Aromatic radical anions as possible intermediates in the nucleophilic aromatic substitution (SNAr): An EPR study

Article abstract of DOI:10.1039/a903407b

The reactions among halonitrobenzenes or polynitrobenzenes and alkoxides, thiolates or tertiary amines have provided the evidence that in a S_NAr reaction type a single electron transfer from the nucleophile to the aromatic substrate, to generate two radical species within the solvent cage, can take place to some extent. The detection of radical intermediates by EPR spectroscopy, in several S_NAr reactions, is reported.

Full text of DOI:10.1039/a903407b

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Aromatic radical anions as possible intermediates in the
nucleophilic aromatic substitution (SNAr): an EPR study
Loris Grossi* and Samantha Strazzari1
Dipartimento di Chimica Organica “A. Mangini”, Università di Bologna, Viale Risorgimento 4,
I-40136, Bologna, Italy. E-mail: grossi@ms.fci.unibo.it; Fax: ϩ39 051 2093654
Received (in Cambridge) 28th April 1999, Accepted 13th July 1999
The reactions among halonitrobenzenes or polynitrobenzenes and alkoxides, thiolates or tertiary amines have
provided the evidence that in a SNAr reaction type a single electron transfer from the nucleophile to the aromatic
substrate, to generate two radical species within the solvent cage, can take place to some extent. The detection of
radical intermediates by EPR spectroscopy, in several SNAr reactions, is reported.
Table 1 Hyperfine coupling constants of the detected aminoxylsa
Introduction
It is commonly accepted2 that nucleophilic aromatic substitu-
Tertiary amines
Radicalb
hfc/G
tion reactions involving activated substrates and good leaving
groups proceed by a two-step mechanism: the first step is the
covalent addition of a nucleophile to a substituted or unsubsti-
tuted ring carbon atom of the aromatic substrate, leading to an
anionic σ-complex known as the Meisenheimer complex; the
second step is the departure of the leaving group to form the
substituted product, Scheme 1.
N(Me)2CH2CH2OH
N(Me)2CH2CH2CH2OH
a6H = 12.50
aN = 15.25
NEt3
N(Et)2CH2CH2OH
a4H = 9.75
aN = 14.50
NPr3
a4H = 9.85
aN = 14.60
NEt(Pri)2
aH = 4.55
a2H = 10.45
aN = 14.95
NEt(Pri)2
a2H = 4.25
aN = 14.55
Scheme 1
a Typically at 0 ЊC or room temperature. b The g-values (2.0054
0.0002) have been evaluated by comparison with the g-factor of the
diphenylpicrylhydrazyl (DPPH) (2.0037).
However, the polar mechanism of several organic reactions,
including SN2 and SNAr, has been reconsidered3 several times
and a composite multi-step mechanism within the solvent cage,
with an initial single electron transfer (SET) between the react-
ants, has been suggested.3 In principle, the mechanism could
move from polar to SET, depending on many factors, such as
the nature of the two reactants (the redox potentials) and the
polarity of the solvent.
radical-coupling within the solvent cage should lead to the
corresponding Meisenheimer complex,7,8 whilst, if the radical
pair escapes from the solvent cage, the free radical species could
become detectable.6,8 These arguments led us to investigate,
by EPR spectroscopy, the SNAr reaction of many polynitro-
benzenes and their halo derivatives with a large variety of
nucleophiles such as alkoxides, thiolates and tertiary amines.
The spectroscopic results, as well as the identification among the
reaction products of species whose formation can be accounted
for only by the occurrence of radical processes, support the
hypothesis that a SET mechanism could make a contribution to
the whole process, depending on the substrate/nucleophile pair.
In 1970 Bunnett4 showed that the SET pathway could be
involved also in some cases of nucleophilic aromatic substitu-
tion: in fact, the corresponding radical anion once formed, for
instance by promoting this process with light, could lead to the
substituted product via an SRN1 mechanism, as shown in
Scheme 2.
Ϫ
RX ϩ eϪ → RX
᝽
Ϫ
Ϫ
ؒ
RX
R ϩ X
᝽
Ϫ
Ϫ
ؒ
R ϩ Nu
RX ϩ RNu
RNu
᝽
Results and discussion
EPR studies
RX Ϫ ϩ RNu
Ϫ
᝽
᝽
Scheme 2
It has been previously reported8 that the reaction between
nitroaromatic derivatives and tertiary amines, conducted dir-
ectly in the cavity of the EPR spectrometer, leads to the
detection of dialkyl aminoxyls, Table 1, together with
the appropriate aromatic radical anions. The formation of the
former species could be explained by invoking a dealkylating
process that the intermediate tertiary aminium radical cations
can undergo, followed by oxidation of the resultant aminyl
radical, Scheme 3. This finding8 could support the involvement
Since then many other examples have been reported5 in
which the initial step of this process involves the stimulated
formation of the aromatic radical anion. For example, the first
step in the SNAr process among several nitroarenes, their halo
and nitrile derivatives, and both the hydroxide ion6,7 and
tertiary amines8 has been argued to be the formation of a π
electron donor–acceptor complex followed by transfer of an
electron to yield a radical pair.6–10 Thus, the occurrence of the
J. Chem. Soc., Perkin Trans. 2, 1999, 2141–2146
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Table 2 Substrates analysed by EPR spectroscopya
Substrate
Alkoxide
Thiolate
Tertiary amine
a) ButOϪ
b) ButSϪ
c) NEt3
a) ButOϪ
a) ButOϪ
a) ButOϪ
d) CH3SϪ
e) PriSϪ
c) NEt3
f) NPr3
b) ButSϪ
g) N(Me)2CH2CH2OH
h) N(Et)2CH2CH2OH
i) N(Me)2CH2CH2CH2OH
b) ButSϪ
c) NEt3
f) NPr3
g) N(Me)2CH2CH2OH
h) N(Et)2CH2CH2OH
i) N(Me)2CH2CH2CH2OH
d) CH3SϪ
e) PriSϪ
c) NEt3
h) N(Me)2CH2CH2OH
b) ButSϪ
l) CH3OϪ
m) EtOϪ
a) ButOϪ
d) CH3SϪ
n) EtSϪ
o) PrSϪ
e) PriSϪ
b) ButSϪ
c) NEt3
f) NPr3
g) N(Me)2CH2CH2OH
h) N(Et)2CH2CH2OH
i) N(Me)2CH2CH2CH2OH
p) NEt(Pri)2
l) CH3OϪ
m) EtOϪ
a) ButOϪ
b) ButSϪ
c) NEt3
f) NPr3
g) N(Me)2CH2CH2OH
h) N(Et)2CH2CH2OH
i) N(Me)2CH2CH2CH2OH
l) CH3OϪ
b) ButSϪ
c) NEt3
f) NPr3
g) N(Me)2CH2CH2OH
h) N(Et)2CH2CH2OH
i) N(Me)2CH2CH2CH2OH
a Solutions in CH3CN, THF or CH3CN–THF mixture (70:30). The molar ratio between the substrates and the nucleophiles is 1:1.
Scheme 3
of a SET mechanism for such a type of reaction; but, to
strengthen this hypothesis it was necessary to verify whether
with different nucleophiles the intermediate radical anions were
still detectable. Experiments on nitroaromatic substrates with
well known nucleophiles such as alkoxides and thiolates were
then conducted, Table 2.
The reactants, in CH3CN, THF, or CH3CN–THF (70:30)
solution, were mixed directly in the EPR sample tubes and the
samples analysed at low temperature: all the substrates investi-
gated enabled the detection of the corresponding aromatic
radical anions, Table 3. The hyperfine coupling (hfc) of the
radical anions reported in Table 3 show the typical values of
ion-pairs11 (see for example radical 2a in which the hyperfine
coupling for Naϩ is observed): that accounts for the unsymmet-
Fig. 1 (a) EPR spectrum, at Ϫ50 ЊC, of 7a in CH3CN solution;
rical distribution of the spin density.11–13 For radical 1a, how-
ever, we observed a symmetric spin distribution:14 we may argue
that it is not possible to rule out contact ion-pairing for the
p-dinitrobenzene radical anion12 as a result of the fast counter-
ion exchange between the two p-nitro groups which averages
completely the hyperfine splitting (hfs) pattern.12,13
(b) computer simulated spectrum.
detection of radical anion intermediates, Fig. 1, for a large
number of substrates with different nucleophiles, was of course
supporting the hypothesis for the involvement of a SET
process.
In a few cases the EPR experiments showed also the incom-
ing formation of other radical species: for some substrates,
when the reaction mixture was kept at ca. Ϫ50 ЊC for 6–7 hours,
it was possible to detect the radical anions corresponding to the
product of the previously occurred SNAr reactions. For
instance, when the 2-chloro-1,4-dinitrobenzene (5) and the
As reported in the literature,12 the hfs pattern can change
depending on the nature of the counter-ion. In particular, we
observe smaller coupling constants for the ion-pairs where the
tertiary aminium radical cations are involved.12
Although we did not have the unequivocal proof needed to
draw a definitive mechanism for this class of reactions, the
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Table 3 Hyperfine coupling constants of the detected radical anionsa
Radical anionb
hfc/G
aN(1) = aN(4) = 1.80
aH(2) = aH(6) = 1.11
aH(3) = aH(5) = 1.11
aN(1) = 9.38
aN(3) = 0.26
aH(2) = 3.25
aH(4) = 4.45
aH(5) = 1.10
aH(6) = 4.15
aNa = 0.22
aN(1) = 3.54
aN(3) = aN(5) = 0.71
aH(2) = 4.25
aH(4) = 3.08
aH(6) = 1.96
Fig. 2 (a) EPR spectrum, at Ϫ50 ЊC, of 5a in CH3CN solution;
(b) EPR spectrum, at Ϫ50 ЊC, of 5b in CH3CN solution.
formed in THF, products which indicated the presence of a
pent-4-enoxyl radical intermediate were found.
aN(4) = 10.18
aH(2) = aH(6) = 3.38
aH(3) = aH(5) = 1.13
The use of sodium pent-4-enoxide represents an unequivocal
probe3d to establish if the formation of radicals is involved in
such a type of reaction. In fact, if free pent-4-enoxyl radicals
are formed during the reaction course, they may rapidly cyclise
to tetrahydrofurfuryl radicals before decaying15,16 and then
products containing the tetrahydrofurfuryl framework may
be obtained in a reaction proceeding with radical character
(see Scheme 4).
aN(1) = 0.16
aN(4) = 1.87
aH(3) = 1.27
aH(5) = 1.31
aH(6) = 0.60
aN(2) = 0.13
aN(4) = 1.76
aH(3) = 1.26
aH(5) = 1.40
aH(6) = 0.54
aCl = 0.036
aN(3) = 2.05
aN(5) = 0.125
aH(2) = 1.325
aH(4) = 1.35
aH(6) = 0.65
a Typically at Ϫ50 ЊC. b The g-values (2.0045 0.0002) have been evalu-
ated by comparison with the g-factor of the DPPH (2.0037). c The
coupling with the counter-ion has been observed only for the reactions
with sodium thiolates.
sodium propane-2-thiolate mixture was investigated, in addi-
tion to the radical anion 5a, Fig. 2a, the 2-chloro-1-
(isopropylsulfanyl)-4-nitrobenzene radical anion, 5b, [aN = 9.78
G, a2H = 3.25 G, aH = 1.18 G, aCl = 0.28 G, g = 2.0045] was
detected after ca. 7 hours, Fig. 2b.
Identical behaviour was observed in the reactions of 5 with
both EtSϪ and PrSϪ: it was possible to detect the 2-chloro-1-
(ethylsulfanyl)-4-nitrobenzene and the 2-chloro-1-(propyl-
sulfanyl)-4-nitrobenzene radical anions respectively, with
hyperfine coupling constants like those found for 5b.
Note, that these spectroscopic results show that in these
processes a nitro group instead of the chlorine atom acts as
the leaving group, reflecting the para-orienting attitude of the
nitro-ring substituents.
Product studies
Scheme 4
To support these spectroscopic results, and then the hypoth-
esised involvement of a SET pathway, we conducted product
analysis for some representative reactions. Along with the
products of the substitution reaction, compounds exclusiv-
ely due to the occurrence of free radical processes were
identified.
From the crude reaction mixture, we recovered unreacted
starting material along with the 1-(pent-4-enyloxy)-2,4-dinitro-
benzene (ca. 21% yield), i.e. the product of the nucleophilic
substitution, the pent-4-en-1-ol (ca. 30%) and the 2-methyltetra-
hydrofuran (GC-MS detectable quantity), i.e. the product of
the cyclization reaction of the intermediate pent-4-enoxyl
radical. No evidence for the formation of the product arising
from the trapping of the cyclised pent-4-enoxyl radical by the
nitroarene radical anion was found.
i) SNAr reaction between 1-chloro-2,4-dinitrobenzene and
sodium pent-4-enoxide. When the reaction between 1-chloro-
2,4-dinitrobenzene and sodium pent-4-enoxide3d was per-
J. Chem. Soc., Perkin Trans. 2, 1999, 2141–2146
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Actually, the recombination of both the pent-4-enoxyl and
the tetrahydrofurfuryl radicals with the nitroarene radical
anion leading to the corresponding substituted products can be
reasonably assumed to arise from the in-cage and the out-of-
cage reaction respectively (Scheme 4). In fact, since the in-cage
recombination must be complete before the radicals can escape
their cage (with a diffusion coefficient of ca. 109 sϪ1),17 the
cyclization of the alkenoxyl radical, even though fast (kc of ca.
108 sϪ1),15 can be assumed not to compete with the in-cage coup-
ling reaction which leads to the 1-(pent-4-enyloxy)-2,4-dinitro-
benzene. Thus, only the cage-escaped pent-4-enoxyl radical can
lead to the tetrahydrofurfuryl radical: this process is also
expected to be faster than the out-of-cage recombination of
the alkenoxyl radical with the nitroarene radical anion. The
tetrahydrofurfuryl radical could eventually recombine with
6a leading to the cyclised substituted product, but in fact
we did not recover any nitroarylmethyltetrahydrofuran. This
finding could be explained by admitting that the out-
of-cage recombination of the tetrahydrofurfuryl radical with
6a is slower than the hydrogen abstraction reaction of the
alkyl intermediate. It should be noted, however, that, accord-
ing to the low quantity of 2-methyltetrahydrofuran recovered,
the SET route represents a minor pathway in this reaction, as
could be predicted since the single electron oxidation of an
alkoxide is expected to be a difficult process.
Furthermore, the EPR detection of 5a and 5b in the reaction
of 5 with i-PrSϪ suggests that a concurrent radical pathway can
contribute to the formation of the substituted product presum-
ably through a SRN1 mechanism.19–21
For SNAr processes where the thioanions are involved, this is
also supported by the comparison of the yields obtained for the
processes between 1-chloro-2,4-dinitrobenzene and sodium
2-methylpropane-2-thiolate performed in the absence (72.1%)
and in the presence (46.4%) of an inhibitor of radicals such as
benzoquinone.
Conclusions
Our experimental results support the hypothesis that a SET
process, leading to a radical pair within the solvent cage, could
be involved as a concomitant first step in the mechanism of the
SNAr reaction of different polynitrobenzenes. The radical
species if stabilised for instance by extensive delocalization, as
for the presence of nitro substituents,3a can escape from the
solvent cage and become detectable by EPR. The SET process
should also be favoured when the substrate is easily reduced, i.e.
when it is characterised by a small negative reduction poten-
tial3d as for nitroaromatic compounds. At the present time, we
believe that the SNAr product can be formed mainly through
the classical pathway, i.e. the formation of the Meisenheimer
complex, via the combination of the two radical species in the
solvent cage. In particular, when alkoxides are involved, since
the single electron oxidation of these nucleophiles is dis-
favoured on account of the electronegativity of oxygen, we
expect the polar pathway to be prevailing. On the other hand,
the ability of thioanions to act as one-electron donors and their
efficiency as nucleophiles in aromatic SRN1 reactions4,9,19,20 (see
Scheme 2), suggests that the concurrent SET pathway in these
processes occurs to a major extent. This is also supported by the
decrease of the yield of the SNAr reaction between 1-chloro-
2,4-dinitrobenzene and sodium 2-methylpropane-2-thiolate in
the presence of benzoquinone.
ii) SNAr reaction between 1-chloro-2,4-dinitrobenzene and
sodium 2-methylpropane-2-thiolate. When the reaction between
1-chloro-2,4-dinitrobenzene and sodium 2-methylpropane-2-
thiolate was performed in CH3CN, the 1-(tert-butylsulfanyl)-
2,4-dinitrobenzene was identified as the main reaction
product, but the GC-MS analysis of the crude reaction mix-
ture also revealed the presence of a significant quantity of
t
ؒ
di(tert-butyl) disulfide, i.e. the product of the Bu S radical
coupling.
iii) SNAr reaction between 1-chloro-2,4-dinitrobenzene and tri-
ethylamine. Only a few examples of SNAr reactions of aromatic
compounds by tertiary amines have been reported,18 probably
because of the low reactivity of tertiary amines due to steric
hindrance. When we performed the reaction of 1-chloro-2,4-
dinitrobenzene with triethylamine in CH3CN, we obtained the
N,N-diethyl-2,4-dinitroaniline, i.e. the substituted product, in
ca. 5% yield (it is worth noting that we conducted this reaction
in very mild conditions compared to those reported in the lit-
erature18 for which an overall yield of only ca. 11% is reported).
The difficulty of the formation of the Meisenheimer adduct,
and its slow dealkylation, can be considered to be responsible
for the low yield of the SNAr reaction. On the other hand, the
slow recombination of the radical pair in the solvent cage
allowed, in principle, the two radicals to escape the cage itself
and EPR evidence of both radical anions and aminoxyls could
thus be obtained.8
Experimental
Materials
The tertiary amines were commercial products, all distilled
before use. CH3SNa, EtSNa, PrSNa, i-PrSNa, t-BuSNa, CH3-
ONa, EtONa and t-BuOK were Fluka or Aldrich products,
used as received. Sodium pent-4-enoxide was prepared as fol-
lows: 0.5 g of pent-4-en-1-ol were added dropwise to a stirred
solution of anhydrous THF and Na wires; the reaction mixture
was gently refluxed under nitrogen atmosphere for ca. 45 min-
utes, then the THF solution of the alkoxide was transferred in a
flask by means of a long double-tipped deflecting needle and
used without further purification. THF was distilled from
sodium–benzophenone just prior to use and stored under
nitrogen. CH3CN was dried over molecular sieves and distilled
under nitrogen before use. The 1,4-dinitrobenzene, 1,3-dinitro-
benzene, 1,3,5-trinitrobenzene, 1-chloro-4-nitrobenzene and
1-chloro-2,4-dinitrobenzene were commercially available and
were recrystallised twice from ethanol before use. The 2-chloro-
1,4-dinitrobenzene was synthesised according to the literature
method.22 The 1-chloro-3,5-dinitrobenzene was prepared from
the corresponding 3,5-ditroaniline by diazotisation and follow-
ing Sandmeyer reaction.23
Mechanistic interpretation
The obtained results seem consistent with a possible contribu-
tion of a SET process to the first step in any reaction between a
nucleophile and a polynitrobenzene. The radical pair can in fact
combine within the solvent cage and lead to the corresponding
Meisenheimer complex, or, escaping from the solvent cage,
form two free radical species, which could be detectable by
EPR. The fate of these free radical species depends upon sev-
eral factors and the most common route of decay is the occur-
rence of typical free radical processes (coupling, H-abstraction,
cyclization, β-scission, etc.) as confirmed by the formation
of side-products such as disulfides or cyclic derivatives. The
recombination of the two radicals can eventually occur also
through an out-of-cage reaction, though this pathway is quite
unlikely.
EPR experiments
EPR spectra were recorded with a Varian E-104 spectrometer,
equipped with a variable temperature apparatus, at Ϫ50/
Ϫ70 ЊC, except for the spectra of the aminoxyls which were
recorded at 0 ЊC or room temperature. All the experimental
spectra were simulated by means of a computer program, to
confirm the assignment of the hfc.
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The EPR samples were prepared in “H-shaped” quartz
tubes: this particular shape allowed to keep the two reactants
separate and to mix them just prior to introduce the sample-
tube in the spectrometer cavity. A solution (typically 10Ϫ2 M)
of a nitroaromatic substrate in THF, CH3CN or CH3CN–
THF mixture, was introduced in one of the two branches of
the EPR tube and analogously the nucleophile, dissolved in
the minimum amount of the solvent, was introduced in the
other (1:1 molar ratio between the reagents). The solution
was degassed by means of the freeze–pump–thaw technique
and the tube sealed off. The two reagents were then mixed
and the mixture immediately analysed at the EPR spectro-
meter.
When the reaction, in the same experimental conditions, was
repeated in the presence of a small amount of benzoquinone
(0.20 g, 1.8 mmol), an overall yield of 46.4% of the SNAr
product was obtained.
Reaction of 1-chloro-2,4-dinitrobenzene with triethylamine. A
mixture of 1-chloro-2,4-dinitrobenzene (2.00 g, 10 mmol) and
triethylamine (2.02 g, 20 mmol) in 50 mL of CH3CN was
refluxed under nitrogen for 6 hours. After the addition of 20
mL of H2O, the reaction mixture was extracted with diethyl
ether and the extract dried over Na2SO4. The solvent was
removed and the crude product was purified by silica gel
column chromatography using light petroleum–diethyl ether
(2:1) as the eluent. 0.13 g of N,N-Diethyl-2,4-dinitroaniline
(5.4%) were obtained: mp 80 ЊC (lit.24 79–80 ЊC), 1H-NMR
(CDCl3, 200 MHz) δ 1.23 (t, 6H, 2CH3), 3.37 (q, 4H, 2CH2),
7.06 (d, H6, Ar), 8.19 (m, H5, Ar), 8.63 (d, H3, Ar).25
Product analysis
General procedure. The SNAr reactions were carried out as
follows: a CH3CN or THF solution (ca. 0.20 M) of the reagents
(1:1 molar ratio, except for the reaction with the tertiary amine,
where an excess of the nucleophile, 1:2, was employed) was
stirred under nitrogen and refluxed for a variable period,
depending on the reactivity of the nucleophile. The reaction
mixture was then quenched with H2O and extracted with Et2O
(3 × 20 mL). The combined organic layers were dried over
Na2SO4 and the solvent was removed under reduced pressure.
Acknowledgements
This work was supported by Ministry of the University and
Technological Research (MURST).
References
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The products were characterised by H-NMR and 13C-NMR
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(Carlo Erba QMD 1000 GC-MS Spectrometer equipped with a
methyl silicon plus 5% phenyl silicon capillary column). All
compounds were identified by comparison of their retention
times with those of authentic samples and by their mass
spectra.
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R. Bacaloglu, A. Blaskò, C. A. Bunton, E. Dorwin, F. Ortega and
C. Zucco, J. Am. Chem. Soc., 1991, 113, 238; (g) R. Bacaloglu,
A. Blaskò, C. A. Bunton and F. Ortega, Atual. Fis.-Quim. Org. 1st,
1991, 165; (h) R. Bacaloglu, A. Blaskò, C. A. Bunton, F. Ortega and
C. Zucco, J. Am. Chem. Soc., 1992, 114, 7708.
(t, 2H, CH ), 5.08 (m, 2H, CH ᎐), 6.82 (m, 1H, CH᎐), 7.26 (d,
᎐
2
᎐
2
H6, Ar), 8.43 (m, H5, Ar), 8.72 (d, H3, Ar); 13C-NMR (CDCl3,
50.3 MHz) δ 29.5 (CH2), 30.7 (CH2), 70.4 (CH2), 114.4 (CH,
Ar), 115.3 (CH ᎐), 122.8 (CH, Ar), 129.5 (CH, Ar), 138.0
᎐
2
(CH᎐), 139.1 (quat, Ar), 142.5 (quat, Ar), 154.4 (quat, Ar).
᎐
Anal. Cald. for C11H12N2O5: C, 52.38; H, 4.80; N, 11.11. Found:
C, 52.32; H, 4.75; N, 11.16%.
8 L. Grossi, Tetrahedron Lett., 1992, 33, 5645.
Reaction of 1-chloro-2,4-dinitrobenzene with sodium 2-methyl-
propane-2-thiolate. 1-Chloro-2,4-dinitrobenzene (0.80 g, 3.9
mmol) was dissolved in 20 mL of CH3CN and stirred under
nitrogen. 0.45 g of Sodium 2-methylpropane-2-thiolate (4
mmol) were then added and, after an additional hour of stirring
at room temperature, the reaction was quenched with H2O. The
reaction mixture was washed with Et2O, the organic phase was
dried over Na2SO4 and the solvent removed. The solid residue
was then crystallised from methanol (2 × 7 mL) to yield yellow
crystals of 1-(tert-butylsulfanyl)-2,4-dinitrobenzene (0.72 g,
72.1%; mp 104–107 ЊC): 1H-NMR (CDCl3, 200 MHz) δ 1.48 (s,
9H, CMe3), 7.91 (d, H6, Ar), 8.34 (m, H5, Ar), 8.67 (d, H3, Ar);
13C-NMR (CDCl3, 50.3 MHz) δ 31.6 (CMe3), 50.2 (quat,
CMe3), 120.3 (CH, Ar), 125.8 (CH, Ar), 135.6 (CH, Ar), 140.4
(quat, Ar), 146.2 (quat, Ar), 152.1 (quat, Ar). Anal. Calcd. for
C10H12N2O4S: C, 46.87; H, 4.72; N, 10.93; S, 12.51. Found: C,
46.92; H, 4.68; N, 10.85; S, 12.54%.
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