Intramolecular and Intermolecular Kinetic Isotope Effects (KIE) in the Nitrosoarene Ene Reaction:  Experimental Evidence for Reversible Intermediate Formation

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-d₆ (k_H/k_D = 3.0) and gem-TME-d₆ (k_H/k_D = 4.0) show large intramolecular primary isotope effects. In contrast, the intramolecular competition in cis-TME-d₆ (k_H/k_D = 1.5) and the intermolecular competition for the TME-d₀/TME-d₁₂ 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 (¹O₂) 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.