MECHANISM OF ACID-CATALYZED PHOTOADDITION OF METHANOL TO 3-ALKYL-2-CYCLOHEXENONES

Contrary to a previous report, it is concluded that formation of metha nol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protona tion of the intermediate enone pi,pi triplet excited state and not by protonation of a relatively long-lived ground state transcyclohexenon e. A rate constant for protonation of the triplet state of 3-methyl-2- cyclohexenone by sulfuric acid of 1.7 x 10(9) M(-1) s(-1) was determin ed by laser flash photolysis in ethyl acetate. Based on quantum effici encies of product formation, a rate constant of ca. 10(8) M(-1) s(-1) was estimated for protonation of the enone tripler by acetic acid, whi ch is too small to cause measurable reduction in the triplet state lif etime in the mM concentration range used in the preparative studies. T he intermediate carbocation can be trapped by methanol, or revert to s tarting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed proc ess, there is an acid concentration at which the yields of products ar e optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation cou ld be quenched using 1-methylnaphthalene and cyclopentene as tripler q uenchers; in the latter case, formation of [2 + 2] photoadducts was ob served to compete with formation of methanol adducts. Quenching rate c onstants were determined by laser flash studies.