| Abstract
| - The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH2−CHCH2) hasbeen examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisenrearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from thewell-established radical pair mechanism. The excited singlet state properties (absorption spectra,fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallelset 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−CHCH2) were also examined. The model compounds 4 and the reactiveallyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of, the rate constant for homolytic cleavage from S1 of the allyl ethers to give the radical pair. Thedifference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtainvalues offor all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2orders 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 methanoland cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantumyields of reaction (Φr) for several of the more reactive ethers demonstrate that neither these values norrate constants of reaction () derived from them are reliable measures of the actual excited-stateprocess. In fact, thevalues are significantly lower than theones, indicating that the radical pairsundergo recombination to generate starting material. Finally, therate constants were found to parallela trend for the change in bond dissociation energy (ΔBDE) for the O−C (allyl) bond of the allyl ethers,indicating that other possible substituent effects are of minor importance.
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