Zur Photochemie von Allylaryläthern. 55. Mitteilung über Photoreaktionen

Abstract

On the Photochemistry of Ally1 Aryl Ethers SummaryThe photochemical reactions of different allyl aryl ethers (Scheme 3) were investigated in hydrocarbons (Chap. 3.1) and in alcoholic solvents (Chap. 3.2). The composition of the photoproducts depended very much on the nature of the solvent. Irradiation (3-95 h) of different methyl substituted allyl aryl ethers ( 1 , 3 , 5 , 7 and 11) with a low pressure mercury lamp (AEmiss.=254 nm; 6 or 15 Watt) under argon (quartz vessel) resulted in the formation of 2-, 3-and 4-substituted phenols, dienones and products of consecutive reactions (Tables 1-4 and 6). The results suggested that all products were formed by homolytic cleavage of the C-0 bond in the singlet state of the ethers to intermediate radical-geminates (Scheme 5) followed by radical recombination of the two fragments. No products were formed by concerted processes (Table 5, Schemes 5 and 6).Upon irradiation of allyl aryl ethers lacking alkyl substituents at position 4 (1 and 5) in protic solvents, mainly 2-and 4-allylated phenols were obtained (Tables 1 and 4); 3-allylated phenols were formed only in small amounts (0.02%). However, in aromatic hydrocarbons or cyclohexane 3-allylated phenols were obtained from 1, 5 and 11 in significant amounts (3-11%; Tables 1, 4 and 6). E.g., upon irradiation of allyl-2,6-dimethylphenyl ether (5) in toluene, the main photoproduct was 6-allyl-2,6-dimethy1-2,4-cyclohexadien-1 -one (6) besides 3-and 4-ally1-2,6-dimethylphenol (23 and 24). Irradiation of 5 in methanol afforded 23 and 6 only in traces, whereas 24 was the main product.') 54. Mitt.: Siehe [I]. 2,

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

“…At these conversions the mass balance was always above 90%; the yields reported are normalized to 100%. The photochemistry of 3a : X = H and 3d : X = 4-CH 3 in both methanol and cyclohexane for the former, but only in cyclohexane for the latter, has been reported previously; as indicated in Table , agreement with the literature yield values is good. For 3c : X = 3-CH 3 O, literature values are available in ethanol …”

“…For 3d : X = 4-CH 3 one of the products was found to be 3-allyl-4-methylphenol ( 12 ), expected from secondary photochemistry of the cyclohexadienone 13 , eq 7. This product was also observed previously on photolysis in methanol . In this case, the initially formed photo-Claisen product cannot enolize to the phenol.…”

“…Thus, a significant fraction of the singlet radical pairs generated undergo recombination to starting material. This fact has been previously demonstrated for the photo-Claisen reaction both by CIDNP studies , and by the observation of 1,3-migration in unsymmetrical (deuterium- or methyl-substituted) allyl derivatives . Such migrations do not occur in the concerted thermal Claisen rearrangement…”

“…In this case, the initially formed photo-Claisen product cannot enolize to the phenol. Similar 2,5-cyclohexadienone derivatives have always been proposed as the primary photoproducts in Claisen reactions and have been isolated from irradiations of substituted allyl aryl, benzyl aryl, and aryl naphthylmethyl ethers. Products analogous to 12 are probably formed from others of the para-substituted compounds 3 , but if so, we were unable to isolate them.…”

Discovery of a Small-Molecule-Dependent Photolytic Peptide

“…Electron spin resonance (ESR) analysis further supported the radical formation process. Upon UV irradiation, YM-53601 displayed an ESR signal consistent with the formation allyl and phenoxy radicals ( g = 2.00382) (Figure D). The ESR signals from derivative 1 were weaker than that from YM-53601 (Figure E), which is in good agreement with its inability to photolyze SQS.…”