Stereochemistry in the reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with beta,beta dimethyl-p-methoxystyrene. Are open biradicals the reaction intermediates?

The reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with beta,beta-dimethyl-p-methoxystyrene (1) in chloroform affords four adducts: the ene, two stereoisomeric [4 + 2]/ene diadducts, and a minor product that is probably the double Diels-Alder diadduct. In methanol, only one regioisomeric methoxy adduct is formed. The stereochemistry of the reaction was examined by specific labeling of the anti methyl group of 1 as CD(3). In chloroform, the ene adduct is formed with >97% synselectivity, while the [4 + 2]/ene diadducts are formed with 20% loss of stereochemistry at the methyl groups. In methanol, the methoxy adducts are formed with almost complete loss of stereochemistry. A mechanism involving open biradicals is inconsistent with the experimental results. It is likely that the reaction proceeds through the formation of an aziridinium imide and an 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 the formation of a [4 + 2] cycloadduct, which reacts with a second molecule of MTAD in an ene-type mode to afford two stereoisomeric [4 + 2]/ene diadducts. In methanol, solvent captures the zwitterionic intermediate and forms the methoxy adduct. The relative distribution of the products in 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 major products.

Isotope effects and syn selectivity in the ene reaction of triazolinedione with conjugated enones: aziridinium imide or an open intermediate mechanism?

The ene reaction of 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) with phorone-d(6) (5-d(6)) shows an inverse beta-secondary isotope effect and with mesityl oxide-d(3) (7-d(3)) proceeds with 93% syn selectivity. These results are consistent with a mechanism involving the formation of an aziridinium imide intermediate in the rate-determining step. An open biradical or dipolar intermediate is excluded by them.

Site specificity in the photooxidation of some trisubstituted alkenes in thionin-supported zeolite Na-Y. On the role of the alkali metal cation

The less substituted side of some geminal dimethyl trisubstituted alkenes becomes significantly more reactive if photooxygenation takes place in thionin-supported zeolite Na-Y. These results are mainly attributed to a synergistic interaction between the alkali metal cation, the alkene, and the oxygen in the transition state of perepoxide formation.