Reactions of the halonium ions of carenes and pinenes: An experimental and theoretical study

Abstract

The reactions of vinylcyclopropane (+)-2-carene (1) and vinylcyclobutanes (-)-β-pinene (7), (-)-α-pinene (11), and (-)-nopol (12) with electrophilic halogens in the presence of oxygen and nitrogen nucleophiles in various solvents have been investigated. The halonium ion intermediates that were presumably formed were very reactive and led to opening of the conjugated cyclopropane or cyclobutane. Reactions of chloramine-T trihydrate with compound 1 in acetonitrile gave amidine 13 and diazepine 14. Reactions of chloramine-T trihydrate with pinenes in methylene chloride gave allylic tosylamines 22, 16B and 24. Mechanisms to explain the observations are proposed and supported by ab initio and Density Functional Theory calculations on the carenes and pinenes in this report and their bromonium ion intermediates. For comparisons, the relative extent of conjugation with the bromonium ion moiety of these, as well as select cyclohexene and cyclohexadiene systems and their corresponding bromonium ions, were optimized at the B3LYP/cc-pVDZ level of theory, and then these geometries were analyzed using the absolute hardness index at the Hartree-Fock/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels of theory. Additionally, Natural Population Analysis charges were calculated for these systems using Moller-Plessett Second-Order Perturbation Theory electron densities and the aug-cc-pVDZ basis set. Combining the results of these theoretical methods with analysis of structural details of their optimized geometries gives much electronic structure insight into the extent of conjugation of bromonium ions of the carenes and pinenes reported here, and places them in relative context of more traditional conjugated and non-conjugated bromonium ion systems. In particular, bromonium ions of compounds 1, 7, and 11 display structural distortions, charge delocalizations and hardness values comparable with those of traditional conjugated cyclohexadienes, with possible reasons for subtle differences presented.