Bodwell et al.
this area has been limited almost exclusively to systems
that contain just a lone benzene ring. To the best of our
knowledge, there has been no prior comprehensive,
systematic experimental or theoretical study of the effect
of distortion from planarity on the aromatic character of
any polycyclic aromatic system.7 In view of the strong
current interest in nonplanar polycyclic aromatic mol-
ecules such as Buckybowls8 and fullerenes,3,9 the results
of such a study could be particularly relevant, especially
if X-ray crystal structures could be obtained.
Very recently we reported the syntheses of a variety
of [n](2,7)pyrenophanes (1b,c, 2b-d , 3, 4) in which the
pyrene moiety is strongly distorted from planarity.10
Pyrenophanes 1a and 2a have not yet been isolated.
X-ray crystal structures were determined for most of
these pyrenophanes and this opened the door to an
investigation of the effect of bending on the aromaticity
of the pyrene system, which manifests itself in the
surfaces of several fullerenes (e.g. D5h-C70,11 D5d-C80,12 D6h-
C8413) and Buckybowls (e.g. pinakene14). Having observed
that the parent [n](2,7)pyrenophanes are the most dif-
ficult pyrenophanes to crystallize in a form suitable for
X-ray crystallography,10c the 1,n-dioxa[n](2,7)pyrenophanes
1 were selected for the present investigation. We now
report the synthesis of a series of 1,n-dioxa[n](2,7)-
pyrenophanes 1b-g and their X-ray crystal structures.
The structural information serves as an excellent starting
point for further analysis of the extent of changes of the
cyclic π-electron delocalization in the pyrene system as
it becomes increasingly nonplanar. This is done here
through the application of quantitative measures of
aromaticity: HOMA15 (Harmonic Oscillator Model for
Aromaticity, a geometry-based index) and NICS16 (Nuc-
leus-Independent Chemical Shift, a magnetism-based
index). Of the many easily accessible quantitative defini-
tions of aromaticity,2d it was anticipated2f,g that these two
models would be the most efficient in accurately deducing
the extent of stabilization due to the cyclic delocalization
effect. Additionally, the values of magnetic susceptibility
calculated for deformed pyrenes can also be used as an
independent indicator of the extent of the changes of
π-electron delocalization involved.17
(7) Except for naphthalene, for which geometry-based investigations
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Discu ssion
Syn th esis. Our approach to the [n](2,7)pyrenophane
skeleton is based on the comprehensively studied valence
isomerization between [2.2]metacyclophane-1,9-dienes 5
and 10b,10c-dihydropyrenes 6 (Scheme 1)18 and the
observation that when the internal substituents are
hydrogen atoms (R ) H), dehydrogenation of trans-6
occurs readily to give pyrene 7.19 Presumably due to the
length of the standard synthetic routes to [2.2]metacy-
clophane-1,9-dienes,20 this valence isomerization/dehy-
drogenation (VID) protocol does not appear to have been
utilized in any target oriented synthesis of a pyrene
derivative prior to our involvement in this area. By
introducing a third bridge of variable length between the
5 and 13 positions of the metacyclophanediene system
5, we have been able to demonstrate that the parent
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R. J . Chem. Soc., Perkin Trans. 2 1997, 1907-1910. (d) Cioslowski,
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(20) Mitchell, R. H. Heterocycles 1978, 11, 563-586.
2090 J . Org. Chem., Vol. 68, No. 6, 2003