Bond dissociation energies of the N-H bond and rate constants for the reaction with alkyl, alkoxyl, and peroxyl radicals of phenothiazines and related compounds

Article abstract of DOI:10.1021/ja992904u

The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compo

Full text of DOI:10.1021/ja992904u

11546
J. Am. Chem. Soc. 1999, 121, 11546-11553
Bond Dissociation Energies of the N-H Bond and Rate Constants for
the Reaction with Alkyl, Alkoxyl, and Peroxyl Radicals of
Phenothiazines and Related Compounds
†
†
,†
†
Marco Lucarini, Pamela Pedrielli, Gian Franco Pedulli,* Luca Valgimigli,
‡
‡
Didier Gigmes, and Paul Tordo
Contribution from the Dipartimento di Chimica Organica “A. Mangini”, UniVersit a` degli Studi di
Bologna, Via S. Donato 15, I-40127 Bologna, Italy, and Laboratoire Structure et R e´ actiVit e´ des Esp e` ces
Paramagn e´ tiques, case 521, CNRS UMR 6517, Chimie, Biologie et Radicaux Libres,
UniVersit e´ s d’Aix-Marseille I et III, Centre de S. J e´ r oˆ me, AV. Escadrille Normandie Niemen,
F-13397 Marseille Cedex 20, France
ReceiVed August 10, 1999. ReVised Manuscript ReceiVed September 27, 1999
Abstract: The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of
phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic
aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N-H group,
persistent aminyl radicals. Equilibration of each of these radicals with the parent amine and a reference compound
having an easily abstractable hydrogen allowed us to determine, by using EPR spectroscopy, the N-H Bond
Dissociation Energies (BDE) of the amines. These are characterized by low BDE values (in some cases lower
than the O-H bond strength of R-tocopherol, i.e 78.3 kcal/mol) and therefore are very good hydrogen atom
transfer reagents. To check the efficiency of tricyclic amines as antioxidants and as polymerization inhibitors,
absolute rate constants were determined for their reaction with alkyl, alkoxyl, and peroxyl radicals by using
competitive techniques in the first two cases and by autoxidation studies under controlled conditions in the
last one. All amines have been found to be highly reactive toward these radicals which makes them very good
autoxidation and polymerization inhibitors.
Introduction
place during the preparation and workup of acrylic monomers
or during their storage.
The mechanism by which aromatic amines behave as au-
toxidation inhibitors in nonacidic media is believed to involve
the initial transfer of a hydrogen atom from the amino group to
the peroxyl radicals carrying on the autoxidative chain reaction
7
,8
Phenothiazines and related derivatives are commonly used
in medicinal chemistry for their neuroleptic and antihistaminic
properties; indeed, molecules bearing the phenothiazine nucleus
such as promazine, chlorpromazine, triflupromazine, prometazine,
1
etc. are widely represented in pharmaceutics of current use.
(eq 1).
Beside experiencing interesting pharmaceutical properties, phe-
nothiazines and, more generally, tricyclic aromatic amines can
act as antioxidants for a wide variety of easily oxidizable
substrates including lubricants, rubber, polymers, and biological
•
•
Ar N-H + ROO f Ar N + ROOH
(1)
2
2
2
-4
materials.
In particular phenothiazine has been found to
9
•
In acidic media an electron transfer to ROO with formation
inhibit the autoxidation of methyl linoleate and phenoxazine to
retard lipid peroxidation in rat brain. It has also been suggested
that the pharmacological activity of phenothiazines might be
somehow related to their antioxidant or radical trapping ability.
Another important application of phenothiazine and related
compounds concerns their use as polymerization inhibitors to
stop the reactions of radical polymerization which may take
of the radical cation of the amine has instead been suggested.
Despite the great potential and practical interest for this class
of molecules not much is known about their homolytic reactivity
and data available in the literature are often contradictory. We
wish to report here the results of a detailed investigation aiming
to assess the thermochemical and kinetic aspects of the reaction
of phenotiazines and related tricyclic aromatic amines toward
three common classes of free radicals: alkyl, alkoxyl, and
peroxyl.
5
6
†
Universit a` degli Studi di Bologna.
Universit e´ s d’Aix-Marseille I et III.
‡
(1) Goodman Gilman, A.; Rall, T. W.; Nies, A. S.; Taylor, P. The
Pharmacological Basis of Therapeutics, 8th ed.; Pergamon Press: New
York, 1990.
Beside phenothiazine (1), the molecules investigated in this
study are phenoxazine (2), phenoselenazine (3), iminostilbene
(
2) Murphy, C. M.; Ravner, H.; Smith, N. L. Ind. Eng. Chem. 1950, 42,
479-2489.
3) Fukuzumi, K.; Ikeda, N.; Egawa, M. J. Am. Oil Chem. Soc. 1976,
3, 623-627.
4) Yamamura, T.; Suzuchi, K.; Yamaguchi, T.; Nishiyama, T. Bull.
Chem. Soc. Jpn. 1997, 70, 413-419.
5) Cini, M.; Fariello, R. G.; Bianchetti, A.; Moretti, A. Neurochem. Res.
994, 19, 283-288.
6) Slater, T. F. Biochem. J. 1968, 106, 155-160.
(
4), diphenylamine (5), some ring-substituted phenothiazines
2
(
(6-10), and the N-methylated cyclic amines 11-13.
5
(
(7) Levy, L. B. J. Polym. Sci. 1985, 23, 1505-1515. Levy, L. B. J.
Polym. Sci., Part A 1992, 30, 569-576.
(8) Nicolson, A. Plant/Oper. Prog. 1991, 10, 171-183.
(9) Kabanova, I. A.; Dubinskaya, A. M.; Yurchenko, N. I.; Gol’denberg,
V. I.; Kinet. Katal. (Engl. Trans.) 1987, 28, 816-821.
(
1
(
1
0.1021/ja992904u CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/24/1999

Bond Dissociation Energies of the N-H Bond
J. Am. Chem. Soc., Vol. 121, No. 49, 1999 11547
•
to a reference radical (A ) by using EPR spectroscopy.
hν
•
BuOOBu
9
8 2BuO
(2)
(3)
•
•
BuO + Ar NH f BuOH + Ar N
2
2
•
•
BuO + AH f BuOH + A
(4)
(5)
k5
•
•
Ar NH + A y z Ar N + AH
2
2
Results
k-5
When phenothiazine (1) was reacted with photolytically
produced alkoxyl radicals in deoxygenated benzene solutions
inside the cavity of an EPR spectrometer the only paramagnetic
product observed was the aminyl radical (1a) originating from
hydrogen abstraction from the N-H group.10 This highly
persistent radical is characterized by a nitrogen coupling constant
of 7.08 G, two large proton splittings (2.80, 3.75 G) due to the
coupling of the unpaired electron with the hydrogens in positions
2
9
kt
•
2Ar N
8 products
(6)
2
2
k′
t
A•
2
98 products
(7)
(8)
2kt′′
•
•
Ar N + A
98 products
2
The reference compound AH may be either a phenol or
another amine whose, previously calibrated, O-H or N-H BDE
1
, 9 and 3, 7, two smaller splittings (1.02, 0.81 G) attributed to
the hydrogens in 2, 8 and 4, 6, and a g-factor of 2.0046.
Oxidation of 1 with m-chloroperbenzoic acid afforded instead
the corresponding nitroxide radical 1b that cannot be mistaken
for 1a since it is characterized by a larger nitrogen (9.23 G)
and smaller proton splittings (2.23 and 0.59 G for hydrogens 1,
-
1
value does not differ by more than 2.5 kcal mol from that of
the investigated amine. The two equilibrating radicals were gen-
erated, within the cavity of an EPR spectrometer, by continuous
photolysis of deoxygenated benzene solutions containing the
amine and the reference compound together with a small amount
of the radical photoinitiator di-tert-butyl peroxide. The overall
reaction scheme is shown in eqs 2-8.
The molar ratio of the two equilibrating radicals [Ar2N ]/
A ] was obtained from the EPR spectra either by double
integration of appropriate lines or by comparison with computer-
simulated spectra when the lines from the two species strongly
overlapped. The equilibrium constant, K5, was determined by
introducing in eq 9
3
(
, 7, 9 and 2, 4, 6, 8, respectively) and a larger g-factor
2.0055).11 A similar behavior was shown by the related amines
2
-5 and by the ring-substituted phenothiazines 6-10. The
•
spectroscopic parameters measured for these radicals under the
•
[
present conditions are reported in the Experimental Section and
are in agreement with data previously reported10 for the majority
of them. In the case of the aminyl radical from iminostilbene
(4), for which no experimental data could be found in the
literature, the assignment of the hyperfine splitting constants to
the ring protons was made by analogy with those for the radicals
derived from 1 and 2.
•
[
Ar N ][AH]
2
0
K )
(9)
5
•
These experiments show that, in nonpolar aprotic media, the
reaction of aromatic amines with alkoxyl radicals leads to
hydrogen abstraction from the N-H bond to form stable and
persistent aminyl radicals. Hydrogen abstraction is also the key
step in the reaction sequences proposed to explain both the
[
A ][Ar NH]
2
0
the initial concentrations of amine, [Ar2NH]0, and reference
compound, [AH]0. To ensure that at the time of measurement
no significant depletion of amine and AH has occurred, high
concentrations of the reactants (0.1-1 M) were used.
Since the values given by eq 9 represent the equilibrium
constant K5 only if the hydrogen transfer reaction (eq 5) takes
place rapidly relative to the decay of the aminyl radicals (eqs
3,4
antioxidant power and the ability to act as polymerization
7,8
inhibitor of these compounds. Since one of the more important
factors determining the ease of this reaction is the N-H bond
strength, we have first measured the nitrogen-hydrogen bond
dissociation energies for derivatives 1-10.
13
6
-8), different experimental conditions were employed by
changing either the initial concentrations of Ar2NH and AH or
the rate of initiation by using different amounts of peroxide or
by partially cutting off the light. In all these measurements the
same value of the equilibrium constant for any given couple of
compounds was obtained within experimental error. In those
•
•
cases where the Ar2N and A radicals were so persistent (for
instance with the couple phenoxazine-BHA shown in Figure
1
) that after interrupting the irradiation no appreciable decay
of the EPR signals was observed for hours, the condition that
equilibration is faster than decay was obviously guaranteed.
The BDE’s were calculated from the equilibrium constants
by means of eq 10 obtained assuming that the entropy change
Bond Dissociation Energies of the N-H Bond. These were
determined by an equilibration method that recently has been
extensively used in our laboratory for the measurement of the
BDE values of the O-H bond of phenols.1
2-14
In the present
BDE(Ar N - H) ) BDE(A - H) + ∆H° =
2
case the method consists of measuring the equilibrium constants
for the hydrogen atom transfer reaction (eq 1) from an amine
BDE(A - H) - RT ln K (10)
5
(
10) Clarke, D.; Gilbert, B. C.; Hanson, P. J. Chem. Soc., Perkin Trans.
1977, 517-525.
11) Chiu, M. F.; Gilbert, B. C.; Hanson, P. J. Chem. Soc. (B) 1970,
700. Neugebauer, F. A.; Bamberger, S. Chem. Ber. 1974, 107, 2362.
∆S° for the hydrogen transfer reaction is negligible. Since the
2
(
(12) Lucarini, M.; Pedulli, G. F.; Cipollone, M. J. Org. Chem. 1994,
59, 5063-5070.
1

11548 J. Am. Chem. Soc., Vol. 121, No. 49, 1999
Lucarini et al.
Reactivity toward Alkyl Radicals. The rate of hydrogen
abstraction by primary alkyl radicals in benzene has been
measured at 298 K by the radical clock method. This is based
on competition kinetics between a known monomolecular
process used as reference and the bimolecular reaction to be
2
1
timed. The competing monomolecular processes employed
22
were the 5-hexo-cyclization of the 1-hexenyl radical, kr ) 2.3
5
-1
23,24
×
10 s at 298 K, the neophyl radical rearrangement,
kr
3
-1
)
1.1 × 10 s at 298 K, and the similar 1,2-aryl migration of
2
-methyl-2-(2-naphthyl)-1-propyl radical that has been recently
25
calibrated in our laboratory and which provided a rate constant
4
-1
of kr ) 1.4 × 10 s at 298 K, thus intermediate between those
Figure 1. Room temperature EPR spectrum of the radicals obtained
of the two other clocks employed.
-
2
from a benzene solution of phenoxazine (2) (1 × 10 M) and 2,6-di-
-
2
tert-butyl-4-methoxyphenol (BHA) (5.4 × 10 M) containing some
di-tert-butyl peroxide (0.1 M), irradiated for a few seconds.
hν
•
Bu SnSnBu
9
8 2Bu Sn
(11)
(12)
3
3
3
Table 1. Bond Dissociation Energies of Aromatic Amines in
Benzene Solutions
•
•
Bu Sn + RBr f Bu SnBr + R
3
3
BDE/kcal mol-
1
ref 15
amine
R
1
, R
9
R
3
, R
7
kr
R•
98 R′
•
(13)
1
2
3
4
5
6
7
8
9
79.3 ( 0.3
77.2 ( 0.3
80.4 ( 0.4
82.4 ( 0.5
85.8 ( 0.7
77.7 ( 0.4
76.2 ( 0.3
78.1 ( 0.4
79.8 ( 0.4
81.0 ( 1.0
82.3
79.7
kH
9
•
•
Ar NH + R
8 Ar N + RH
(14)
(15)
2
2
84.6
87.5
•
•
Me
Ar NH + R′ f Ar N + R′H
2 2
OMe
CMe
Cl
3
The alkyl radicals were generated by photolyzing oxygen free
solutions of the corresponding bromide in the presence of
hexabutyldistannane that were then left to react with the aromatic
amines (eqs 11-15). Experimental conditions were chosen to
avoid significant consumption of the phenothiazines during the
reaction
1
0
NO
2
validity of this assumption is not warranted, especially when
the equilibrating couple is made by an amine and a phenol, we
studied the temperature dependence of the equilibrium constant
K5 in the case of the representative couple phenothiazine and
R-tocopherol in the temperature range 290-345 K. The results
[
RH]
k [Ar NH] ) k
r
(16)
H
2
[R′H]
-
1
of these measurements, ∆Η° ) -1.13 ( 0.83 kcal mol and
S° ) -1.25 ( 2.98 cal mol-1 K , clearly indicate that the
-1
The reaction products RH and R′H were analyzed by means
of GC and the rate constants for hydrogen abstraction, kH, were
obtained by using eq 16. When using the neophyl and the
2-methyl-2-(2-naphthyl)-1-propyl radical clocks, besides tert-
butylbenzene and isobutylbenzene or 2-tert-butylnaphthalene
and 2-isobutylnaphthalene, very minor side products, identified
on the basis of their mass spectra as 1-phenyl-2-methylpropene
and 1-(2-naphthyl)-2-methylpropene, respectively, were also
detected. These are known to arise from disproportionation of
∆
entropic contribution is so small that it can be safely neglected,
similarly to what was previously found for equilibrating
1
2
phenols.
The BDE values obtained by the above procedure are
collected in Table 1, which also reports for comparison some
data available in the literature which were calculated by
Bordwell and co-workers15 from thermochemical cycles by using
pKa values and reversible oxidation potentials determined by
cyclic voltammetry. The present values are lower by about 2 to
•
24,25
the rearranged radicals R′ ,
and their concentration has been
considered in the denominator of eq 16 when calculating the
rate constant for hydrogen abstraction from the amine. In all
cases the measured product ratio was linear with the amino
substrate concentration so that rate constants for hydrogen atom
3
kcal/mol with respect to those data.
An examination of Table 1 and of its pictorial equivalent
Figure 2) shows that phenothiazine (1), phenoxazine (2), and,
to a lesser extent, phenoselenazine (3) have low N-H bond
dissociation energies even when compared with the most active
radical trapping phenols such as R-tocopherol, galvinol, and
,4,6-trimethoxyphenol. Amines lacking a heteroatom like
iminostilbene (4) and diphenylamine (5) are instead character-
ized by stronger nitrogen-hydrogen bonds.
It is also worth pointing out that substitution of the ring
hydrogens of phenothiazine with electron-donating groups
significantly decreases the BDE value, while electron withdraw-
ing groups make hydrogen abstraction more energetically
(
(16) Burton, G. W.; Doba, T.; Gabe, E. J.; Hughes, L.; Lee, F. L.; Prasad,
L.; Ingold, K. U. J. Am. Chem. Soc. 1985, 107, 7053-7065.
(
17) Bordwell, F. G.; Cheng, J. P. J. Am. Chem. Soc. 1991, 113, 1736-
2
1
743. Bordwell, F. G.; Zhang, X. M. J. Org. Chem. 1990, 55, 6078-6079.
(18) Bordwell, F. G.; Zhang, X.-M.; Satish, A. V. J. Am. Chem. Soc.
1994, 116, 6605-6610. Bordwell, F. G.; Zhang, X.-M. J. Phys. Org. Chem.
995, 8, 529-535.
19) Lind, J.; Shen, X.; Eriksen, T. E.; Mer e´ nyi, G. J. Am. Chem. Soc.
1
(
1990, 112, 479-482.
(
(
(
20) Arnett, E. M.; Flowers, R. A. Chem. Soc. ReV. 1993, 9.
21) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317-323.
22) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc.
13,16-20
demanding, similarly to what was observed with phenols.
1
981, 103, 7739.
(
23) Franz, J. A.; Barrows, R. D.; Camaioni, D. M. J. Am. Chem. Soc.
(
13) Lucarini, M.; Pedrielli, P.; Pedulli, G. F.; Cabiddu, S.; Fattuoni, C.
1984, 106, 3964-3967.
J. Org. Chem. 1996, 61, 9259-9263.
(24) Burton, A.; Ingold, K. U.; Walton, J. C. J. Org. Chem. 1996, 61,
3778-3782.
(
14) Pedrielli, P.; Pedulli, G. F. Gazzetta 1997, 127, 509-512.
(15) Bordwell, F. G.; Zhang, X. M.; Cheng, J. P. J. Org. Chem. 1993,
(25) Leardini, R.; Lucarini, M.; Pedulli, G. F.; Valgimigli, L. J. Org.
Chem. 1999, 64, 3726-3730.
5
8, 6410-6416.

Bond Dissociation Energies of the N-H Bond
J. Am. Chem. Soc., Vol. 121, No. 49, 1999 11549
double double bond of olefines. A large number of these data
have been measured by Fischer and were found in the range
4
6
-1 -1 28
1
0 -10 M s . Since the rate constants for the addition to
double bonds are comparable or larger than the rates of hydrogen
transfer to alkyl radicals, only in a few cases can the polym-
erization be efficiently inhibited by small amounts of phenothi-
azines or phenols as stabilizing agents. This is in agreement
7
,8
with the reported finding that the majority of these inhibitors
are more efficient in the presence of some oxygen. Under these
conditions, the few alkyl radicals spontaneously produced during
the storage or distillation of the monomers will be transformed
in the corresponding peroxyl radicals which will both add much
more slowly to the double bonds and react much faster with
phenothiazines or phenols.
It is also worth noting that the rate constant, kH, for the
abstraction of a hydrogen atom from the methyl group of the
N-methylated amines 11-12 is similar to those measured for
the hydrogen abstraction from the N-H group of amines 1-3
and 6-9.
Reactivity toward Alkoxyl Radicals. The rate of hydrogen
abstraction from the investigated aromatic amines by alkoxyl
radicals has been studied by competition kinetics using the
hydrogen abstraction from (TMS)SiH for which the rate constant
for the reaction with tert-butoxyl radicals has been reported as
1 × 10 M s . The tert-butoxyl radicals were generated
by UV irradiation at 298 K of the reaction mixture containing
di-tert-butyl peroxide, an internal standard, and different
amounts of the silane and of the cyclic amine.
8
-1 -1 29
Figure 2. BDE values of the N-H bond of aromatic amines in benzene
solution.
Table 2. Absolute Rate Constants for the Abstraction of the N-H
Hydrogen Atom from Aromatic Amines by Primary Alkyl Radicals
•
•
(
R ) and the tert-Butoxyl Radical (Me
3
CO ) at 298 K in Benzene
The mixture was analyzed by GC and CG-MS before and
after irradiation and the desired rate constants were obtained
Solution
3
0
(M-1 s-1) (R•)
clocka
(M-1 s-1) (Me CO•)
3
from the loss of the starting hydrogen donors. The results,
reported in Table 2, clearly show that these molecules,
consistently with their low values of N-H bond dissociation
energies, are very reactive toward hydrogen abstraction by
butoxyl radicals, their rate constants being similar to those
measured for the most reactive phenols and close to the diffusion
control limit.
amine
k
H
k
H
4
9
1
2
3
4
5
6
7
8
9
1
1
6.0 × 10
â-L, H
2.3 × 10
5
9
4.8 × 10
H
N
1.1 × 10
4
9
2.2 × 10
1.1 × 10
8
7.8 × 10
6
4
6
4
4
4
5
5
b
1.3 × 10
2.1 × 10
2.7 × 10
8.0 × 10
3.6 × 10
1.6 × 10
1.1 × 10
6.0 × 10
N
â-L
Η
â-L
â-L
â-L
H
9
1.0 × 10
9
1.9 × 10
•
•
BuO + Ar NH f BuOH + Ar N
(17)
2
2
1
2
•
•
BuO + (TMS) Si-H f BuOH + (TMS) Si (18)
c
9 d
3
3
R-tocopherol
3.1 × 10
a
H ) 1-hexenyl; N ) neophyl; â-L ) 2-methyl-2-(2-naphthyl)-1-
•
propyl radical. b Measured at 373 K (see ref 24). c Reference 26.
Ar N f products
(19)
(20)
2
d
Reference 27.
•
(
TMS) Si f products
3
abstraction could be obtained straightforwardly. With diphen-
ylamine (5) data were taken from a previous paper by Burton
et al. reporting a kinetic study of the reaction of alkyl radicals
with several 4-substituted diphenylamines.
In principle these measurements might be affected by the back
formation of some amine from the aminyl radical by reaction
with the silane (eq 21) which is a good hydrogen donor. Thus,
the amount of residual amine actually determined might be
larger and that of the silane lower than expected on the basis of
reactions 17 and 18 only.
24
The results of these measurements, reported in Table 2, show
that phenothiazines and related compounds, being characterized
4
5
-1 -1
by kH values in the range 10 -10 M s , are good trapping
agents for alkyl radicals similar to the most effective phenols
whose reactivity toward alkyl radicals has been recently
measured in our laboratory by the same experimental tech-
nique.26 For instance, the room temperature rate constants of
hydrogen abstraction from R-tocopherol and 2,4,6-trimethylphe-
•
•
Ar N + (TMS) Si-H f Ar NH + (TMS) Si (21)
2
3
2
3
To check the importance of reaction 21, the evolution with
time of the EPR signal of the phenothiazinyl radical (1a) was
5
4
-1 -1
nol in benzene are 6.0 × 10 and 6.4 × 10
M
s ,
followed both in the absence and in the presence of (TMS) SiH.
3
respectively.
(27) Valgimigli, L.; Banks, J. T.; Ingold, K. U.; Lusztyk, J. J. Am. Chem.
To predict the efficiency of these aromatic amines as
polymerization inhibitors these rate constants should be com-
pared to the rate of addition of carbon-centered radicals to the
Soc. 1996, 117, 9966-9971.
(28) Fischer, H. In Free Radicals in Biology and EnVironment; Minisci,
F., Ed.; NATO ASI Series; Kluwer Academic Publishers: Dordrecht, 1997;
pp 63-78.
(26) Franchi, P.; Lucarini, M.; Pedulli, G. F.; Valgimigli, L.; Lunelli, B.
(29) Chatgilialoglu, C.; Rossini, S. Bull. Soc. Chim. Fr. 1988, 298.
(30) Ingold, C. K.; Shaw, F. R. J. Chem. Soc. 1927, 2918.
J. Am. Chem. Soc. 1999, 121, 507-514.

11550 J. Am. Chem. Soc., Vol. 121, No. 49, 1999
Lucarini et al.
Since no substantial decay was found, also in the presence of
silane up to a concentration ca. 0.1 M, we may be confident
that the rate constants reported in Table 2 are correct.
•
•
2
BuO + Ar NCH BuOH + Ar NCH
(22)
2
3
2
No data are instead reported for the reaction of tert-butoxyl
radicals with N-methylated amines (eq 22) since, in this case,
hydrogen atom transfer from the silane to the aminoalkyl radical
cannot be neglected.
Reactivity toward Peroxyl Radicals. The rate constants for
the reaction of phenothiazines and related derivatives with
peroxyl radicals (eq 23) were obtained by studying the inhibition
by these compounds of the thermally initiated autoxidation of
styrene and measuring the rate of oxygen uptake during the
induction period where the autoxidation is strongly retarded by
the aromatic amine.
Figure 3. Oxygen consumption observed during the AIBN initiated
autoxidation at 50 °C of styrene in benzene in the presence of 1.0 ×
-
5
10 M phenoxazine 2 (b) and of its N-deuterated analogue.
•
•
-d[O2] k R [styrene]
p
i
ROO + Ar NH f ROOH + Ar N
(23)
2
2
)
(24)
dt
nk [Ar NH]
inh 2
The AIBN initiated autoxidation of styrene was carried out
at 50 °C in a sealed quarz tube containing an air-saturated
solution of styrene in benzene, in the presence of various
amounts of the tricyclic compounds or of R-tocopherol, which
was used as a reference chain breaking antioxidant. The reaction
was allowed to occur inside the thermostated cavity of an EPR
spectrometer and the oxygen consumption in solution was
During autoxidation a broad EPR spectrum developed reach-
ing a maximum approximately at the end of the induction period.
The intensity of this spectrum remains constant during the period
where oxygen is rapidly consumed and finally rapidly decays
near the end of the autoxidation, when the concentration of
oxygen becomes negligible. This spectrum could be unambigu-
ously assigned to the nitroxide radical from the tricyclic amine,
which reaches a concentration never larger than 15% of the
concentration of the starting amine. Since its time evolution
seems to indicate that the nitroxide is not responsible for the
antioxidant activity of these amines, its role, if any, in the
inhibition process is not clear.
3
1
monitored by EPR, as described in previous papers, by
following the variation with time of the spectral line width of
a nitroxide spin probe added to the solution. The spin probe
employed was tetramethylpiperidine N-oxide (TEMPO), which
was added to the reaction mixture in a concentration low enough
-
6
-5
(
usually in the range 5 × 10 to 1 × 10 M) to avoid
interference with the autoxidation. The three lines of TEMPO,
which are initially broadened by Heisenberg spin exchange with
molecular oxygen, become sharper as the concentration of
oxygen decreases. Since both the EPR line width and the square
root of the signal height are linearly related with the oxygen
concentration in solution, the rate of oxygen consumption can
be easily measured by this method. Experimentally, the signal
intensity of the first spectral line of TEMPO was measured at
fixed time intervals and was then converted into the molar
concentration of dissolved oxygen by a calibration curve. An
advantage of this method, besides its simplicity, is that the
formation of transient paramagnetic species during the different
stages of autoxidation may in some cases be directly detected.
As an example, Figure 3 shows the time dependence of the
oxygen concentration during the initiated autoxidation of styrene
in the presence of phenoxazine (2) as autoxidation inhibitor. A
well-defined inhibition period is initially observed, whose length
depends on the initiation rate, Ri, and on the concentration and
stoichiometric factor n of the antioxidant,32 the latter being the
number of peroxyl radicals trapped by one molecule of
antioxidant. The slope of the plot during the inhibition period,
given by eq 24, provides the ratio of the rate constant for
hydrogen abstraction by the peroxyl radicals (kinh) and the rate
constant for the propagation of the autoxidative chain reaction
The absolute values of the rate constant for the hydrogen atom
abstraction by peroxyl radicals from the amines were obtained
by comparing the rate of autoxidation of styrene in the presence
of a given amine with that measured with R-tocopherol as
inhibitor and using as reference for kinh(R-TOH) the value of
6
-1 -1
16
3.2 × 10 M
s
reported at 30 °C by Ingold and co-workers.
Since, however, our measurements were performed at 50 °C,
the above value was recalculated as 4.1 × 10 M
6
-1 -1
s
at the
latter temperature by assuming a log A of 8 for this bimolecular
reaction. The results of the determinations, reported in Table
3, show that phenothiazine is an excellent antioxidant being
twice as fast as R-tocopherol in trapping peroxyl radicals and
being characterized by a stoichiometric factor very close to two.
Even the other heteroaromatic amines behaved as extremely
effective antioxidants. Indeed, phenoselenazine (3), which was
the least active, showed about the same reactivity toward peroxyl
radicals as R-tocopherol. Substitution of the protons in positions
3 and 7 with electron donors, such as methoxy and tert-butyl
groups, makes the reaction with peroxyl radicals even faster.
In the case of iminostilbene (4) and diphenylamine (5), which
behaved instead as relatively poor antioxidants, no induction
period could be observed when they were used as inhibitors of
styrene autoxidation. In these cases the inhibition rate constants
were obtained by making measurements at different amine
concentration and analyzing the data using the method proposed
(kp).
3
3
by Darley-Usmar et al.
(
31) Cipollone, M.; Di Palma, C.; Pedulli, G. F. Appl. Magn. Reson.
The most striking result was, however, obtained with phe-
1
992, 3, 99. Pedulli G. F.; Lucarini, M.; Pedrielli, P.; Sagrini, M.; Cipollone,
7
M. Res. Chem. Intermed. 1996, 22, 1. Lucarini, M.; Pedulli, G. F.;
Valgimigli, L. J. Org. Chem. 1998, 63, 4497-4499.
noxazine (2) for which a rate constant as high as 2.9 × 10
(32) Howard, J. A. In Free Radicals; Kochi, J. K., Ed.; Wiley: New
(33) Darley-Usmar, V. M.; Hersey, A.; Garland, L. G. Biochem. Pharm.
1989, 38, 1465-1469.
York, 1973; Chapter 12, Vol. 2.

Bond Dissociation Energies of the N-H Bond
J. Am. Chem. Soc., Vol. 121, No. 49, 1999 11551
3
4
Table 3. Stoichiometric Factors, n, and Rate Constants for
Hydrogen Atom Transfer to Peroxyl Radicals, kinh, of Aromatic
Amines Determined at 50 °C in Benzene Solution
assumed the rate constant reported for benzylperoxyl radicals
8
-1 -1
-1 -1
is 3 × 10 M s , which gives kp ) 60 M s , i.e., a value
several orders of magnitude lower than that obtained for the
arylamines 1-9.
amine
n
k
inh/M- s-1
1
amine
n
k
inh/M-1 s-1
7
6
1
2
3
4
5
6
1.8 8.8 × 10
7
8
9
11
2.0 5.0 × 10
7
6
4
4
7
6
5
2.9 × 10
1.6 1.2 × 10
Discussion
2.5 3.2 × 10
3.7 × 10
1.8 5.1 × 10
a
a
- - 60b
The use of tricyclic heteroaromatic amines as both autoxi-
dation and polymerization inhibitors has been described in the
literature since 1950. This behavior can be well understood on
the basis of the experimental data of Table 1 reporting the N-H
bond dissociation energy values for this class of compounds.
Actually bond strengths lower than 80 kcal mol were always
found, and with phenoxazine and some ring-substituted phe-
nothiazines (6-8) the nitrogen-hydrogen bond is even weaker
6
1.5 × 10
R-tocopherol
2
4.1 × 10
5
1.7 (1.4 ( 0.7) × 10
Determined from the slope of the inhibited autoxidation by using
the method of ref 33. Determined from the oxidizability of 11 (see
text). The value reported represents a higher limit for kinh
a
b
-
1
.
Table 4. Reversible Oxidation Potentials of Amines 1 and 7 and
N-Alkylated Amines 11 and 13, Measured in ACN vs SCE
-
1 12,13
than the O-H bond in R-tocopherol (78.2 kcal mol ),
of the more effective phenolic antioxidants known.
one
1
7
11
13
E1/2(ox)/V
0.58
0.36
0.68
0.47
Besides the base structure, the presence of substituents in
positions conjugated with the heterocyclic nitrogen also plays
an important role in determining the strength of the N-H bond,
electron-donating groups, such as alkyls or alkoxyls, producing
a weakening of this bond and electron-withdrawing groups, such
as chlorine and NO2, a strengthening of the bond. This behavior,
similar to that found in phenols, can be attributed to the
conjugative effect of the substituent which leads to destabiliza-
tion of the starting amine with electron donors and to stabiliza-
tion of it with electron acceptors. The aminyl radical, formed
by hydrogen atom abstraction from the amine, is expected to
be slightly stabilized, through delocalization of the unpaired
M-1 s-1 was obtained (seven times larger than for R-tocopherol)
and a stoichiometric factor of 5. At present we have no
reasonable explanation of this value, since all other compounds
show stoichiometric factors close to 2, as expected. This high
value of n was always obtained when using solutions of the
inhibitor freshly prepared, while with older samples lower values
were sometime obtained, presumably because of depletion of 2
by air oxidation.
The very high values of kinh obtained for tricyclic aromatic
amines might suggest that some process other than hydrogen
abstraction is responsible for their antioxidant activity. Particu-
larly, electron transfer from amines to peroxyl radicals with
formation of peroxyl anions and of the amine radical cations
could be involved, even though benzene is not the more
appropriate solvent for this process to take place. To check this
possibility the autoxidation experiments were carried out in the
presence of the N-methylated derivatives 11-13. These com-
pounds, having oxidation potentials (see Table 4) determined
by cyclic voltammetry, very close to those of the parent
protonated amines, should display a similar antioxidant activity
if electron transfer is the key step in the inhibition reaction.
Indeed, none of the N-methylated derivatives were able to inhibit
the AIBN initiated autoxidation of styrene, when used in
concentrations comparable to those of 1-10, this indicating that
the presence of the N-H hydrogen is a necessary prerequisite
to observe antioxidant activity with these cyclic amines.
Additional support for the conclusion that kinh represents the
rate of hydrogen transfer from amines to peroxyl radicals was
obtained by repeating styrene autoxidation experiments using
N-deuterated phenoxazine as antioxidant. As shown in Figure
1
2-20
electron, by both donors and acceptors.
Kinetically these amines exhibit an extraordinarily high
reactivity toward peroxyl radicals, which makes them very good
autoxidation inhibitors. Some of them are characterized by
inhibition rate constants even larger than that of R-tocopherol.
The data of Tables 2 and 3 allow us to rationalize also the reason
phenothiazines are good polymerization inhibitors especially in
the presence of small amounts of air. Under these conditions
the oxygen present will react at a diffusion-controlled rate with
the alkyl radicals spontaneously produced during the handling
of monomeric olefines, transforming them into the correspond-
ing peroxyl radicals which will both add more slowly to the
double bonds to polymerize and react faster with phenothiazines.
A comparison of the rate constants for hydrogen abstraction
from the tricyclic amines by peroxyl radicals and by primary
alkyl radicals provides evidence that the exothermicity of the
H-transfer is not the only important factor determining the rates
of these processes. The rate constants for the reaction of the
amines with peroxyl radicals (where a ROO-H bond having a
-
1
strength of 89 kcal mol is formed) are about 2 orders of
magnitude larger than those for the reaction with primary alkyl
radicals (where the RCH2-H bond formed has a strength of
3
, the deuterated compound is a worse inhibitor than the
protonated one; from the slopes of the inhibited traces of oxygen
consumption a deuterium kinetic isotope effect (kH/kD) of 4.2
-
1
1
00 kcal mol ) despite the fact that the latter reaction is more
-1 35
exothermic by 11 kcal mol . The higher reactivity of peroxyl
radicals toward the N-H bond of amines can be instead
explained, according to the theory proposed by Zavitsas,36 on
the basis of the magnitude of the triplet repulsion terms in the
transition state which will be lower for H atom transfer between
oxygen and nitrogen rather than between oxygen and carbon.
A lower energy of activation is expected in the former case
was obtained, a value very close to that found by Ingold and
co-workers for R-tocopherol.16
To obtain the value of the rate constant for hydrogen transfer
from N-alkylated phenothiazine to peroxyl radicals we studied
the autoxidation reaction of the N-methyl derivative 11 initiated
by AIBN. From the rate of oxygen consumption, determined
in benzene at 50 °C by the EPR method described above, we
32
1/2
-3
could obtain the oxidizability, kp/(2kt) , of 11 as 3.5 × 10
(
34) Howard, J. A.; Ingold, K. U. Can. J. Chem. 1967, 45, 793.
Kenisberg, T. P.; Arika, N.; Mitskevich, N. I. Dokl. Akad., Nauk, SSSR
1980, 24, 817.
-
1/2 -1/2
M
s
. Here kp, i.e., the rate constant of propagation, is
the rate constant for hydrogen atom transfer from the oxidizable
•
(35) Kerr, J. A. In Handbook of Chemistry and Physics, 74th ed.; CRC
Press: Boca Raton, 1993; pp 9-123.
substrate 11 to the related peroxyl radical Ar2NCH2OO , while
2
kt is the rate constant for the bimolecular decay of the peroxyl
(
36) Zavitsas, A. A. J. Am. Chem. Soc. 1972, 94, 2779-2789. Zavitsas,
radicals from 11. As an upper limit of the latter one it was
A. A. J. Am. Chem. Soc. 1991, 113, 4755.

11552 J. Am. Chem. Soc., Vol. 121, No. 49, 1999
Lucarini et al.
Figure 4. Plot of the logarithm of the rate constants for the hydrogen transfer from amines to alkyl (left) and peroxyl (right) radicals versus the
corresponding N-H bond dissociation energies.
3
8
because the N-O antibonding repulsion would be low as a result
of the weak R2N-OOR bond (the BDE value is not known but
is believed to be smaller than for the R2N-OR bond, i.e., 30-
chased from Aldrich and were used as received. Phenoselenazine,
39
40
3
,7-dimethoxyphenothiazine, 3,7-di-tert-butylphenothiazine, 3,7-
dichlorophenothiazine, 3,7-dinitrophenothiazine, and 10-methylphe-
41
42
noxazin,43 were prepared according to literature procedures.
-
1 37
4
0 kcal mol ). On the other hand, the N-C antibonding
1
,9-Dimethylphenothiazine. A solution of 1,1′-dimethyldiphenyl-
amine (5 g; 0.025 mol) prepared as previously described, sulfur (1.6
repulsion should be important given the high BDE value of the
4
4
-
1
NH2-CH3 bond (84.6 kcal mol ).
-
3
g; 0.05 mol), and iodine (0. 175 g; 0.7 × 10 mol) dissolved in 9 mL
of o-dichlorobenzene is refluxed for 4 h, under inert atmosphere. During
the reaction hydrogen sulfide develops. The o-dichlorobenzene is
eliminated by distillation under reduced pressure (40 °C, 1 mbar).
Column chromatography of the solid residue (eluant: acetone/pentane
It is even more remarkable that the rate constant for hydrogen
abstraction from the CH3 group of the N-methylated phenothi-
-1 -1
azine 11 (kH e 60 M s ) by peroxyl radicals is instead about
orders of magnitude lower than that for the reaction with alkyl
3
4
-1 -1
radicals (kH ) 1.6 × 10 M s ). This behavior, reversed with
1/12) affords 1,9-dimethylphenothiazine (1 g; 18%). Mp 129 °C. R
f
+
+
0
9
7
6
.6. SM (70 eV): m/e (rel intensity) 227 (M , 100%), 194 (M - SH,
5%). Anal. Calcd for C14 13NS: C 73.98; H 5.77; N 6.17. Found: C
3.89; H 5.84; N 6.11. NMR H (200 MHz, (CD
H), 6.2 (s, 1H), 2.3 (s, 6H). NMR 13C (50.32 MHz; (CD
respect to the reactivity of the same radicals with phenothiazine
H
1
, can be explained by admitting that in this case the relative
1
)
3 2
SO), δ 7.1-6.4 (m,
SO), δ 141
kinetic constants are basically determined by the exothermicity
of the process, which is larger for the reaction with alkyl
radicals, rather than by a different contribution to the transition
state energy of the triplet repulsion term. This estimate is based
on the reported BDE values for the RCH2-CH2R (86.6 kcal
3 2
)
(
2C), 129.7 (2CH), 125.1 (2CH), 123.3 (2C), 122.8 (2CH); 118.9 (2C),
1
6.9 (2CH
0-Methyl-3,7-dimethoxyphenothiazine. Dimethyl sulfate (0.62 g;
.9 mmol) is added to a dioxane solution (8 mL) containing 3,7-
dimethoxyphenothiazine (0.4 g; 1.54 mmol) and potassium carbonate
(1.55 g; 13.1 mmol). After this solution is stirred under reflux for 3.5
h, more dimethyl sulfate (0.4 g; 3.17 mmol) is added and the reaction
mixture is refluxed for another 20 h. Then the mixture is poured into
warm water (20m1) and stirred for 8 h. The precipitate formed is
collected by filtration, dried, and purified by silica column chroma-
tography (chloroform) to give 10-methyl-3,7-dimethoxyphenothiazine
H15NO S (273): C 65.93; H
2
.49; N 5.13. Found: C 65.72; H 5.56; N 5.07. NMR H (100 MHz,
DMSO), δ 6.79 (s, 6H), 3.68 (s, 6H), 3.20 (s, 3H). NMR C (50.32
MHz, DMSO), δ 154.66 (2C), 139.36 (2C), 123.13 (2C), 114.70 (2CH),
3
).
1
4
-1
-1
37
mol ) and RCH2-OOR (72.6 kcal mol ) bonds which, being
of comparable strengths, are expected to give rise to antibonding
repulsions of similar magnitude in the transition states of the
two reactions.
Finally, to check if the kinetic constants for hydrogen transfer
to peroxyl, alkyl, and alkoxyl radicals are linearly correlated
with the corresponding bond dissociation energies as in phenolic
(
5
0.18 g; 43%). Mp 168 °C. Calcd for C15
antioxidants,1
3,26
the logarithm of the rate constants for the
1
reaction of these amines with the three classes of radicals was
plotted against the N-H BDE values.
13
As shown in Figure 4, the correlation between kinetic and
thermodynamic data is excellent when the attacking species are
peroxyl and alkyl radicals, with a couple of exceptions, i.e.,
3 3
112.79 (2CH), 112.41 (2CH), 55.44 (2OC-H ), 35.08 (CH ).
Kinetic Measurements. In a typical experiment 200 µL of a solution
of the amine (0.1-1 M) containing either neophyl bromide or 6-bromo-
1
-hexene (0.005-0.01 M) and hexa-n-butylditin (0.01 M) was sealed
1,9-dimethylphenothiazine (6) and iminostilbene (4) where steric
in a quartz tube, after being deoxygenated by bubbling nitrogen. The
reaction mixtures were then irradiated for 30-120 min at the desired
temperature in a thermostated photoreactor, built in our laboratories,
equipped with a 125 W high-pressure mercury lamp and the products
were analyzed by gas chromatography. For each amine 4 to 7
measurements were made with different concentration and the reaction
products ratio [UH]/[RH] was plotted versus the amine concentration
to obtain the k /k ratio by linear regression of the experimental data.
crowding due to the methyl groups in the peri positions and
the distorted geometry adopted to accommodate the seven-
membered ring, respectively, are expected to slow the approach
of the attacking radical to the N-H hydrogen atom. The same
correlation does not hold with alkoxyl radicals whose reactivity
toward phenothiazines seems independent of the energy of the
bond to be broken (plot not shown). The more obvious
explanation for this behavior is that the very high rate constants
characterizing this reaction are essentially controlled by diffusion
of the reactants in solution and therefore independent of the
BDE of the N-H bond being broken.
H
r
(
38) Podolesov, B. D.; Jordanovska, V. B. Croat. Chem. Acta 1970, 42,
1-63.
39) Craig, J. C.; Rogers, W. P.; Warwick, G. P. Aust. J. Chem. 1955,
6
(
8, 252-257.
(40) Greci, L.; Mar’in, A.; Stipa, P.; Carloni, P. Polym. Degrad. Stab.
1
995, 50, 305-312.
Experimental Section
(
41) Strell, M.; Rupprecht, M. German Patent No. 938669, 1956; Chem.
Abstr. 1959, 57, 8173a.
Materials. Solvents were of the highest purity grade commercially
available and were used as received. Phenothiazine, phenoxazine,
iminostilbene, diphenylamine, and 10-methylphenothiazine were pur-
(
42) Bodea, C.; Raileanu, M. Studii Cercetari Chim. 1957, 8, 303-313;
Chem. Abstr. 1960, 58, 22657 g.
(43) M u¨ ller, P.; Buu-Ho ¨ı , N. P.; Rips, R. J. Org. Chem. 1959, 24, 37-
9.
3
(
37) Zavitsas, A. A.; Chatgilialoglu, C. J. Am. Chem. Soc. 1995, 117,
(44) Frye, C. L.; Vincent, G. A.; Hauschildt, G. L. J. Am. Chem. Soc.
1966, 88, 2727-2730.
1
0645-10654.

Bond Dissociation Energies of the N-H Bond
J. Am. Chem. Soc., Vol. 121, No. 49, 1999 11553
Autoxidation experiments were performed on air-saturated solutions
iterative least-squares fitting procedure based on the systematic
application of the Monte Carlo method was performed to obtain the
experimental spectral parameters of the two species including their
relative intensities.
Cyclic Voltammetry. Oxidation potentials were measured in deoxy-
of styrene (1.5-5.2 M) in benzene containing the desired amine (5 ×
-
6
-4
-3
-2
1
0
to 1 × 10 M), AIBN (1 × 10 to 4 × 10 M) as thermal
-6
12
initiator, and the stable nitroxide TEMPO (ca. 5 × 10 M). The
solutions were introduced (ca. 200 µL) into a capillary tube with the
internal diameter of ca. 1.85 mm and a second capillary tube (external
diameter of 1.60 mm) sealed at one end was introduced into the sample
tube to leave very little dead volume space. The tube was sealed and
put into the EPR cavity kept at 50 °C and the first spectrum was
recorded after ca. 1 min to allow for temperature equilibration time.
genated anhydrous acetonitrile containing tetrabutylammonium hexa-
-
1
fluorophosphate (0.1 M) as supporting electrolyte and the amine (10
3
1
M). Cyclic voltammograms were recorded using a three-electrode
potentiostat coupled to a digital acquisition system based on a personal
computer. The voltammetric experiments were carried out in a standard
cell fitted with a working platinum disk electrode (Tacussel EDI,
diameter:2 mm), a platinum wire auxiliary, and a SCE reference
electrode (Tacussel XR 110). The potentiostat and the dedicated
software were designed by G. Gronchi and Y. Berchadsky in the
“Laboratoire Structure et R e´ activit e´ des Esp e` ces Paramagn e´ tiques” and
in the “Laboratoire d’Electrochimie Organique et d'Instrumentation”
at the University of Aix-Marseille III.
Initiation rates, R
experiments by the inhibitor method using R-tocopherol as reference
i
, were determined for each condition in preliminary
1
6
antioxidant: R ) 2[R-tocopherol]/τ.
i
The EPR spectra were recorded on a Bruker ESP 300 spectrometer
by using the following settings: microwave frequency 9.74 GHz, power
.4 mW, modulation amplitude 0.7 G, center field 3320 G, sweep time
1 s, and time constant 81 ms.
6
8
For measuring the kinetic parameters for the reaction of tert-butoxyl
radical with phenothiazine (and the related compounds investigated)
ESR Spectral Parameters of the Aminyl (1a-10a) and Nitroxide
Radicals (1b-10b). The following hyperfine splitting constants
(expressed in gauss ) 0.1 mT) and g-factors have been measured in
benzene at room temperature. The assignment of the proton splittings
to the various positions is based on those reported in the literature for
aminyl10 and nitroxide radicals.
1a: 7.08 (N), 2.80 (H1,9), 0.81 (H2,8), 3.75 (H3,7), 1.02 (H4,6), g
2.0046. 2a: 7.72 (N), 3.02 (H1,9), 0.72 (H2,8), 3.79 (H3,7), 0.95 (H4,6),
g 2.0036. 3a: 7.07 (N), 2.93 (H1,9), 0.85 (H2,8), 3.94 (H3,7), 1.22 (H4,6),
g 2.0102. 4a: 7.46 (N), 3.37 (H1,10), 1.06 (H2,9), 4.24 (H3,8), 1.12 (H4,7),
solutions of di-tert-butyl peroxide (0.2-0.4 M), the tricyclic amine (ca.
-
2
1
× 10 M), tris(trimethylsilyl)silane (1 M) as reference hydrogen
donor, and tert-butylbenzene as internal GC standard, in benzene, were
degassed and sealed under nitrogen in quartz ampules. The reaction
mixture was photolyzed at 298 K for 15-30 min in a thermostated
photoreactor equipped with a 125 W high-pressure mercury lamp and
the disappearance of the products was analyzed by GC. For each
compound the results were averaged over 3-5 measurements with
different amine/silane concentrations.
11
o m p
2.17 (H5,6), g 2.0031. 5a: 8.86 (N), 3.71 (4H ), 1.53 (4H ), 4.34 (2H ),
Determination of the BDE Values. Deoxygenated benzene solutions
containing the amine under investigation (0.01-1 M), an appropriate
reference phenol (0.1-1 M), and di-tert-butyl peroxide (0.1 M) were
sealed under nitrogen in a suprasil quartz EPR tube sitting inside the
thermostated cavity of an EPR spectrometer. Photolysis was carried
out by focusing the unfiltered light from a 500 W high-pressure mercury
lamp on the EPR cavity. The temperature was controlled with a standard
variable-temperature accessory and was monitored before and after each
run with a copper-constantan thermocouple.
g 2.0032. 6a: 6.67 (N), 2.00 (6H), 1.01 (H2,8), 2.78 (H3,7), 1.15 (H4,6),
g 2.0046. 7a: 7.39 (N), 2.80 (H1,9), 0.49 (H2,8), 0.49 (6H), 1.02 (H4,6),
g 2.0046. 8a: 7.40 (N), 2.80 (H1,9), 0.82 (H2,8), 0.15 (18H), 1.10 (H4,6),
g 2.0049. 9a: 7.02 (N), 2.97 (H1,9), 0.80 (H2,8), 0.42 (2Cl), 1.13 (H4,6),
g 2.0052. 10a: 6.11 (N), 1.85 (H1,9), 0.15 (H2,8), 1.71 (2N), 0.88 (H4,6),
g 2.0050.
1b: 9.23 (N), 2.23 (H1,3,7,9), 0.59 (H2,4,6,8), g 2.0055. 2b: 8.97 (N),
2.39 (H1,3,7,9), 0.51 (H2,4,6,8), g 2.0050. 3b: 9.30 (N), 2.16 (H1,3,7,9), 0.65
(H2,4,6,8), g 2.0070. 4b: 12.61 (N), 1.51 (H1,10), 0.50 (H2,4,7,9), 1.71 (H3,8),
•
•
The molar ratio of the two equilibrating radicals [Ar
2
N ]/[ArO ] was
0.69 (H5,6), g 2.0058. 5b: 9.65 (N), 1.90 (4H
g 2.0055.
o o p
), 0.81 (4H ), 1.82 (2H ),
obtained from the EPR spectra and used to determine the equilibrium
•
constant, K, by introducing in the equation K ) ([Ar
2
N ][ArOH])/
•
Acknowledgment. Financial support from CNR (Rome) and
from the University of Bologna and MURST (Research project
“Free Radicals and Radical Ions in Chemical and Biological
Processes”) is gratefully acknowledged. This work is dedi-
cated to Dr. Keith U. Ingold in the occasion of his 70th birth-
day.
([Ar
2
NH][ArO ]), the initial concentrations of the reference phenol
, and the amine under investigation [Ar NH] . Initial concentra-
[
ArOH]
0
2
0
tions were chosen to avoid significative consumption during the course
of the experiment.
Relative radical concentrations were determined by comparing the
double integrals of at least two lines of the equilibrating species or,
when strong line overlap was present, by comparison of the digitized
experimental spectra with computer simulated ones. In these cases an
JA992904U