41516-23-8Relevant academic research and scientific papers
Substituent effects on the rate constants for the photo-claisen rearrangement of allyl aryl ethers
Pincock, Alexandra L.,Pincock, James A.,Stefanova, Roumiana
, p. 9768 - 9778 (2002)
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 khoms, the rate constant for homolytic cleavage from S1 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 khoms 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 (Φr) for several of the more reactive ethers demonstrate that neither these values nor rate constants of reaction (khomr) derived from them are reliable measures of the actual excited-state process. In fact, the khomr values are significantly lower than the khoms ones, indicating that the radical pairs undergo recombination to generate starting material. Finally, the khoms rate constants were found to parallel a 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.
