(c) T. F. Markle and J. M. Mayer, Angew. Chem., Int. Ed., 2008,
47, 738; (d) C. Costentin, M. Robert, J.-M. Saveant and C. Tard,
Angew. Chem., Int. Ed., 2010, 49, 3803.
5 (a) H. Yin, L. Xu and N. A. Porter, Chem. Rev., 2011, 111, 5944;
(b) R. Amorati, M. Lucarini, V. Mugnaini, G. F. Pedulli, F. Minisci,
F. Recupero, F. Fontana, P. Astolfi and L. Greci, J. Org. Chem.,
2003, 68, 1747; (c) R. Amorati, S. Menichetti, E. Mileo,
G. F. Pedulli and C. Viglianisi, Chem.–Eur. J., 2009, 15, 4402;
(d) G. W. Burton and K. U. Ingold, J. Am. Chem. Soc., 1981,
103, 6472; (e) M. C. Foti, J. Pharm. Pharmacol., 2007, 59, 1673.
6 (a) V. Roginsky and E. A. Lissi, Food Chem., 2005, 92, 235;
(b) M. C. Foti, E. R. Johnson, M. R. Vinqvist, J. S. Wright,
L. R. C. Barclay and K. U. Ingold, J. Org. Chem., 2002, 67, 5190;
(c) M. C. Foti, C. Daquino, I. D. Mackie, G. A. DiLabio and
K. U. Ingold, J. Org. Chem., 2008, 73, 9270; (d) G. Litwinienko
and K. U. Ingold, J. Org. Chem., 2003, 68, 3433; (e) M. C. Foti,
C. Daquino and C. Geraci, J. Org. Chem., 2004, 69, 2309;
(f) M. C. Foti, C. Daquino, G. A. DiLabio and K. U. Ingold,
Org. Lett., 2011, 13, 4826.
Fig. 2 Decay of the 519 nm absorbance of dpphꢀ (0.1 mM) following
the addition of 5 (2.7 mM) in MeCN. The inset shows the observed
rate constants measured during the fast dpphꢀ decay.
7 Phenol 4 was obtained by reductive amination of 5-hydroxy-2-
methoxybenzaldehyde with piperidine. Phenols 5 and 6 were
obtained by Betti reaction of hydroquinone or 4-methoxyphenol
and paraformaldehyde and piperidine. See ESIw for further details.
8 (a) M. C. Foti, R. Amorati, G. F. Pedulli, C. Daquino, D. A. Pratt
and K. U. Ingold, J. Org. Chem., 2010, 75, 4434; (b) R. Amorati,
P. Franchi and G. F. Pedulli, Angew. Chem., Int. Ed., 2007, 46, 6336;
(c) R. Amorati and G. F. Pedulli, Org. Biomol. Chem., 2011, 10, 814.
9 (a) D. W. Snelgrove, J. Lusztyk, J. T. Banks, P. Mulder and
K. U. Ingold, J. Am. Chem. Soc., 2001, 123, 469; (b) R. Amorati
and L. Valgimigli, Org. Biomol. Chem., 2012, 10, 4147;
(c) M. C. Foti, Int. J. Chem. Kinet., 2012, 44, 524.
10 The large difference between the KSEs measured with ROOꢀ and
with dpphꢀ for compound 7 is due to the fact that the oxidizable
substrates (styrene and cumene) are also weak H-bond acceptors.
In the case of dpphꢀ kinetics, pure solvents can be employed.
11 The HAT mechanism has been reformulated.1a See also:
(a) G. A. DiLabio and E. R. Johnson, J. Am. Chem. Soc., 2007,
129, 6199; (b) R. Amorati, P. T. Lynett, L. Valgimigli and D. A. Pratt,
Chem.–Eur. J., 2012, 18, 6370; (c) R. Amorati, G. F. Pedulli,
D. A. Pratt and L. Valgimigli, Chem. Commun., 2010, 46, 5139.
12 (a) W. W. Yao, H. M. Peng, R. Webster and P. M. W. Gill,
J. Phys. Chem. B, 2008, 112, 6847; (b) V. V. Pavlishchuk and
A. W. Addison, Inorg. Chim. Acta, 2000, 298, 97.
13 M. J. Frisch, et al., Gaussian03, RevisionD.02, —full reference in ESIw.
14 P. Mulder, H.-G. Korth, D. A. Pratt, G. A. DiLabio,
L. Valgimigli, G. F. Pedulli and K. U. Ingold, J. Phys. Chem. A,
2005, 109, 2647.
15 (a) E. Solon and A. Bard, J. Am. Chem. Soc., 1964, 86, 1926;
(b) M. C. Foti and C. Daquino, Chem. Commun., 2006, 3252.
16 We exclude involvement of the H-bonded dimer (–OHꢄ ꢄ ꢄN) because
the observed rate constants are too large for being associated with it
which is expected to be present in solution at low concentrations.
17 K3 = Ka(4)/Ka(NH+) where NH+ is the ammonium ion of
Scheme 1 Reaction of 5 and 6 with dpphꢀ in MeCN at 298 K.
In conclusion, our work shows that the dpphꢀ radical
behaves very differently from ROOꢀ in MeCN. The relatively
high Eo of dpphꢀ activates MS-EPT/PT reactions with phenols
5 and 6 or PT–ET with 4 which are not observed with ROOꢀ
because of its low Eo (0.17 V, see Table 2).19,20 In fact, we
calculate from the data in Table 2 that the ET process from 6
to ROOꢀ is endergonic by about 17 kcal molꢁ1 while in the
case of dpphꢀ only by 4.6 kcal molꢁ1. Therefore, caution must
be exercised when the antioxidant ability of phenols is eval-
uated through their reactivity with dpphꢀ in polar solvents.
Our results also show that peroxyl radicals are ineffective in
the H-atom abstraction from phenols H-bonded to N-bases.
This implies that the H-bond that tyrosines may form with
basic residues in proteins protect them from the attack of
peroxyl radicals formed during oxidative stress.
4. Given that in MeCN pKa(4)
E ) E
27 and pKa(NH+
pKa(Et3NH+) = 18.8 then K3 = 6 ꢃ 10ꢁ9; [40] = K31/2[4] E
10ꢁ7 M (for pKa data in MeCN see ref. 18). The characterization of
the transient species formed in MeCN during the reaction between
dpphꢀ and 4–6 by ESR spectroscopy was not possible because of
their low persistency.
18 (a) A. Kutt, I. Leito, I. Kaljurand, L. Soovali, V. M. Vlasov,
¨
¨
L. M. Yagupolskii and I. A. Koppel, J. Org. Chem., 2006, 71, 2829;
(b) I. Kaljurand, A. Kutt, L. Soovali, T. Rodima, V. Maemets,
I. Leito and I. A. Koppel, J. Org. Chem., 2005, 70, 1019.
¨
¨
¨
Notes and references
1 (a) D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy,
C. A. Kent, B. C. Westlake, A. Paul, D. H. Ess, D. G. McCafferty and
T. J. Meyer, Chem. Rev., 2012, 112, 4016; (b) T. J. Meyer, M. H. V.
Huynh and H. H. Thorp, Angew. Chem., Int. Ed., 2007, 46, 5284.
2 S. Y. Reece, J. M. Hodgkiss, J. Stubbe and D. G. Nocera, Philos.
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3 J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010,
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4 (a) T. Maki, Y. Araki, Y. Ishida, O. Onomura and Y. Matsumura,
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H. Nagao, A. G. DiPasquale, O. P. Lam, M. A. Lockwood,
K. Rotter and J. M. Mayer, J. Am. Chem. Soc., 2006, 128, 6075;
19 Eo of ROOꢀ calculated herein is much lower than that reported in
the literature for the cumylperoxyl radical in MeCN (0.90 V
vs. NHE).20 This large Eo of ROOꢀ would imply, however, an
unrealistically high acidity for the hydroperoxide in MeCN since
the ROO–H BDE, Eo and pKa are linked together by Hess’s Law.3
If Eo (ROOꢀ/ROOꢁ) vs. NHE in MeCN was 0.90 V using the value
of ROO–H BDE E 85.6 kcal molꢁ1 in MeCN, then pKa(ROOH)
would be estimated to be ca. 14.6, i.e. more acidic than CF3COOH
(pKa = 20.6 in MeCN) or other common organic acids.18 On the
other hand, our Eo value gives a pKa(ROOH) of 26.9.
20 S. Fukuzumi, K. Shimoosako, T. Suenobu and Y. Watanabe,
J. Am. Chem. Soc., 2003, 125, 9074.
c
11906 Chem. Commun., 2012, 48, 11904–11906
This journal is The Royal Society of Chemistry 2012