| Abstract
| - The reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with β,β-dimethyl-p-methoxystyrene(1) in chloroform affords four adducts: the ene, two stereoisomeric [4 + 2]/ene diadducts, and aminor product that is probably the double Diels−Alder diadduct. In methanol, only one regioisomericmethoxy adduct is formed. The stereochemistry of the reaction was examined by specific labelingof the anti methyl group of 1 as CD3. In chloroform, the ene adduct is formed with >97% synselectivity, while the [4 + 2]/ene diadducts are formed with 20% loss of stereochemistry at themethyl groups. In methanol, the methoxy adducts are formed with almost complete loss ofstereochemistry. A mechanism involving open biradicals is inconsistent with the experimentalresults. It is likely that the reaction proceeds through the formation of an aziridinium imide andan open zwitterionic intermediate. The aziridinium imide leads to the formation of the ene adduct.The open zwitterion, which has sufficient lifetime to rotate around the C−C bond, leads to theformation of a [4 + 2] cycloadduct, which reacts with a second molecule of MTAD in an ene-typemode to afford two stereoisomeric [4 + 2]/ene diadducts. In methanol, solvent captures thezwitterionic intermediate and forms the methoxy adduct. The relative distribution of the productsin chloroform depends on the reaction temperature. Lower temperatures favor the ene reaction(entropically favorable), whereas at higher temperatures the [4 + 2]/ene diadducts become the majorproducts.
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