Nondegenerate equilibrium in (2-propyl)cycloalkyl cations. Comparison of (2-propyl)cyclopentyl and (2-propyl)cyclohexyl cation using 13C NMR spectroscopy, equilibrium isotope effects and quantum chemical calculations

Abstract

Tertiary (2-propyl)cyclopentyl and (2-propyl)cyclohexyl carbocations were investigated by 13C NMR spectroscopy in superacid solution. Both ions undergo fast nondegenerate 1,2-hydride shifts to the corresponding 2-cycloalkyl-2-propyl cations. The direction of these equilibria depends on the size of the ring. The more stable isomer of the (2-propyl)cyclopentyl cation has the formal positive charge at the endocyclic carbon atom, while the more stable isomer of the (2-propyl)cyclohexyl cation has the formal charge at the exocyclic carbon atom. The dynamic NMR results were confirmed by NMR spectroscopic measurement of the equilibrium isotope effects and rationalized by quantum chemical calculations.