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.
✍ Scribed by Harold Booth; J. Mark Dixon; Khedhair A. Khedhair
- Publisher
- Elsevier Science
- Year
- 1992
- Tongue
- French
- Weight
- 763 KB
- Volume
- 48
- Category
- Article
- ISSN
- 0040-4020
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✦ Synopsis
A bstracr: The ring inversion equilibrium in carbomethoxycyclohexanne is compared with those in 2-carbomethoxytetrahydropyran and 2-carbomethoxypiperidine through a variable temperature nmr study of the positions of equilibria. The derived A%, values (kcal mol-t) of -1.24 (cyclohexane), -1.69 (tetrahydropyran) and -0.54 (piperidine) can be rational&d in terms of competition between steric effects and endo-anomeric effects of the ring heteroatom.
Variable temperature studies have also given A%, values (kcal mol-l) for ring inversions in cyanocyclohexane, 2-cyanotetmhydropyran and 2cyanopiperidine as -0.18, +0.36 and +2.22. Steric effects are small for CN. and the trend in AH' values is consistent with the expected increase in stabilising endo-anomeric effect along the series C, 0. N.
In the ring inversion equilibrium (1 t 2) for a tetrahydropymn substituted at 2 by an electronegative group XR, both conformations are stabilised by the exo-anomeric effect illustrated, in which an exocyclic lone pair orbital overlaps with the antibonding orbital of an anticoplanar carbon-heteroatom bond.2,3 The concept has been supported3 by values of bond lengths from crystallographic studies, and by values of coupling constants from IH and t3C nmr spectra of solutions.4
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