3122
J.A. Mesa et al. / Tetrahedron 67 (2011) 3119e3123
Table 2
4.2. Buffer solutions
Observed rate constants (k) and stoichiometric values (n) for the reaction of TSPTM
trianion radical with ascorbic acid, pyrogallol, and catechol in aqueous solution
(a) pH¼12.67ꢃ0.01: to an aqueous solution of KCl (0.2 m)
(50 mL) was added another solution of NaOH (0.2 M) (12 mL). (b)
pH¼12.98ꢃ0.01: to an aqueous solution of KCl (0.2 M) (50 mL) was
added another solution of NaOH (0.2 M) (132 mL).
Compound
k (Mꢂ1 sꢂ1
)
n
Ascorbic acid
Pyrogallol
Catechol
33
1.5
0.05
3.5
6e7
d
4.3. Cyclic voltammetries
Now the generated tetraanion TSPTM4ꢂ is rapidly and quantita-
tively protonated by water to H-TSPTM trianion due to the acid
a
Cyclic voltammetries were carried out in a standard thermo-
stated three-electrode cell. A platinum (Pt) disk with 0.093 cm2
area was used as the working electrode and a Pt wire as the counter
electrode. The reference electrode was a saturated calomel elec-
trode (SCE), submerged in a salt bridge of the same electrolyte,
which was separated from the test solution by a Vycor membrane.
Solution of TSPTM (1) radical (w10ꢂ3 M) in water containing lith-
ium perchlorate (0.1 M) as background electrolyte was studied. The
volume of all test solutions was 50 mL. Electrochemical measure-
ments were performed under an Argon atmosphere (25 ꢀC) using
an Eco Chemie Autolab PGSTAT100 potentiostategalvanostat con-
trolled by a computer with a Nova 1.5 software. Cyclic voltammo-
character of the aqueous solutions and, therefore, the tetraanion is
not observed in UVevis spectroscopy unlike what it happens with
PTM in THF, and HNTTM and TNPTM in non-aqueous solvents.
While ascorbic acid presents a rapid TSPTM trianion radical-
scavenging reaction and pyrogallol exhibits a significantly moder-
ate reaction, the reaction of catechol is noticeably slow. The order of
activity of the antioxidants is ascorbic acid>pyrogallol>catechol,
which is well correlated with their onset anodic peak potentials
(Table 1). This selectivity in the activity of the TSPTM trianion
radical with ascorbic acid, pyrogallol, and catechol in aqueous
medium is also a consequence of its moderate oxidant power and,
therefore of its reduction potential. As strong acids, the functional
hydroxysulfonyl groups in the TSPTM radical are completely dis-
grams were recorded at scan rates ranging from 20 to 200 mV sꢂ1
.
associated in water, and the presence of net negative charges eSO3
ꢂ
4.4. Kinetic measurements
decreases significantly the electron-withdrawing properties of
these substituents, thereby decreasing the oxidant power of the
trivalent carbon atom. It is also valuable to mention the high stoi-
chiometric values obtained in the reactions of TSPTM trianion
radical with ascorbic acid (3:1, radical/acid) and mainly with py-
rogallol (6 or 7:1, radical/pyrogallol) (Table 2). It is known that
ascorbic acid scavenges 2 equiv of perchlorotriphenylmethyl (PTM)
radical in THF and pyrogallol scavenges 3 equiv of either HNTTM or
TNPTM radicals in CHCl3/MeOH (2:1), roughly one electron per
reactive hydroxyl. Table 2 shows that ascorbic acid reduces 3 equiv
of TSPTM trianion radical and pyrogallol 6 or 7 equiv of TSPTM
radical. All these results indicate that the stoichiometry of these
electron transfer reactions largely depend on the applied solvent.
The kinetic parameters were obtained by EPR. Freshly prepared
solutions of TSPTM (1) (245
degassed water were mixed (1:1, v/v), and TSPTM (1) (223
catechol (45 M) in degassed water were mixed (1:1, v/v) in the
m
M) and ascorbic acid (49
mM) in
mM) and
m
spectrometer cell (molar ratios between TSPTM and antioxidant,
w5:1) The EPR spectra were recorded at different intervals.
4.5. Synthesis of tris(2,3,5,6-tetrachlorophenyl-4-
hydroxysulfonyl)methane (aH-TSPTM)
A
mixture of tris(2,3,5,6-tetrachlorophenyl)methane (2.0 g;
3.04 mmol) and fuming H2SO4 (SO3, 65%) (60 mL) was stirred
(95e110 ꢀC, 5 days). The solution was then poured into cracked ice
(200 mL) and kept overnight. The precipitate was separated by fil-
tration, dried (100 ꢀC), and chromatographed by column on silicagel
3. Conclusion
elutingwith ethyl acetate/MeOH (5:3, v/v)affordedaH-TSPTM(2.4 g;
In conclusion, this manuscript reports the synthesis of a stable
radical of the PTM series very soluble in water. Due to the presence
of sulfonic groups in the molecule, aqueous solutions of radical 1
are strongly acidic. It has been characterized by UVevis and EPR
spectroscopy in aqueous solution. It is electrochemically reduced in
a quasi-reversible one-electron process to a tetraanion, a charged
species very stable in degassed basic aqueous solution, and char-
acterized by UVevis spectroscopy. TSPTM trianion radical has the
ability to react in water with ascorbic acid and pyrogallol by elec-
tron transfer processes and it reacts scarcely with catechol. Now we
have focused our attention to test the chemosensor properties of
TSPTM radical with other natural and synthetic antioxidants to
know if radical 1 is able to discriminate catechol or pyrogallol
moieties from those antioxidants.
89 %) as a white ugly solid: UVevis (H2O) lmax/nm (3 )
;/Lmolꢂ1 cmꢂ1
304 (3640), 312 (4305);IR (KBr, cmꢂ1): 3439 (OH), 2800e2900 (CeH,
alf-H),1651 (aromaticring),1311,1289,1203,1118,1055, 807, 786, 731,
699, 612, 509; 1H NMR (D2O, 300 MHz):
d
7.130 (s,1H,
a
-H); 13C NMR
(D2O, 500 MHz)
d 141.4, 140.0, 137.2, 136.4, 133.3, 132.2, 30.8; ES-
HRMS (ꢂ) calcd for [C19HCl12O9S3Na]2ꢂ 458.7434, found m/2
458.7449; calcd for[C19H2Cl12O9S3]2ꢂ 447.7524, found m/2447.7563;
calcd for [C19H2Cl12O6S2]2ꢂ 407.2701, found m/2 407.2717; calcd for
[C19HCl11O6S2]2ꢂ 388.7871, found m/2 388.7886; calcd for
[C19Cl12O9S3]3ꢂ 298,1658, found m/3 [C19Cl12O9S3]3ꢂ 298.1673.
4.6. Synthesis of tris(2,3,5,6-tetrachlorophenyl-4-
hydroxysulfonyl)methyl (TSPTM) radical
To a stirred solution of aH-TSPTM (508 mg; 0.64 mmol) in THF/
4. Experimental
H2O (5:1) (60 mL) at room temperature, an aqueous solution of KOH
(1 M) (2.5 mL) was added and the strong red solution was stirred
overnight. Chloranil (0.528 g; 2.1 mmol) was added in one portion
and the solution was stirred further (6 h). Afterward, the solution
was evaporated at reduced pressure and the residue was digested in
ethyl acetate/methanol (2:1). The insoluble fraction, separated by
filtration and dried, gave a pale red solid (592 mg), which was dis-
solved in MeOH and treated with Amberlite IRA-120 (1.48 g). The
mixture was stirred at room temperature (12 h). The Amberlite was
separated by filtration and the solution was dried to give TSPTM
4.1. General procedures
IR spectra were recorded with a FT-IR spectrophotometer, 1H
and 13C NMR spectra were collected on a 400 MHz spectrometer at
room temperature, electronic spectra with a single cell UVevis
spectrophotometer and the EPR spectra with an EMX-Plus 10/12
spectrometer. Electrospray mass spectra (ESIMS) were recorded on
a LC/MSD-TOF.