Synthesis of novel donor -π- acceptor chromophores with dipicolinate as acceptor

Article abstract of DOI:10.1080/00397910802531963

Three novel donor - acceptor (D - A) chromophores, with dipicolinate as acceptor, have been synthesized. All the compounds were characterized by 1H NMR, infrared (IR) spectroscopy, mass spectrometry (MS), and elemental analysis.

Full text of DOI:10.1080/00397910802531963

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Synthesis of Novel
Donor–π–Acceptor
Chromophores with
Dipicolinate as Acceptor
Haibo Xiao a , Xiaoming Tao a , Hui Li a & Guanghao
Shi a
a Department of Chemistry , Shanghai Normal
University , Shanghai, China
Published online: 19 Mar 2009.
To cite this article: Haibo Xiao , Xiaoming Tao , Hui Li & Guanghao Shi (2009)
Synthesis of Novel Donor–π–Acceptor Chromophores with Dipicolinate as Acceptor,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:8, 1472-1477, DOI: 10.1080/00397910802531963
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Synthetic Communications1, 39: 1472–1477, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802531963
Synthesis of Novel Donor–p–Acceptor
Chromophores with Dipicolinate as Acceptor
Haibo Xiao, Xiaoming Tao, Hui Li, and Guanghao Shi
Department of Chemistry, Shanghai Normal University, Shanghai, China
Abstract: Three novel donor–p–acceptor (D–p–A) chromophores, with dipicolinate
as acceptor, have been synthesized. All the compounds were characterized by H
NMR, infrared (IR) spectroscopy, mass spectrometry (MS), and elemental analysis.
1
Keywords: Acceptor, chromophore, dipicolinate, synthesis
INTRODUCTION
The luminescence of lanthanide (LnIII) coordination complexes is unique
because of their high color purity, long lifetimes, and insensitivity to
environmental quenching.[1] The sensitization of LnIII by the two-photon
antenna effect may lead to less harmful and deep-penetrating bioimaging
applications.[1–3] Although studies on biological imaging from lantha-
nides (by ultraviolet, UV, absorption) or induced by two-photon excited
fluorescence (from organic systems) have been widely reported,[4] rela-
tively few complexes that combine the advantages of two-photon sensiti-
zation and high-purity LnIII emission can be found. Pyridine-2,6-
dicarboxylic (dipicolinic) acid and its derivatives are highly useful triden-
tate
ligands.
Pyridine-2,6-dicarboxylic
acid
forms
nine-
coordinating luminescent complexes with lanthanides, providing high
quantum yields of phosphorescence, and these are used as luminescent
tags, labels, and barcodes.[5–9] Despite the simplicity of the preparation
of tridentate ligands based on pyridine-2,6-dicarboxylic acid, fabrication
Received July 25, 2008.
Address correspondence to Haibo Xiao, Department of Chemistry, Shanghai
Normal University, Shanghai, China. E-mail: xiaohb@shnu.edu.cn
1472

Novel Donor–p–Acceptor Chromophores
1473
of complex systems based on its derivatives necessitates derivatization of
the pyridine ring. Consequently, substantial efforts have been concen-
trated on the preparation of functionalized pyridine-2,6-dicarboxylic acid
derivatives.[10,11] To the best of our knowledge, there has not been any
report on dipicolinates or its derivatives as acceptors in donor–p–
acceptor (D–p–A) chromophores.
In this article, we describe the synthesis of three novel D–p–A chro-
mophores containing dipicolinate. The arylamine group in chromophore
functions as an electron donor (D), the dipicolinate moiety acts as an
electron acceptor (A), and the C¼C or C¼N double bond acts as a con-
jugation bridge. Therefore, a chromophore is a polar molecule that shows
D–p–A character, which is known to be the structural basis for an effi-
cient two-photon absorber.[12] The target compounds are the possible
candidates to be used to sensitize LnIII by the two-photon antenna effect.
RESULTS AND DISCUSSION
The target compounds 6, 7, and 8 were synthesized according to the steps
outlined in Scheme 1. Direct functionalization of pyridine-2,6-dicarboxy-
lates cannot be performed by conventional substitution reactions, because
Scheme 1. Synthesis of three D–p–A chromophores containing dipicolinate.

1474
H. Xia et al.
of the resistance of the pyridine ring in pyridine-2,6-dicarboxylic acid deriva-
tives to electrophilic and nucleophilic attacks.[8] Shelkov and Melman
reported the synthesis of 4-(hydroxyalkyl) pyridine-2,6-dicarboxylates
through free-radical reactions.[8] These reactions proceeded with complete
regioselectivity, and no detectable amounts of isomeric products were
observed, despite the presence of two electron-withdrawing methoxycarbo-
nyl groups known to favor 3-substitution. The key intermediate, dimethyl
4-formylpyridine-2,6-dicarboxylate 5, was synthesized by oxidation of
4-(hydroxymethyl) pyridine-2,6-dicarboxylate, and the formyl group was
employed to synthesize a C¼C or C¼N double bond as the conjugation
bridge. The synthesis of dimethyl 4-formylpyridine-2,6-dicarboxylate 5 from
4-(hydroxymethyl)pyridine-2,6-dicarboxylate using MnO2 resulted in poor
yield. The application of CrO3 was used as an alternative for this reaction
and gave 5 in 75% yield.
In conclusion, we have been successful in utilizing dimethyl 4-formyl-
pyridine-2,6-dicarboxylate 5 for the syntheses of novel D–p–A chromo-
phores. Compounds 5, 6, 7, and 8 are new and have been characterized
by spectral and analytical data.
EXPERIMENTAL
Melting points were determined with an XT4A apparatus and are
uncorrected. Magnetic resonance spectra were recorded on a Bruker
DRX 400 spectrometer, using CDCl3 as solvent and tetramethylsilane
(TMS) as internal standard. Infrared (IR) spectra were obtained on a
Nicolet Avatar 370 DTGS spectrometer. Mass spectra were obtained
on a VG12-250 mass spectrometer. Elemental analyses were obtained
on a Perkin-Elemer instrument.
All reagents and solvents were commercial reagents of analytical
grade and were used as received. Further purification and drying by stan-
dard methods were employed and they were distilled prior to use when
necessary. Reactions were monitored by thin-layer chromatography
(TLC), on silica gel (grade 60 PF254)–protected glass sheets, and the spots
were detected by exposure to a UV lamp at k254 nm for a few seconds.
Compound 4 was prepared starting from 2,6-dimethylpyridin
according to the literature method.[8,13]
Dimethyl 4-formyl-pyridine-2,6-dicarboxylate (5)
A mixture of CrO3(1 g, 1 mmo1), pyridine (1.58 g, 2 mmo1), and CH2Cl2
(100 ml) was stirred at room temperature for 20 min. A solution of 4

Novel Donor–p–Acceptor Chromophores
1475
(0.45 g, 2 mmol) in CH2C12 (10 ml) was added dropwise over 30 min at
25–30ꢀC. The mixture was stirred for 5 h at room temperature. The
organic phase was washed with l mol=L HC1 and water successively
and dried over MgSO4. The crude product was purified by silica-gel col-
umn chromatography, eluting with petroleum=ethyl acetate to afford 5
1
ꢀ
(0.33 g). Yield 75%. Mp 186–188 C. H NMR (CDC13, d ppm): 10.2
(s, 1H, CHO), 8.7 (s, 2H, PyH), 4.1 (s, 6H, OCH3). EI-MS: m=z 223
(Mþ). Anal. calcd. for C10H9NO5: C, 53.82; H, 4.06; N, 6.28. Found:
C, 53.79; H, 4.05; N, 6.25.
Trans-dimethyl-4-[40-(N,N-diphenylamino)-styry1]-pyridin-2,6-
dicarboxylate (6)
To a 50-ml, two-necked flask, 0.115 g 5 (0.5 mmol), 0.323 g phosphonium
salt (0.6 mmol), and 100 ml anhydrous tetrahydrafuran (THF) were
added under a nitrogen atmosphere. The reaction mixture was cooled
to 0ꢀC in an ice bath. A THF solution (15 ml) of 0.024 g NaH was
dropped into the flask. After the reaction mixture had been stirred for
24 h at room temperature, the mixture was washed with water. The
organic phase was dried over MgSO4, then evaporated to give yellow
solid. The crude product was purified by chromatography on silica gel,
using CH2Cl2/hexane as the eluent, to give pure product in 50% yield
(119 mg). Mp 193–195 ꢀC. IR (KBr) 2953.9, 2925.3, 2851.7, 1711.6,
1585.0, 1491, 1356, 1278.5, 1249.9, 1115.1, 992.5, 967.9, 759.6,
1
694.2 cmꢁ1. H NMR (CDC13, d ppm): 8.35 (s, 2H, PyH), 7.4–7.46 (m,
2H), 7.28–7.34 (m, 4H), 7.15 (d, J ¼ 8.4 Hz, 4H), 7.03–7.13 (m, 4H),
6.95–7.0 (d, J ¼ 16.8 Hz, 2H), 4.0 (s, 6H). EI-MS: m=z 464 (Mþ). Anal.
calcd. for C29H24N2O4: C, 74.98; H, 5.21; N, 6.03. Found: C, 75.03; H,
5.16; N, 6.06.
Trans-dimethyl-4-[40-(N,N-dimethylamino)-styry1]-pyridin-2,6-
dicarboxylate (7)
Compound 7 used the same method as 6. Yield 45%, mp 200–202 ꢀC. IR
(KBr): 2953.9, 2921.2, 2855.8, 2802.7, 1752.5, 1711.6, 1609.5, 1589.1,
.
1446.1, 1258.1, 1237.7, 1184.6, 1155.9, 992.5,955.7, 804.5 cmꢁ1 1H
NMR (CDC13, d ppm): 8.35 (s, 2H, PyH), 7.4–7.5(m, J ¼ 8.0 Hz,
16.0 Hz, 3H), 6.90 (d, J ¼ 16.0 Hz, 1H, Py–CH¼), 6.75 (d, J ¼ 8.0 Hz,
2H, ArH), 4.0 (s, 6H, OCH3), 3.0 (s, 6H, NCH3). EI-MS: m=z 340
(Mþ). Anal. calcd. for C19H20N2O4: C, 67.05; H, 5.92; N, 8.23. Found:
C, 67.04; H, 5.89; N, 8.23.

1476
H. Xia et al.
Dimethyl 4-((E)-[4-(dimethylamino]phenylimino]methyl)pyridine-2,6-
dicarboxylate (8)
To a 100- ml flask, 50 mg (0.224 mmol) 5 and 30 ml anhydrous ethanol
were added. After the mixture was stirred for 30 min at room tempera-
ture, 30.5 mg (0.224 mmol) N,N-dimethylaniline were added. The reac-
tion mixture was refluxed for 4 h and then cooled to room
temperature. The resulting precipitate was filtered and recrystallized from
ethanol to give yellow crystals in 85% yield (65 mg). Mp 192–193 (C. IR
(KBr): 2949.8, 2884.5, 2806.8, 1752.5, 1719.8, 1625.8, 1576.8, 1442.0,
1339.8, 1245.8, 1204.9, 1160.0, 996.6, 816.8, 780.0 cmꢀ1 1H NMR
.
(CDC13, d ppm): (8.8(s, 2H, PyH), 8.6 (s, 1H, –CH¼N), 7.4 (d,
J ¼ 8.8 Hz, 4H, ArH), 4.1 (s, 6H, OCH3), 3.1 (s, 6H, NCH3). EI-MS:
m=z 341 (Mþ). Anal. calcd. for C18H19N3O4: C, 63.33; H, 5.61; N,
12.31. Found: C, 63.34; H, 5.58; N, 12.30.
ACKNOWLEDGMENTS
This work was financially supported by the Shanghai Municipal
Education Committee (No. 06DZ010) and Shanghai Normal University
(No. SK200839).
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