to study the face-to-face geometry have been particularly
challenging. Often, the face-to-face geometry is dominant
but the models are sufficiently flexible that edge-to-face
interactions can also occur. In other cases, the model systems
are too rigid and fix the arene surfaces in an unfavorable
geometry.4a
Scheme 1. Synthesis of the Molecular Balances 1-4 via the
Diels-Alder Reaction between Maleimides 5 and Fused
Bicyclic Dienes 6a
We have designed a new molecular balance based on the
rigid bicyclic N-arylimide framework that restricts the
arene-arene interaction to a face-to-face geometry.9 Balances
1-3 adopt distinct folded and unfolded conformations due
to restricted rotation about a central Caryl-Nimide bond.
Molecular modeling studies predicted that the benzene ring
of the phenyl ether arm was perfectly positioned in the
folded-conformer to form an effective offset face-to-face
interaction with the arene shelf (Figure 1, left). More imp-
ortantly, the modeling studies also predicted that the phenyl
ether arm cannot adopt an edge-to-face geometry in the
folded-conformer because it is held too closely (Figure 1,
right) to the arene shelf (<4.0 Å) to allow edge-to-face
interactions, which normally require 5-6 Å (centroid to
centroid).10 In the unfolded conformer, the phenyl ether arm
is far from the arene shelf and cannot interact with the arene
shelf. Thus, the strength of the face-to-face arene-arene
interaction can be assessed by measuring the ratio of folded
a Balances 1-3 are shown in the folded state.
soluble in a wide range of organic solvents allowing study
of the influence of solvent on the face-to-face arene-arene
interaction.
A major difficulty in studying and quantifying arene-arene
interactions is that two arene rings can adopt a continuum
of different face-to-face and edge-to-face geometries (Figure
1).6 Accordingly, model systems have been developed to
1
to unfolded conformers by H NMR. Balances 1-3 differ
in the size of their arene shelves. Molecular models predicted
that the larger pyrene and phenanthrene shelves of 1 and 2
could form effective arene-arene interactions with the
phenyl ether arm. In the case of balance 3, the benzene shelf
is too short to form stacking interactions, and thus, balance
3 was used as a control system to assess the influence of
additional attractive or repulsive interactions between the arm
and the arene shelf.
The study of balances 1-3 was facilitated by their facile
and modular synthesis (Scheme 1). The rigid bicyclic
framework was assembled in one step via a Diels-Alder
reaction between N-aryl maleimide 5 and cyclic diene 6. In
each case, the reaction proceeded in high yields (>95%).11
In the case of the benzene and phenanthrene balances 2-4,
the cyclic dienes (6b and 6c) were commercially available.
The pyrene-based diene 6a was synthesized in two steps from
pyrene.12
First, crystallographic studies were carried out in order to
confirm the nature of the arene-arene interactions in the new
balances in the folded structures. Crystals of 2 were obtained
from various solvents. However, balance 2 consistently
crystallized in the unfolded conformer (Supporting Informa-
tion). The structure of unfolded 2 confirmed the endo-
stereochemistry of the balance, and the inability of the
Figure 1. Representation of the geometric constraints in the folded
conformers of systems 1-4. The proximity of the phenyl arm to
the arene shelf allows for effective formation the face-to-face
geometry but prevents the formation of an edge-to-face geometry.
independently study the edge-to-face7 and face-to-face
arene-arene geometries.8 The development of model systems
(5) Mckay, S. L.; Haptonstall, B.; Gellman, S. H. J. Am. Chem. Soc.
2001, 123, 1244.
(9) This framework has been used to measure C-H-arene interactions,
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Spectrosc. Lett. 2004, 37, 493. (b) Grossmann, G.; Potrzebowski, M. J.;
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W.; Nazarov, V.; Golovko, V. New J. Chem. 2003, 27, 1095. (c) Kishikawa,
K.; Yoshizaki, K.; Kohmoto, S.; Yamamoto, M.; Yamaguchi, K.; Yamada,
K. J. Chem. Soc., Perkin Trans. 1 1997, 1233.
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1986, 234, 1005. (b) Jennings, W. B.; Farrell, B. M.; Malone, J. F. Acc.
Chem. Res. 2001, 34, 885.
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