INTRAMOLECULAR ANODIC CARBON-CARBON BOND-FORMING REACTIONS OF OXIDIZED PHENOL INTERMEDIATES LEADING TO SPIRODIENONES - STRUCTURAL EFFECTS ON REACTIVITY AND EVIDENCE FOR A PHENOXONIUM ION INTERMEDIATE

Anodic oxidation of 4-phenylphenol in methanol leads to 4-methoxy-4-ph enylcyclohexa-2,5-dienones whereas anodic oxidation of 4-(2-alkenylphe nyl)-phenols leads to spirocyclic 2,5-cyclohexadienones in competition with methanol addition to the 4-position. Using 4-phenylphenol and (4 -(2-propenylphenyl)phenol as model systems, the optimum conditions for solvent addition versus carbon-carbon bond formation have been studie d. The yield of the anodic cyclization reaction shows a dramatic depen dence on olefin structure. Whereas 4-(2-propenylphenyl)phenol gives th e spirocyclic 2,5-cyclohexadienone in high yield, 4-(2-vinylphenyl)phe nol affords the analogous product in only 16 % yield. This low yield o f intramolecular carbon-carbon bond-forming reactions can be markedly improved if the vinyl substituent is forced closer to the 4-position o f the phenol by the buttressing effect of a o-methyl group. Coloumetri c studies as well as the oxidation chemistry of a (4-hydroxyphenyl)(2- propenylaryl)methane derivative support the involvement of a phenoxoni um ion as the intermediate in these carbon-carbon bond-forming reactio ns. Finally, non-oxidative generation of a phenoxonium ion by reaction of -hydroxy-4-(2-propenylphenyl)2,5-cyclohexadieneone with methanesul fonyl chloride/triethylamine leads to spirodienones related to those i solated in the anodic oxidation chemistry. Although a slightly acidic media is critical for obtaining good yields of spirodienones for the p ropenyl system, anodic oxidation of the trimethylsilyl derivatives of the phenol allows this reaction to be performed in neutral or slightly basic media.