Syntheses of shuttlecock- and bowl-equipped phenylazopyridines and photomodulation of their coordination ability to Zn-porphyrin

Article abstract of DOI:10.1016/j.tetlet.2009.02.126

Shuttlecock- and bowl-equipped 4-(phenylazo)pyridine derivatives, which bear substituents that allow the pyridine moiety to protrude in the trans form but hinder it in the cis form, have been designed and synthesized. These molecules show cis/trans photoisomerization despite the presence of bulky substituents. ¹H NMR titration with Zn-porphyrin showed that the trans isomers coordinate to Zn-porphyrin much stronger than the cis isomers.

Full text of DOI:10.1016/j.tetlet.2009.02.126

Tetrahedron Letters 50 (2009) 2106–2108
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses of shuttlecock- and bowl-equipped phenylazopyridines
and photomodulation of their coordination ability to Zn-porphyrin
b
Kazuya Suwa a, Joe Otsuki a, , Kei Goto
*
a College of Science and Technology, Nihon University, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
b Interactive Research Center of Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Shuttlecock- and bowl-equipped 4-(phenylazo)pyridine derivatives, which bear substituents that allow
the pyridine moiety to protrude in the trans form but hinder it in the cis form, have been designed
and synthesized. These molecules show cis/trans photoisomerization despite the presence of bulky sub-
stituents. 1H NMR titration with Zn-porphyrin showed that the trans isomers coordinate to Zn-porphyrin
much stronger than the cis isomers.
Received 29 December 2008
Revised 13 February 2009
Accepted 17 February 2009
Available online 21 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Azobenzene and its derivatives are expected to be applied to
molecular switches and devices, making use of differences in the
properties such as structures, absorption spectra, and dipole mo-
ments between the two isomers, trans and cis forms.1 Phenylazo-
pyridine derivatives, which bear a pyridyl ring in place of a
phenyl ring of azobenzenes, are utilized in a field of coordination
chemistry because of their coordination ability to a metal ion.
We previously reported a supramolecular photoswitches wherein
the fluorescence from Zn-porphyrin can be controlled by photoir-
radiation making use of the difference in axial coordination ability
between the trans and the cis isomers of 3-(phenylazo)pyridine
derivatives,2 which incorporate a bulky group in such a way that
the association with Zn-porphyrin is hindered in the cis configura-
tion. However, the cis isomer did not dissociate from Zn-porphyrin
completely. In this work, to solve this problem, we designed and
synthesized shuttlecock- and bowl-equipped 4-(phenylazo)pyri-
dine derivatives, TbetNNPy 5 and BmtNNPy 10, which bear a
tetra(tert-butylethynyl)-1,10:30,100-terphenyl group and a 4-tert-bu-
tyl-2,6-bis[(2,200,6,600-tetramethyl-m-terphenyl-20-yl)methyl]phenyl
group3, respectively. For these phenylazopyridine ligands, it was
expected that the cis isomers mostly lose the coordination ability
to Zn-porphyrin but the trans isomer is able to coordinate to it be-
cause the pyridine-N of the cis isomer is covered by the shuttle-
cock- and bowl-shaped framework but that of the trans isomer is
not, as shown schematically in Figure 1. We then investigated
the difference in coordination ability between the trans and the
cis isomers of these compounds.
tween 2 and 4-aminopyridine with NaOH5 in 10% yield. Terphenyl
azopyridine 4 was synthesized by Suzuki–Miyaura cross-coupling
reaction between 3 and 3,5-dichlorophenylboronic acid6 in 66%
yield. Finally, TbetNNPy 5 was synthesized by Sonogashira cross-
coupling reaction7 between 4 and t-butylacetylene in 72% yield.
This linear route was employed because a more convergent route,
in which the incorporation of the 3,5-bis(tert-butylethynyl)phenyl
group to 1 was followed by a coupling of the resulting amino com-
pound and 4-aminopyridine5, was unsuccessful because the latter
coupling of the amino compounds did not proceed.
BmtNNPy 10 was synthesized as shown in Scheme 2. Com-
pound 6 was prepared according to a reported procedure.8–12 Nitr-
osopyridine 1-oxide 7 was prepared from 4-nitropyridine 1-oxide
by reduction to the hydroxylamino compound followed by oxida-
tion with H2SO4/KMnO4.13 The coupling reaction of 6 and 7 under
basic conditions with K2CO3 in DMF produced azoxy compound 8
in 57% yield (route 1). Only the pyridine-O in 8 was reduced to pro-
14
duce azoxy compound 9 when PCl3 was used. The subsequent
TbetNNPy 5 was synthesized as shown in Scheme 1. 1,3-Dibro-
mo-2-nitrosobenzene 2 was obtained by the oxidation of 2,6-
dibromoaniline 1 by m-chloroperbenzoic acid (mCPBA)4 in 50%
yield. Azo compound 3 was prepared by a coupling reaction be-
* Corresponding author. Tel./fax: +81 3 3259 0817.
E-mail address: otsuki@chem.cst.nihon-u.ac.jp (J. Otsuki).
Figure 1. Supramolecular photoswitch using 4-(phenylazo)pyridines bearing a
shuttlecock- and a bowl-shaped framework.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.126

K. Suwa et al. / Tetrahedron Letters 50 (2009) 2106–2108
2107
N
N
N
N
PyHo
PyH
m
Cl
Cl
N
N
N
NO
NH2
N
N
Cl
Cl
Br
Br
Br
Br
b
Br
Br
c
d
a
1
2
3
TbetNN Py 5
4
Scheme 1. Preparation of TbetNNPy (5). Reagents and conditions: (a) mCPBA, CH2Cl2, 20 °C, 4 h, 50%; (b) 4-aminopyridine, 50% NaOH aq, benzene, reflux, 10%;
(c) m-dichlorophenylboronic acid, Pd(PPh3)4, K2CO3, DME, reflux, 66%; and (d) (i) PdCl2(CH3CN)2, 2-dicyclohexylphosphino-20,40,60-triisopropylbiphenyl, CsCO3, anhydrous
CH3CN, and (ii) tert-butylacetylene, 72%.
O
N
N
N
N
N
b
N
c
O
O
route 1
N
N
PyH
o
a
N
N
PyH
m
8
9
NO
N
NH2
N
N
O
N
O
6
7
d
BmtNNPy 10
12
N
N
e
route 2
11
Scheme 2. Preparation of BmtNNPy (10). Reagents and conditions: (a) K2CO3, DMF, 50 °C, overnight, 57%; (b) PCl3, CHCl3, reflux, 2 h, 100%; (c) (i) Zn, AcOH, THF, rt, 6 h, (ii) Zn,
rt, 16 h, 11%; (d) AcOH, CHCl3, 50 °C?rt?70 °C, overnight, 93%; (e) TiCl4, In, dry THF, rt, 5 min, 31%.
reduction of 9 with Zn/AcOH afforded 10 in 11% yield.15 The cou-
pling of 6 and 7 under acidic conditions (route 2) in the presence
16
of AcOH in CHCl3 produced azo pyridine oxide 11 in 93% yield
without formation of azoxy compound 8. The reduction of pyri-
dine-O in 11 with TiCl4/In17 produced BmtNNPy 10 in 31% yield.
In these final steps from 9 or 11 to 10, compound 12 formed in
6–16% yield via cyclization between the azo-N adjacent to the pyr-
idyl group and the benzyl carbon, along with a trace amount of 6.
Route 2 has been found to be better than route 1 because steps are
shorter and the yield of 10 is higher.
Large pp* absorption bands for trans-TbetNNPy and trans-
BmtNNPy appear at about 330 nm, as a shoulder to the absorption
of the terphenyl group, and 348 nm, respectively, while small and
broad np* bands appear at 450 or 469 nm, respectively, which are
shown in Supplementary data (Figs. S1 and S2). These absorption
bands are characteristic to azobenzene derivatives. Upon UV light
Figure 2. 1H NMR upfield shifts in C6D6 for trans- and cis-TbetNNPy 5 (2.5 mM)
(
pp*) irradiation to a solution of trans-TbetNNPy or trans-BmtNNPy
upon addition of Zn-tetraphenylporphyrin ((a) 0, (b) 0.1, (c) 0.2, (d) 0.3, (e), 0.4, (f)
0.5, and (g) 0.6 mM).
(e.g., in toluene at 25 °C), the trans isomer was converted to the cis
isomer. The cis isomer was converted back to the trans isomer by
irradiating visible light (np
*). Thus, TbetNNPy and BmtNNPy be-
haved as photochromic compounds regardless of the bulky
substituents.
much as those of the trans isomer (Fig. S3). In contrast, in the case
of TbetNNPy, the chemical shifts for the trans isomer were shifted
upfield significantly, while those for the cis isomer were shifted only
a little (Fig. 2). For BmtNNPy, although the resonance for the ortho
protons cannot be compared because they disappear on addition
of the porphyrin, the chemical shift change for the meta protons of
the trans isomer was much lager than that of the cis isomer
(Fig. 3). Plots of these chemical shifts of pyridine protons vs. the con-
centration of Zn-porphyrin clearly indicate the difference in coordi-
nation ability between the two isomers (Fig. S4). The association
To investigate the difference in coordination ability to Zn-por-
phyrin between the trans and the cis isomers, we carried out 1H
NMRtitrationoftheazoligandswithZn-porphyrininC6D6. Wemon-
itored chemical shift changes for pyridine protons of TbetNNPy and
BmtNNPy, in addition to 4-(phenylazo)pyridine (4-PhNNPy) as a
model, upon incremental addition of Zn-porphyrin to mixed solu-
tions of the trans and the cis isomers. For 4-PhNNPy, the signals of
pyridine protons of the cis isomer were shifted upfield almost as

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K. Suwa et al. / Tetrahedron Letters 50 (2009) 2106–2108
Supplementary data
The synthesis of new compounds: 3, 4, 5, 8, 9, 10, 11 and 12, the
absorption changes due to the photoisomerization for 5 and 10,
and plots of chemical shifts for pyridine protons versus the concen-
tration of Zn-porphyrin are available. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.02.126.
References and notes
1. (a) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001; (b)
Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003,
125, 5612; (c) Yagai, S.; Karatsu, T.; Kitamura, A. Chem. Eur. J. 2005, 11,
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2. (a) Otsuki, J.; Narutaki, K.; Bakke, J. M. Chem. Lett. 2004, 33, 356; (b) Otsuki, J.;
Narutaki, K. Bull. Chem. Soc. Jpn. 2004, 77, 1537.
3. Goto, K.; Okazaki, R. Liebigs Ann. Recl. 1997, 2393.
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Tartakovsky, V. A. Russ. Chem. Bull. Int. Ed. 2002, 51, 1849.
Figure 3. 1H NMR upfield shifts in C6D6 for trans- and cis-BmtNNPy 10 (2.5 mM)
upon addition of Zn-5,15-bis(30,50-di-tert-butylphenyl)-2,8,12,18-tetraethyl-
3,7,13,17-tetramethylporphyrin19 ((a) 0, (b) 0.6, (c) 1.2, (d) 1.8, and (e) 2.4 mM).
5. Brown, E. V.; Granneman, G. R. J. Am. Chem. Soc. 1975, 97, 621.
6. Cravotto, G.; Beggiato, M.; Penoni, A.; Palmisano, G.; Tallari, S.; Leveque, J.;
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constants of the trans and cis isomers of 4-PhNNPy with Zn-tet-
raphenylporphyrin (ZnTPP) were estimated as 1300 and 920 Mꢀ1
,
respectively, while those of the trans and cis isomers of TbetNN-
Py were 860 and 120 Mꢀ1 18
.
For BmtNNPy, the association
9. Field, J. E.; Hill, T. J.; Venkataraman, D. J. Org. Chem. 2003, 68, 6071.
10. Tashiro, M.; Yamato, T. J. Org. Chem. 1985, 50, 2939.
constants of the trans and cis isomers with Zn-5,15-bis(30,50-di-
tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporp-
hyrin19,20 were estimated as 2000 and 100 Mꢀ1, respectively.
These results clearly indicate that the affinities of the trans
and cis isomers of 4-PhNNPy toward Zn-porphyrins are similar,
while those of BmtNNPy and TbetNNPy depend heavily on
whether they are in the trans or in the cis configuration.
In conclusion, we have synthesized shuttlecock-equipped
(TbetNNPy) and bowl-shaped (BmtNNPy) phenylazopyridine
derivatives, which bear substituents that allow the pyridine moiety
to protrude in the trans form but hinder it in the cis form. These
molecules show cis/trans photoisomerization by irradiation. The
cis isomers only weakly interact with Zn-porphyrins, while the
trans isomers coordinate to Zn-porphyrins significantly. Work is
in progress toward the application of these compounds to photore-
sponsive switches.
11. Goto, K.; Holler, M.; Okazaki, R. Tetrahedron Lett. 1996, 37, 3141.
12. Tan, B.; Goto, K.; Kobayashi, J.; Okazaki, R. Chem. Lett. 1998, 10, 981.
13. Gowenlock, B. G.; Cameron, M.; Boyd, A. S. F. J. Chem. Res. (S) 1995, 358.
14. Jones, R. A.; Roney, B. D.; Sasse, W. H. F.; Wade, K. O. J. Chem. Soc. (B) 1967, 106.
15. Demmark, S. E.; Fan, Y. J. Am. Chem. Soc. 2002, 124, 4233.
16. Peters, M. V.; Stoll, R. S.; Goddard, R.; Buth, G.; Hecht, S. J. Org. Chem. 2006, 71,
7840.
17. Yoo, B. W.; Choi, K. H.; Choi, K. I.; Kim, J. H. Synth. Commun. 2003, 33, 4185.
18. The association constants were estimated by a least-squares method using the
equilibria of the association of the trans and the cis ligands with ZnTPP:
Kt ¼ Pt=ðP0 ꢀ Pt ꢀ PcÞðt0 ꢀ PtÞ
Kc ¼ Pc=ðP0 ꢀ Pt ꢀ PcÞðc0 ꢀ PcÞ
Here Pt and Pc are the concentrations of the trans-azo ligandꢁZn-porphyrin and
the cis-azo ligandꢁZn-porphyrin, respectively, P0 is the total concentration of Zn-
porphyrin, t0 and c0 are the total concentration of the trans- and cis-azo ligands,
respectively. The value of Kt was obtained with experiments with pure trans
isomers. The value of Kc was obtained from titration of mixtures of trans and
cis isomers, assuming the limiting values of chemical shift changes of pyridyl
protons in the ligand in the cis isomer,
D
dc1, are the same as those in the trans
Acknowledgment
isomer
D
dt1
.
19. Osuka, A.; Liu, B.; Maruyama, K. J. Org. Chem. 1993, 58, 3582.
20. Zn-5,15-bis(30,50-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramet-
This work was supported by the Japan Ministry of Education,
Science, Technology, Culture, and Sports through the High-Tech
Research Center Project for Private Universities.
hylporphyrin was used for its better solubility in C6D6 and its less overlap of 1
NMR spectrum with that of BmtNNPy, as compared to ZnTPP.
H