Temperature dependence of the kinetic isotope effect and tunneling contribution in the innermolecular reaction of phenylcarbene inside a hemicarcerand

The mobility and kinetic isotope effect (KIE) for the inner phase acetal insertion of phenylcarbene 1 have been probed inside the partially deuterated hemicarcerand d₄₀-5 and the parent 5. Hemicarcerand d₄₀-5 has OCH₂O spanners in its upper and OCD₂O spanners in its lower hemisphere. Phenylcarbene was generated photochemically from incarcerated phenyldiazirine 4. At low temperature, hemicarceplex 5⊙4 shows carceroisomerism and different populations of carceroisomers. In the photolysis of d₄₀-5⊙4, C-H bond insertion exceeded 50%, which supports that transient 1 is rotationally mobile within its life-time. The KIE in the solid state increases with decreasing temperature. The Arrhenius parameters A_H/A_D= 4.2±03 and the linearity of the Arrhenius plot of KIE suggests that both isotopes tunnel considerably over the whole experimental temperature range (0-196°C). In solution, the KIE shows unusual temperature dependence. The KIEs are explained by postulating three reaction phases 5 with different C-H bond reactivity: (1) the reaction phase in solution (2) the reaction phase in the solid state and (3) the reaction phase in a frozen solution. The latter is characterized by partially inward rotated α-CH₂ groups of the four O(CH₂)₄O bridges of 5.