Thermochemical and Kinetic Study of the Carbocation Ring Contraction of Cyclohexylium to Methylcyclopentylium

Abstract

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Ring contraction mechanisms in substituted cyclohexylium cations

“…The large reaction barrier (20.1 kcal/mol) [10] calculated by Vrček et al [8] for the ring contraction of 1,2-dimethyl-1-cyclohexylium led us to ponder the influence of substituents on ring contraction reaction pathways given that, for 1, the experimentally-determined (E a = 7.4 ± 1 kcal/mol) [11] and our calculated (E a = 6.9 kcal/mol) [9,12] barriers are significantly lower. The main difference between the two reactions is that the latter is driven from a secondary (cyclohexylium) carbocation toward a tertiary (methyl-cyclopentylium) carbocation product for energetic reasons, whereas the former starts and ends with tertiary carbocations and the energetic driving forces are not present.…”

“…[9], we employed the PBE density functional with 6-311++G(2d, 2p) basis sets to calculate the ring isomerizations of 2 and 3. These two systems neatly highlight differences in energetics and structure when either an electron donating or withdrawing group is added to a cyclohexylium cation, 1.…”

“…1) [7,9]. These can be described as appearing either flat or puckered about the charge center, with the third having the CH 2 a to the charge center lying out-of-plane of the ring.…”

“…Most recently, we published details of the mechanism for isomerization of cyclohexylium (1) to methyl-cyclopentylium, see Fig. 1 [9]. We used a QCISD(T)-based approach, which produces kinetic parameters for the reaction that are in excellent agreement with the experiment values, to characterize the structures and energetics of all of the structures along the reaction coordinate.…”

“…We used a QCISD(T)-based approach, which produces kinetic parameters for the reaction that are in excellent agreement with the experiment values, to characterize the structures and energetics of all of the structures along the reaction coordinate. It was found that the reaction profile is very much dependent upon both the method employed (including density functional) and on basis set quality [9]. For example, the B3LYP and MP2 methods, two commonly used computational approaches, predict incorrect structures and mechanisms (compared to QCISD(T)), and poor kinetic parameters for the reaction.…”