the monomers to prepare cyclotriveratrylene with a rigid bowl-shape, the reaction
sites were the 4- and 5-positions of 1,2-dialkoxybenzenes because the reactions
at the 3- and 6-positions are unfavorable due to steric hindrance (Scheme 4b). When
1,3-dialkoxybenzenes are used as monomers to obtain calix[4]resorcinarenes, four
resorcinarene molecules are connected at their 4- and 6- positions (Scheme 4c). Pillar
[n]arenes, with highly symmetrical cyclic structures, were first reported by Ogoshi
et al. in 2008 [18]; they are connected by a methylene bridge at the 2- and 5-positions
as a result of highly reactive reaction sites and a low steric hindrance (Scheme 4d).
Therefore, considering a change in the relative positions of the two hydroxyl
groups of the dihydroxybenzene building blocks, we wondered whether it is possible
to use 2,2
0 -dialkoxy biphenyls as the monomer to obtain new biphenarenes because
the reaction sites of the 3,3
0 -positions (para position of dialkoxy) have a high
electron density and a low steric hindrance. As expected, the other type of
2,2
0 -dialkoxy disubstituted biphen[n]arenes (22BPns, n = 4–8) were successfully
synthesized in 2017 by our group [26] (Scheme 5a). In addition, we tried to obtain
the pillar-shaped hosts 3,3
0 -biphen[n]arenes (33BPns) based on the 3,3
0 -diethoxy
biphenyl linked by a methylene bridge at the 4,4
0 -positions. However, no cyclic
oligomers were detected after many attempts. One possible reason is that both the
ortho- and the para-positions (marked as red arrows in Scheme 5b) to the ethoxy
groups are efficient reaction sites, making the reaction quite complex. The synthetic
method of 22BPns is similar to that of 44BPns. 22BPns can be directly prepared
from 2,2
0 -dialkoxy biphenyl and paraformaldehyde in 1,2-dichloroethane at 25
C,
with BF 3 O(Et) 2 as the catalyst. Changing the relative position of the disubstituted
dialkoxy substituents resulted in the formation of macrocycles containing 4–8
Scheme 5 (a) Synthesis of 22BPns (n = 4–8). (b) Attempted synthesis of 33BPns [26]
12 Biphen[n]arenes: Synthesis and Host–Guest Properties
315
sites were the 4- and 5-positions of 1,2-dialkoxybenzenes because the reactions
at the 3- and 6-positions are unfavorable due to steric hindrance (Scheme 4b). When
1,3-dialkoxybenzenes are used as monomers to obtain calix[4]resorcinarenes, four
resorcinarene molecules are connected at their 4- and 6- positions (Scheme 4c). Pillar
[n]arenes, with highly symmetrical cyclic structures, were first reported by Ogoshi
et al. in 2008 [18]; they are connected by a methylene bridge at the 2- and 5-positions
as a result of highly reactive reaction sites and a low steric hindrance (Scheme 4d).
Therefore, considering a change in the relative positions of the two hydroxyl
groups of the dihydroxybenzene building blocks, we wondered whether it is possible
to use 2,2
0 -dialkoxy biphenyls as the monomer to obtain new biphenarenes because
the reaction sites of the 3,3
0 -positions (para position of dialkoxy) have a high
electron density and a low steric hindrance. As expected, the other type of
2,2
0 -dialkoxy disubstituted biphen[n]arenes (22BPns, n = 4–8) were successfully
synthesized in 2017 by our group [26] (Scheme 5a). In addition, we tried to obtain
the pillar-shaped hosts 3,3
0 -biphen[n]arenes (33BPns) based on the 3,3
0 -diethoxy
biphenyl linked by a methylene bridge at the 4,4
0 -positions. However, no cyclic
oligomers were detected after many attempts. One possible reason is that both the
ortho- and the para-positions (marked as red arrows in Scheme 5b) to the ethoxy
groups are efficient reaction sites, making the reaction quite complex. The synthetic
method of 22BPns is similar to that of 44BPns. 22BPns can be directly prepared
from 2,2
0 -dialkoxy biphenyl and paraformaldehyde in 1,2-dichloroethane at 25
C,
with BF 3 O(Et) 2 as the catalyst. Changing the relative position of the disubstituted
dialkoxy substituents resulted in the formation of macrocycles containing 4–8
Scheme 5 (a) Synthesis of 22BPns (n = 4–8). (b) Attempted synthesis of 33BPns [26]
12 Biphen[n]arenes: Synthesis and Host–Guest Properties
315