The camphenehydro (methylcamphenilyl) and isobornyl (bornyl) cations. III. Oxidative decarboxylation of carboxylic acids with lead tetraacetate

Abstract

The oxidative decarboxylation of exo- and endo-bornane-2-carboxylic acid, exo- and endo-2,3,3- trimethylnorbornane-2-carboxylic acid and a-campholenylcarboxylic acid [3-(2',2',3'-trimethyl- cyclopent-3'-eny1)propanoic acid] with lead tetraacetate in benzene and dimethyl sulphoxide (each containing pyridine) in the presence, and absence, of cupric acetate has been investigated. The mode of formation of almost all the products can be satisfactorily rationalized in terms of the initial generation of radicals. In the case of exo- and endo-bornane-2-carboxylic acid, the derived bornyl radical forms organolead and organocopper derivatives that decompose in three ways: (1) by heterolysis by the direct route to give the equilibrating isobornyl and camphenehydro cations, (2) by a cyclic cis-elimination to give bornylene and (3) by an SNi process to give isobornyl and bornyl acetates. exo- and endo-2,3,3-Trimethylnorbornane-2-carboxylic acid yield the 2,3,3-trimethyl- norborn-2-yl radical which is converted into the equilibrating camphenehydro and isobornyl cations either by one-electron oxidation by lead species or via organolead or organocopper derivatives which undergo heterolysis. Processes involving cyclic cis-elimination and SNi substitution may also operate in the organometallic derivatives derived from the tertiary radical. a-Campholenylcarboxylic acid yields the a-campholenyl radical which, in the absence of cupric acetate, undergoes in the main cyclization to give the 2,3,3-trimethylnorbornyl radical. The resulting mixture of products is similar to that obtained from exo- and endo-2,3,3-trimethylnorbornane-2- carboxylic acid. In the presence of cupric acetate, the a-campholenyl radical is trapped at least to the extent of 50% in benzene and 80% in dimethyl sulphoxide to give the corresponding organo- copper derivative which undergoes a cyclic elimination to give 2,3,3-trimethyl-4-vinylcyclopentene and may undergo heterolysis by the x-route to give the camphenehydro and isobornyl cations. A marked change in the composition of the products on changing the solvent from benzene to dimethyl sulphoxide is observed only in the case of a-campholenylcarboxylic acid in the presence of cupric acetate.