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantum yields of reaction (Phi(r)) for several of the more reactive ethers demonstrate that neither these values nor rate constants of reaction derived from them are reliable measures of the actual excited-state process. In fact, the values are significantly lower than the ones, indicating that the radical pairs undergo recombination to generate starting material. Finally, the rate constants were found to parallel a trend for the change in bond dissociation energy (deltaBDE) for the O-C (allyl) bond of the allyl ethers, indicating that other possible substituent effects are of minor importance.

Singlet excited state rate constants have been measured for both the heterolytic and homolytic photocleavage of 3- and 4-methoxy and 3- and 4-cyano (1-naphthylmethyl)trimethylammonium chlorides, 6–10. The results are interpreted in terms of the meta effect or changes in charge distribution upon excitation and the competition between bond cleavage, electron transfer, and hydrogen atom transfer in the contact pairs resulting from the two types of cleavage.

Irradiation of benzonitrile in 2,2,2-trifluoroethanol (TFE) with 254-nm light from low-pressure mercury lamps results in the formation of four addition products, the stereoisomers of 6-cyano-2-(2,2,2-trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22−25. The proposed mechanism begins by formation of the 6-cyanobicyclo[3.1.0]hex-3-en-2,6-diyl biradical/zwitterion from S1 followed by both endo and exo protonation by TFE at C6. Deuterium labeling demonstrated that the resulting 6-cyanobicyclo[3.1.0]hex-3-en-2-yl cation underwent a rapid degenerate 1,4-sigmatropic rearrangement with inversion of configuration at the migrating carbon before being trapped by the nucleophilic solvent. Irradiation of p- and m-methylbenzonitrile in the same way gave six major addition products. Three of them, 32, 33, and 35 were 6-cyano-2-(2,2,2-trifluoroethoxy)-4-methylbicyclo[3.1.0]hex-3-enes and the other three, 32, 33, and 35, were 6-cyano-2-(2,2,2-trifluoroethoxy)-2-methylbicyclo[3.1.0]hex-3-enes. The proposed mechanism is again by TFE endo and exo protonation of the first formed biradical/zwitterion followed by trapping of the cations by the solvent. The 1,4-sigmatropic rearrangement of the cations now stops at the most stable structures, the endo- and exo-6-cyano-2-methylbicyclo[3.1.0]hex-3-en-2-yl cations, and all of the products are derived from them.

No abstract