Lifting of the degeneracy in semibullvalenes by remote and direct substituents: A quantitative study using variable-temperature carbon-13 NMR spectroscopy

Citation
M. Heubes et al., Lifting of the degeneracy in semibullvalenes by remote and direct substituents: A quantitative study using variable-temperature carbon-13 NMR spectroscopy, J ORG CHEM, 66(6), 2001, pp. 1949-1960
Citations number
86
Categorie Soggetti
Chemistry & Analysis","Organic Chemistry/Polymer Science
Journal title
JOURNAL OF ORGANIC CHEMISTRY
ISSN journal
00223263 → ACNP
Volume
66
Issue
6
Year of publication
2001
Pages
1949 - 1960
Database
ISI
SICI code
0022-3263(20010323)66:6<1949:LOTDIS>2.0.ZU;2-7
Abstract
A series of six 1,5-(ethylmethyl)semibullvalenes (1a = 1a ', 2 = 2 ', 3 = 3 ') and two 4(2)-substituted semibullvalenes (4 = 4 '), each undergoing Cop e equilibria between nondegenerate valence tautomers, was investigated by c arbon-13 NMR spectroscopy at a range of temperatures in several. different solvents. Gompper's treatment of substituent perturbation was extended, spe cifically accounting for the effects of the substituents on chemical shifts , to allow the determination of the thermodynamic parameters for these skew ed equilibria. These new treatments were used to determine the population d ifference (f - f ') between the valence tautomers and the perturbation ther modynamic quantities DeltaH(P), DeltaS(P), and DeltaG(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 w ith the ethyl group on the cyclopropane ring. Despite considerable effort, the slow-exchange limit could not be reached in any of our other remotely s ubstituted semibullvalenes. Provided that the ethyl group always prefers th e cyclopropyl position as in 3a, the 1-ethyl-5-methylsemibullvalenes la, 2, and 3 are more stable by DeltaH(P) = 0.7-1.7 kJ mol-l than their Valence t automers 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 (C-4) rather than on the cycloprop ane ring (Cz) and are more stable than 4a ' and 4b ' by DeltaH(P) = 4.8 and 7.0 kJ mol(-1), respectively.