Zur Photochemie von Allylaryläthern

The 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 (lambda(Emiss.) = 254nm; 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-O 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. Ethers alkylated at position 4 (3 and 7) yielded 3-allylated phenols after irradiation in hydrocarbons and in methanol (Tables 2 and 3). The time independent equilibration of deuterium labelling in the allyl chain of dienone d3-6 obtained upon irradiation of 2,6-dimethylphenyl-2’, 3’, 3’-trideuterioallyl ether (2’,3’, 3’-d3-5) (cf. Table 5) demonstrated that the photolysis of aromatic allyl ethers did not occur by a [1s,3s]-sigmatropic process (cf. Chap. 3.1.4.2). For the photochemical formation of 3-allylated phenols, the following two mechanisms may be envisaged: 1. According to CIDNP measurements [12] at least one portion of the 3-allylated phenols 14, 20 and 23 is produced via a direct recombination of the triplet-geminate, which is formed from the singlet-radical-geminate via intersystem crossing (Scheme 5, pathway d). 2. Formation of 4-allyl-2,5-cyclohexadien-1-ones (III, IV and 12) could occur via a singlet-geminate (Scheme 5, pathway c). These dienones undergo photochemical excitation and give bicyclic intermediates, which after further photoexcitation are finally transformed into the 3- allylated phenols 14, 20 and 23 (cf. Scheme 6, pathway g, h and i). This path allows the formation of significant amounts of 3-allylated phenols (3- 11%) during photo-Claisen-rearrangement of allyl phenyl ethers lacking a substituent at position 4. The lifetimes of the initially formed 4-monosubstituted dienones III and IV in hydrocarbons were long enough to permit photochemical isomerization to 3- allylated phenols. In protic solvents however, a fast enolization of 3 and IV to 4-allylated phenols is expected, so that the photochemical isomerization is interrupted. The presence of bases which catalyse this heterolytic enolization further suppress the photochemical isomerization (Table 1). The very small amount (0,01- 0,1%) of 3-allylphenols, which were still formed after the irradiation of allyl aryl ethers lacking a substituent at position 4 in protic solvents or under basic conditions, must be produced by direct radical recombination within the triplet-geminate (cf. Scheme 5, pathway m and n). Free phenoxy and allyl radicals were also formed from the triplet-geminate (cf. ESR. experiments, Chap. 5). The former yielded the observed phenols after hydrogen abstraction from the solvent. A crossover experiment with tritiated ether 3’-t-3 and ether 1 suggested that recombination of free phenoxy and allyl radicals during photolysis did occur only to a very small extent (2 - 0,1%; cf. Chap. 4). The photochemical transformation of ((E)- or (Z)-2’-buteny1-2,6-dimethylphenylether ((E)- and (Z)-11, respectively) to (E)- or (Z)-6-(2’-butenyl)-2,6-dimethyl-2,4-cyclohexadien-1-one ((E)- and (Z)-26, respectively) at 7° showed, that the configurational integrity of the allyl radical was maintained (90-95%) until the recombination had occurred (Table 6)