Electronic Structure of Cyclohexa-1,2,4-triene
A R T I C L E S
Scheme 2
into the dipole 4c as compared to that of 5a into 5c. Since
pyrylium ions are attacked by nucleophiles in position 2, 4, or
6,26 the ether 12 provides experimental evidence for the strong
polar nature of the ground state of 4 (Scheme 2), which is in
line with the results of the calculations presented below. A
product derived from the reaction of KOt-Bu with 4 at the
centers bearing the oxygen atom was not detected, which may
be explained by the instability of such a compound. As a 2H-
pyran, the possibly formed acetal could have undergone an
electrocyclic ring opening,27 and the resulting aldehyde might
1
have been subject to further reactions. Indeed, the H NMR
spectra of the mixtures containing 12 display aldehyde signals
of substantial intensity (see Supporting Information).
3. Computational Details
contrast to 8 and 9, the tetrabromide 10 is a stable compound
and thus is useful as a storable precursor of 9.
For a reliable description of systems which possess diradical
character, a multireference treatment is essential in most cases.
Since the planar diradical species appeared to be important for
the understanding of the chemistry of cyclic allenes, MR-CI+Q
and CASPT2 single point energy calculations were employed
in the present work. The geometric parameters of the stationary
points were optimized using analytical gradients of the complete
active space SCF (CASSCF) method and of the density
functional theory (DFT). Since the calculated MR-CI+Q
energies obtained from the DFT optimized geometries were
always slightly lower (0.4-1.0 kcal/mol), we will concentrate
on these results. To obtain the minima of a given multiplicity,
we performed optimizations without symmetry constraints. The
geometric parameters of the different planar singlet species were
optimized with Cs symmetry constraints.
For the geometry optimizations, we employed the B8828a or
B328b exchange expressions in combination with the correlation
functionals by Lee, Yang, and Parr (LYP).29 The unrestricted
ansatz was used for all DFT computations (UDFT). The natures
of the various stationary points were checked by the number of
imaginary frequencies. The DFT approach was also utilized to
obtain zero point vibrational energy corrections. The influence
of the nuclear motion and temperature effects were incorporated
in the standard approach.30 All DFT calculations as well as (PU)-
MP2 and the CCSD(T) single point calculations were performed
with the Gaussian 98 program package.30
It turned out that 9, dissolved in furan or styrene, did not
react with potassium tert-butoxide (KOt-Bu). The low acidity
of the methylene group of 9 may be caused by the antiaromatic
nature of the anion resulting on deprotonation. On addition of
18-crown-6 to the reaction mixtures, 9 was consumed, but no
products were observed that could have formed by cycloaddition
of 4 with furan or styrene. The only new compound, also
detected as a product of the reaction of 9 dissolved in benzene-
d6 with KOt-Bu in the presence of 18-crown-6 and the absence
of furan and styrene, was identified to be 4-tert-butoxy-4H-
pyran (12) (Scheme 2). The lability of 12 prevented its isolation,
but a high-resolution mass spectrum and the NMR spectra leave
no doubt as to its structure. Particularly indicative are the signals
of an AA′MM′X system at δ 6.60 (H2, H6), 5.01 (H3, H5),
and 4.52 (H4) in the 1H NMR spectrum in CDCl3 (see
Supporting Information).
Efficient cycloadditions with activated olefins are a feature
of cyclohexa-1,2-diene (1) and most of its known derivatives,1,2
among them 2,10-12 5,20 and 7.10,22 Cycloadducts of 4 with furan
and styrene should be stable enough for direct observation. Thus,
their absence as well as the formation of 12 indicate the special
character of 4. Nucleophiles attack unpolar six-membered cyclic
allenes such as 1,1 7,22 and its 1,1-dimethyl derivative23 at the
central allene carbon atom exclusively. However, the dihydro
derivative of 4, the unsymmetrical isopyran, reacts at all three
allene carbon atoms with KOt-Bu24 and at the central or one
terminal allene carbon atom with enolates.25 The involvement
of the allene termini has been interpreted as indication of a
polarization of this cyclic allene in terms of a zwitterionic
structure.20 That 5 takes up KOt-Bu exclusively at the allene
terminus bearing the oxygen atom, giving rise to an acetal, is
evidence for an even greater contribution of the zwitterion 5c
to the ground state of 5. Yet, if furan or styrene is present, 5
prefers to undergo cycloaddition.20 This reactivity order could
be reversed in the case of 4, that is, 4 might favor nucleophiles
over activated olefins since the polar character of 4 should be
enhanced relative to that of 5 because of the greater gain in
aromatic stabilization energy on conversion of the allene 4a
In the CASSCF computations, an eight electron in eight
orbital active space ([8,8]-CAS) was used for all species. For
the planar structures which possess Cs symmetry, it consists of
(26) Ellis, G. P. In ComprehensiVe Heterocyclic Chemistry, 1st ed.; Katritzky,
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Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
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J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
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