No abstract

The regioselectivities and the reactivities (relative rates) for the ene reaction of the enophile 4-nitronitrosobenzene (ArNO) with an extensive set of regiochemically defined acyclic and cyclic olefins have been determined. These experimental data establish that the ArNO enophile attacks the olefinic substrate along the novel skew trajectory, with preferred hydrogen abstraction at the corner (twix regioselectivity). This is in contrast to the isoelectronic species singlet oxygen ((1)O(2)), which abstracts at the higher substituted side of the double-bond (cis effect), and triazolindione (TAD), which undergoes the ene reaction at the more crowded end (gem effect). Ab initio computations (B3LYP/6-31+g) for the ene reaction of the ArNO with 2-methyl-2-butene reveal that the steric effects between the aryl group of the enophile and the substituents of the olefin dictate the skew trajectory. These computations identify the aziridine N-oxide (AI) as a bona fide intermediate in this ene reaction, whose formation is usually rate-determining and, thus, irreversible along the skew trajectory (twix selectivity). The reversible generation of the AI becomes feasible when conformational constraints outweigh steric effects, as manifested by enhanced twin regioselectivity.

No abstract

The regioselectivity for the intrazeolite photooxygenation of several trisubstituted alkenes with geminal dimethyl groups was examined. The length of the alkyl chain at the lone position was varied, and as end groups, the phenyl or the cyclohexyl functionalities were chosen. The general trend for all alkenes is a significant increase of the reactivity at the twin position compared to the photooxygenation in solution. For the cyclohexyl-substituted alkenes, it was found that the regioselectivity is nearly independent of the alkyl chain length. However, for the phenyl-substituted alkenes, the ene reactivity of the allylic methylene hydrogen atoms at the lone position and the twix/twin regioselectivity depend significantly on the distance of the phenyl group from the double bond. These trends are discussed in terms of cation-pi interactions and conformational effects. Intramolecular and intermolecular isotope effects in the intrazeolite photooxygenation of deuterium-labeled alkenes suggest that a perepoxide-type intermediate is formed in the rate-determining step. Type I photooxygenation that involves reaction of the radical cations of the alkenes with superoxide ion are unlikely.

The intramolecular and intermolecular kinetic isotope effects (KIE) have been determined for the nitrosoarene ene reaction with deuterium-stereolabeled 2,3-dimethyl-2-butenes (TME). trans-TME-d(6) (k(H)/k(D) = 3.0) and gem-TME-d(6) (k(H)/k(D) = 4.0) show large intramolecular primary isotope effects. In contrast, the intramolecular competition in cis-TME-d(6) (k(H)/k(D) = 1.5) and the intermolecular competition for the TME-d(0)/TME-d(12) pair (k(H)/k(D) = 1.98) show considerably smaller, but mechanistically significant kinetic isotope effects. The latter fact is rationalized in terms of reversible formation of a three-membered-ring intermediate, namely the aziridine N-oxide, or a similar unsymmetrical, polarized diradical in the first step of the reaction. Such reversibility has also been implied earlier for triazolinedione (TAD) and singlet oxygen ((1)O(2)) with deuterium-stereolabeled 2-butenes, but of the three enophiles, ArNO is the most sensitive toward reversibility, which is due to its moderate reactivity and its high steric demand.