N-trifluoroacetyl arenesulfenamides, effective precursors for synthesis of unsymmetrical disulfides and sulfenamides

Article abstract of DOI:10.1016/j.tet.2003.09.080

N-Trifluoroacetyl arenesulfenamides (3) were effective precursors for the synthesis of unsymmetrical disulfides (4) and sulfenamides (5). Reactions of 3 with a variety of aromatic thiols at room temperature were generally complete within 5 min and gave unsymmetrical diaryl disulfides in high yields. Aralkyl disulfides were isolated in high yields from the reaction of 3 with aliphatic thiols. The nucleophilic substitution reactions of 3 with amines proceeded smoothly and provided N-substituted sulfenamides in good to excellent yields.

Full text of DOI:10.1016/j.tet.2003.09.080

Tetrahedron 59 (2003) 9655–9659
N-Trifluoroacetyl arenesulfenamides, effective precursors for
synthesis of unsymmetrical disulfides and sulfenamides
*
Ming Bao and Masao Shimizu
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba,
Ibaraki 305-8565, Japan
Received 8 May 2003; accepted 24 September 2003
Abstract—N-Trifluoroacetyl arenesulfenamides (3) were effective precursors for the synthesis of unsymmetrical disulfides (4) and
sulfenamides (5). Reactions of 3 with a variety of aromatic thiols at room temperature were generally complete within 5 min and gave
unsymmetrical diaryl disulfides in high yields. Aralkyl disulfides were isolated in high yields from the reaction of 3 with aliphatic thiols. The
nucleophilic substitution reactions of 3 with amines proceeded smoothly and provided N-substituted sulfenamides in good to excellent
yields.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
derived from the reaction of phthalimides or hydantoins
with sulfenyl chlorides, can be used as sulfenyl-transfer
reagents to prepare other sulfenamides. However, gener-
ation of sulfenyl chlorides requires chlorination of disulfides
or thiols with hazardous, poisonous, and corrosive chlorine
gas, the use of which does not reflect the modern trend
toward using safer reagents.
Disulfides and sulfenamides have attracted considerable
interest because of their practical applications in bio-
chemistry, pharmacological chemistry, and industrial
chemistry as well as their utility as synthetic reagents.1
The development of an efficient method for the preparation
of disulfides and sulfenamides is still a challenge in organic
synthesis. Although symmetrical disulfides can be prepared
by the direct transformation of thiols, the synthesis of
unsymmetrical disulfides usually requires the construction
of sulfenyl derivatives in advance, followed by nucleophilic
substitution with thiols. Over the past three decades, many
sulfenyl derivatives have been prepared for the synthesis of
unsymmetrical disulfides: dithioperoxyesters,2 thio-
sulfinates and thiosulfonates,3 S-alkylthiosulfates and
S-arylthiosulfates (Bunte salts),4 40-nitroarenesulfen-
anilides,5 thioimides (thiophthalimides, thiosuccinimides,
and thiomaleimides),6 thionitrites,7 alkylthiodialkyl-
sulfonium salts,8 sulfenyl thiocarbonates,9 sulfenyl thio-
cyanates,10 and S-alkylthioisothioureas.11 However, using
these sulfenyl compounds to synthesize unsymmetrical
disulfides often has drawbacks. For example, because
rapid thiol–disulfide exchange occurs under the typical
reaction conditions (high temperature, acidic conditions),
pure unsymmetrical diaryl disulfides are not always
obtained.
We have recently reported the efficient preparation of
N-trifluoroacetyl arenesulfenamides by a chlorine-free
procedure.14 Herein, we report the use of N-trifluoroacetyl
arenesulfenamides as effective precursors for the synthesis
of unsymmetrical disulfides and sulfenamides.
2. Results and discussion
2.1. Synthesis of unsymmetrical disulfides
In our initial studies, we investigated nucleophilic
substitution reactions of N-acetyl-2-(methoxycarbonyl)-
benzenesulfenamide (1), N-benzoyl-2-(methoxycarbonyl)-
benzenesulfenamide (2), and N-(trifluoroacetyl)-2-
(methoxycarbonyl)benzenesulfenamide
benzenethiol in dichloromethane at room temperature
(3a)
with
(Scheme 1). After the reaction mixture of 1 or 2 with
N-Sulfenylphthalimides12 and 3-sulfenylhydantoins,13
Keywords: synthesis; unsymmetrical disulfides; sulfenamides;
N-trifluoroacetyl arenesulfenamides.
*
Corresponding author. Tel.: þ81-29-861-4575; fax: þ81-29-861-4487;
e-mail: m.shimizu@aist.go.jp
Scheme 1.
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.09.080

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M. Bao, M. Shimizu / Tetrahedron 59 (2003) 9655–9659
benzenethiol was stirred for 5 h, phenyl 2-(methoxy-
carbonyl)phenyl disulfide (4a)15 was obtained in 37 or
62% yield, respectively, along with the symmetrical
disulfides 2,20-di[(methoxycarbonyl)phenyl] disulfide
and diphenyl disulfide. However, 4a was obtained in a
high yield (89%) from the reaction of 3a with
benzenethiol in a short time (Table 1, entry 1, reaction
time: 5 min.).
in satisfactory yields (85 and 72%, respectively; entries 9
and 12), the latter reaction required high temperature
to afford 4k in such a yield. These results indicated
that the reactivity of 3 in the nucleophilic substitution
reaction with an aliphatic thiol depends on the
electronic nature of R1 of 3; electron-withdrawing groups
depressed the reactivity of 3 (entries 2 and 12).
2.2. Synthesis of sulfenamides
Several other N-trifluoroacetyl arenesulfenamides (3) were
used to synthesize unsymmetrical disulfides (4) (Table 1).
When 3a was treated with the aliphatic thiol 1-dodecane-
thiol in dichloromethane at room temperature for 1 h,
dodecyl 2-(methoxycarbonyl)phenyl disulfide (4b)15 was
obtained in only 13% yield, and a large amount of starting
material 3a was recovered (69%). Evidently, the low yield
was due to the relatively low nucleophilic reactivity of
1-dodecanethiol compared to that of benzenethiol, and the
reaction required a higher temperature to reach completion.
Indeed, when the reaction was carried out in acetone at
reflux for 1 h, the desired product (4b) was obtained in
excellent yield (90%, entry 2). The reaction of N-(trifluoro-
acetyl)benzenesulfenamide (3b) with 4-methylbenzenethiol
in dichloromethane at room temperature was complete
within 5 min and gave phenyl 4-methylphenyl disulfide
(4c)16 in 94% yield (entry 3). Interestingly, the reaction of
3b with 1-dodecanethiol in dichloromethane was complete
within 1 h even at room temperature and gave dodecyl
phenyl disulfide (4d)17 in 98% yield (entry 4). Reactions of
N-(trifluoroacetyl)-2-bromobenzenesulfenamide (3c),
N-(trifluoroacetyl)-4-methylbenzenesulfenamide (3d), and
N-(trifluoroacetyl)-4-nitrobenzenesulfenamide (3e) with
aromatic thiols were similar to the reaction of 3a with
benzenethiol (entry 1) and gave unsymmetrical
disulfides18–21 in yields ranging from 73 to 98% (entries
5–8, 10, and 11). The reactivity of 3d (bearing an electron-
donating group on the aromatic ring) or 3e (bearing an
electron-withdrawing group on the aromatic ring) was
similar to that of 3a or 3b. Although 3d and 3e reacted
with 1-dodecanethiol to give dodecyl 4-methylphenyl
disulfide (4h) and dodecyl 4-nitrophenyl disulfide (4k)
First, we investigated the nucleophilic substitution reaction
of 3a with a variety of amines to evaluate the scope of the
reaction (Table 2). When the reaction of 3a with
benzylamine was carried out in THF at reflux for 6 h, the
corresponding product, N-benzyl-2-(methoxycarbonyl)-
benzenesulfenamide (5a),22 was obtained in 99% yield
(entry 1). However, a cyclized product, N-benzyl-1,2-
benzisothiazolin-3-one (6),22 was obtained as a by-product
when the reaction was carried out in methanol or toluene at
reflux (entries 2 and 3). The use of 4-chlorobenzylamine in
place of benzylamine in the substitution reaction of 3a
afforded N-(4-chlorobenzyl)-2-(methoxycarbonyl)benzene-
sulfenamide (5b)22 in 73% yield (entry 4). The reaction of
3a with butylamine proceeded smoothly and afforded a high
yield of the desired product, N-butyl-2-(methoxycarbonyl)-
benzenesulfenamide (5c) (89%, entry 5). A good yield of
N-cyclohexyl-2-(methoxycarbonyl)benzenesulfenamide
(5d)23 was obtained from the reaction of 3a with
cyclohexylamine (76%, entry 6). Sterically bulky amines,
such as 2-phenyl-2-propylamine, showed lower reactivity in
the substitution reaction of 3a and gave N-(2-phenyl-2-
propyl)-2-(methoxycarbonyl)benzenesulfenamide (5e)22 in
only 40% yield even when the reaction was conducted for
21 h (entry 7). The use of a secondary amine, pyrrolidine,
also afforded the desired product, N-[(2-(methoxycar-
bonyl)benzenesulfenyl]pyrrolidine (5f),22 in high yield
(87%, entry 8). However, in the case of reaction of 3a
with anilines, such as 4-methylaniline and 4-chloroaniline,
the substitution reaction did not occur, owing to poor
reactivity of the aromatic amines, and the starting material
3a was recovered.
Table 1. Synthesis of unsymmetrical disulfides (4) by the reaction of N-trifluoroacetyl arenesulfenamides (3) with thiolsa
Entry
3
R1
R2
Solvent
Temperature
4
Yield (%) of 4b
1
3a
3a
3b
3b
3c
3c
3d
3d
3d
3e
3e
3e
2-CO2Me
2-CO2Me
H
H
2-Br
Ph
CH2Cl2
Acetone
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Acetone
rt
Reflux
rt
rt
rt
rt
rt
rt
rt
4a
4b
4c
4d
4e
4f
4c
4g
4h
4i
89
90
94
98
96
98
94
91
85
78
73
72
2c
3
CH3(CH2)11
4-MeC6H4
CH3(CH2)11
Ph
4-MeC6H4
Ph
4-ClC6H4
CH3(CH2)11
Ph
4-MeC6H4
CH3(CH2)11
4c
5
6
7
2-Br
4-Me
4-Me
4-Me
4-NO2
4-NO2
4-NO2
8
9c
10
11
12c
rt
rt
Reflux
4j
4k
a
Reactions were carried out in dichloromethane at room temperature for ca. 5 min, unless otherwise specified.
Isolated yield.
The reaction was carried out for 1 h.
b
c

M. Bao, M. Shimizu / Tetrahedron 59 (2003) 9655–9659
9657
Table 2. N-Substituted sulfenamides derived from the reaction of N-trifluoroacetyl arenesulfenamides (3) with aminesa
Entry
3
R1
R2
PhCH2
PhCH2
PhCH2
4-ClC6H4CH2
CH3(CH2)3
cyclohexyl
PhMe2C
R3
Solvent
Time (h)
5
Yield of 5b (%)
1
3a
3a
3a
3a
3a
3a
3a
3a
3f
2-CO2Me
2-CO2Me
2-CO2Me
2-CO2Me
2-CO2Me
2-CO2Me
2-CO2Me
2-CO2Me
4-CO2Me
4-NO2
H
H
H
H
H
H
H
THF
6
5
4
7
4
4
21
3
2
4
5a
5a
5a
5b
5c
5d
5e
5f
99
61
74
73
89
76
40
87
86
96
61
2c
3d
4
MeOH
Toluene
THF
THF
THF
THF
THF
Toluene
Toluene
Toluene
5
6
7e
8
–(CH2)4–
9
10
11
PhCH2
PhCH2
PhCH2
H
H
H
5g
5h
5i
3e
3g
2-NO2
4
a
A mixture of N-trifluoroacetyl sulfenamide (0.5 mmol) and 1.2 equiv. of amine in 6 mL of solvent was stirred at reflux for the period indicated.
Isolated yield.
2-Benzyl-1,2-benzisothiazolin-3-one (6) was isolated in 20% yield.
6 was isolated in 4% yield.
The starting material 3a was recovered in 45% yield.
b
c
d
e
From the above results, it seems that THF was an effective
reaction conditions in a short time, thiol–disulfide exchange
did not occur during reaction. The reaction of 3 with amines
proceeded smoothly in THF or toluene and provided
N-substituted sulfenamides in good to excellent yields.
solvent for the substitution reaction of 3 with amines.
However, further attempts to synthesize sulfenamides using
other N-trifluoroacetyl arenesulfenamides in THF gave none
of the desired products. The pKa values of 3 measured by
titration showed that para-substituted arenesulfenamides
(3e, 3f) were more acidic than ortho-substituted ones (3a,
3d) (Table 3). This result suggests that the N–H proton of
3e or 3f is easily transferred to benzylamine to form a salt in
THF.
4. Experimental
4.1. General
Melting points were determined on a Mettler FP90
1
microscopic plate and are uncorrected. H and 13C NMR
Table 3. pKa values of N-trifluoroacetyl arenesulfenamides (3)
spectra were obtained with a JEOL LA-500 spectrometer,
and chemical shifts (d) are reported in parts per million
relative to internal tetramethylsilane (1H NMR) or CDCl3
(13C NMR). IR spectra were recorded on a JASCO FTIR-
5300 spectrophotometer. HRMS analysis was performed by
the Analytical Center at the National Institute of Advanced
Industrial Science and Technology with a Hitachi M-80B
mass spectrometer. N-Trifluoroacetyl arenesulfenamides (3)
were prepared by the method described in our previous
paper.14
3
pKa
3a
3d
3e
3f
7.9
7.6
6.3
6.8
7.0
3g
When the substitution reactions of 3e, 3f, and 3g with
benzylamine were carried out in toluene (a nonpolar
solvent) at reflux for 2–4 h, the corresponding products24,25
were obtained in good yields (entries 9–11). These results
showed that the proton transfer from 3 to amine was
inhibited in nonpolar solvent.
4.2. General procedure for the synthesis of
unsymmetrical disulfides (4)
A thiol (0.6 mmol) was added to a solution of 3 (0.5 mmol)
in 6 mL of dichloromethane at room temperature. The
mixture was stirred for ca. 5 min, and then the solvent was
removed under reduced pressure. The crude product was
chromatographed on a silica gel column with dichloro-
methane/hexane (2:1) mixture as an eluent.
3. Conclusion
N-Trifluoroacetyl arenesulfenamides (3) were readily pre-
pared by a chlorine-free procedure. These stable, crystalline
solids were effective precursors for the synthesis of
unsymmetrical disulfides and N-substituted sulfenamides.
Reactions of 3 with a variety of aromatic thiols in
dichloromethane at room temperature were generally
complete within 5 min and gave unsymmetrical diaryl
disulfides in high yields. Aralkyl disulfides were isolated in
high yields from the reaction of 3 with aliphatic thiols. Since
the reaction of 3 with thiols proceeded under neutral
4.2.1. 2-Bromophenyl 4-methylphenyl disulfide (4f). Pale
yellow oil with bp 808C/6.7 Pa. 1H NMR (500 MHz,
CDCl3): d 7.69 (dd, J¼8.0, 1.5 Hz, 1H), 7.50 (dd, J¼8.0,
1.2 Hz, 1H), 7.37 (dt, J¼8.2, 2.2 Hz, 2H), 7.28 (ddd, J¼8.0,
7.3, 1.2 Hz, 1H), 7.10 (dt, J¼8.2, 2.2 Hz, 2H), 7.05 (ddd,
J¼8.0, 7.3, 1.5 Hz, 1H), 2.30 (s, 3H); 13C NMR (125 MHz,
CDCl3): d 137.6, 137.5, 132.8, 132.5, 129.9, 128.1, 128.0,

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M. Bao, M. Shimizu / Tetrahedron 59 (2003) 9655–9659
127.7, 127.1, 121.0, 21.0; IR (neat): 3054, 2919, 1574, 1489,
1443, 1427, 1017, 804, 747 cm21; HRMS: calcd for
C13H11BrS2: 309.9486, 311.9465; found: 309.9480,
311.9473. Anal. calcd for C13H11BrS2: C, 50.16; H, 3.56;
found: C, 50.55; H, 3.41.
4.4. Determination of pKa values
The compound (3, 0.3 mmol) was dissolved in 50% aqueous
methanol, and the solution was titrated with 0.06 M sodium
hydroxide. The pH of the solution was measured after each
addition (0.1 mL) of titrant (all measurements were made at
room temperature). The pKa values were calculated from the
pH readings.
4.2.2. Dodecyl 4-methylphenyl disulfide (4h). Colorless
1
oil with bp 808C/8.0 Pa. H NMR (500 MHz, CDCl3): d
7.43 (dt, J¼8.7, 2.5 Hz, 2H), 7.13 (dt, J¼8.3, 2.5 Hz, 2H),
2.72 (t, J¼7.3 Hz, 2H), 2.33 (s, 3H), 1.65 (quint, J¼7.3 Hz,
2H), 1.35–1.24 (m, 18H), 0.88 (t, J¼7.3 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d 136.9, 134.2, 129.7, 128.3, 38.9, 31.9,
29.63, 29.57, 29.5, 29.3, 29.2, 28.7, 28.5, 22.7, 21.0, 14.1;
IR (neat): 2924, 2853, 1489, 1464, 1017, 912, 804, 741,
486 cm21; HRMS: calcd for C19H32S2: 324.1945; found:
324.1962. Anal. calcd for C19H32S2: C, 70.31; H, 9.94;
found: C, 70.62; H, 9.71.
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4.2.3. Dodecyl 4-nitrophenyl disulfide (4k). Pale yellow
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(500 MHz, CDCl3): d 8.18 (dt, J¼8.9, 2.5 Hz, 2H), 7.66 (dt,
J¼8.9, 2.5 Hz, 2H), 2.76 (t, J¼7.3 Hz, 2H), 1.66 (quint,
J¼7.3 Hz, 2H), 1.39–1.27 (m, 18H), 0.88 (t, J¼7.3 Hz,
3H); 13C NMR (125 MHz, CDCl3): d 147.3, 146.1, 125.8,
124.0, 39.1, 31.9, 29.63, 29.55, 29.5, 29.3, 29.1, 28.9, 28.4,
22.7, 14.1; IR (neat): 3098, 2926, 2853, 1458, 1518, 1468,
1339, 1109, 1078, 851, 741 cm21; HRMS: calcd for
C18H29NO2S2: 355.1640; found: 355.1635. Anal. calcd for
C18H29NO2S2: C, 60.80; H, 8.22; N, 3.94; found: C, 61.13;
H, 8.26; N, 3.86.
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4.3.1. N-Butyl-(2-methoxycarbonyl)benzenesulfenamide
(5c). Colorless oil. 1H NMR (500 MHz, CDCl3): d 8.00 (dd,
J¼7.6, 1.4 Hz, 1H), 7.83 (dd, J¼8.3, 1.2 Hz, 1H), 7.52 (ddd,
J¼8.3, 7.2, 1.4 Hz, 1H), 7.14 (ddd, J¼7.6, 7.2, 1.2 Hz, 1H),
3.91 (s, 3H), 2.97 (q, J¼7.1 Hz, 2H), 2.59 (br s, 1H), 1.58
(quint, J¼7.4 Hz, 2H), 1.39 (sext, J¼7.4 Hz, 2H), 0.92 (t,
J¼7.4 Hz, 3H); 13C NMR (125 MHz, CDCl3): d 166.9,
149.5, 132.4, 131.3, 123.8, 123.5, 122.5, 52.1, 51.6, 32.8,
20.1, 13.9; IR (neat): 3339, 2955, 2928, 2861, 1709, 1460,
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;
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1
hexane). H NMR (500 MHz, CDCl3): d 7.98 (dt, J¼8.6,
1.8 Hz, 2H), 7.37–7.29 (m, 7H), 4.12 (d, J¼5.8 Hz, 2H),
3.90 (s, 3H), 3.11 (t, J¼5.8 Hz, 1H); 13C NMR (125 MHz,
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126.7, 121.9, 56.4, 52.0; IR (neat): 3329, 3029, 2949, 2851,
1717, 1593, 1435, 1277, 1109, 760 cm21; HRMS: calcd for
C15H15NO2S: 273.0823; found: 273.0832. Anal. calcd for
C15H15NO2S: C, 65.91; H, 5.53; N, 5.12; found: C, 65.82;
H, 5.39; N, 5.03.
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19. 4-Chlorophenyl 40-methylphenyl disulfide (4g): see Ref. 16.

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