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