7,7-Dipyridyl-2,6-di-tert-butyl-p-quinone methides: Synthesis, complexation with silver(I) ion, and oxidation to their pyridine N-oxide

Article abstract of DOI:10.1246/cl.2006.74

A series of 7,7-dipyridyl-p-quinone methides are prepared from the corresponding dipyridylketones with 4-lithio-2,6-di-tert-butylphenoxide followed by dehydration. 7,7-Bis(2-pyridyl)-p-quinone methide formed the binuclear Ag(I) complex in which the quinon

Full text of DOI:10.1246/cl.2006.74

74
Chemistry Letters Vol.35, No.1 (2006)
7,7-Dipyridyl-2,6-di-tert-butyl-p-quinone Methides:
Synthesis, Complexation with Silver(I) Ion, and Oxidation to Their Pyridine N-Oxide
Kouzou Matsumoto,ꢀ Junya Ikeda, Hiroyuki Kurata, Takeshi Kawase, and Masaji Oda
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
(Received October 4, 2005; CL-051267; E-mail: kmatsu@chem.sci.osaka-u.ac.jp)
A series of 7,7-dipyridyl-p-quinone methides are prepared
Li
Br
from the corresponding dipyridylketones with 4-lithio-2,6-
di-tert-butylphenoxide followed by dehydration. 7,7-Bis(2-
pyridyl)-p-quinone methide formed the binuclear Ag(I) complex
in which the quinone methide rings are deformed to the boat
form. Oxidation of 7,7-dipyridyl-p-quinone methides gave the
corresponding bis(pyridine N-oxide)s.
b
a
t-Bu
t-Bu
t-Bu
t-Bu
OLi
OH
Py
Py
OH
C7
Py
Py
Py
Py
C4
c
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
C1
p-Quinone methides (p-QMs), a structural hybrid between
p-benzoquinone and p-quinodimethane, are featured by their
highly dipolar properties arising from the less symmetric struc-
tures and the tendency of aromatization of the central six-mem-
bered ring.1 Highly dipolar ꢀ-electron systems have attracted
considerable attention in relation to near-infrared dyes and non-
linear optics.2 Although the parent p-QM is a highly reactive
compound,3 7,7-diaryl p-QMs4 are fairly stable, whose proper-
ties are well tunable by the substituents on the aryl groups. In this
context, 7,7-dipyridyl p-QMs would be interesting molecules,
because of the properties of pyridine such as basicity, oxidation
to N-oxide, and complexation with the various transition met-
als.5 Contrary to the numerous studies on 7,7-diphenyl p-QM de-
rivatives, 7,7-dipyridyl p-QMs have not been studied. In order to
explore the potential utility of the structural feature of p-QM for
the novel functional dyes, we now report the synthesis and prop-
erties of a series of 7,7-dipyridyl p-QMs 1a–1c, their bis(pyri-
dine N-oxide)s 2a–2c, and the structural feature of silver(I) com-
plex of 1a (Chart 1).
OH
3a−3c
O
O
1a−1c
dipolar structure
Scheme 1. (a) t-BuLi (3.0 equiv.), ether, 0 ꢁC, 1 h; (b) Py2C=O (0.25
equiv.), ꢂ78 ꢁC, 1 h then rt, overnight (46% for 3a (Py = 2-pyridyl),
32% for 3b (Py = 3-pyridyl), 87% for 3c (Py = 4-pyridyl)); (c) POCl3
(5.0 equiv.), pyridine, reflux, overnight (62% for 1a, 70% for 1b, 82%
for 1c).
was accomplished by heating with POCl3 in pyridine in good
yields. p-QMs 1a–1c are stable, orange crystalline substances.7
Table 1 summarizes the selected spectral data of 1a–1c to-
gether with 7,7-diphenyl p-QM 4.8 p-QMs 1a–1c showed the
relatively low field shifts at C4 as well as high field shifts at
C7 in their 13C NMR chemical shifts. The C=O stretching fre-
quency of 1a–1c were observed at higher wavenumber than that
of 4, and the slightly low field shifts at C1 was also observed.
These results indicate that 1a–1c take smaller contribution of
the dipolar structure than 4 due to the electron-withdrawing
effect of the pyridyl groups. The cyclic voltammograms of 1a–
1c showed two reduction potentials, which were more positive
than those of 4. p-QMs 1a–1c exhibit the absorption maxima
at slightly shorter wavelength than 4.
Scheme 1 shows the synthesis of dipyridyl p-QMs 1a–1c.
Reactions of 4-lithio-2,6-di-tert-butylphenoxide6 with 0.25
equiv. of the corresponding dipyridyl ketones afforded diols 3a–
3c in moderate to good yields. Dehydration of 3a–3c to 1a–1c
Figure 1 shows the ORTEP drawing of 2-pyridyl p-QM 1a.9
The bond lengths of C1–O1 and C4–C7 are 1.223 and
ꢀ
1.370(4) A, respectively. p-QM 1a takes a propeller conforma-
tion in which the p-QM moiety is almost planar while 2-pyridyl
groups are twisted by the angle of 52.9 and 52.3ꢁ. In comparison
with 7,7-diphenyl-2,6-dimethyl p-QM,10 1a exhibits slightly
shorter C=O bond length also suggesting the larger contribution
of quinoid structure.
Although no change in absorption spectrum of 1a was ob-
served by the addition of silver(I) complexes, the prominent
changes in 1H NMR chemical shifts were observed.11 This
N
N
N
N
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
O
1a (2-pyridyl)
1b (3-pyridyl)
1c (4-pyridyl)
2a (pyridine N-oxide)-2-yl
2b (pyridine N-oxide)-3-yl
2c (pyridine N-oxide)-4-yl
4
Chart 1.
Table 1. Selected spectral data of p-QMs 1a–1d
13C NMR chemical shiftsa ꢁ/ppm
C4
Reduction potentialsc/V
E1 E2
Compounds
ꢂ(C=O)b/cmꢂ1
ꢃmaxd/nm (log ")
C1
C7
red
red
1a
1b
1c
4
186.31
186.01
186.03
185.84
132.55
132.02
132.14
129.54
149.41
146.57
147.08
155.63
1611
1609
1610
1601
ꢂ1:43
ꢂ1:40
ꢂ1:27
ꢂ1:55
ꢂ1:83
ꢂ2:00e
ꢂ1:63
ꢂ2:21e
349 (4.45)
360 (4.47)
346 (4.46)
365 (4.50)
aIn CDCl3. bKBr disk. cV vs Ag/Agþ, in 0.1 M n-Bu4NClO4/DMF, sweep rate 100 mV sꢂ1, at 25 ꢁC, Fc/Fcþ = þ0:11 V. dIn cyclohexane. ePeak potential.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.1 (2006)
75
p-QM moiety; this absorption of 2c in acetonitrile (ꢃmax
413 nm) disappeared by protonation at the oxygen atoms of pyr-
idine N-oxide groups with one drop of 2 M H2SO4. On the other
hand, although 2-pyridyl derivative 2a also exhibits the absorp-
tion at around 400 nm, its intensity is quite weak. This weakness
of the absorption intensity would be due to the steric congestion
around C7 decreasing the coplanarity between the QM ring and
pyridine N-oxide groups. On the contrary, the absorption of 3-
pyridyl derivative 1b (ꢃmax 365 nm) is blue-shifted by oxidation
into 2b (ꢃmax 346 nm).
In summary, we prepared the series of dipyridyl p-QMs
1a–1c. 2-Pyridyl derivative 1a forms the binuclear silver(I)
complex in which the distortion of the QM ring is increased
by the complexation. The difference of the properties dependent
on the position of the nitrogen atoms appears by the oxidation
into the corresponding N-oxides 2a–2c. The preparation of metal
complex of 1b and 1c, and other 7,7-bis(heteroaryl) p-QMs are
now under investigation.
Figure 1. ORTEP drawing of 1a (50% probability). Selected bond lengths
ꢀ
(A): C1–O1 1.223(4), C1–C2 1.481(4), C1–C6 1.480(4), C2–C3 1.338(4),
C3–C4 1.453(4), C4–C5 1.457(4), C4–C7 1.370(4), C5–C6 1.344(4).
References and Notes
1
Reviews: a) A. B. Turner, Quart. Rev. 1964, 18, 347. b) H.-U. Wagner,
R. Gompper, The Chemistry of the Quinoid Compounds, ed. by S.
Patai, Wiley & Sons, London, 1974, p. 1145. c) A. A. Volod’kin,
V. V. Ershov, Russ. Chem. Rev. 1988, 57, 336. d) M. G. Peter, Angew.
Chem., Int. Ed. Engl. 1989, 28, 555. e) P. Wan, B. Barker, L. Diao, M.
Fischer, Y. Shi, C. Yang, Can. J. Chem. 1996, 74, 465.
2
3
a) R. Gompper, H.-U. Wagner, Angew. Chem., Int. Ed. Engl. 1988, 27,
1437. b) D. J. Williams, Angew. Chem., Int. Ed. Engl. 1988, 23, 690.
a) G. J. Leary, Wood Sci. Technol. 1980, 14, 21. b) S. M. Shevchenko,
A. G. Apushkinskii, Russ. Chem. Rev. 1992, 61, 1. c) D. C. Thompson,
J. A. Thompson, M. Sugumaran, P. Moldeus, Chem.-Biol. Interact.
1992, 86, 129.
Figure 2. ORTEP drawing of silver(I) complex of 1a along c axis (50%
ꢂ
probability). tert-Butyl groups and BF4 ions are omitted for clarity.
4
5
For example: a) A. Bistrzycki, C. Herbst, Chem. Ber. 1903, 36, 2333.
b) K. H. Meyer, Justus Liebigs Ann. Chem. 1920, 420, 134. c) R.
Gompper, H.-U. Wagner, Tetrahedron Lett. 1968, 9, 165.
We recently synthesized the molecules having polypyridyl groups for
supramolecular architectrure: a) K. Matsumoto, Y. Harada, T. Kawase,
M. Oda, Chem. Commun. 2002, 324. b) K. Matsumoto, M. Kannami,
M. Oda, Tetrahedron Lett. 2003, 44, 2861. c) K. Matsumoto, M.
Kannami, M. Oda, Chem. Lett. 2004, 33, 1096. See also: d) P. J. Steel,
C. J. Sumby, Chem. Commun. 2002, 322. e) P. J. Steel, C. J. Sumby,
Inorg. Chem. Commun. 2002, 5, 323.
a) H. Kurata, T. Tanaka, M. Oda, Chem. Lett. 1999, 28, 749. b) H.
Kurata, Y. Takehara, T. Kawase, M. Oda, Chem. Lett. 2003, 32, 538.
c) H. Kurata, T. Shimoyama, K. Matsumoto, T. Kawase, M. Oda, Bull.
Chem. Soc. Jpn. 2001, 74, 1327.
Spectroscopic data for the p-QMs 1a–1c and their N-oxides 2a–2c are
described in the Supporting Information.
ꢁ
ꢀ
Selected bond lengths (A) and angles ( ): Ag1–Ag2 3.099(2), Ag1–N1
2.184(7), Ag1–N3 2.163(7), Ag2–N2 2.213(7), Ag2–N4 2.189(8), C1–O1
1.25(1), C1–C2 1.49(1), C2–C3 1.34(1), C3–C4 1.45(1), C4–C7 1.36(1),
Ag2–Ag1–N1 67.3(2), Ag2–Ag1–N3 100.7(2), N1–Ag1–N3 157.4(2),
Ag1–Ag2–N2 99.7(2), Ag1–Ag2–N4 65.2(2), N2–Ag2–N4 156.0(3).
changes of the chemical shifts were saturated by 1.0 equiv. of
silver(I) ion, which suggested the formation of a 1:1 complex
of 1a and silver(I) ion. Single crystals of silver(I) complex of
1a suitable for X-ray crystallographic analysis were obtained
from a solution of 1a in acetonitrile–ether involving an equiva-
lent of AgBF4.12 The molecular structure (Figure 2) reveals that
the complex adopts a binuclear coordination structure connected
with two molecules of 1a. Atomic distances of the Ag1–N1,
Ag1–N3, Ag2–N2, and Ag2–N4 are in the range of 2.16–
6
7
8
9
´
´
B. Koutek, M. Pısova, M. Soucek, O. Exner, Collect. Czech. Chem.
Commun. 1976, 41, 1676.
ꢀ
ꢀ
2.21 A and Ag1–Ag2 is 3.10 A. Although this short silver–silver
contact is often observed in many binuclear silver(I) complexes,
it is not thought to be indicative of any particular intermetallic
interactions.13 Compared with 1a, it should be noted that the
quinone methide rings are deformed to the boat form by the
complexation.
Crystal data for 1a: C25H28N2O, MW ¼ 372:51, monoclinic, space
group P21=c (No. 14), a ¼ 11:986ð3Þ, b ¼ 8:406ð4Þ, c ¼ 22:550ð3Þ
ꢁ
A, ꢄ ¼ 96:61ð1Þ , V ¼ 2256:9ð10Þ A , Z ¼ 4, Dcalcd ¼ 1:096 g cmꢂ3
,
ꢀ
ꢀ 3
T ¼ 296 K, Of the 5787 reflections which were collected, 5414 were
unique (Rint ¼ 0:012) used in refinement. R1 ¼ 0:063 (2024 data
I > 2ꢅðIÞ), wR2 ¼ 0:252 (all data), GOF ¼ 1:00, CCDC-289129.
10 T. W. Lewis, I. C. Paul, D. Y. Curtin, Acta Crystallogr. 1980, B36,
70.
11 The change of the chemical shifts of 1a by silver(I) ion are described in
the Supporting Information.
12 Crystal data for the silver(I) complex of 1a: C50H56Ag2B2F8N4O2,
MW ¼ 1134:36, monoclinic, space group P21=c (No. 14), a ¼
The N-oxides 2a–2c were prepared by oxidation of 1a–1c
with MCPBA in CH2Cl2 at 0 ꢁC in good yields (84% for 2a,
83% for 2b, and 92% for 2c) as stable yellow (2a) or orange
(2b and 2c) crystalline substances.7 In the UV–vis measurement,
4-pyridyl derivative 2c exhibits the most prominent effect,
compared with the corresponding dipyridyl p-QM 1c. Thus,
the solution of 1c in CH2Cl2 exhibits the longest absorption
maxima at 348 nm which are red-shifted up to 412 nm by oxida-
tion into 2c. This bathochromic effect would be attributable to
the effective conjugation between pyridine N-oxide groups and
10:87ð1Þ, b ¼ 18:70ð3Þ, c ¼ 30:68ð4Þ A, ꢄ ¼ 95:82ð4Þꢁ, V ¼
ꢀ
6205ð51Þ A , Z ¼ 4, Dcalcd ¼ 1:214 g cmꢂ3, T ¼ 200 K, Of the 58518
ꢀ 3
reflections which were collected, 13963 were unique (Rint ¼ 0:044)
used in refinement. R1 ¼ 0:073 (5367 data, I > 2ꢅðIÞ), wR2 ¼ 0:248
(all data), GOF ¼ 0:99. CCDC-289130.
13 F. A. Cotton, X. Feng, M. Matusz, R. Poli, J. Am. Chem. Soc. 1988,
110, 7077.
Published on the web (Advance View) December 10, 2005; DOI 10.1246/cl.2006.74