Solvolysis of 4-Halogeno-4-Alkyl-2,6-di-tert-butylcyclohexa-2,5-dienones Induced by Positive Halogen Donors as Electrophiles

Positive halogen donors such as N-iodosuccinimide (NIS) induce solvolysis of dienones 1, as model 4-halogenocyclohexa-2,5-dienones, in different hydroxylic solvents (ROH), yielding the 4-RO-cyclohexa-2,5-dienones (2). The rate of the solvolysis with NIS is highly dependent on the structure of ROH. The problem of such dependency is overcome by running the reaction in ROH diluted with MeCN, a polar aprotic solvent, in place of pure ROH; the rate of the reaction in the ROH-MeCN solvent mixture is almost independent of the structure (or the polarity) of ROH, and the reaction is completed faster or markedly faster than in neat ROH. The results suggest that the solvolysis rate is controlled by the polarity of the solvent system, although the hydrogen-bond acceptability of MeCN for dilution also accelerates the reaction. A mechanism for the solvolysis is proposed, involving electrophilic attack of a positive halogen donor at the halogen atom of 1, generating the 4-oxocyclohexa-2,5-dienyl cation intermediates (8) via the rate-limiting polar transition states.