Synthetic and mechanistic studies of ene reactions of carbonyl enophiles

In a synthetic study of a new carbonyl enophile, the ene reactions of dimethyl dioxosuccinate (81) with olefins were investigated and it was found the 81 reacts smoothly with most of the olefins investigated under thermal conditions to give the expected products in good yields. Sequential ene reactions to form cyclic products were also investigated and it was found that under catalysis with tin tetrachloride, the ene reaction product from methylenecyclohexane (8) and 81, dimethyl 2-oxo-3-hydroxyl-3-(1$\sp\prime$-cyclohexenylmethyl)succinate (89) undergoes another intramolecular ene reaction to give substituted cyclopentane products in moderate yield. The thermal ene reaction of 81 with vinyl heteroatom-substituted olefins, vinyl ethers, vinyl acetates, enamines, enols were also investigated.The mechanism of ene reactions of a selected group of carbonyl enophiles with 8 and 1-methylenetetralin (129) were studied by measurement of inter- and intramolecular hydrogen-deuterium isotope effects. In the thermal ene reactions of both 81 and diethyl oxomalonate (3) with 8, both inter- and intramolecular isotope effects are primary and substantial, consistent with concerted mechanisms. In the tin tetrachloride catalyzed reaction of 3 with 8, and in the reaction of acetylium hexachloroantimonate (CH$\sb3$CO$\sp{+}$SbCl$\sb{6-}$) with 8, the observed isotope effects are small in both inter- and intramolecular cases, which is inconsistent with either simple stepwise or concerted mechanism. To account for the observed anomalous isotope effects, a general quantitative kinetic model is proposed in which the starting materials form an intermediate which can partition between product formation, dissociation back to starting materials, and equilibration. The model predicts that depending on the extent of reversal in the formation of the intermediate and extent of equilibration of the intermediate between its different isomers, a range of inter- and intramolecular isotope effects can be observed even when the real isotope effect in the hydrogen transfer step is primary and substantial. Application and test of this model in the ene reaction of acetic anhydride with 129 catalyzed by zinc chloride was demonstrated.U of I OnlyETDs are only available to UIUC Users without author permissio