Lifting of the Degeneracy in Semibullvalenes by Remote and Direct Substituents: A Quantitative Study Using Variable-Temperature Carbon-13 NMR Spectroscopy
A series of six 1,5-(ethylmethyl)semibullvalenes (1a ⇋ 1a‘, 2 ⇋ 2‘, 3 ⇋ 3‘) and two 4(2)-substituted semibullvalenes (4 ⇋ 4‘), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solve...
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Published in | Journal of organic chemistry Vol. 66; no. 6; pp. 1949 - 1960 |
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Main Authors | , , , , , , |
Format | Journal Article |
Language | English |
Published |
WASHINGTON
American Chemical Society
23.03.2001
Amer Chemical Soc |
Subjects | |
Online Access | Get full text |
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Summary: | A series of six 1,5-(ethylmethyl)semibullvalenes (1a ⇋ 1a‘, 2 ⇋ 2‘, 3 ⇋ 3‘) and two 4(2)-substituted semibullvalenes (4 ⇋ 4‘), each undergoing Cope equilibria between nondegenerate valence tautomers, was investigated by carbon-13 NMR spectroscopy at a range of temperatures in several different solvents. Gompper's treatment of substituent perturbation was extended, specifically accounting for the effects of the substituents on chemical shifts, to allow the determination of the thermodynamic parameters for these skewed equilibria. These new treatments were used to determine the population difference (f − f ‘) between the valence tautomers and the perturbation thermodynamic quantities ΔH P, ΔS P, and ΔG P. The slow-exchange limit was reached for the parent 1,5-(ethylmethyl)semibullvalenes 3a ⇋ 3a‘ from which it was established that the preferred valence tautomer is 3a with the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely substituted semibullvalenes. Provided that the ethyl group always prefers the cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes 1a, 2, and 3 are more stable by ΔH P = 0.7−1.7 kJ mol-1 than their valence tautomers 1a‘, 2‘, and 3‘. In the directly substituted semibullvalenes (4 ⇋ 4‘), the preferred valence tautomers 4a and 4b have the bromine atom or the nitrile group on the vinyl position (C4) rather than on the cyclopropane ring (C2) and are more stable than 4a‘ and 4b‘ by ΔH P = 4.8 and 7.0 kJ mol-1, respectively. |
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Bibliography: | istex:64078F16A8D28D3C0394A2E8486724D7DA926DF0 ark:/67375/TPS-QJSZ6LDN-1 Dedicated to Professor Leo A. Paquette on the occasion of his 65th birthday ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0022-3263 1520-6904 |
DOI: | 10.1021/jo001062a |