Fluorescent sensing system for palladium(II) based on the Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2011.01.118

In this paper, we report a fluorescent sensing system based on the palladium-catalyzed Heck reaction between N-methyl vinylpyridinium and 4-bromo-N,N′-dimethylaniline. Generation of a new fluorophore as a product enhances fluorescence and permits selective detection of palladium(II) among other metal species.

Full text of DOI:10.1016/j.tetlet.2011.01.118

Tetrahedron Letters 52 (2011) 1512–1514
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Tetrahedron Letters
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Fluorescent sensing system for palladium(II) based on the Heck reaction
⇑
Suh young Yu, Hyun-Woo Rhee, Jong-In Hong
Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we report a fluorescent sensing system based on the palladium-catalyzed Heck reaction
between N-methyl vinylpyridinium and 4-bromo-N,N0-dimethylaniline. Generation of a new fluorophore
as a product enhances fluorescence and permits selective detection of palladium(II) among other metal
species.
Received 3 December 2010
Revised 20 January 2011
Accepted 26 January 2011
Available online 1 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Palladium metal is widely used in the fields of environmental
and material sciences;1,2 for example, it is frequently used in cata-
lytic converters, dental crowns, fuel cells, and so on.3 Moreover, it
plays an important role in synthetic chemistry as a catalyst. In sev-
eral carbon–carbon bond-forming reactions such as Heck, Suzuki,
and Sonogashira coupling reactions, palladium compounds play
pivotal roles in accelerating the reaction rate.4 Organic synthesis
that uses palladium as the catalyst is widely applied in the produc-
tion of medicinal substances. However, palladium metal species
may be left over even after rigorous purification; they can harm
the human body.5 Similar to its previously mentioned drawbacks
in medicinal applications, it can cause harm to the human body
when employed in dental applications or used as a catalyst in con-
trolling automobile emissions.
Even very low doses of palladium can trigger allergic reactions
in susceptible individuals. Palladium salts may cause eye and skin
irritation. Moreover, continued exposure to palladium species can
induce more serious symptoms such as inhibition of enzyme reac-
tions, coordination to the thymine moiety in DNA, and accumula-
tion in body organs.6 Therefore, it is important to develop
efficient tools to detect palladium species.
Currently, spectroscopic methods such as plasma emission
spectroscopy, atomic absorption spectroscopy, X-ray fluorescence,
and ICP-MS are commonly used techniques for quantifying palla-
dium with high sensitivity. These methods, however, require
expensive instruments and trained operators.7 Therefore, continual
researches have been conducted for the effective detection of pal-
ladium species. For example, Koide and co-workers developed high-
performance fluorescent sensors for palladium detection based on
a fluorescein derivative. Especially, Koide co-workers have de-
signed excellent fluorescent sensors for palladium detection based
on a fluorescein derivative. The sensors exhibit bright green fluo-
rescence with high sensitivity and good selectivity upon the addi-
tion of palladium.8 However, Koide’s system has a disadvantage in
that it detects platinum in addition to palladium and requires
reducing agents for palladium(II). Several fluorescent probes that
can overcome these problems have been developed.9
Herein, we report a fluorescence sensing system that can selec-
tively detect palladium among other metals through the palla-
dium-catalyzed Heck reaction. The Heck reaction is a typical
palladium-catalyzed carbon–carbon bond-forming reaction be-
tween an unsaturated halide and an alkene. Scheme 1 shows our
main strategy. The sensing system is composed of two simple mol-
ecules; N-methylvinylpyridinium unit (A) and 4-bromo- N,N0-
dimethylaniline (B). Although both
A and B do not exhibit
fluorescence originally, after the addition of palladium, the two
are connected through a vinyl group to generate a fluorophore C.
Therefore, the system is able to detect the presence of both Pd(0)
and Pd(II) because Pd(II) can be reduced to Pd(0) by reducing
agents.8a
Compound B is commercially available, and N-methyl vinylpy-
ridinium iodide (A) was synthesized by the methylation of 4-vinyl-
pyridine using excess iodomethane and K2CO3 (Scheme 2).
We first examined the fluorescent emission spectra of the reac-
tion mixture before and after the Heck reaction between A and B
using palladium(II) species, a reducing agent (tri-o-tolylphosphine,
C
21H21P), and triethylamime (TEA).10,11 When using palladium
Scheme 1. Fluorescent sensing protocol of palladium species. Compound C is
generated via Heck reaction between N-methyl-vinylpyridinium (A) and 4-
bromo-N,N0-dimethylaniline (B).
⇑
Corresponding author.
E-mail address: jihong@snu.ac.kr (J.-I. Hong).
a
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.118

S. young Yu et al. / Tetrahedron Letters 52 (2011) 1512–1514
1513
Scheme 2. Synthesis of A.
Figure 1. Typical emission spectra (kex = 450 nm) of the sensing system ([A] = [B]
= 100 M in DMF, 2 equiv TEA, 0.5 equiv Pd(OAc)2, 0.5 equiv tri-o-tolylphosphine).
l
The reaction proceeded at 65 °C for 3 h.
Figure 3. (a) Reaction rate analysis at room temperature. Spectra shown above are
for 100
lM of A and B in DMF with palladium catalyst concentrations of 1, 5, 10, 20,
50, and 100
lM (0.01, 0.05, 0.1, 0.2, 0.5, and 1 equiv) in DMF. The fluorescence was
monitored at 30-min intervals and collected at 565 nm. (b) Fluorescence intensity
of samples with various concentrations after 3.5 h. The fluorescence intensity data
show a linear correlation with the concentration of palladium in the range of 1 lM
acetate (Pd(OAc)2), after the reaction proceeded for 3 h at 65 °C, a
40-fold increase was observed in the fluorescence emission inten-
sity at kmax of 565 nm (Fig. 1). This result indicates that C was gen-
erated through the Heck reaction between A and B. C shows its
maximum emission intensity at 565 nm.12 The presence of C was
further detected by GC/MS (see Supplementary data).
(0.01 equiv) to 50
lM (0.5 equiv) in DMF, which enabled the calculation of the
detection limit.
This sensing system was found to be highly selective for palla-
dium. Figure 2 shows the fluorescence intensity of the reaction
mixture toward various metal acetates and other metal species,
which are known as catalytic metal species. We used 10–15 equiv
of metal species other than palladium (20–30 times the concentra-
tion of Pd); however, the reaction mixture containing these metal
ions showed a slight increase in the fluorescence intensity. The
fluorescence intensity of the reaction mixture using 0.5 equiv of
palladium was 5- to 38-fold larger than that using other metal spe-
cies. Specifically, platinum, whose physical and chemical proper-
ties are similar to those of palladium, hardly induced any
fluorescence change.
Next, we measured the time-dependent fluorescence change at
various concentrations of palladium at room temperature (Fig. 3a).
The maximum fluorescence intensity increased steadily and was
dependent on the concentration of palladium. The highest emis-
sion intensity was observed in the case of 0.5 equiv (50
palladium.
lM) of
Linear dependence of the reaction rate on the palladium con-
centration was observed at room temperature, from which the
detection limit (signal to noise ratio of 3) of the sensing system
was determined to be 5.45 lM after 3.5 h (Fig. 3b).
Recently, Ahn and co-workers emphasized the importance of
direct and convenient detection of palladium(II) ions without
reducing agents.9a Concurring with this view, we tested whether
the sensing system is able to detect palladium(II) with or without
a ligand (tri-o-tolylphosphine). A similar fluorescence enhance-
ment was observed (Fig. 4) in both cases, and the molecule C could
be confirmed by GC/MS (Supplementary data). The proposed
mechanism for the ligand-free Heck reaction with palladium ace-
tate is also described in Supplementary data.13
In summary, we have developed a palladium(II) sensing system
with two simple molecules: N-methyl vinylpyridinium (A) and 4-
bromo-N,N0-dimethylaniline (B). Although both A and B do not ex-
hibit any fluorescence originally, they produce a fluorophore
through the Heck reaction. Although the reaction system is accel-
erated on heating, it can detect palladium(II) even at room temper-
ature. The developed fluorescent sensing system is highly selective
for palladium(II) among other metal species and is able to detect
palladium(II) with or without the presence of any ligand.
Figure 2. Fluorescence intensity upon addition of Pd (0.5 equiv), metal acetates
(Co(OAc)2ꢀ4H2O, Hg(OAc)2, KOAc, NaOAc, and Pb(OAc)2, each 10 equiv, 1 mM in
DMF) and transition metal species (Cu2O, AuI, PtCl2, and AgNO3, each 15 equiv,
1.5 mM in DMF) at 565 nm.

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S. young Yu et al. / Tetrahedron Letters 52 (2011) 1512–1514
References and notes
1. (a) Chidambara, D.; Hennebel, T.; Taghavi, S.; Mast, J.; Boon, N.; Verstraete, W.;
Lelie, D. V. D.; Fitts, J. P. Environ. Sci. Technol. 2010, 44, 7635–7640; (b) Antolini,
E. Energy Environ. Sci. 2009, 2, 915–931; (c) Hrdlicka, J. A.; Seames, W. S.; Mann,
M. D.; Muggli, D. S.; Horabik, C. A. Environ. Sci. Technol. 2008, 42, 6677–6682;
(d) Li, Q.; Page, M. A.; Mariñas, B. J.; Shang, J. K. Environ. Sci. Technol. 2008, 42,
6148–6153; (e) Zereini, F.; Wiseman, C. L. E.; Püttman, W. Environ. Sci. Technol.
2007, 41, 451–456.
2. (a) Ismail, A. A.; Bahnemann, D. W.; Robben, L.; Yarovyi, V.; Wark, M. Chem.
Mater. 2010, 22, 108–116; (b) MacQuarrie, S.; Horton, J. H.; Barnes, J.;
McEleney, K.; Loock, H. P.; Crudden, C. M. Angew. Chem., Int. Ed. 2008, 47,
3279–3282; (c) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M.
Science 2001, 293, 2227–2231.
3. (a) Shu, J.; Grandjean, B. P. A.; Neste, A. V.; Kaliaguine, S. Can. J. Chem. Eng. 1991,
69, 1036–1060; (b) Rushforth, R. Platinum Met. Rev. 2004, 48, 30–31; (c)
Mazumder, V.; Sun, S. J. Am. Chem. Soc. 2009, 131, 4588–4589.
4. (a) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320–2322; (b) Mizoroki,
T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581; (c) Miyaura, N.; Suzuki,
A. Chem. Rev. 1995, 95, 2457–2483; (d) Sonogashira, K.; Tohda, Y.; Hagihara, N.
Tetrahedron Lett. 1975, 50, 4467–4470; (e) Cho, E. J.; Senecal, T. D.; Kinzel, T.;
Zhang, Y.; Watson, D. A.; Buchwald, S. L. Science 2010, 328, 1679–1681.
5. Garrett, C. E.; Prasad, K. Adv. Synth. Catal. 2004, 346, 889–900.
6. (a) Downey, D. Contact Dermatitis 1989, 21, 54; (b) Marcusson, J. A. Contact
Dermatitis 1996, 34, 320–323; (c) Liu, T. Z.; Lin, T. F.; Chiu, D. T. Y.; Tsai, K. -J.;
Stern, A. Free Radic. Biol. Med. 1997, 23, 155–161; (d) Shultz, M. D.; Lassid, J. P.;
Gooch, M. G.; Evans, B. R.; Woodward, J. Biochem. Biophys. Res. Commun. 1995,
209, 1046–1052.
7. (a) Martin, M.; Evans, B.; O’Neill, H.; Woodward, J. Appl. Optics 2003, 42, 6174–
6178; (b) Gal’tsev, P. A.; lokhin, B. S.; Levunin, S. L.; Khorev, V. I. At. Energ. 1991,
69, 692–693; (c) Rao, R. N.; Talluri, M. V. N. K. J. Pharm. Biomed. Anal. 2007, 43,
1–13; (d) Jakubowski, N.; Feldmann, I.; Stuewer, D. J. Anal. At. Spectrom. 1997,
12, 151–157.
8. (a) Song, F.; Garner, A. L.; Koide, K. J. Am. Chem. Soc. 2007, 129, 12354–12355;
(b) Garner, A. L.; Koide, K. J. Am. Chem. Soc. 2008, 130, 16427–16473; (c) Garner,
A. L.; Koide, K. Chem. Commun. 2009, 86–88; (d) Garner, A. L.; Song, F.; Koide, K.
J. Am. Chem. Soc. 2009, 131, 5163–5171; (e) Song, F.; Carder, E. J.; Kohler, C. C.;
Koide, K. Chem. Eur. J. 2010. ASAP.
9. (a) Santra, M.; Ko, S.-K.; Shin, I.; Ahn, K. H. Chem. Commun. 2010, 46, 3964–
3966; (b) Li, H.; Fan, J.; Du, J.; Guo, K.; Sun, S.; Liu, X.; Peng, X. Chem. Commun.
2010, 46, 1079–1081.
Figure 4. Fluorescence emission of the sensing system with or without reducing
agents. Palladium(II) was introduced in the form of Pd(OAc)2 at 65 °C for 3 h. The
maximum fluorescence intensity was observed at 535 nm without a ligand and at
565 nm with a ligand. Inset: structure of ligand, tri-o-tolylphosphine.
Acknowledgments
This work was supported by the NRF grant funded by the MEST
(Grant No. 2009-0080734) and. S.Y.Y thanks the Ministry of Educa-
tion for the BK fellowship. H.-W.R is the recipient of POSCO TJ Park
Postdoctoral Fellowship.
10. Wu, Q.; Anslyn, E. V. J. Am. Chem. Soc. 2004, 126, 14682–14683.
11. Other emission spectra of the sensing system in the case of using palladium(II)
chloride, dichlorobis(triphenylphosphine) palladium(II), bis(benzonitrile)-
dichloropalladium(II), and bis(acetonitrile)palladium(II) dichloride are shown
in Supplementary data.
Supplementary data
Supplementary data (NMR, GC/MS spectra and fluorescence
data (PDF)) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2011.01.118.
12. Abdel-Halim, S. T.; Awad, M. K. J. Mol. Struct. 2009, 920, 332–341.
13. Yao, Q.; Kinney, E. P.; Yang, Z. J. Org. Chem. 2003, 68, 7528–7531.