Cyclohexadienyl cations. 7. Kinetics and mechanism of the acid-catalyzed dienol-benzene rearrangement

Photochemical and Acid-Catalyzed Rearrangements of 4-Carbomethoxy-4-methyl-3-(trimethylsilyl)-2,5-cyclohexadien-1-one

The synthesis of 4-carbomethoxy-4-methyl-3-(trimethylsilyl)-2,5-cyclohexadien-1-one (1) in 60% overall yield from benzaldehyde is described. Irradiation (366 nm) of 1 in benzene solution gave products of type A photorearrangement; e.g., diastereomers of the 4-(trimethylsilyl)- and 5-(trimethylsilyl)bicyclo[3.1.0]hex-3-en-2-ones 8 and 9. Bicyclohexenones 9a and 9b could not be isolated, but underwent acid-catalyzed protiodesilylative rearrangements on attempted chromatography (silica gel) to give a 1:1 mixture of (E)- and (Z)-4-(carbomethoxymethylmethylene)cyclopent-2-en-1-ones 12 and 13. Irradiation (366 nm) of either 12 or 13 resulted in photoisomerization to a photostationary state that was also a 1:1 mixture. Irradiation of 8a or 8b gave equivalent mixtures of phenols 14 and 15 by way of the type B oxyallyl zwitterion 17. The available experimental evidence suggests that both 9a and 9b undergo regiospecific photorearrangement to phenol 16 with no trace of 3-methyl-4-carbomethoxyphenol (19), the product of ipso substitution of the Me(3)Si group at C(4). Phenol 15 was isolated in 65% yield from the photoreaction of 1 in benzene with 20 equiv of CF(3)CO(2)H. The acid-catalyzed rearrangement of 1 to 3-carbomethoxy-4-methylphenol (21) occurs in 91% yield by way of CO(2)Me group rearrangement to C(3) to give the Me(3)Si-stabilized carbocation 23.