EPR studies of nitrogen-centred free radicals. Part 50.1 Unusual decomposition behaviour of isolable stable thioaminyl free radicals at high temperature

Article abstract of DOI:10.1039/a800163d

Thermal stabilities of N-(4-nitrophenylthio)-N-(4-tert-butyl-2,6-diphenylphenyl)aminyl 1a, N-(4-nitrophenylthio)-N-(2,4,6-triphenylphenyl)aminyl 2a, N-(2,4-dichlophenylthio)-N2,4,6-triphenylphenyl)aminyl 2b and N-(4-nitrophenylthio)-N-(2-tert-butyl-4,6-di

Full text of DOI:10.1039/a800163d

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EPR studies of nitrogen-centred free radicals. Part 50.1 Unusual
decomposition behaviour of isolable stable thioaminyl free radicals
at high temperature
Yozo Miura* and Masayoshi Momoki
Department of Applied Chemistry, Faculty of Engineering, Osaka City University,
Sumiyoshi-ku, Osaka 558, Japan
Thermal stabilities of N-(4-nitrophenylthio)-N-(4-tert-butyl-2,6-diphenylphenyl)aminyl 1a,
N-(4-nitrophenylthio)-N-(2,4,6-triphenylphenyl)aminyl 2a, N-(2,4-dichlorophenylthio)-N-2,4,6-
triphenylphenyl)aminyl 2b and N-(4-nitrophenylthio)-N-(2-tert-butyl-4,6-diphenylphenyl)aminyl 3a
have been studied. When 1a, 2a and 2b are heated in degassed benzene at 80 ЊC for 24–30 h, they almost
completely decompose, giving two non-radical compounds. On the other hand, 3a is thermally much
more stable and the complete decomposition requires heating for 8 days. On the basis of the product
analysis study the thermal decomposition mechanism is proposed and the reason why 3a is so stable
at high temperature is discussed.
phenylphenyl)aminyls 2, and N-(arylthio)-N-(2-tert-butyl-2,6-
Introduction
diphenylphenyl)aminyls 3 are isolable radicals.9–11 Although
these radicals persist for a long time in solution at room tem-
perature without decomposition, slow decomposition occurs at
high temperatures. For example, when 1 and 2 were heated in
benzene at 80 ЊC for 24–30 h, they were almost completely
decomposed to non-radical compounds. In contrast, aminyl 3 is
much more stable than 1 and 2, and the complete decom-
position required heating for 8 days. To clarify their thermal
decomposition mechanism and to elucidate the reason why only
3 is thermally much more stable, the product analyses for the
decomposition were performed, and we have found that the first
step of the decomposition is an intramolecular homolytic
attack of the nitrogen radical centre to one of the ortho posi-
tions of the 2- and 6-phenyl group attached to the anilino group.
To our knowledge, such a decomposition behaviour has not
been found for isolable stable free radicals, and this is an
important finding in the study of stable free radical chemistry.
Herein we describe the thermal stabilities of 1, 2 and 3, the
product analyses for the thermal decomposition and the
thermal decomposition mechanism.
Free radicals are inherently transient species because there is an
electron vacancy in the highest occupied molecular orbital.
However, electronic stabilization and steric protection would
render them persistent. The most extreme examples are isolable
stable free radicals represented by aminoxyl radicals, nitronyl
aminoxyls, N,N-diphenyl-1-picrylhydrazyl (DPPH) and verd-
azyls,2 etc. In recent years, isolable stable free radicals have
attracted much attention in the studies on organic magnetism3
and ‘living’ free radical polymerization.4
As part of the program toward organic magnetism,5 we have
searched for a new class of isolable free radicals. Thioaminyl
˙
radicals (ArNSArЈ), a new class of stable free radicals, are elec-
tronically stabilized by the conjugative delocalization of the
unpaired electron from the nitrogen to the sulfur (Scheme 1).
– +
•
• •
• • •
–N–S–
–N–S–
Scheme 1
However, they are not so long-lived because irreversible homo-
lytic reactions occurring at the anilino ortho and para positions
Results and discussion
destroy the radicals.6,7 For example, PhNSPh is decomposed via
˙
The thermal decomposition for the isolable stable thioaminyl
free radicals was carried out in oxygen-free benzene at 80 ЊC.
The radicals used in this study are N-(4-nitrophenylthio)-N-(4-
tert-butyl-2,6-diphenylphenyl)aminyls 1a,11 N-(4-nitrophenyl-
thio)-N-(2,4,6-triphenylphenyl)aminyls 2a,9 N-(2,4-dichloro-
phenylthio)-N-(2,4,6-triphenylphenyl)aminyls 2b9 and N-(4-
nitrophenylthio)-N-(2-tert-butyl-2,6-diphenylphenyl)aminyls
3a,11 which are more stable and easily obtained than the others.
The reactions were carried out under oxygen-free conditions to
avoid secondary reactions of the products with oxygen. Com-
plete decomposition of the thioaminyl radicals was clearly indi-
cated by disappearance of the characteristic colour of the rad-
icals [1a, purple (583 and 492 nm); 2a, yellowish green (637 and
513 nm); 2b, bluish green (637 and 535 nm); 3a, green (641 and
512 nm)]. When the characteristic colours almost disappeared
(24 h–8 days), the reaction mixtures were inspected by TLC.
Since column chromatographic separation of the products on
silica gel or alumina caused complete decomposition of some
products (6 and 8) during chromatography, the isolation was
performed with a preparative GPC instrument. The structures
of the products isolated were elucidated on the basis of IR,
C᎐N coupling to give p-benzoquinone diimine in 40% yield as a
main non-radical product.8 Therefore, if the anilino benzene
ring can be completely protected from such reactions, they may
be isolable radicals. We have prepared a variety of sterically
protected thioaminyls and found that N-(arylthio)-N-(4-tert-
butyl-2,6-diphenylphenyl)aminyls 1, N-(arylthio)-N-(2,4,6-tri-
Ph
Ph
.
.
Ph
NSAr
NS
NO2
Ph
Ph
1a
2
a Ar = 4-NO2C6H4
b Ar = 2,4-Cl2C6H3
.
Ph
NS
NO2
Ph
3a
J. Chem. Soc., Perkin Trans. 2, 1998
1185

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Table 1 Thermal decomposition of thioaminyl radicals in oxygen-free
benzene at 80 ЊC
Entry
Radical
t/ha
Product (% yield)b
1
2
3
4
1a
2a
2b
3a
24
28
30
5 (50), 6 (43)
7a (49), 8a (48)
7b (43), 8b (48)
11 (48)
192
a The time required for the almost complete decomposition of radical.
b Isolated yield.
1H NMR and mass spectra. The results are summarized in
Table 1.
When 1a was heated in oxygen-free benzene for 24 h, it was
almost completely decomposed. TLC analysis of the reaction
mixture showed formation of two products (Scheme 2). One
Ph
SAr
N
80 °C
1a
1a
H
in benzene
.
4
Fig. 1 1H NMR spectrum of 5. Expansion of the aromatic region is
shown.
Ph
Ph
Ph
SAr
2
N
+
NHSAr
2
80 °C, in benzene
Ph
a
5
6
Ph
Ph
Ph
SAr
N
b
Ar = 4-NO2C6H4
+
Ph
NHSAr
Scheme 2
c
f
8
7
product (less mobile) was shown to be the known compound, 6,
d
e
1
by comparison of the IR and H NMR spectra with those of
the authentic sample. Compound 6 is the corresponding pre-
cursor of 1a. The other, more mobile, product was an unknown
compound, 5. The IR spectra showed no absorption due to the
NH stretching vibration around 3300 cmϪ1. The mass spectrum
gave a strong peak at m/z 452 as the molecular ion (Mϩ) which
corresponds to 1a Ϫ H, indicating that the product is formed by
a Ar = 4-NO2C6H4
b Ar = 2, 4-Cl2C6H3
Scheme 3
group yields an intermediate cyclohexadienyl radical 4. Dispro-
portionation between 4 and 1a gives an equivalent amount of
5 and 6. The formation of 7 and 8 can be rationalized in terms
of the same mechanism.
1
hydrogen-atom abstraction from 1a. The H NMR spectrum
1
(Fig. 1) was assigned as follows on the basis of the 2D H
COSY NMR spectrum: a singlet at 1.45 ppm was assigned to
the tert-butyl protons. The doublets with J 8.8 Hz at 6.60 and
7.95 ppm were assigned to Hg and Hh, the doublets with J 2.0
Hz at 7.32 and 8.12 ppm were assigned to Ha and Hb and the
doublets with J 7.3 Hz at 7.55 and 8.14 ppm were assigned to Hc
and Hf. The triplets with J 7.3 Hz at 7.37 and 7.45 ppm were
assigned to Hd and He. On the other hand, the absorption due
to the 1-phenyl protons (Hi) was observed as a broad signal
spread over 6.7–7.6 ppm. As discussed below, this broadening is
due to the occurrence of an unpaired electron in the phenyl
group. On the basis of the above spectroscopic results and the
satisfactory elemental analysis the compound was identified to
be 5.
Radicals 2a and 2b gave nearly identical results, (Scheme 3).
Thus, upon heating at 80 ЊC for 28 h, 2a almost completely
decomposed to give 7a and 8a in 49 and 48% yields, respect-
ively, and upon heating for 30 h 2b almost completely
decomposed to give 7b and 8b in 43 and 48% yields, respect-
ively. Their structures were confirmed on the basis of the IR, 1H
NMR and mass spectra and the elemental analyses.
Although homolytic aromatic substitution of transient
radicals such as phenyl, methyl and benzoyloxyl radicals are
well established,12 it is surprising that stable free radicals such as
1 and 2 undergo homolytic aromatic substitution. The EPR
study for 1–3 showed the spin densities on the 2- and 6-phenyl
groups are nearly zero. We assume that the reason why such a
homolytic aromatic substitution can occur in 1 and 2 is that it is
an intramolecular reaction favourable in entropy. Of course, we
confirmed that a homolytic aromatic substitution between 1 or
2 and benzene used as the solvent never occurs even at high
temperatures.
As mentioned above, the 1H NMR spectra of 5 and 7 gave a
broad signal for the 1-phenyl protons (Hi), as found in Fig. 1.
However, the broad signal became more and more sharp with a
lowering in temperature and at Ϫ50 ЊC considerably sharp
NMR signals were observed. Such a broadening does not occur
in the 1H NMR spectra of carbazole itself and 1-phenyl-3-tert-
butylcarbazole 10. When molecules are contaminated with
paramagnetic species or molecules are partly converted to
paramagnetic species by oxidation or bond homolysis, a signifi-
cant line broadening is observed in the NMR measurements
because a strong magnetic interaction between the unpaired
electron spin and protons reduce the relaxation times of pro-
The decomposition mechanism of 1a is shown in Scheme 2.
An intramolecular homolytic attack of the nitrogen radical
centre to one of the ortho positions of the 2- and 6-phenyl
1186
J. Chem. Soc., Perkin Trans. 2, 1998

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is speculated that the broadening of the phenyl protons in the
Ph
NMR spectra of 5 and 7 is due to the formation of phenyl and
cyclohexadienyl radicals.
NH
To detect the intermediate radicals, the EPR measurements
of a benzene solution of 5 were carried out at room temper-
ature, and a 1:1:1 triplet with aN = 0.934 mT and g = 2.0055
was detected. Since the aN and g values are the same as those for
1 (aN = 0.932 mT; g = 2.0055),11 the radical observed was iden-
tified as 1. It seems that this radical is formed by hydrogen-atom
abstraction by 9 from hydrogen donors.
Thioaminyl 3a showed a quite different behaviour for the
thermal stability. Upon heating for one day at 80 ЊC, the charac-
teristic blue colour due to 3a remained strong, and for almost
complete disappearance, a further 7 days, at least, were
required. TLC analysis showed formation of at least six
compounds, and only 11 could be isolated in 48% yield by the
preparative GPC method.
Why is only 3a so persistent at high temperature? The reason
lies in its particular structure. First, one of the ortho positions is
occupied by a tert-butyl group which is inert towards hydrogen-
atom abstraction. Secondly, the phenyl group attached to the
remaining ortho position is twisted by 87Њ from the anilino ben-
zene ring, as indicated by the X-ray crystallographic analysis.11
This great twisting of the phenyl group makes intramolecular
homolytic attack by the nitrogen radical centre very difficult.
On the other hand, 1a and 2b are shown to adopt a different
conformation by the X-ray crystallographic analyses.9,11
Although one of the ortho phenyl groups is certainly seriously
twisted from the anilino benzene ring by 64 (1a) or 87Њ (2b),
the remaining phenyl group is only twisted by 45 (1a) or 49Њ
(2b). At this angle, it is possible for intramolecular homolytic
attack by the nitrogen radical centre to occur. Accordingly,
if the ortho positions of the 2- and 6-phenyl groups are pro-
tected from such a reaction, the resulting radicals may become
thermally very stable.
Ph
NHS
NO2
Ph
11
10
.
X
Y
12
tons. It is speculated that there is a slow interconversion
between 5 and 5Ј and between 7 and 7Ј at room temperature, as
shown in Scheme 4. A homolytic cleavage of the C᎐N bond in
.
H
N
.
SAr
R
R
NSAr
.
.
9
Conclusions
N
SAr
R
R
SAr
We have investigated the thermal decomposition of 1–3 and
found that decomposition takes place via an intramolecular
homolytic aromatic substitution reaction. Accordingly, when
radicals have a structure represented by 12, an identical decom-
position reaction may take place, even though they are quite
stable and isolable radicals.
N
5' (or 7')
5 (or 7)
Experimental
All mps were determined on a Yanagimoto micro melting point
apparatus and are uncorrected. IR spectra were run for samples
as KBr pellets on a JASCO A-202 spectrophotometer. 1H
NMR spectra were recorded with a JEOL α-400 spectrometer
(400 MHz) with SiMe4 as internal standard; J values are given
in Hz. Mass spectra were taken with JEOL JMS-HX 100 spec-
trometer at 70 eV using a direct inlet system. Isolation of the
products was performed on an LC-908 recycling preparative
HPLC (Japan Analytical Industry Co., LTD) using chloroform
as the eluent. TLC analyses were carried out on Merck 60 F254
neutral aluminium oxide sheets. Thioaminyl radicals 1a, 2a, 2b
and 3a were prepared by previous methods.9–11
.
.
.
R
SAr
H
R
NSAr
N
.
9'
R = But (5) or Ph (7)
Thermal decomposition of 1, 2 and 3 in benzene
Scheme 4
Thioaminyl radical crystals (0.265 mmol) and benzene (20 cm3)
were put in a glass tube. The glass tube was then degassed by
freeze–pump–thaw cycles and sealed off from the vacuum sys-
tem. The glass tube was immersed in a water-bath kept at 80 ЊC
until the characteristic purple (1a), yellowish green (2a), bluish
green (2b) or blue colour (3a) almost completely disappeared.
The solvent was evaporated under reduced pressure and the
resulting residue was subjected to preparative GPC. The yields
of the products were determined by weight. Compounds 6, 8a,
the carbazole ring yields an aminyl and phenyl biradical. The
aminyl radical centre intramolecularly attacks the remaining
phenyl group to yield a cyclohexadienyl radical. In phenyl and
cyclohexadienyl radicals the unpaired electron spin strongly
interacts with protons, as indicated by the large aH value.†,13,14 It
† The aH values for phenyl and cyclohexadienyl radicals are reported
to be 1.74 (o-H), 0.59 (m-H), 0.19 (p-H) and 0.913 (1- and 5-H), 0.264
(2- and 4-H), 1.356 (3-H), 4.81 mT (6-H), respectively.
1
8b and 11 were identified by comparison of the IR and H
J. Chem. Soc., Perkin Trans. 2, 1998
1187

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NMR spectra with the authentic samples. The structures of 5,
7a and 7b were determined on the basis of IR, H NMR and
mass spectra and elemental analyses.
N-(4-Nitrophenylthio)-2-tert-butyl-2,4-diphenylaniline 11
1
Yield 48% (57.8 mg, 0.127 mmol), yellow plates (EtOH); mp
157–159 ЊC (lit.,11 158–160 ЊC).
N-(4-Nitrophenylthio)-1-phenyl-3-tert-butylcarbazole 5
Yield 50% (60.0 mg, 0.133 mmol), yellow micro prisms
(MeOH); mp 80–83 ЊC; m/z 452 (Mϩ, 41%), 299 (50), 284 (100),
242 (96) and 57 (33); δH(CDCl3) 1.45 (9 H, s, But), 6.60 (2 H, d,
J 8.8, Hg), 6.7–7.6 (5 H, br s, Hi), 7.32 (1 H, d, J 2.0, Ha or Hb),
7.37 (1 H, t, J 7.3, Hd or He), 7.45 (1 H, t, J 7.3, Hd or He), 7.55
(1 H, d, J 7.3, Hc or Hf), 7.95 (2 H, d, J 8.8, Hh), 8.12 (1 H, d,
J 2.0, Ha or Hb) and 8.14 (1 H, d, J 7.3, Hc or Hf) (Found:
C, 74.24; H, 5.39; N, 6.16. C28H24N2O2S requires C, 74.30; H,
5.36; N, 6.19%).
References
1 Part 49, Y. Miura, M. Momoki, H. Nakatsuji and Y. Teki, J. Org.
Chem., in the press.
2 A. R. Forrester, J. M. Hay and H. R. Thomson, Organic Chemistry
of Stable Free Radicals, Academic Press, London and New York,
1968; E. G. Rozantsev, Free Nitroxide Radicals, Plenum Press,
New York and London, 1970; L. B. Volodarsky, V. A. Reznikov and
V. I. Ovcharenko, Synthetic Chemistry of Stable Nitroxides, CRC
Press, Boca Raton, 1994.
3 Proceedings of the 5th International Conference on Molecule-Based
Magnets, Osaka, Japan, 1996 (Mol. Cryst. Liq. Cryst., 1997, 305, 1;
306, 1).
N-(4-Nitrophenylthio)-4-tert-butyl-2,6-diphenylaniline 6
Yield 43% (52.1 mg, 0.115 mmol), yellow plates (EtOH); mp
148–149 ЊC (lit.,11 146–148 ЊC).
4 C. J. Hawker, Acc. Chem. Res., 1997, 30, 373.
5 Y. Teki, K. Itoh, A. Okada, H. Yamakage, T. Kobayashi, K. Amaya,
S. Kurokawa, S. Ueno and Y. Miura, Chem. Phys. Lett., 1977, 270,
573; Y. Miura, T. Issiki, Y. Ushitani, Y. Teki and K. Itoh, J. Mater.
Chem., 1996, 6, 1745 and references cited therein.
N-(4-Nitrophenylthio)-1,3-diphenylcarbazole 7a
6 Y. Miura and M. Kinoshita, Bull. Chem. Soc. Jpn., 1997, 50, 1142;
Y. Miura, H. Asada and M. Kinoshita, Bull. Chem. Soc. Jpn., 1977,
50, 1855; Y. Miura, Y. Katsura and M. Kinoshita, Bull. Chem. Soc.
Jpn., 1979, 52, 1121; Y. Miura, H. Asada and M. Kinoshita, Bull.
Chem. Soc. Jpn., 1980, 53, 720; Y. Miura, A. Yamamoto, Y. Katsura
and M. Kinoshita, J. Org. Chem., 1980, 45, 3875.
Yield 49% (62.0 mg, 0.131 mmol), yellow micro prisms
(MeOH); mp 93–96 ЊC; m/z 472 (Mϩ, 4.4%) and 319 (100);
δH(CDCl3) 6.61 (2 H, d, J 8.8, o-H of SAr), 6.8–7.6 (5 H, br s,
2-Ph), 7.37–7.50 (5 H, m, Hd, He and m- and p-H of 3-Ph), 7.54
(1 H, d, J 2.0, Ha or Hb), 7.60 (1 H, d, J 7.3, Hc or Hf), 7.74 (2 H,
d, J 7.3, o-H of 3-Ph), 7.97 (2 H, d, J 8.8, m-H of SAr), 8.18
(1 H, d, J 7.3, Hc or Hf) and 8.33 (1 H, d, J 2.0, Ha or Hb)
(Found: C, 76.20; H, 4.31; N, 5.89. C30H20N2O2S requires C,
76.24; H, 4.27; N, 5.93%).
7 Y. Miura, A. Yamamoto and M. Kinoshita, Bull. Chem. Soc. Jpn.,
1981, 54, 3215.
8 L. Benati, P. C. Montevecchi and P. Spagnolo, J. Chem. Soc., Perkin
Trans. 1, 1982, 3049; C. Balboni, L. Benati, P. C. Montevecchi and
P. Spagnolo, J. Chem. Soc., Perkin Trans. 1, 1983, 2111.
9 Y. Miura and A. Tanaka, J. Chem. Soc., Chem. Commun., 1990,
441; Y. Miura, A. Tanaka and K. Hirotsu, J. Org. Chem., 1991, 56,
6638.
N-(4-Nitrophenylthio)-2,4,6-triphenylaniline 8a
Yield 48% (60.9 mg, 0.128 mmol), yellow prisms (EtOH–
10 Y. Miura, Y. Kitagishi and S. Ueno, Bull. Chem. Soc. Jpn., 1994, 67,
benzene); mp 148–149 ЊC (lit.,9 149–150 ЊC).
3282.
11 Y. Miura, T. Fuchikami and M. Momoki, Chem. Lett., 1994, 2127;
Y. Miura, M. Momoki, T. Fuchikami, Y. Teki, K. Itoh and
H. Mizutani, J. Org. Chem., 1996, 61, 4300.
12 M. J. Perkins, in Free Radicals, ed. J. K. Kochi, Wiley, New York,
1973, vol. 2, p. 231.
13 P. H. Kasai, E. Hedaya and E. B. Whipple, J. Am. Chem. Soc., 1969,
91, 4364; P. H. Kasai, P. A. Clark and E. B. Whipple, J. Am. Chem.
Soc., 1970, 92, 2640.
14 M. Kira, H. Sugiyama and H. Sakurai, J. Am. Chem. Soc., 1983,
105, 6436.
N-(2,4-Dichlorophenylthio)-1,3-diphenylcarbazole 7b
Yield 43% (56.2 mg, 0.113 mmol), colorless prisms (MeOH–
ethyl acetate); m 145–146 ЊC; m/z 497 (11%), 496 (5), 495 (Mϩ,
14) and 319 (100); δH(CDCl3) 6.05 (1 H, d, J 8.8, o-H of SAr),
6.6–7.6 (5 H, br s, 1-Ph), 6.91 (1 H, dd, J 8.8 and 2.0, m-H of
SAr), 7.22 (1 H, d, J 2.0, m-H of SAr), 7.33–7.50 (5 H, m, Hd,
He and m- and p-H of 3-Ph), 7.53 (1 H, d, J 2.0, Ha or Hb), 7.60
(1 H, d, J 7.8, Hc or Hf), 7.73 (2 H, d, J 8.3, o-H of 3-Ph), 8.17
(1 H, d, J 7.8, Hc or Hf) and 8.32 (1 H, d, J 2.0, Ha or Hb)
(Found: C, 72.32; H, 4.13; N, 2.55. C30H19Cl2NS requires: C,
72.57; H, 3.87; N, 2.82%).
N-(2,4-Dichlorophenylthio)-1,3-diphenylaniline 8b
Yield 48% (64.3 mg, 0.129 mmol), colorless needles (EtOH–
benzene); mp 144–145 ЊC (lit.,9 142–143 ЊC).
Paper 8/00163D
Received 5th January 1998
Accepted 19th February 1998
1188
J. Chem. Soc., Perkin Trans. 2, 1998