engleski

The Ring Enlargement Process in the 2-Cyclopentyl-2-Propyl Cation

In order to investigate the five membered ring enlargement in dammarenyl carbocation, which is the key step in the phytosterol biosynthesis, rearrangement processes in the simplest model carbocation, the 2-cyclopentyl-2-isopropyl cation 1, were followed by means of 13C NMR spectroscopy. Te cation 1 was prepared in SbF5-SO2ClF from the corresponding alchol precursors, using the molecular beam method. 13C NMR spectrum of the starting cation 1 at –130 °C indicates that the ring expansion has not occurred during the cation preparation. At this temperature the cation 1 undergoes rapid nondegenerate hydrid shift between two structures1A and 1B. At about –100 °C the ring enlagement process takes place. The product obtained is 1, 2-dimethylcyclohexyl cation 3 in which the degenerate rapid hydrid shift occurs over a low barrier. The formation of product 3 can be rationalized if presumed that the rearrangement proceeds by the mechanism presented in Scheme 1. The first step is the expansion of the five membered ring of the less stable isomer 1B involved in fast preequilibrium 1A<->1B. The outcome is the formation of the relatively unstable secondary cation 2. The next step, the 1, 2-methyl shift in 2 leads to the formation of the cation 3. The computational simulation of the reaction 1A->3 was carried out and the relative energies between the species involved in the rearrangement process were obtained on HF/6-31G(d) and on B3LYP/6-31G(d) levels of theory

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