Protonated cyclopropane as an intermediate in cation–olefin cyclizations. Ab initio and density functional theory investigations

Abstract

In order to investigate the mechanism of cation–olefin cyclizations, the model calculations for the cyclization of cis-5,6-dimethyl-5-hexenyl cation were studied with both ab initio and density functional theory methods. The geometry optimization of the cis-5,6-dimethyl-5-hexenyl cation reveals its spontaneous rearrangement into protonated cyclopropane intermediates, for which four conformers were identified on the potential energy surface of C 8H 15 + ions. The present study demonstrates that two of these conformers are suitably preorganized to undergo chair- and boat-like cation–olefin cyclizations. The result is the formation of the chair cyclohexyl cation and the protonated cyclopropane moiety embedded into the cyclohexyl cation in boat conformation. The analysis of the intrinsic reaction coordinates show that the formation of the latter species via the boat-like pathway is preferred by 2.0 kcal/mol. To appreciate the influence of the leaving group, the cyclization reactions of protonated cis-1,2-dimethyl-6-hydroxyhexene were also studied by the MP2 and MP4 methods, using the 6-31G ∗ and 6-311+G ∗ basis sets. In this model system, the neutral water molecule was used to simulate the leaving group during the cyclization step in which the cyclohexyl cation–water complexes were generated. The MP4(SDQ)/6-31G ∗//MP2/6-31G ∗ computed energy barriers are almost identical for the chair- and boat-like cyclization reactions of the appropriately preorganized reactants. The boat-like cyclization pathway, preferred only by 0.2 kcal/mol, again leads to the incorporation of the protonated cyclopropane into the cyclohexyl cation–water complex in boat conformation. To evaluate the cyclization energy barriers when the preorganization of the reactant is not taken into account, the lowest conformer of protonated alcohol has to be considered. In this case, the chair-like cyclization pathway is preferred by a small margin of 0.8 kcal/mol for the formation of the C–C hyperconjugated chair cyclohexyl cation–water complex. The use of a larger basis set and the inclusion of polarization functions has little, if any, effect on the relative stability of cation–water complexes. The MP2 and MP4 methods appear to be well suited for investigations of cation–olefin cyclizations for the model systems proposed in this study. In contrast to intermolecular cation–olefin additions, which are barrier-less processes, the present study demonstrates that the MP2 and, to an even greater extent, the MP4 method, reveal distinct energy barriers for intramolecular cation–olefin cyclizations.