Photo-Claisen Rearrangement. The Photochemical Rearrangement of Allyl Phenyl Ethers

Abstract

Two years ago, Woodward and Hoffmann presented an ingenious theory in an attempt to interpret the stereochemical cours of the so-called "electrocyclic" and "sigmatropic" transformations.1) They suggested1b) that the photo-Claisen rearrangement reported to give a para-rearrangernent product2) would serve as an example of the [3,5] sigmatropic transformation.Another example of the photo-Claisen rearrangement had previously been reported to give only a pararearrangement product.3)Although this interpretation of the photo-Claisen rearrangement was highly attractive, the following facts seemed to us conflicting enough to warrant a thorough reinvestigation of the photo-rearrangement of allyl phenyl ethers:(1) An eight-membered cyclic transition state these systems seems sterically impossible.(3) Several related systems, phenyl esters,4) acyl anilides,5) phenoxyacetic acid,6) and ethyl phenyl carbonate,7) have been reported to rearrange photochemically to ortho-and para-rearrangement products.The recent publication of a communication8) dealing with an investigation closely related to ours prompts us to publish our own results. ExperimentalMaterials. The allyl phenyl ethers for the photoreaction were purified by rectification until no appreciable impurity was observed by glc.Reaction Procedure. A 5% solution of a sample in isopropyl alcohol was irradiated with an immersiontype 10W low-pressure mercury lamp (Ricoh Kagaku Sangyo Co., UVL300Q) under a nitrogen atmosphere. After irradiation, most of the solvent was removed under reduced pressure, and the residual mixture was analyzed by glc.Identification of Products. o-and p-Allylphenols, 2-allyl-3,5-dimethylphenol, trans-o-and -p-but-2-enylphenols, and o-and p-(1-methyl-prop-2-enyl)phenols were identified by a comparison of their retention times with those of authentic sample. 4-Allyl-3,5-dimethylphenol was isolated from the reaction mixture by means of preparative glc and was identified spectrometrically. Two unknown peaks in the gas chromatogram of the reaction mixture from traps-but-2-cnyl phenyl ether were tentatively assigned to cis-o-and -p-but-2-enylphenols on the basis of their having been formed by the irradiation of their trans-isomers.Analyses. The samples were analyzed by the use of a Hitachi K-23 gas chromatograph. A 1-meter Celite-545 (60-80mesh) column with 33% Silicon KF-54 was used. The results in the table are given as the percentage ratio of the peak area. Results and DiscussionAs shown in the table, all the compounds rearranged to give ortho-and para-rearrangement products in addition to the parent phenols of the starting ethers. Some unidentified products were also present. trans-But-2-enyl phenyl ether was shown to give o-and p-but-2-enyl and o-and p-(1-methylprop-2-enyl)phenols.The results are consistent with those of Schmid2) and clearly indicate that the rearrangement does not proceed *1 A preliminary account of this investigation has been presented at

Photochemical Activation of Tertiary Amines for Applications in Studying Cell Physiology

“…A radical mechanism could also be possible, because the rearrangement could be effected in acetonitrile, in which homolytic cleavage of the C–N bond predominates (vide infra). Additionally, some evidence − suggests that the photo-Claisen rearrangement is mechanistically related to the photo-Fries rearrangement, which is known to proceed via a radical mechanism. , More work is necessary to better understand the rearrangement mechanism.…”

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

“…Along these lines, the ‘on-water’-effect was explored to reduce the reaction temperatures, too [ 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 , 91 , 92 , 93 , 94 ]. In addition, the use of alternative energy inputs, such as photochemistry [ 95 , 96 , 97 , 98 ], or microwave irradiation [ 99 , 100 , 101 , 102 , 103 , 104 ], that do not solely rely on a thermal activation led to further improvements.…”

Photo-Claisen Rearrangement of Allyl Phenyl Ether in Microflow: Influence of Phenyl Core Substituents and Vision on Orthogonality

“…Here, the main advantage is the possibility to generate para-substituted products besides the ortho-substitution pattern (for details on mechanism of photo-Claisen rearrangement, one is referred to refs. [17][18][19][20]). Yet, it should be noted as well that this generates an extra separation issue.…”