Radical-nucleophilic substitution (SRN1) reactions. Part 7. Reactions of aliphatic α-substituted nitro compounds

Article abstract of DOI:10.1039/b103350f

α-Nitrothiocyanates R₂C(SCN)NO₂ have been prepared by oxidative addition of thiocyanate anion to nitronate anions and undergo S_RN1 substitution reactions by loss of thiocyanate with nitronate anions, phenylsulfinate, azide and p-nitro- and p-chloro-benzenethiolates in dipolar aprotic solvents. 2-Nitro-2-thiocyanatopropane and other 2-substituted-2-nitropropanes [Me₂C(X)NO₂ with X = I, Br, Cl, NO₂, PhSO₂] react with thiolates by S_RN1 reactions and/or redox reactions to give disulfides by a polar abstraction or chain SET (S_ET2) mechanisms. The products are dependent on the nucleophilicity of the thiolates, the polarisability of the α-substituent, the solvent and the presence of light catalysis, radical traps or strong electron acceptors. 2-Substituted-2-nitropropanes [Me₂C(X)NO₂ with X = N₃, NO₂, CN, p-NO₂-C₆H₄-N=N] undergo S_RN1 substitutions with thiolates by loss of nitrite. 2-Substituted-2-nitropropanes Me₂C(X)NO₂ and thiolates only yield disulfides in methanol due to solvation of the nitro groups.

Full text of DOI:10.1039/b103350f

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Radical-nucleophilic substitution (SRN1) reactions. Part 7.1
Reactions of aliphatic ꢀ-substituted nitro compounds.
Suleiman I. Al-Khalil, W. Russell Bowman,* Katherine Gaitonde,
Madeleine A. Marley (née Nagel) and Geoffrey D. Richardson
2
Department of Chemistry, Loughborough University, Loughborough, Leicestershire,
UK LE11 3TU
Received (in Cambridge, UK) 17th April 2001, Accepted 29th May 2001
First published as an Advance Article on the web 22nd June 2001
α-Nitrothiocyanates R2C(SCN)NO2 have been prepared by oxidative addition of thiocyanate anion to nitronate
anions and undergo SRN1 substitution reactions by loss of thiocyanate with nitronate anions, phenylsulfinate, azide
and p-nitro- and p-chloro-benzenethiolates in dipolar aprotic solvents. 2-Nitro-2-thiocyanatopropane and other
2-substituted-2-nitropropanes [Me2C(X)NO2 with X = I, Br, Cl, NO2, PhSO2] react with thiolates by SRN1 reactions
and/or redox reactions to give disulfides by a polar abstraction or chain SET (SET2) mechanisms. The products
are dependent on the nucleophilicity of the thiolates, the polarisability of the α-substituent, the solvent and the
presence of light catalysis, radical traps or strong electron acceptors. 2-Substituted-2-nitropropanes [Me2C(X)NO2
with X = N , NO , CN, p-NO –C H –N᎐N] undergo SRN1 substitutions with thiolates by loss of nitrite.
᎐
3
2
2
6
4
2-Substituted-2-nitropropanes Me2C(X)NO2 and thiolates only yield disulfides in methanol due to solvation of the
nitro groups.
steps using radical traps [e.g. di-tert-butylaminoxyl (DTBN)
Introduction
or oxygen] or strong electron acceptors [e.g. oxygen or p-
Aliphatic α-substituted nitro compounds undergo a variety of
interesting single electron transfer (SET) and radical reactions
and have been the subject of extensive study.2 In our contri-
bution to this study, we have been interested over some time in
the SET mechanisms of the reactions between α-substituted
nitro compounds and nucleophiles.1–9 We have also studied
the synthesis and the SET mechanisms of reactions of new
α-substituted nitro compounds1–10 which include α-nitro-
sulfides,3 α-nitrothiocyanates,4 α-nitroazides5 and ω-alkenyl-
α-halogeno-nitroalkanes.6 Coupled with these studies, we have
also investigated the SET mechanisms of targeted nucleophiles
(thiolates7,8 and N-anions of nitroimidazoles9 and other nitro-
diazoles1) with α-substituted nitro compounds. Our interest has
extended to the biological activity of these compounds which
act as antibacterial and antifungal agents, several of which are
used as commercial preservatives.10 The mode of action is due
to inhibition of enzymes by oxidation of thiol groups in the
active sites.10 This mode of action has focused much of our
studies on the reactions between α-substituted nitro com-
pounds and thiolates.1–9 In this paper we report the synthesis
and reactions of α-nitrothiocyanates which have been reported
in a preliminary communication.4 We also report the related
and continuing study of the mechanisms of reaction between
α-substituted nitro compounds and thiolates,3–6 some of which
have also been published in preliminary communications.7,8
Two routes have been observed for the SRN1 reactions
between α-substituted nitro compounds [R2C(X)NO2] and
nucleophiles as shown in Scheme 1. Most of these SRN1 reac-
tions are initiated by light-induced SET in a charge-transfer
complex between the nitro compound and the nucleophile
(NuϪ).2 Depending on the nature of the α-substituent, either
the α-substituent (X) (Route A) or the nitro group (Route B)
leaves from the intermediate radical anions. The route of dis-
sociation of the radical anions depends on the bond strength,
leaving group ability and overlap of the nitro π*-molecular
orbital (MO) holding the unpaired electron of the radical anion
with the C–X σ*-MO. Evidence for these steps in the mechan-
ism is commonly obtained from inhibition of the propagation
dinitrobenzene (DNB)].1–9
We have also obtained strong evidence for much of the
mechanism by using EPR spectroscopy to study the structure
and reactivity of the intermediate radical anions trapped in
solid matrices at low temperature.5,9,11,12 The nitro compounds
efficiently undergo electron capture to give rise to radical anions
which undergo dissociation by either route A or route B to yield
ؒ
ؒ
the intermediate radicals [R2C( )–NO2 or R2C( )–X]. Addition
of nucleophiles to these radicals to yield new radical anions has
also been observed for Route A [eqn. (2b)].12 We used the EPR
spectroscopic technique to predict that α-nitrothiocyanates
would react by Route A because dissociation of the radical
anions [Me2C(SCN)NO2]Ϫ to yield 2-nitro-2-propyl radicals
ؒ
was observed [eqn. (2a)].4,11 SRN
1 reactions of α-nitro-
thiocyanates were carried out to prove the accuracy of the EPR
spectroscopic conclusion.
Results and discussion
Mechanisms in the synthesis of ꢀ-nitrothiocyanates
We first attempted the synthesis using normal conditions for
SRN1 reactions between 2-bromo-2-nitropropane and thiocyan-
ate anions (Scheme 1 with R = Me, X = Br and Nu = SCN).
No traces of the desired product could be detected and
unaltered 2-bromo-2-nitropropane (100%) was recovered. Even
the use of entrainment with a good electron donating nucleo-
phile, such as the anion of 2-nitropropane, failed.2 This tech-
nique uses the good electron-donator to initiate the reaction
[eqn. (1) in Scheme 1] to yield the required radical anion
[Me2C(SCN)NO2]Ϫ which then enters the propagation steps
ؒ
[eqns. (2a)–(2c) with NuϪ = ϪSCN]. Only 2,3-dimethyl-2,3-
dinitrobutane, the normal product from SRN1 reaction between
2-bromo-2-nitropropane and the anion of 2-nitropropane,
was obtained as a product, indicating that the anion of 2-
nitropropane adds much faster than thiocyanate anions to the
intermediate 2-nitro-2-propyl radical. Thiocyanate adds easily
to the 2-nitro-2-propyl radical, as evidenced from the oxidative
DOI: 10.1039/b103350f
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This journal is © The Royal Society of Chemistry 2001
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Scheme 1 SRN1 mechanisms for α-substituted nitro compounds.
as indicated by EPR spectroscopic studies. Alternative routes of
dissociation of the intermediate radical anion of 1 by loss of
nitrite or cyanide were not observed by EPR spectroscopy or in
the liquid phase SRN1 reactions. The SRN1 reaction using the
anion of 2-nitropropane gave a good yield of the expected 2,3-
dimethyl-2,3-dinitrobutane 3. The use of the normal diagnostic
studies (absence of light or inhibition by an atmosphere of oxy-
gen or the addition of sub-stoichiometric amounts of DNB or
DTBN) gave reduction in the yield of 3 but not as much as
normally observed. This lower than predicted inhibition can be
explained by alternative SET non-chain mechanisms which are
common for reactions involving the anion of 2-nitropropane.2
The formation of the substitution product from phenylsulfinate
(PhSO2Ϫ) was completely inhibited with a good yield of
unaltered 2-nitro-2-thiocyanatopropane 1 indicating clearly
only an SRN1 mechanism. The reaction with azide anions was
troublesome and a low yield could be observed when using
HMPA as solvent. The low yield is probably explained by the
lack of stability of 2-azido-2-nitropropane.
Scheme 2 Oxidative addition of thiocyanate and nitronate anions 1,
R = Me (40%) and 2, R2C = c-C6H10 (43%).
addition reactions shown in Scheme 2. The results suggest that
SET between thiocyanate anions and 2-bromo-2-nitropropane
is poor. The EPR spectroscopic studies show that the radical
anion of 2-thiocyanato-2-nitropropane dissociates rapidly [eqn.
(2a) with R = Me and Nu = SCN], suggesting that dissociation
is faster than SET to the starting material [eqn. (2c) with
R = Me and Nu = SCN], thereby inhibiting the propagation
steps of the SRN1 reaction. Similarly, bromide and chloride
anions have not been observed to undergo SRN1 reactions.
Similar lack of reactivity for thiocyanate in SRN1 reactions
has been observed in reactions between thiocyanate and
halogenoquinolines.13
1-Nitro-1-thiocyanatocyclohexane 2 was also reacted with
the anion of 2-nitropropane to give a good yield of the corre-
sponding SRN1 product (60%) as well as 2,3-dimethyl-2,3-
dinitrobutane 3 (30%) (Scheme 3). Again, loss of thiocyanate
Two α-nitrothiocyanates, 2-nitro-2-thiocyanatopropane 1
and 1-nitro-1-thiocyanatocyclohexane 2, were successfully pre-
pared using oxidative addition (Scheme 2). Normally in this
synthetic procedure,1–5,14 a solution of potassium ferricyanide is
added to a solution of the nitronate anion and the nucleophile.
However, this procedure gave large amounts of 2,3-dimethyl-
2,3-dinitrobutane from the oxidative addition of the nitronate
anion of 2-nitropropane to the 2-nitro-2-propyl radical. This
indicates that the addition of nitronate anion to the inter-
mediate α-nitroalkyl radical is faster than the addition of thio-
cyanate, which concurs with the findings from the above
attempted SRN1 reactions. In order to overcome this problem, a
solution of the nitronate anion was added to a stirred solution
of thiocyanate and ferricyanide. The synthesis of 1-nitro-1-
thiocyanatocyclohexane 2 also yielded cyclohexanone (5%) as
an impurity. Ketones are common impurities in the oxidative
addition reactions. Thiocyanate is an ambident anion but no
signs of addition via the N-anion (isothiocyanate) were
observed. We have shown that nucleophile addition to 2-nitro-
2-propyl radicals is under kinetic control and that the more
nucleophilic centre in ambident anions reacts fastest.3,9
Scheme 3 Reaction conditions: i. DMSO, 4 h, light catalysis, nitrogen
atmosphere.
was observed. The SRN1 mechanism shown in Scheme 1 (Route
A) explains the substitution reactions observed for these
reactions.
Reaction between 2-nitro-2-thiocyanatopropane 1 and thiol-
ates gave a more complex mixture of products. The thiolate
anions of p-chlorobenzenethiol and p-nitrobenzenethiol both
gave the corresponding 2-nitro-2-(phenylsulfanyl)propane as
the major product with loss of thiocyanate. The yield of
1-methyl-1-nitroethyl p-chlorophenyl sulfide was reduced and
the yield of 1-methyl-1-nitroethyl p-nitrophenyl sulfide was
almost reduced to zero when inhibitors were added (see
Table 1). The yield of unaltered 1 also increased when inhibitors
were used, indicating an SRN1 reaction. Notably, the yield of
disulfide increased, indicating an alternative non-chain mechan-
ism of formation which is discussed more fully in the next
section.
SRN1 reactions of ꢀ-nitrothiocyanates
2-Nitro-2-thiocyanatopropane 1 was reacted with a range of
anions known to undergo SRN1 reactions (Table 1). All gave the
expected substitution of thiocyanate anion by the nucleophile,
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Table 1 Reactions: Me2C(SCN)NO2 + NuϪ → Me2C(Nu)NO2 + ϪSCN
% Yield
Nucleophile
Conditionsa
Me2C(Nu)NO2
Me2C(SCN)NO2
Other
Ϫ
Me2C᎐NO2
2 h; 2 h, DMF
72; 51
42; 38
34
34
49
0; 0
0; 0
0
0
0
᎐
2 h, dark; 2 h, O2
2 h, DNB (5 mol%)
2 h, DTBN (10 mol%)
2 h
Ϫ
PhSO2
2 h, dark; 2 h, O2
2 h, DNB (5 mol%)
2 h, DTBN (10 mol%)
HMPA, 90 min; 5 h
2 h
2 h, dark; 2 h, O2
2 h, DNB (40 mol%)
2 h, DTBN (40 mol%)
15 min
0; 0
0
0
8; 8
37
20; 12
17
26
17
0
5
40; 37
40
35
39; 27
0
10; 41
34
11
18
62
36
Ϫ
N3
p-ClC6H4SϪ
14b, 16c
16b, 26c; 4b, 10c
26b, 8c
0b, 0c
p-NO2C6H4SϪ
9b, 3c
15 min, O2
15 min, DNB (20 mol%)
15 min, DTBN (20 mol%)
15b, 4c
20b, 4c
0
36
15b, 3c
a All reactions were carried out in DMSO, photolysed with tungsten ‘white light’ (350 nm, 2 × 150 W), under an atmosphere of nitrogen, unless
otherwise stated. ‘Dark’ refers to reactions carried out without light catalysis and the reactions flask was wrapped in aluminium foil to further
exclude light. b % Yield of corresponding disulfide. c % Yield of 2,3-dimethyl-2,3-dinitrobutane.
Reaction between 2-nitro-2-thiocyanatopropane 1 and the
anion of diethyl 2-ethylmalonate 5 gave an efficient redox reac-
tion with the formation of dimeric products 3 and 6 (Scheme 4).
1. SRN1 mechanism. (Scheme 1, Route A with R = Me and
Nu = RS): 1-Methyl-1-nitroethyl sulfide products [Me2C(S-
R)NO2], chain reaction, light catalysed, inhibited by radical
traps and strong electron acceptors, only in dipolar aprotic
solvents, no reaction in protic solvents, favoured by weakly
nucleophilic thiolates, favoured by poor leaving groups (non-
polarisable).
Scheme 4 Reaction conditions: i. DMSO, 5 h, photolysis by tungsten
‘white light’ lamps (2 × 150 W), nitrogen atmosphere.
We propose a non-SET mechanism in which 5 abstracts thio-
cyanate from 1 to yield the anion of 2-nitropropane and diethyl
2-ethyl-2-thiocyanatomalonate. The anion of 2-nitropropane
and 1 undergo an SRN1 reaction to yield 3 (Table 1) and
diethyl 2-ethyl-2-thiocyanatomalonate and the anion of diethyl
2-ethylmalonate 5 also undergo SET by either an SRN1 or a
non-chain mechanism. Attempts to prepare diethyl 2-ethyl-2-
thiocyanatomalonate from 5 gave only the dimer 6, indicating
that it is easily formed. Reactions of the anion of diethyl
2-ethylmalonate 5 have been reported to give similar results as
well as SRN1 reactions with other α-substituted nitro com-
pounds.2,3a,15 An attempted SRN1 reaction between 1 and the
Scheme 5 Non-chain non-SET abstraction mechanism.
2. Non-SET abstraction mechanism. (Scheme 5), also termed
X-philic: Disulfide products, non-chain reaction, not light cat-
alysed, not inhibited by radical traps and strong electron
acceptors, best in protic solvents, favoured by strongly nucleo-
philic thiolates, favoured by α-substituents which are strongly
polarisable and have weaker bond strengths. The nitronate
anion formed reacts further by an SRN1 mechanism [eqn. (11)]
to yield 2,3-dimethyl-2,3-dinitrobutane 3 which is inhibited by
radical traps and strong electron acceptors and is less favoured
in protic solvents.
anion of diethyl phosphite, a commonly used anion in SRN
reactions, failed and gave intractable products.
1
Reactions between 2-substituted-2-nitropropanes and thiolate
anions
The reaction between 2-nitro-2-thiocyanatopropane and
more weakly nucleophilic thiolates, p-chloro- and p-nitro-
benzenethiolates, yielded both 1-methyl-1-nitroethyl sulfides
and disulfides, whereas the more nucleophilic benzenethiolate
anion only gave disulfide (36–52%). The yield of disulfide was
unchanged when inhibitors were added and no SRN1 product
was formed, i.e. a non-chain reaction. In order to explain
these reactions for 2-nitro-2-thiocyanatopropane, a compari-
son with other unpublished or preliminary results is instruc-
tive. The reactions between 2-substituted-2-nitropropanes and
thiolates of heterocyclic thiols, pyridine-2-thiol, pyrimidine-
2-thiol, benzothiazole-2-thiol, 1-methylimidazole-2-thiol and
4,5-dihydro-1,3-thiazole-2-thiol have been reported in full.3b
We have proposed three mechanisms for these reactions
which have been reported in full papers3–6 or in preliminary
communications.7,8 Distinguishing features are summarised
below:
3. SET redox mechanism. (Scheme 6), termed SET2 (substitu-
tion, electron transfer, bimolecular16): Disulfide products, chain
reaction, light catalysed, inhibited by radical traps and strong
electron acceptors, takes places in dipolar aprotic or protic
solvents, favoured by strongly nucleophilic thiolates in protic
solvents and unclear in dipolar aprotic solvents, favoured by
poor leaving groups (non-polarisable). The nitronate anion also
reacts as detailed in mechanism 2 above.
A similar disparity of mechanism has been reported for the
reactions between α-substituted nitro compounds and tributyl-
tin hydride (Bu3SnH). In these radical reactions, the difficulty
is determining whether there is SET between the tributyltin
ؒ
radicals (Bu3Sn ) and α-substituted nitro compounds to
yield the tributyltin cations (Bu3Sn+) and the radical anions
J. Chem. Soc., Perkin Trans. 2, 2001, 1557–1565
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Table 2 Reactions between 2-substituted-2-nitropropanes and thiolates in dipolar aprotic solvents
Me2C(X)NO2 + RSϪ → Me2C(SR)NO2 + RSSR + 3 + XϪ
% Yields
X
I
R (of RSϪ)
Conditionsa
Me2C(SR)NO2
RSSR
‘dimer’ 3
Me2C(X)NO2
b
b
o-NO2C6H4
p-ClC6H4
p-MeC6H4
p-NO2C6H4
p-ClC6H4
Ph
p-MeC6H4
Bn
Ph
DMF, 30 min
0
0
0
89
0
0
0
0
0, 0
0
0
0
0
0
0
0
0
97
75
78
0
98
79
DMF, 4 hc
DMF, 30 minc
76
0
b
b
Br
DMF, 4 hcd
DMF, 2 hc
70
87
90
86
36, 52
38
26
38
37
30
3
0
0
0
0
b
DMF, 2 hc
97
47
86
35, 50
41
11
12
13
22
3
DMF, 4 hc
DMF, 4 hc
SCN
DMSO, 10 min, 2 h
DMSO, 10 min, dark
DMSO, 10 min, O2
DMSO, 10 min, DNBe
DMSO, 10 min, DTBNe
DMSO, 2 h
DMSO, 2 h, dark, O2
DMSO, 2 h, dark, DNBe
DMSO, 2 h, dark, DTBNe
DMF, 5 h
10, 0
0
25
9
6
Bn
15 (15)f
31, 0
18 (22)f
39 (19)f
57
3
3
<2
14
44, 32
52
87
0
3
b
Cl
o-NO2C6H4
p-ClC6H4
Ph
34
17, 35
0
b
DMF, 20 min, 4 h
DMF, 4 h
DMF, 4 h
0, 0
0
0
13
28
Bn
0
NO2
o-NO2C6H4
p-ClC6H4
DMF, 24 h, 3 h
55, 36
69
0
32, 0
18
26
<2
0
59
0, 0
0
0
0
0
0
DMF, 18 hc
0
b
b
b
b
DMF, 18 h, O2
30
29, 92
33
36
50
91
0
48, 23
12
43
18
58
Ph
DMF, 2 h, 16 hc
DMF, 2 h, dark, O2
DMF, 2 h, dark, DNBe
DMF, 2 h, dark, DTBNe
DMF, 4 hc
10,b
4
6
6,b
23
16
<2
50
Bn
p-ClC6H4
Ph
15
0
b
b
SO2Ph
DMF, 4 h
DMF, 4 h,c 5 h
b, 12
0
8
38, 38
56
9
78
54
DMF, 5 h, dark, O2
DMF, 5 h, dark, DNBe
DMF, 5 h, dark, DTBNe
0
c
b
Bn
DMF, 4 h,
0
a All reactions were photolysed with tungsten ‘white light’ (350 nm, 2 × 150 W) under an atmosphere of nitrogen unless otherwise stated. The molar
ratio of thiolate anions : Me2C(X)NO2 = 2 : 1 unless otherwise stated. ‘Dark’ refers to reactions carried out without light catalysis and the reactions
flask was wrapped in aluminium foil to further exclude light. b % Yield was not measured. c The molar ratio of thiolate anions : Me2C(X)NO2 = 1 : 1.
d Ref. 3a. e 20% Molar equiv. f Polymer.
[R2C(X)NO2]Ϫ or abstraction of the α-substituent by Bu3Sn
and/or has weaker C–X bond strength (I > Br > SCN >
Cl > NO2 > SO2Ph). We have shown that formation of Me2-
C(SR)NO2 by the SRN1 reactions is catalysed by light and
inhibited by radical traps and strong electron acceptors (Table 1
and ref. 3).
ؒ
ؒ
via an SH2 mechanism.17 In the Bu3SnH reactions there is
abstraction of the α-substituent by the Bu3Sn radical (SH2)
instead of nucleophile polar abstraction as reported in this
present study (SN2 on X).
ؒ
The formation of disulfide is not inhibited by radical traps or
strong electron acceptors nor light catalysed when the α-
substituent is strongly polarisable and the thiolate strongly
nucleophilic, e.g. reactions between benzenethiolate and
Me2C(SCN)NO2 or Me2C(NO2)2. The yields of disulfide in
reactions between 2-bromo-2-nitropropane and benzene-,
phenylmethane- and p-chlorobenzenethiolates were unaltered
within experimental error when carried out under nitrogen and
oxygen. These results indicate that these redox reactions pro-
ceed by the non-SET abstraction mechanism (Scheme 5). How-
ever, when either the α-substituent is less polarisable (and/or
Reactions between 2-substituted-2-nitropropanes and thiolate
anions in dipolar aprotic solvents
The results of reactions between a range of 2-substituted-2-
nitropropanes and thiolates in dipolar aprotic solvents are pre-
sented in Table 1 (2-thiocyanato-2-nitropropanes) and Table 2.
The general trend of the mechanism and reactivity is clear.
Disulfide formation is favoured as the thiolate becomes more
nucleophilic (RSϪ with R = Bn > Ph > p-ClC6H4 > o- or p-
NO2C6H4) and as the α-substituent becomes more polarisable
Scheme 6 Chain SET redox mechanism.
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has a stronger C–X bond strength), e.g. SO2Ph, or the nucleo-
phile is strongly nucleophilic, e.g. phenylmethanethiolate,
there is some inhibition, e.g. reactions between phenylmeth-
anethiolate and Me2C(SCN)NO2, thereby indicating a chain
SET redox mechanism SET2 (Scheme 6). Other reactions are less
clear, e.g. the reaction between Me2C(SO2Ph)NO2 and benz-
enethiolate, which possibly suggests a mixture of mechanisms.
This third mechanism, the chain SET redox SET2 mechanism
(Scheme 6), was first proposed by Russell and co-workers for
the reactions between α-substituted nitroalkanes and enolate
anions which gave dimers of the enolates.16 We also proposed
this mechanism in a preliminary communication8 to explain the
formation of disulfide. The SET shown in eqn. (13) should be
favoured when the leaving group X is poor, thereby increasing
nitrite to yield 1-azido-1-methylethyl p-chlorophenyl sulfide
(70%) and 1-(1-azidocyclohexyl) p-chlorophenyl sulfide (53%)
respectively.5 The reaction between p-chlorobenzenethiolate
and the α-nitro-ketone, 3-methyl-3-nitrobutan-2-one [Me2C-
(NO2)COMe] gave an intractable mixture of products and was
not further investigated. However, reaction between 2-cyano-2-
nitropropane 7 and benzenethiolate gave a good yield (53%) of
the SRN1 product 8 with the expected loss of nitrite (Scheme 7).
Scheme 7 Reagents and conditions: HMPA, 3 h, photolysis by tungsten
‘white light’ lamps (2 × 150 W), nitrogen atmosphere.
the lifetime of the intermediate [Me2C(X)NO2]Ϫ radical anion.
ؒ
No reaction was obtained with the less nucleophilic p-chloro-
benzenethiolate.
Reactions between (1-methyl-1-nitroethyl)(p-nitrophenyl)-
diazene 9 and thiolates also gave good yields of SRN1 substi-
tution products 10 with loss of nitrite (Scheme 8). The results
Likewise, a strongly nucleophilic thiolate will react much faster
and thereby intercept the radical anion before it has had time
ؒ
to dissociate [eqn. (13)]. Thiyl radicals (RS ) are known to
add to thiolate anions to form detectable disulfide radical
anions [(RSSR)Ϫ ] [eqn. (14)] and have been observed by
ؒ
EPR spectroscopy.18 Evidence for the intermediacy of radical
anions of disulfides in redox reactions between thiolate anions
and 1-benzyl-1,4-dihydropyridine-3-carboxamide (BNAH) has
also been reported.19 The chain reaction is completed by
SET between the disulfide radical anion and the starting
Me2C(X)NO2 [eqn. (15)].
Blank reactions show that the oxidation of thiolate to
disulfide in an atmosphere of oxygen is very slow and not sig-
nificant in inhibition reactions. However, certain factors cloud
the analysis of results, e.g. p-dinitrobenzene is a slow oxidant of
thiolates to disulfide and gives higher yields of disulfide than
expected. Some of the 2-substituted-2-nitropropanes react to
yield the corresponding SRN1 product Me2C(SR)NO2 which
reacts with further thiolate to yield disulfide, e.g. reaction
between Me2C(NO2)2 and benzenethiolate. In most reactions,
some of the ‘dimer’, 2,3-dimethyl-2,3-dinitrobutane 3 was
formed. This formation resulted from reaction between the
starting 2-substituted-2-nitropropane and the nitronate anion,
by abstraction of the α-substituent (Scheme 5). Formation of
the dimer 3 was largely inhibited when radical traps or strong
electron acceptors were used, indicating the SRN1 mechanism
for its formation.
In the non-SET abstraction mechanism we have assumed the
arylsulfanyl derivatives (ArS–X) formed by abstraction react
rapidly with a second equivalent of thiolate to yield disulfides.
Sulfanyl iodides (RSI), bromides (RSBr), chlorides (RSCl),
nitrites (RSNO2) and sulfonates (RS–SO2Ph) are all known to
react rapidly with thiolate to yield disulfides. In the reaction
between 2-thiocyanato-2-nitropropane and phenylmethane-
thiolate the intermediate sulfanylthiocyanate (BnS–SCN) is
known to eliminate thiocyanic acid (HSCN) to yield thiobenz-
aldehyde.20 Attempts to isolate thiobenzaldehyde have been
generally unsuccessful; it either polymerises or can be trapped
by suitable dienes.20 We isolated a polymer in each reaction
which we assume results from thiobenzaldehyde (Table 1).
Our mechanistic proposal for the non-chain non-SET mech-
anism (Scheme 5) is supported by evidence in the literature
which indicates that thiolates react by polar abstraction of the
α-substituent by a non-SET mechanism with a range of acti-
vated halogeno compounds to yield disulfides, e.g. α-nitro-
benzyl halides,21 α-halogenoketones,22 α-halogenosulfones23
and halogenoalkynes.24
Scheme 8 Reagents and conditions: i. DMF, 18 h, photolysis by
tungsten ‘white light’ lamps (2 × 150 W), nitrogen atmosphere.
did not appear to follow the trend of nucleophilicity of
the thiolates. However, reaction with the more nucleophilic
phenylmethanethiolate yielded only disulfide and the p-nitro-
phenylhydrazone of acetone (20%). Interestingly, the attempted
SRN1 reaction between 9 and the anion of 2-nitropropane
reported in the literature25 gave no SRN1 substitution product
but instead gave a redox reaction to yield the ‘dimer’ of nitro-
propane 3. Reactions between 2-methyl-2-nitropropane
(Me3CNO2) and thiolates gave only disulfide products with
no SRN1 substitution [p-chlorobenzenethiolate (24 h), RSSR
(34%); benzenethiolate (16 h), RSSR (55%); phenylmethane-
thiolate (8 h), RSSR (77%)]. The yields of these redox reactions
reflect the nucleophilicity (or reduction ability) of the thiolates.
No products resulting from the 2-methyl-2-nitropropane were
detected. We propose that the mechanism is probably a non-
chain SET reaction [Scheme 6, eqns. (12), (14) and (15)].
Reactions between 2-substituted-2-nitropropanes and thiolate
anions in protic solvents
In sharp contrast to the reactions in dipolar aprotic solvents,
only disulfide products, and no SRN1 products, were obtained in
protic solvents (MeOH or MeOH–water) (Table 3). With the
polarisable α-substituents (I, Br, SCN), high yields of disulfide
were obtained even with weakly nucleophilic thiolates such as
p-nitro- and p-chloro-benzenethiolate. These reactions were not
light catalysed nor inhibited by radical traps and strong elec-
tron acceptors, e.g. Me2C(Br)NO2 and p-nitrobenzenethiolate
or p-chlorobenzenethiolate and Me2C(SCN)NO2 and p-
nitrobenzenethiolate or p-chlorobenzenethiolate. All of these
reactions gave major amounts of the SRN1 products in dipolar
aprotic solvents (ref. 3a and Table 1).
We also determined the reactions between thiolates and 2-
substituted-2-nitropropanes for which the intermediate radical
anions in SRN1 reactions are known to dissociate with loss of
nitrite (Scheme 1, Route B). Reactions of 2,2-dinitroprop-
ane can be regarded as loss of the α-substituent or nitrite.
We have previously shown that the reactions between 2-
azido-2-nitropropane and 2-azido-2-nitrocyclohexane and
p-chorobenzenethiolate gave SRN1 substitutions with loss of
J. Chem. Soc., Perkin Trans. 2, 2001, 1557–1565
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Table 3 Reactions between 2-substituted-2-nitropropanes and thiolates in MeOH Me2C(X)NO2 + RSϪ → RSSR + Me2C᎐NO2Ϫ + XϪ
᎐
% Yield
RSSR
X
R (of RSϪ)
Conditionsa
Me2C(X)NO2
I
Br
p-ClC6H4
p-NO2C6H4
5 min
94
98
74, 75
77; 67
85
59, 69
95
71
70
68
96
0
0
0, 0
0; 0
0
0, 0
0
0
MeOH–H2O (85 : 15), 48 h
15 min
15 min, dark; 15 min, O2
15 min, DNBb
15 min, DTBNb
MeOH–H2O (85 : 15), 5 min
20 min
p-ClC6H4
20 min, DNBb
20 min, DTBNb
5 min
0
0
0
Ph
SCN
p-NO2C6H4
MeOH–H2O (85 : 15), 20 min
15 min
39, 60
77
0, 0
0
15 min, dark; 15 min, O2
15 min, DNBb
15 min, DTBNb
20 min
20 min, dark; 20 min, O2
20 min, DNBb
20 min, DTBNb
4 h
69, 73; 72
78
55
60
55; 53
55
0, 0; 0
0
0
0
0; 0
0
0
p-ClC6H4
p-ClC6H4
56
45
Cl
0
4 h, dark; 4 h, O2
32, 33
26
28
0, 0
0
0
4 h, DNBb
4 h, DTBNb
20 min
36
21
18
30
6
12
22
0
20 min, O2
20 min, DNBb
20 min, DTBNb
2 h
NO2
p-ClC6H4
44
39
2 h, dark; 2 h, dark, O2
2 h, DNBb; 2 h, dark, DNBb
1 h
12; 7
6
34
55; 70
55
31
1 h, dark; 2 h, O2
1 h, DTBN
4 h
6; 23
33
38, 28
11
50
28
47; 43
47
62, 53
50
50
32
Ph
4 h, dark, O2
4 h, dark, DNBb
4 h, dark, DTBNb
a All reactions were carried out in MeOH, photolysed with tungsten ‘white light’ (350 nm, 2 × 150 W), under an atmosphere of nitrogen and with a
molar ratio of RSϪ : Me2C(X)NO2 = 2 : 1, unless otherwise stated. ‘Dark’ refers to reactions carried out without light catalysis and the reactions flask
was wrapped in aluminium foil to further exclude light. b DTBN or DNB at 20% molar equiv.
With less polarisable α-substituents with stronger C–X bonds
(e.g. Cl, NO2), the reactions were much slower and unaltered
starting 2-substituted-2-nitropropanes were also measured, e.g.
Me2C(Cl)NO2 or Me2C(NO2)2 and p-chlorobenzenethiolate.
These reactions also showed inhibition and light catalysis,
suggesting that a chain redox SET2 mechanism (Scheme 6)
was predominant. The inhibition of the reaction between
Me2C(NO2)2 and benzenethiolate was less clear.
We have explained this clear difference in mechanism by
solvation of the nitro group.3b,8 SRN1 reactions have been
reported largely for dipolar aprotic solvents.2 The effect of solv-
ation on nucleophiles in SRN1 reactions has been shown to be
significant.26 However, the solvation of thiolates is similar in
protic and dipolar aprotic solvents because of the size and
polarisability of the large sulfur atom,27,28 and therefore, the
effect of solvation must lie with the nitro compounds. Similarly,
the nucleophilicity of thiolates is almost the same in protic and
dipolar aprotic solvents.28 The rate of dissociation of proto-
nated radical anions of p-nitrobenzyl halides is slower than that
of the non-protonated radical anion, e.g. the dissociation of the
protonated radical anion of p-nitrobenzyl bromide is 60 times
slower.29 We have predicted that H-bonding should have similar
effects and have suggested that the effect of protic solvation is
two-fold.8 Firstly, the methanol solvates the intermediate rad-
ical anion 11 in the SRN1 or SET2 mechanisms as shown in
Scheme 9. The H-bonding draws electron density from the nitro
group and hence from the σ* MO of the C–X bond and thereby
Scheme 9 Solvated species in SRN1 and abstraction mechanisms.
slows the rate of dissociation. For dissociation to occur, a
transition state needs rearrangement of the molecular orbitals
to give higher electron density in the C–X bond to facilitate
dissociation by loss of XϪ. Secondly, as shown in Scheme 9, for
the non-SET abstraction reaction, solvation of the nitro group
in the starting material, and more significantly in the transition
state where charge on the oxygen atoms is higher, will strongly
lower transition state energy for abstraction of the α-
substituent by thiolate. Further evidence for our initially pro-
posed mechanism has been reported for the reactions between
α-bromo nitro compounds and thiolates in
a synthetic
protocol.30
1562
J. Chem. Soc., Perkin Trans. 2, 2001, 1557–1565

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spectroscopy which indicated cyclohexanone as a minor con-
Experimental
taminant. Purification by preparative TLC (silica gel with
carbon tetrachloride and hexane (60 : 40) as eluant) gave 2 as a
yellow oil (4.02 g, 43%) (Found: C, 45.6; H, 5.6; N, 15.0; S, 16.8.
C7H10N2SO2 requires C, 45.16; H, 5.37; N, 15.05; S, 17.20%);
νmax(neat)/cmϪ1 2160 and 1550; δH (60 MHz) 1.50–1.70 (6 H, m)
and 2.15–2.45 (4 H, m). A repeat reaction using diethyl ether in
place of dichloromethane gave 34% of 2.
General
DMF and DMSO were distilled at reduced pressure from cal-
cium hydride and stored over molecular sieves. Methanol was
dried using magnesium and iodine. Light petroleum refers to
the bp 40–60 ЊC fraction. Mps were determined on a Koffler
block melting point apparatus and are uncorrected. Elemental
analyses were determined on a Perkin-Elmer 2400 CHN Elem-
ental Analyser in conjunction with a Perkin-Elmer AD-4 Auto-
balance. IR spectra were recorded as solutions of CDCl3 with
tetramethylsilane (TMS) as the internal standard on a Perkin-
Elmer 177 spectrophotometer on NaCl plates. NMR spectra
were recorded at 90 MHz on a Perkin-Elmer R32 spectrometer
or at 60 MHz on a Varian EM 360A spectrometer. Chemical
shifts are given in parts per million (ppm) and J values in
hertz (Hz). Analyses of reaction mixtures using 1H NMR
spectroscopy were carried out using p-dinitrobenzene or
p-dimethoxybenzene as internal standards. TLC using silica gel
as absorbent was carried out with aluminium backed plates
coated with silica gel (Merck Kieselgel 60 F254). Silica gel
(Merck Kieselgel 60 H silica) was used for column chrom-
atography unless otherwise specified. Photolysis of reactions
was carried out using tungsten ‘white light’ lamps (2 × 150 W)
or fluorescent desk lamps (2 × 15 W) which were positioned on
either side of the reaction at a distance of 15 cm. The pro-
cedures for carrying out diagnostic inhibition studies and reac-
tions in the absence of light for SRN1 reactions are fully
described in previous papers.2–9
General procedure for SRN1 reactions between nucleophiles and
2-nitro-2-thiocyanatopropane
Sodium salt of 2-nitropropane. The anion of 2-nitropropane
(1.00 g, 9 mmol) was dissolved in dry DMSO under nitrogen in
a dry flask with one neck covered with a rubber septum. Nitro-
gen gas was passed through the solution for 30 min to complete
deoxygenation. 2-Nitro-2-thiocyanatopropane 1 (550 mg, 3.7
mmol) was added via a syringe through the septum. The colour
of the reaction turned red and remained red throughout the
reaction. The reaction was illuminated by two tungsten ‘white
light’ lamps for 2 h. Ice-cold water was poured into the reaction
and extracted with diethyl ether. The ethereal extracts were
combined and washed with water (7 times) to remove residual
DMSO, dried and evaporated to dryness. The crude product
was recrystallised with ethanol to yield pure colourless crystals
of 2,3-dimethyl-2,3-dinitrobutane 3 (480 mg, 72%), mp 207–
208 ЊC (lit.3 207–208 ЊC); νmax(Nujol)/cmϪ1 1552; δH (60 MHz)
1.73 (s, Me). The mp, IR and 1H NMR spectra and TLC were
identical to those of authentic material.
Sodium benzenesulfinate. Sodium benzenesulfinate (1.00 g,
6 mmol) gave 1-methyl-1-nitroethyl phenyl sulfone (420 mg,
49%), mp 116–118 ЊC (EtOH) (lit.3a mp 116–117 ЊC);
νmax(Nujol)/cmϪ1 1590, 1550 and 760; δH (60 MHz) 1.95 (6 H, s,
Me) and 7.50–7.75 (5 H, m, PhH).
Materials
Literature procedures were used for the synthesis of 2-bromo-
2-nitropropane,31 2-chloro-2-nitropropane,31 2-iodo-2-nitro-
propane,31 2,2-dinitropropane,14 1-methyl-1-nitroethyl phenyl
sulfone,3a 2-azido-2-nitropropane,5 2-cyano-2-nitropropane,14
(1-methyl-1-nitroethyl)(p-nitrophenyl)diazene 9,25 and the
sodium salts of 2-nitropropane,3a diethyl 2-ethylmalonate3a
and all the thiolates.3 The following were synthesised for
comparison as authentic samples: 2,3-dimethyl-2,3-
dinitrobutane 3,3a 2,3-bis(ethoxycarbonyl)-2,3-diethylsuccinic
acid diethyl ester 6,25 1-(1-methyl-1-nitroethyl)-1-nitro-
cyclohexane,3a 1-methyl-1-nitroethyl p-nitrophenyl sulfide,3a
1-methyl-1-nitroethyl o-nitrophenyl sulfide,3a 1-methyl-
1-nitroethyl p-chlorophenyl sulfide,3a 1-methyl-1-nitroethyl
phenyl sulfide3a and all the disulfides.3a
Sodium azide. Sodium azide (500 mg, 7.6 mmol) was reac-
ted with HMPA instead of DMSO for 90 min and for 5 h to
yield crude products which were analysed using 1H NMR
spectroscopy and TLC with an internal standard.
Sodium diethyl 2-ethylmalonate. The general procedure for
SRN1 reactions using 2-nitro-2-thiocyanatopropane 1 (70 mg,
0.48 mmol) and sodium diethyl 2-ethylmalonate (100 mg,
0.48 mmol) for 5 h yielded a crude product which was leached
with hexane. The solution was evaporated to yield pure
2,3-bis(ethoxycarbonyl)-2,3-diethylsuccinic acid diethyl ester 6
(47 mg, 53%). The residual crystals were recrystallised from
ethanol to yield 2,3-dimethyl-2,3-dinitrobutane 3 (40 mg, 95%).
Both compounds were identical to authentic material (mp, IR
and 1H NMR spectra).3a
Oxidative addition reactions
2-Nitro-2-thiocyanatopropane 1. Freshly distilled 2-nitro-
propane (4.45 g, 0.05 mol) was dissolved in a solution of
sodium hydroxide (2.2 g, 1.1 molar equiv.) in water (30 cm3).
The solution was added dropwise to a two phase solution of
sodium thiocyanate (8.10 g, 0.01 mol, 2 equiv.) and potassium
ferricyanide (32.90 g, 0.01 mol, 2 equiv.) in water (30 cm3) and
dichloromethane (70 cm3) at a rate such that the temperature
remained between 20 and 25 ЊC. The reaction mixture was
stirred for 20 min. The ethereal layer was separated and the
aqueous layer extracted with diethyl ether. The ether extracts
were combined, washed with water, dried and evaporated to
dryness to yield a yellow liquid. Fractional distillation gave a
yellow oil of 1 (2.46 g, 40%), bp 66–68 ЊC (1.5 mm Hg) (Found:
C, 32.7; H, 4.3; N, 19.4; S, 22.1. C6H6N2SO2 requires C, 32.86;
H, 4.14; N, 19.20; S, 21.94%); νmax(neat)/cmϪ1 2880, 2170 and
1550; δH (60 MHz) 2.10 (s, Me); m/z (EI) 100 (M+ϪNO2, 100,
100%) and 41 (32).
1-(1-Methyl-1-nitroethyl)-1-nitrocyclohexane
The general procedure for SRN1 reactions using 1-nitro-1-
thiocyanatocyclohexane (550 mg, 3 mmol) and the anion of
2-nitropropane (1.00 g, 9 mmol) for 5 h yielded 2,3-dimethyl-
2,3-dinitrobutane 3 (80 mg, 30%) and 1-(1-methyl-1-nitro-
ethyl)-1-nitrocyclohexane (390 mg, 60%). Both products were
1
identical to authentic material (mp, IR and H NMR spectro-
scopy and TLC).3a
Reactions between 2-substituted-2-nitropropanes and thiolate
anions
The procedures for the preparation of thiolate anions using
sodium methoxide in methanol and for reactions between
2-substituted-2-nitropropanes and thiolate anions are fully
described in reference 3. The solvents, times of reaction, molar
equivalents of reagents, and other conditions are defined in the
Tables. The disulfides are all known compounds and were
1-Nitro-1-thiocyanatocyclohexane 2. The above conditions
were used with nitrocyclohexane (6.45 g) to yield a crude oil
of 2 which was analysed using GLC and IR and H NMR
1
J. Chem. Soc., Perkin Trans. 2, 2001, 1557–1565
1563

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purchased commercially. The 1-methyl-1-nitroethyl aryl sulfides
and disulfides were compared with authentic material.3 In those
studies in which yields were measured using H NMR spec-
troscopy with an internal standard, products were isolated and
characterised in at least one experiment in each series. A sample
procedure for reactions in dipolar aprotic and protic solvents is
detailed below.
C15H14N4O4S requires C, 52.0; H, 4.06; N, 16.18; S, 9.2%);
νmax(neat)/cmϪ1 1535 and 1354; δH (60 MHz) 1.69 (6 H, s, Me),
7.40–7.50 (4 H, m, p-ClC6H4) and 8.01 (4 H, AAЈBBЈq,
p-NO2C6H4); m/z (EI) 196, 192, 155 and 146. Bis(p-chloro-
phenyl) disulfide (44%) was also separated and compared with
authentic material (mp, TLC and IR and 1H NMR spectra).
1
Protic solvents. Reaction between 2-nitro-2-thiocyanatoprop-
ane 1 and the sodium salt of p-chlorobenzenethiol. The general
procedure for reactions between 2-substituted 2-nitropropanes
and thiolates was used with methanol in place of the dipolar
aprotic solvent. The sodium salt of p-chlorobenzenethiol (500
mg, 3 mmol) and 2-nitro-2-thiocyanatopropane 1 (220 mg, 1.5
mmol) yielded a crude product which was analysed by TLC
Dipolar aprotic solvents. Reaction between 2-nitro-2-thio-
cyanatopropane 1 and the sodium salt of p-chlorobenzenethiol.
The sodium salt of p-chlorobenzenethiol (500 mg, 3 mmol) was
dissolved in dry DMSO under nitrogen in a dry flask with one
neck covered with a rubber septum. The reaction mixture was
stirred and nitrogen gas was passed through the solution for 30
min to complete deoxygenation. 2-Nitro-2-thiocyanatopropane
1 (500 mg, 3.6 mmol) was added via a syringe through the
septum. The colour of the reaction turned red and remained
red throughout the reaction. The reaction was illuminated by
two tungsten ‘white light’ lamps for 2 h. Ice-cold water was
poured into the reaction and extracted with diethyl ether. The
ethereal extracts were combined and washed with water (7
times) to remove residual DMSO, dried and evaporated to
dryness. The crude product was leached with hexane and the
residue was recrystallised from EtOH to yield 2,3-dimethyl-2,3-
dinitrobutane 3 (16%). The solution was evaporated to dryness
and the residue recrystallised to yield 1-methyl-1-nitroethyl
p-chlorophenyl sulfide (37%); mp 81–82 ЊC (lit.3a mp 82–83 ЊC);
νmax(Nujol)/cmϪ1 1590 and 1552; δH (60 MHz) 1.83 (6 H, s, Me)
and 7.40 (4 H, AAЈBBЈq, ArH). The filtrate from the recrys-
tallisation was purified by chromatography to yield bis-
(p-chlorophenyl) disulfide (14%). All compounds were identical
to authentic materials (TLC, mp, IR and 1H NMR spectra).3a
Reaction between 2-cyano-2-nitropropane 7 and the sodium
salt of benzenethiol. The general procedure for reactions
between 2-substituted-2-nitropropanes and thiolate anions was
used with 2-cyano-2-nitropropane (200 mg, 1.75 mmol) and the
sodium salt of benzenethiolate (230 mg, 1.74 mmol) in HMPA
(40 cm3) for 3 h. The reaction gave a crude mixture which was
purified by preparative TLC to yield 1-cyano-1-methylethyl
phenyl sulfide 8 as a yellow oil (180 mg, 53%) (Found: C, 67.4;
H, 6.1; N, 8.0; S, 18.4. C10H11NS requires C, 67.76; H, 6.25;
N, 7.90; S, 18.09%); νmax(neat)/cmϪ1 1595, 1150, 1070, 750
and 695; δH (60 MHz) 1.95 (6 H, s, Me) and 7.20–7.40 (5 H, m,
PhH).
Reaction between (1-methyl-1-nitroethyl)(p-nitrophenyl)-
diazene 9 and thiolates. All the reactions were carried out using
the standard conditions in dry DMF with fluorescent lamps
(2 × 15 W) for 18 h using an equimolar ratio of 9 and the
respective thiolate. All products were purified by preparative
TLC using mixtures of diethyl ether and light petroleum as
eluant.
a. Benzenethiolate. [1-Methyl-1-(phenylsulfanyl)ethyl](p-
nitrophenyl)diazene 10 (R = Ph) (54%); mp 124–126 ЊC (from
CCl4) (Found: C, 60.0; H, 5.1; N, 13.5; S, 10.8. C15H15N3O2S
requires C, 59.8; H, 5.0; N, 13.9; S, 10.6%); νmax(neat)/cmϪ1 1535
and 1357; δH (60 MHz) 1.70 (6 H, s, Me), 7.20–7.30 (5 H, m,
PhH) and 8.01 (4 H, AAЈBBЈq, p-NO2C6H4); m/z (EI) 192, 151,
150, 146, 125, 122 and 105. Unaltered starting material 9 (19%)
was also separated, mp 102–104 ЊC (lit.25 104 ЊC).
1
and H NMR spectroscopy using p-dimethoxybenzene as an
internal standard to show pure bis(p-chlorophenyl) disulfide
(60%) and no unaltered 1 or other products. The residue was
recrystallised from hexane to yield pure bis(p-chlorophenyl)
disulfide which was identical to authentic material (TLC, mp,
IR and 1H NMR spectra).
Acknowledgements
We thank the EPSRC for a postgraduate studentship (G. D. R.)
and Mike Harris and Hansa Madha for technical assistance.
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b. o-Nitrobenzenethiolate. [1-Methyl-1-(o-nitrophenylsulf-
anyl)ethyl](p-nitrophenyl)diazene 10 (R = o-NO2C6H4) (16%);
mp 92–93 ЊC (from CCl4) (Found: C, 51.6; H, 4.0; N, 15.9; S,
9.6. C15H14N4O4S requires C, 52.0; H, 4.06; N, 16.18; S, 9.2%);
νmax(neat)/cmϪ1 1535 and 1354; δH (60 MHz) 1.69 (6 H, s, Me),
7.40–7.60 (4 H, m, o-NO2C6H4) and 8.01 (4 H, AAЈBBЈq, p-
NO2C6H4); m/z (EI) 196, 192, 155 and 146. Unaltered starting
material 9 (84%) was also separated.
c. p-Chlorobenzenethiolate. [1-Methyl-1-(p-chlorophenylsulf-
anyl)ethyl](p-nitrophenyl)diazene 10 (R = p-ClC6H4) (25%); mp
92–93 ЊC (from CCl4) (Found: C, 51.6; H, 4.0; N, 15.9; S, 9.6.
17 W. R. Bowman, D. Crosby and P. J. Westlake, J. Chem. Soc., Perkin
Trans. 1, 1991, 73.
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