An experimental and computational study on the reactivity and regioselectivity for the nitrosoarene ene reaction: Comparison with triazolinedione andsinglet oxygen

The regioselectivities and the reactivities (relative rates) for the ene re action of the enophile 4-nitronitrosobenzene (ArNO) with an extensive set o f regiochemically defined acyclic and cyclic olefins have been determined. These experimental data establish that the ArNO enophile attacks the olefin ic substrate along the novel skew trajectory, with preferred hydrogen abstr action at the corner (twix regioselectivity). This is in contrast to the is oelectronic species singlet oxygen (O-1(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 t he enophile and the substituents of the olefin dictate the skew trajectory. These computations identify the aziridine N-oxide (AI) as a bona fide inte rmediate in this ene reaction, whose formation is usually rate-determining and, thus, irreversible along the skew trajectory (twix selectivity). The r eversible generation of the AI becomes feasible when conformational constra ints outweigh steric effects, as manifested by enhanced twin regioselectivi ty.