Werz et al.
employed. In earlier days a donor-acceptor model was used,11
which was refined on the basis of a one-electron model invoking
the interaction of an occupied n(p)-type lone pair of a donor
center and an empty σ* orbital of a chalcogen-carbon bond.12
In the case of three chalcogen atoms in close contact, their
interaction was described as an electron-rich three-center
bond.10,13 With the advent of more sophisticated quantum
chemical methods these interactions have been modeled on the
basis of semiempirical,14 HF-SCF,15 and DFT16 theory. How-
ever, recent studies have shown that for a quantitative description
methods including electron correlation effects are necessary.17,18
To find out if further highly ordered structures in the solid state
arise when the alkyne units are separated by aryl units we
synthesized 4-10 and studied their structural properties.
The rather poor solubility of 4(3), 4(5), and 5 (Chart 2) in
common organic solvents imposed difficulties regarding their
purification. In an effort to overcome this problem we synthe-
sized 6, 7(3)-7(6), and 8(3)-8(6) together with the side
products 9(3) and 9(5) bearing alkyl substituents on the aromatic
rings prone to increase the solubility. In the case of 10 we
present an example where the alkyl groups were tethered to the
bridge.
CHART 1
Recently, we have studied a number of monocyclic systems
mirroring cyclophane topologies: rigid rods of two electron-
rich alkyne units, placed opposite to each other, are tethered
with two hydrocarbon chains of equal or nearly equal chain
length (e.g., 2).3 If the alkyne units were terminated with divalent
chalcogen centers the cyclic systems formed tubular structures
in the solid state. A closer investigation revealed that the
tubes were formed by stacking of rings on top of each other.4
This high order in the solid state was caused by van der Waals
forces between the chalcogen centers4 of various stacks as shown
in 3 (Chart 1). Similar close contacts between two or more
chalcogen atoms have been known for many years5-10 and have
been identified by means of X-ray diffraction studies on single
crystals or by solution NMR studies of carefully designed
molecules. To interpret the short chalcogen-chalcogen dis-
tances, bonding models of different levels of sophistication were
Results and Discussion
Syntheses. To derive the systems 4(3) and 4(5) we employed
a four-component cyclization reaction of the di(lithium) salt of
1,4-di(ethynyl)benzene (11) and the corresponding R,ω-dithio-
cyanatoalkanes 12(n) (n ) 3, 5) following previously published
procedures.3a,b,19 The dithiocyanatoalkanes used in this paper
have been described in the literature.20 The reaction was carried
out in anhydrous THF under argon atmosphere at -40 °C. The
yields of the macrocyclization products were low and amounted
to 7% for 4(3) and 11% for 4(5) (Scheme 1).
In analogy to 4(n) the macrocycle 5 was obtained from 4,4′-
di(ethynyl)biphenyl (13) and 1,5-dithiocyanatopentane (12(5)).20
The yield of 5 was only 3%, and moreover the resulting
cyclophane was nearly insoluble in any of the common solvents.
NMR investigations had to be performed in hot toluene.
In an effort to increase the solubility of larger rings we
attached alkyl groups to the aromatic moieties as depicted in
6-8 (Chart 2). As a starting material for 6 we first prepared
1,4-di(ethynyl)-2,5-di(n-hexyl)benzene (18) (Scheme 2).
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