Chemistry Reference
In-Depth Information
R
1
R
2
R
2
R
2
= Bu
R
1
= octyl
R
1
19
20
21
22
Scheme 8 Macrocycles with four 1,8-anthracene units
R
1
R
1
OH
HO
R
1
HO
R
1
R
1
R
1
R
2
R
2
R
1
R
1
R
1
R
1
R
1
R
1
R
1
= octyl
R
2
= dodecyl
23
24
25
26
Scheme 9 Macrocycles with six 1,8-anthracene units and analog
two octyl groups at the 10-position of 1,2-alternating 1,8-A units, while tetramer 21
has two long linkers at the opposite corners. The enantiomers of these chiral
compounds were resolved by chiral HPLC and showed characteristic CD bands
in the UV-vis region. It is notable that spontaneous resolution took place upon
crystallization of 21 from chloroform. Namely, each single crystal consisted of
either of the enantiomers showing optical activity. Another chiral tetramer 22
consisting of two 1,8-A units, one 1,5-A unit, and one 9,10-A unit was also
synthesized and its enantiomers were resolved [
60
,
61
]. The kinetic measurements
revealed that the barrier to enantiomerization via rotation about the acetylene axes
was 114 kJ/mol at 70
C.
Similarly, cyclic hexamers were constructed from six 1,8-A units. The
1
HNMR
spectrum of hexamer 23 was very symmetric on the NMR time scale (Scheme
9
)
[
62
]. In contrast, the DFT calculation of the alkyl-free derivative of 23 suggested a
parallelogram prism structure as energy minimum to maximize intramolecular
p
p
stacking. These findings indicate that the molecules undergo facile conformational
exchanges between possible parallelogram structures. Hexamer 24 with one long
linker and hexamer 25 with two long linkers also prefer to take parallelogram-type
structures [
62
,
63
]. Hexamer 24 has a chiral structure, and its enantiomers were
resolved by chiral HPLC. Hexameric compound 26, in which three
m
-terphenyl
units were incorporated, was proposed by the group of Sakamoto and Sch¨ lter
