Tunnel effects on inter- and intramolecular hydrogen transfer reactions of transient dihydro- and hexahydrocarbazoles

Abstract

We examine inter- and intramolecular hydrogen-transfer processes in two related metastable dihydrocarbazoles in nonpolar solvents of different viscosity and compare them with similar transfer processes in transient hexahydrocarbazoles. N-ethyldiphenylamine (A′) and N-ethyl-2,6-dimethyldiphenylamine (A) photocyclize in their triplet states, yielding the triplet states of the zwitterionic dihydrocarbazoles 3Z′* and 3Z*, respectively, which subsequently relax to their metastable singlet ground states 1Z′ and 1Z′. In spite of their similarity, the two transients 1Z′ and 1Z stabilize by completely different pathways: the unsubstituted transient 1Z′ is converted into N-ethylcarbazole (C) and an N-ethyltetrahydrocarbazole (THC) by a bimolecular disproportionation reaction. The methylsubstituted intermediate 1Z is converted into a stable dihydrocarbazole (D) by a sigmatropic, intramolecular [1,8]-H-shift and by an intermolecular, mutual hydrogen-exchange reaction within the encounter complex 1(ZZ) which yields two molecules of D. The rates of the intra- and of the intermolecular transfer reaction of 1Z are governed by tunnel effects. The rate of the intramolecular tunnel process does not depend on solvent friction and becomes temperature independent at low temperatures. The rate of the intermolecular, reaction-controlled exchange reaction 1(ZZ) → 2 1D becomes also temperature independent if the solvent is fluid enough. In more viscous solvents the reaction becomes diffusion controlled and, therefore, strongly temperature dependent. The intermolecular disproportionation reaction 2 1Z′ → C + THC is also reaction controlled but no tunnel effects are observed.