Chemistry Reference
In-Depth Information
5
6
7
8
Scheme 4 Typical anthracenophanes and related compound
4 Aromatic Macrocycles with Acetylene Linkers
Macrocyclic compounds with arene units and linear acetylene linkers are novel
types of
-conjugated compounds. The shape and conformational freedom of such
networks strongly depend on the kind of arene units and the direction of linkers
extending from arene units. Not only ethynylene linkers but also 1,3-butadiynylene
linkers and longer polyyne linkers can be used for the acetylene linkers. This
section shows examples of 3D macrocyclic compounds with various arene units
and these linear linkers.
p
4.1 Benzene Macrocycles
For the molecular design of aromatic macrocycles, benzene units (
o
-,
m
-, and
p
-)
are most common as aromatic moieties. For example, planar 2D systems can be
constructed from six
m
-phenylene units having 120
angular orientation or three
o
-phenylene units having 60
angular orientation [
38
]. These structures should
become nonplanar when the number of units is greater or smaller than the ideal
number expected for the planar framework. Selected examples of such nonplanar
phenylene macrocycles are introduced in this section.
Four
o
-phenylene units form nonplanar cyclic structures to satisfy the angular
requirement. The fundamental compound is tetraphenylene, which has a nonplanar
saddle like structure [
39
]. Several twisted compounds were designed by the inser-
tion of linkers between phenylene units (Scheme
5
). Compound 9 was reported in
1968 by Staab et al. [
40
,
41
] and its structure was established by X-ray analysis
[
140
,
141]
. Compound 10 with two long linkers and two short linkers also had a
helical structure and underwent enantiomerization between chiral helical structures
at the barrier of 39 kJ/mol [
42
]. In the X-ray structure of 11 with longer linkers, two
p
-phenylene units were overlapped with a distance of 3.57
˚
[
43
]. Six
o
-phenylene
units form a highly twisted nonplanar structure of 12 [
44
]. Compounds 13 and 14
are oligomers having both
o
-phenylene and
m
-phenylene units [
45
]. The
macrocyclization of a common precursor with three phenylene units gave two
conformational isomers, 13 (
C
2
) and 14 (
C
2h
). Their macrocyclic frameworks are
rigid and stable because no isomerization took place even at 100
C.
