Tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical: a new stable coloured magnetic species as a chemosensor for natural polyphenols.

Article abstract of DOI:10.1039/b209985c

We report the synthesis, electron paramagnetic resonance and electrochemical properties of a novel stable radical of the TTM series. Its strong electron acceptor ability has been tested with (-)-epicatechin, a natural polyphenolic antioxidant.

Full text of DOI:10.1039/b209985c

Tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical: a new stable
coloured magnetic species as a chemosensor for natural polyphenols
a
a
b
c
Josep L. Torres, Begoña Varela, Enric Brillas and Luis Juliá*
a
Departament de Química de Peptids i Proteïnes, Institut d’Investgacions Químiques i Ambientals (CSIC),
Jordi Girona 18-26, 08034, Barcelona, Spain
Departament de Química Física, Universitat de Barcelona, Martí i Franquès 1-11, 08028, Barcelona,
Spain
Departament de Química Orgànica Biològica, Institut d’Investgacions Químiques i Ambientals (CSIC),
Jordi Girona 18-26, 08034, Barcelona, Spain. E-mail: ljbmoh@cid.csic.es; Fax: 34 93 204 59 04; Tel: 34
b
c
93 400 61 07
Received (in Cambridge, UK) 11th October 2002, Accepted 13th November 2002
First published as an Advance Article on the web 25th November 2002
We report the synthesis, electron paramagnetic resonance
and electrochemical properties of a novel stable radical of
the TTM series. Its strong electron acceptor ability has been
tested with (2)-epicatechin, a natural polyphenolic anti-
oxidant.
Organic radicals derived from the TTM series (tris(2,4,6-
trichlorophenyl)methyl radical (1)) are among the most stable
carbon centered radicals due to steric hindrance of the
2
1
substitutes around the trivalent sp -carbon atom. However,
they are very sensitive to redox processes and some of them are
thus characterized as a new type of stable magnetic substrate
exhibiting amphoteric redox behavior which make them good
candidates to react by electron transfer reaction with either
electron donating or accepting reactants to the corresponding
carbanions or carbocations which have comparable stabilities.2
As these reactions can be monitored by electronic spectroscopy
and electron paramagnetic resonance we are interested in
exploring the potential of these radicals as chemosensors of
natural and synthetic polyphenols. Redox properties of these
radicals are chemically modulated with the introduction of
different functional groups in the molecular structure either by
replacement of the aromatic para-chlorines1 or by electro-
Scheme 1
aqueous solution of tetrabutylammonium hydroxide in THF in
an equimolecular ratio and at low temperatures (250 K) (45
min) gave a red solution of the carbanion which is oxidized to
radical 2 in the presence of an excess of 2,3-dichloro-
5,6-dicyanoquinone (Scheme 2). Highly diluted solutions of
carbanion in THF are stable and the absorption spectrum
= 500 nm; e = 23 500 L mol21
max
consists of a broad band (l
cm21).†
Radical 2 is stable in solid form, and in chloroform solution
in the dark its stability is > 99% after two days. The absorption
spectrum of 2 is characterized by two typical radical bands (Fig.
1), with the second band showing hyperfine structure, at l(e) in
b,c
3
philic substitution in the free aromatic meta-positions, in such
a way that each radical species can show a different standard
redox potential in cyclic voltammetry (CV). In this way, a list of
redox chemosensors will be available to use, the suitable radical
depending in each case on the electron donating properties of
polyphenols.
3
mol21 cm21) 385(22 600) and 513(740) and 560(800).
nm(dm
These bands are bathochromically shifted relative to those of
TTM radical (373, 478 and 542 nm). As the stability of the
radical series of TTM is mainly governed by the presence of the
six ortho-chlorine atoms, all of which adopt a propeller-like
The stable free radical DPPH has been widely used to
conveniently test the free radical scavenging activity of the
natural polyphenols by hydrogen abstraction reactions, generat-
ing relatively stable phenoxyl radicals which result in the
6
conformation with very similar torsion angles between the
central carbon atom and the phenyl groups, the bathochromic
shift is in correlation with the extended conjugation due to the
nitro substitution in all meta-positions.
4
formation of ortho-quinones. The method does not measure the
capacity of natural polyphenols to participate in electron
transfer reactions. However, carbon centered radicals derived
from TTM do not directly abstract hydrogen due to steric factors
and when the standard potentials of the radicals are appropriate,
the reactivity will be based on the ability of the radical A to
abstract an electron of the polyphenol R–OH to generate
transient radical cations which subsequently will evolve to
stabilized phenoxyl radicals by removing a proton to the
medium (Scheme 1). In this context, now we report a new free
radical of the TTM series, the tris(2,4,6-trichloro-3,5-dini-
trophenyl)methyl radical (2), its oxidant properties being
analyzed in the presence of very diluted solutions of a natural
polyphenolic antioxidant of the family of flavanols such as
The cyclic voltammogram for the reduction of radical 2 in
CH Cl solution with tetrabutylammonium perchlorate (0.1 M)
2
2
a
c
as electrolyte on Pt displays a quasi-reversible (Ep
2 Ep =
(
2)-epicatechin.
For the preparation of 2, tris(2,4,6-trichlorophenyl)methane
5
(3) was converted by prolonged (6 days) and exhaustive
nitration at 130 °C with fuming sulfuric acid and nitric acid
(
3+1) to tris(2,4,6-trichloro-3,5-dinitrophenyl)methane (4)
Scheme 2 Reagents and conditions: i, HNO +H SO .SO (15%) (1+3); ii,
3
2
4
3
(32%). Treatment of the hydrocarbon precursor 4 with an
4
(a) Bu NOH–THF (b) 2,3-dichloro-5,6-dicyanoquinone.
7
4
CHEM. COMMUN., 2003, 74–75
This journal is © The Royal Society of Chemistry 2003

23
2 2
Fig. 3 (a) EPR spectrum of a solution of radical 2 ( ~ 6 3 10 M) in CH Cl
at room temperature (microwave power, 5 mW; modulation frequency, 100
Fig. 1 Molar electronic absorption spectrum of radical 2 in CHCl
right-side of the graphic shows the molar absortivity enlarged by a factor of
0.
3
. The
kHz; modulation amplitude, 0.1) and amplification skowing 13C couplings.
(b) Computer simulation with the parameters given in the text.
1
Magnetic quenching of 2 by electron transfer reaction has been
satisfactorily tested with (2)-epicatechin. As the antioxidant
activity of natural polyphenols is closely related to their electron
donating properties,10 this strategy opens up a new convenient
way to evaluate natural polyphenols which are complements of
the organism’s antioxidant defence system. Work is now in
progress to test the ability of this stable radical 2 and other
related radical species with different plant polyphenolic anti-
oxidants.
Financial support for this work from the MEC (Spain)
through project BQU2000-0789-C02-02 and PPQ2000-
0688-C05-03 is gratefully acknowledged. We also thank the
EPR service of the Centre d’Investigació I Desenvolupament
.96 V at n = 200 mV s21 and 0.78 V at n = 20 mV s21) one-
0
2
electron redox pair corresponding to the process A· + 1e " A
0
at a redox potential (E ) shown in Table 1. The reversibility of
this electrochemical process is indicative of the stability of both
species. Redox values for the reduction processes of other stable
radicals of the same triphenylmethyl series such as 1 and
4-amino-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl
radical (5) are also listed in Table 1.
(
0
Table 1 Redox potentials (E ) for the reduction of radicals 1, 2 and 5
23
(
2 2 4 4
~ 10 M) in CH Cl solution with 0.1 M Bu NClO on Pt
Radical
E0/V
(CSIC) for recording the spectra.
1
2
5
20.66
0.58
20.83
Notes and references
†
Selected data for 4 and 2. 4: IR (KBr) 3016(w), 1551(s), 1341(s), 1239(w),
a
1208(w), 1018(w), 949(m), 832(m), 783(m), 749(m), 631(m) cm ; HRMS
EI): m/z = 823.675, calc. for C19HCl 12 = 823.679. 2: IR (KBr)
21
Potential vs. SSCE (NaCl-saturated calomel electrode).
(
9 6
N O
1
553(s), 1342(s), 1278(w), 1238(w), 1187(w), 946(w), 836(w), 787(w),
It is worth emphasising the positive value of the peak
686(w) cm21; HRMS (EI): m/z
=
823.676 (M + H) , calc. for
+
potential for the reduction of 2 to its anion as a consequence of
the strong electron acceptor properties of this radical. The
remarkable shift in the redox potential of 2 relative to those of
radicals 1 and 5 opens up the possibility of modulating the redox
properties as a function of the different substituents in the
aromatic structure of these open-shell species. Consequently,
9 6
C19HCl N O12 = 823.679.
1
(a) O. Armet, J. Veciana, C. Rovira, J. Riera, J. Castañer, E. Molins, J.
Rius, C. Miravitlles, S. Olivella and J. Brichfeus, J. Phys. Chem., 1987,
91, 5608; (b) J. Carilla, Ll. Fajarí, L. Juliá, J. Riera and Ll. Viadel,
Tetrahedron Lett., 1994, 35, 6529; (c) L. Teruel, Ll. Viadel, J. Carilla,
Ll. Fajarí, E. Brillas, J. Sañé, J. Rius and L. Juliá, J. Org. Chem., 1996,
61, 6063; (d) J. Carilla, Ll. Fajarí, L. Juliá, J. Sañé and J. Rius,
Tetrahedron, 1996, 52, 7013.
7
radical 2 unlike radical 1 is a good chemosensor of the presence
of (2)-epicatechin. So, the oxidant ability of radical 2 as a good
electron acceptor compound was evaluated by EPR, measuring
the decrease of the intensity of the radical signal from a diluted
solution (100 mM) in choroform–methanol (2+1) in the presence
of variable concentrations of (2)-epicatechin, and the results
were plotted as the molar percentage of reduced radical against
the concentration of (2)-epicatechin (Fig. 2).8
2
Preliminary results on cyclic voltammetry show the amphoteric
character of TTM radical (1), (4-amino-2,6-dichlorophenyl)bis(2,4,6-
trichlorophenyl)methyl radical (5) and bis(2,6-dichlorophenyl)-
(2,4,6-trichlorophenyl)methyl radical exhibiting one reversible reduc-
tion and one reversible oxidation wave (E. Brillas and L. Juliá,
unpublished results).
3 S. Salomé, J. Carilla, Ll. Fajarí, L. Juliá, E. Brillas and A. Labarta,
Tetrahedron, 1995, 26, 7301.
4
5
6
M. S. Blois, Nature, 1958, 181, 1199; W. Brand-Williams, M. E.
Cuvelier and C. Berset, Lebensm.-Wiss. Technol., 1995, 28, 25.
M. Ballester, J. Riera, J. Castañer, C. Rovira and O. Armet, Synthesis,
1
986, 64.
Attempts to get crystals for X ray analysis of radical 2 were
unsuccessful. However, molecular structures of other radicals of the
TTM series, such as radicals 11a and 5, have confirmed that phenyl
rings are twisted around their bonds to the trivalent carbon atom.
Radical 1 has shown to be stable in the presence of (2)-epicatechin in
similar testing analysis.
1c
7
8
Fig. 2 Oxidant activity of radical 2 against (2)-epicatechin. The results are
During hydrogen transfer reactions from catechins and other flavonoids
to radicals such as DPPH, two steps can be distinguished, a first fast step
when the most labile H atoms are abstracted and a second slower step
corresponding to the reaction of oxidation-degradation products. The
byproducts may include partially inactivated polymeric species, partic-
ularly at high polyphenol concentration. This would result in dose-
response curves such as the one depicted in Fig. 2 for the electron
transfer from (2)-epicatechin to radical 2. W. Brand-Williams, M. E.
Cuvelier and C. Berset, Lebensm.-Wiss. Technol., 1995, 28, 25; O.
Dangles, G. Fargeix and C. Dufour, J. Chem. Soc., Perkin Trans. 2,
1999, 1387; O. Dangles, G. Fargeix and C. Dufour, J. Chem. Soc.,
Perkin Trans. 2, 2000, 1653.
plotted as the molar percentage (1 2 I
f
/I
o
o
) 3 100 (I initial peak intensity of
f
the radical signal in the EPR; I final peak intensity) of reduced radical from
a 100 mM solution in chloroform–methanol (2+1) in the presence of
(2)-epicatechin at different concentrations.
X-Band electron paramagnetic resonance (EPR) spectrum of
radical 2 was recorded in degassed CH Cl solution at room
2 2
temperature and showed a narrow band (DHpp = 0.4 G) with
non-resolved nitrogen splitting (a6N < 0.4 G) (Fig. 3). At higher
gain values, the isotropic coupling with the 13C nuclear spins of
the a-carbon atom, the three bridgehead carbon atoms, and the
six ortho-carbon atoms appeared in the spectrum on both sides
of the main spectrum at approximately 30.0 G, 13.0 G and 11.4
9
The spectrum was simulated following the WINSIM program provided
by D. Dulog, Public EPR Software Tools, National Institute of
Environmental Health Sciences, Bethesda, MD, 1996.
9
G.
1
0 S. V. Jovanovic, S. Steenken, M. Tosic, B. Marjanovic and M. G. Simic,
J. Am. Chem. Soc., 1994, 116, 4846; S. V. Jovanovic, Y. Hara, S.
Steenken and M. G. Simic, J. Am. Chem. Soc., 1995, 117, 9881.
We have reported the synthesis, and the electrochemical and
magnetic properties of the stable triphenylmethyl radical 2.
CHEM. COMMUN., 2003, 74–75
75