Experimental Studies of the Anomeric Effect. Part VI. Ring Inversion Equilibria in Cyclohexane, Tetrahydropyran and Piperidine Rings Substituted by a Carbomethoxy or a Cyano Group.

Article abstract of DOI:10.1016/S0040-4020(01)89862-8

The ring inversion equilibrium in carbomethoxycyclohexane is compared with those in 2-carbomethoxytetrahydropyran and 2-carbomethoxypiperidine through a variable temperature nmr study of the positions of equilibria.The derived ΔH⁰_a<*>e values (kcal mol^-1) of -1.24 (cyclohexane), -1.69 (tetrahydropyran) and -0.54 (piperidine) can be rationalised in terms of competition between steric effects and endo-anomeric effects of the ring heteroatom.Variable temperature studies have also given ΔH⁰_a<*>e values (kcal mol^-1) for ring inversions in cyanocylohexane, 2-cyanotetrahydropyran and 2-cyanopiperidine as -0.18, +0.36 and +2.22.Steric effects are small for CN, and the trend in ΔH⁰ values is consistent with the expected increase in stabilising endo-anomeric effect along the series C, O, N.