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.

[4 + 2] Cycloadditions of rigid s-cis dienes to C(60). A synchronous Diels-Alder reaction.

[reaction: see text] The Diels-Alder reaction of rigid s-cis dienes with C(60) occurs by a concerted mechanism, via a symmetrical transition state.

Mechanism of the

Stereochemical studies on [2 + 2] photoaddition of cis-/trans-4-propenylanisole (cis-1 and trans-1) and cis-1-(p-methoxyphenyl)ethylene-2-d(1) (cis-3-d(1)) to C(60) exhibit stereospecificity in favor of the trans-2 cycloadduct in the former case and nonstereoselectivity in the latter. The observed stereoselectivity in favor of the cis-6-d(3) [2 + 2] diastereomer by 12% in the case of the photochemical addition of (E)-1-(p-methoxyphenyl)-2-methyl-prop-1-ene-3,3,3-d(3) (trans-5-d(3)) to C(60) is attributed to a steric kinetic isotope effect (k(H)/k(D) = 0.78). The loss of stereochemistry in the cyclobutane ring excludes a concerted addition and is consistent with a stepwise mechanism. Intermolecular secondary kinetic isotope effects of the [2 + 2] photocycloaddition of 3-d(0) vs 3-d(1), and 3-d(6) as well as 5-d(0) vs 5-d(1), and 5-d(6) to C(60) were also measured. The intermolecular competition due to deuterium substitution of both vinylic hydrogens at the beta-carbon of 3 exhibits a substantial inverse alpha-secondary isotope effect k(H)/k(D) = 0.83 (per deuterium). Substitution with deuterium at both vinylic methyl groups of 5 yields a small inverse k(H)/k(D) = 0. 94. These results are consistent with the formation of an open intermediate in the rate-determining step.

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.

The ene reaction of singlet oxygen with olefins.

The reactions of singlet oxygen (1O2) with cis and trans butenes-1,1,1-d3, at -80 degrees C in Freon-11, show a product isotope effect (kH/kD) of 1.38 and 1.25 respectively. Isomerization of the starting materials or formation of dioxetanes were not observed during the course of the photooxygenation. Together with the isotope effects on the reactions of tetramethylethylene-d6 isomers with singlet oxygen, these results require the reversible formation of a perepoxide or charge transfer intermediate.