Substituent Effects on the Rate Constants for the Photo-Claisen Rearrangement of Allyl Aryl Ethers

Abstract

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.

Control of an Unusual Photo-Claisen Rearrangement in Coumarin Caged Tamoxifen through an Extended Spacer

“…Despite the extensive use of coumarin caged compounds, 3,6,37 only few previous studies have indicated the occurrence of photorearrangement in caged thiols and phenols. 31,34,37 Our present study reports on the prevalence of photo-Claisen rearrangement 33,36 in coumarin ether caged compounds, but no evidence is observed for the implication of such rearrangement in ONB uncaging, which occurs exclusively via an intramolecular cyclization of its nitro group. 6 We attribute the basis of this photorearrangement to the radical-based mechanism of coumarin uncaging in combination with the presence of an ether linkage which, due to its shorter length in nature, brings the transient radical species generated by light exposure into close proximity, thus promoting their recombination.…”

“…The occurrence of these unexpected products from 2 is, in retrospect, explained by its [3,3] bond framework, which is composed of an allyl (BHC)-to-aryl (4OHT) ether, a unique structural feature required for the occurrence of photo-Claisen rearrangement. 33,36 …”

Photocycloaddition of Arenes and Allenes

“…Likewise, an analoguous side product, tentatively assigned to 17a was also isolated from the irradiation of 2-(buta-2,3-dienyloxy)-benzaldehyde 1a at 254 nm in 6 % yield as well as in the thermal conversion of 1a upon heating above 200 °C (vide supra). The formation of this product could be explained by a photoinduced Claisen rearrangement, 31 but to our knowledge, there are no earlier reports on such reactions with allenes.…”

Photo-Induced Micellization of Block Copolymers

“…While several photorearrangements have been observed for N-aryl lactams [72,73], the carboxylic acid derivatives of N-aryl amides [74,75], enol esters [76,77], enol lactones [78], enamides [79], and the sulfonic acid derivative of N-aryl amides [80], a number of publications have reported the photo-Claisen rearrangement [81][82][83] and photo-Fries rearrangement [81,[84][85][86].…”