Formation of acyldisulfide ions from the reaction of sulfur with thiocarboxylate ions, and reactivity towards acyl chlorides in N,N-dimethylacetamide

Article abstract of DOI:10.1039/a606701h

The reactivity of sulfur towards thiocarboxylate ions RC(O)S^- a (R = Ph 1, Me 2, Bu′ 3) has been studied by spectroelectrochemistry in N,N-dimethylacetamide. For 2a-3a, two parallel and partial reactions, for which equilibrium constants have been determined, led to: (i) [RC(O)]₂S₂^- b species and S₃^·-/S₈^2- polysulfide ions and (ii) RC(O)S₂ ions c; only traces of 1c were detected by voltammetry. As previously observed with thiolate ions, our results are consistent with an initial monoelectronic transfer between RC(O)S^- ions and S₂ molecules in equilibrium with S₈, followed by concurrent couplings of RC(O)S^· and S₂^·- radicals. On a preparative scale, when sulfur was added to RC(O)S^- ions 1a,3a the enhanced reactivity of RC(O)S₂^- ions towards acyl chlorides RC(O)Cl (R = C₆H₅ and Bu′, respectively) only yielded diacyl disulfides 1b,3b.

Full text of DOI:10.1039/a606701h

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Formation of acyldisulfide ions from the reaction of sulfur with
thiocarboxylate ions, and reactivity towards acyl chlorides in
N,N-dimethylacetamide
Julie Robert, Meriem Anouti and Jacky Paris*
Laboratoire de Physicochimie des Interfaces et des Milieux Réactionnels,
UFR Sciences et Techniques, Parc de Grandmont, 37200 Tours, France
Ϫ
t
The reactivity of sulfur towards thiocarboxylate ions RC(O)S a (R = Ph 1, M e 2, Bu 3) has been studied
by spectroelectrochemistry in N,N-dimethylacetamide. For 2a–3a, two parallel and partial reactions, for
؊
Ϫ
2Ϫ
which equilibrium constants have been determined, led to: (i) [RC(O)]2S2 b species and S3 /S8 polysulfide
᝽
ions and (ii) RC(O)S ions c; only traces of 1c were detected by voltammetry. As previously observed with
thiolate ions, our results are consistent with an initial monoelectronic transfer between RC(O)S ions and
S molecules in equilibrium with S , followed by concurrent couplings of RC(O)S and S
preparative scale, when sulfur was added to RC(O)S ions 1a,3a the enhanced reactivity of RC(O)S ions
2
Ϫ
ؒ
Ϫ
radicals. On a
2
8
2
᝽
Ϫ
Ϫ
2
t
towards acyl chlorides RC(O)Cl (R = C H and Bu , respectively) only yielded diacyl disulfides 1b,3b.
6
5
Ϫ
Ϫ
In dimethylacetamide (DMA), a dipolar aprotic medium, we
ArS + S
ArS3
(5)
(6)
2
Ϫ
recently showed that sulfur reacts with thiolate ions RS in two
1
Ϫ
Ϫ
Ϫ
simultaneous ways: (i) partial oxidation [reaction (1)] leading
ArS3 + ArS
ArS2
Ϫ
Ϫ
᝽
RS + ³S → ¹RS R + S
(1)
Because of the sulfur–trigonal carbon linkages in both cases
we thought that thiocarboxylate ions, like ArS , should be able
8
2
3

¯
²
Ϫ
to RS R and polysulfide ions; (ii) preponderant reaction (2)
to yield S᎐S bonds. We report here on the reactivity of sulfur
2
Ϫ
towards the following electrogenerated RC(O)S ions in DMA:
Ϫ
Ϫ
t
RS + (x Ϫ 1)/8 S → RS
(2)
R = Ph 1, Me 2, Bu 3. As with characterization of the
8
x
Ϫ
RC(O)SH/RC(O)S systems, reactions were followed by UV–
Ϫ
yielding stable RSx ions (R = alkyl, x = 2–5; R = aryl, x = 2–3).
VIS absorption spectrophotometry coupled with stationary
voltammetry. The results were then applied on a preparative
scale byϪ addition of acyl chlorides RC(O)Cl to initial
2
Ϫ
The ‘thiophilicity’ of RS species appeared to be analogous
3
Ϫ
2Ϫ
to that of a number of ‘S-nucleophiles’ Nu (CN , SO , Ar3P
3
t
ؒ
ؒ ؒ ). Since the 1950s, the rate-determining step has been
[RC(O)S + nS] solutions with R = Ph and Bu .
believed to involve initial opening [reaction (3)] of the S ring
8
k
Results and discussion
+
Ϫ
Nu + S8
NuS–S –S
(3)
6
Ϫ
Characteristics of RC(O)SH/RC(O)S species in DMA
RC(O)SH species are stronger acids than their homologues
followed by fast and successive displacements of sulfur giving
the final product SNu, according to the overall reaction (4).
RCO H, notably in aprotic media; as an example, in dimethyl
2
6
sulfoxide pK (CH COSH) = 6.7 and pK (CH CO H) = 12.6.
A
3
A
3
2
Ϫ
RC(O)S ions 1a–3a were generated in low concentrations
S8 + 8Nu → 8SNu
(4)
Ϫ3
Ϫ3
(
C < 3.6 × 10 mol dm ) by electrolysis, under dry nitrogen
pressure, of RC(O)SH acids (characteristics in Table 1) at a
controlled reduction-wave potential on a gold electrode [reac-
tion (7)].
A very different mechanism involving reactive S molecules in
equilibrium with cyclic S8 was suggested by our group
Scheme 1).
6
2
4
1
(
Ϫ
Ϫ
RC(O)SH + e → RC(O)S + ¹H
(7)
2
¯
²
Ϫ
During oxidation of RCO ions, decarboxylation of the
transient RCO2 acyloxy radicals leads to R alkyl radicals
‘Kolbe reaction’ ). During electrooxidation of RC(O)S [over-
all reaction (10)], the acyl thioradicals RC(O)S dimerize into
2
ؒ
ؒ
7
Ϫ
(
ؒ
8
Scheme 1
diacyl disulfides without decomposition.
Ϫ
Ϫ
Ϫ
Ϫ
ؒ
The key redox system S /S2 should behave like the O /O
RC(O)S Ϫ e → RC(O)S
(8)
(9)
2
᝽
2
2
᝽
5
system in the same type of reactions. After monoelectronic
transfer between RS and S , fast and competing couplings of
entails the simultaneous formation of
RS R, RS and polysulfide ions. If alkyl RS2 , RS3 , RS4
RS5 ions can be obtained from the appropriate additions of
sulfur, ArS2 and ArS3 only result from equilibrium reactions
5) and (6) with less basic, reducing and nucleophilic aromatic
Ϫ
ؒ
2RC(O)S → [RC(O)] S
2
2 2
ؒ
Ϫ
Ϫ
radicals RS and S
2
᝽
Ϫ Ϫ Ϫ
Ϫ
Ϫ
,
2RC(O)S Ϫ 2e → [RC(O)] S
(10)
2
x
2
2
Ϫ
Ϫ
Ϫ
Ϫ
Electrolysis of RC(O)S ions 1a–3a on their well-defined
monoelectronic wave [reaction (10)] entails growth of the
[RC(O)] S wave 1b–3b with an electrical yield >95%. RC(O)S
(
Ϫ
Ϫ
thiolates than RS ions.
2
2
J. Chem. Soc., Perkin Trans. 2, 1997
473

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Table 1 Electrochemical and spectrophotometric characteristics of thiocarboxylic acids RC(O)SH, diacyldisulfides and thiocarboxylate ions
Ϫ3
in N,N-dimethylacetamide. E1/2 at a gold-disc electrode vs. ref. Ag/AgCl, KCl sat. in DMA/N(Et) ClO 0.1 mol dm
4
4
Ϫ
RC(O)SH
RC(O)S
[RC(O)]2S2
a
3
a
3
3
ε
/dm
ε
/dm
max
εmax/dm
Ϫ1 Ϫ1
max
Ϫ1
Ϫ1
Ϫ1
Ϫ1
R
E1/2(R)/V
E1/2(O)/V
~+0.87
~+0.63
~+1.1
λmax/nm
mol cm
E1/2(O)/V λmax/nm
mol cm
E1/2(R)/V
λmax/nm
mol cm
b
C H
CH
Bu
Ϫ0.86
Ϫ0.86
Ϫ0.92
295
257
257
8 800
1 500
1 400
+0.72
+0.31
+0.60
312
262
263
5 000
8 000
7 000
Ϫ1.14
Ϫ1.19
~Ϫ1.5
268
265
258
18 000
3 300
3 000
6
5
a
3
t
a
a
3
Ϫ1
Ϫ1
b
ε/dm mol cm ± 5%. ε ± 10%.
Fig. 1 Evolution of voltammograms during the addition of sulfur to a
Fig. 2 Evolution of voltammograms during the addition of sulfur to
Ϫ3 Ϫ3
Ϫ3
Ϫ3
solution of thiobenzoate ions C = 3.0 × 10 mol dm . y = 8[S ] /C = 0
a solution of trimethylthioacetate ions C = 3.60 × 10 mol dm .
8
0
(
1); 0.13 (2); 0.26 (3); 0.43 (4); 0.60 (5); 0.94 (6). Rotating gold-disc
y = 8[S
8 0
] /C = 0 (1); 0.14 (2); 0.28 (3); 0.42 (4); 0.56 (5); 0.70 (6); 0.84 (7);
Ϫ1
electrode Ω = 1000 rev min ; diameter = 2 mm. E vs. reference Ag/AgCl
KCl sat. in DMA/N(Et) ClO 0.1 mol dm
0.98 (8); 1.12 (9).
Ϫ3
.
4
4
ions 1a–2a can be recovered by electroreduction of the
Ϫ
diacyl disulfides. Based on their redox behaviour, RC(O)S /
Ϫ
1
[
RC(O)]2S2 mimic RS /R S systems. The electrochemical
2 2
Ϫ
and spectrophotometric characteristics of RC(O)S and [RC-
O)] S obtained in the course of reaction (10) are listed in
(
2
2
Table 1.
Ϫ
Reactivity of RC(O)S ions with sulfur
The study was carried out in the same way with 1a–3a ions:
spectra and voltammograms were recorded at (20.0 ± 0.5) ЊC
during the progressive addition of a concentrated solution of
Ϫ3
Ϫ3
Ϫ
3
sulfur (8.0 × 10 mol dm ) to [RC(O)S ] = C(V = 40 cm ) at
0
ratio y = 8[S ] /C.
a) With R = C H , the absorption of RC(O)S (λ = 312
nm) remained unchanged and sulfur was completely recovered
8
0
Ϫ
(
6
5
max
Ϫ3
Ϫ3
Fig. 3 Evolution of UV–VIS spectra during the reaction of sulfur with
trimethylacetate ions. Same conditions as for Fig. 2. Thickness of cell =
0.1 cm.
(
Fig. 1, C = 3.0 × 10 mol dm , 0 < y < 0.94) from solution,
according to the values of its diffusional limiting current i(R )
1
1
,4
[
E1/2 (R) = Ϫ0.40 V vs. ref.†] after calibration. However, as
soon as S was added (curve 2, y = 0.13) an oxidation wave
8
2Ϫ 1,4
8
8
3
Ϫ1
Ϫ1
of S8
λ
(λ
= 515 nm, ε
= 3800 dm mol cm ;
3 Ϫ1 Ϫ1
max1
max1
appeared at less anodic potentials [E1/2(O) ≈ +0.35 V] than that
8
8
Ϫ
max2 = 310 nm, εmax2 = 11 800 dm mol
because of the equilibrium reaction (11). Note that S8 ions
affect the specific absorption of X at 330 nm.
cm ) increase
2Ϫ
of RC(O)S [E1/2(O) = +0.72 V] and slightly increased with
1,4
increasing y.
t
Ϫ3
Ϫ3
(
b) With R = Bu and C = 3.6 × 10 mol dm , Figs. 2 and 3
show the i = f(E) and A = f(λ) evolutions as a function of y
0 < y < 1.12); (i) for y = 0.14 (curves 2) the oxidation wave
Ϫ
2Ϫ
2S3᝽
+ S2
S8
(11)
(
Ϫ
of RC(O)S [E1/2(O) = +0.60 V] is almost totally displaced
∆E1/2 ≈ Ϫ350 mV) with total consumption of sulfur [absence
Ϫ
(
c) The addition of sulfur to CH C(O)S ions entailed the
3
(
Ϫ
same qualitative behaviour as for 3a; however, the growth of
of i(R )]; blue S
ions are detected at low concentrations in
1
3
3
᝽
Ϫ
1
,4
3 3 Ϫ1 Ϫ1
A
617(S
3
) was weaker in proportion, and without the presence
᝽
the spectra (λ
= 617 nm, ε
= 4390 dm mol cm ) and
max
max
2Ϫ
of S8 and X species gave two absorption bands (λmax1 = 336
nm, λmax2 = 467 nm; A /A ≈ 6.0) as shown in Fig. 4.
These results are in agreement with the competing and par-
another X species absorbs at λmax = 330 nm; (ii) for y > 0.14
1
2
(
curves 3–8) the first reduction wave R of sulfur appears and
1
Ϫ 3
the characteristic absorptions of S3᝽
(λ
= 617 nm) and
max
tial formations of diacyl disulfides [RC(O)] S 2b–3b and acyl
disulfide ions RC(O)S2 2c–3c when sulfur is added to alkyl-
thiocarboxylate ions 2a–3a, as was the case in the course of
2
2
Ϫ
9
†
All potentials are expressed in comparison to the reference electrode
Ϫ3
1,10
Ag/AgCl, KCl sat. in DMA/N(Et) ClO 0.1 mol dm
.
sulfur–arylthiolate interactions.
4
4
4
74
J. Chem. Soc., Perkin Trans. 2, 1997

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Ϫ
Table 2 Half-wave potentials of reduction and oxidation of [RC(O) ]S /RC(O)S systems in the absence or presence of sulfur
2
2
a
a
b
E1/2 (R)/V
E1/2 (O)/ϪV
E1/2(R)/V
S8
E1/2 (O)/V
Ϫ
R
[RC(O)]2S2
RC(O)S
|∆E1/2|/V
RC(O)S2
|∆E1/2|/V
C H5
Ϫ1.14
Ϫ1.19
~Ϫ1.5
+0.72
+0.31
+0.60
1.86
1.50
2.1
Ϫ0.40
Ϫ0.40
Ϫ0.40
+0.35
+0.09
+0.18
0.75
0.49
0.58
6
CH3
t
Bu
a
b
Ϫ
E1/2 values of Table 1. E1/2 values for y = 8[S ] /[RC(O)S ] ≈ 1.
8
0
0
Fig. 4 Evolution of UV–VIS spectra during the addition of sulfur to a
Fig. 5 Relative increase in the first reduction wave of sulfur as a func-
Ϫ3
Ϫ3
solution of thioacetate ions C = 2.85 × 10 mol dm . y = 8[S ] /C = 0
t
8
0
tion of [RC(O) S ]/[S ] ratio. R = Ph 1; Me 2; Bu 3.
2
2
8 0
(1); 0.12 (2); 0.24 (3); 0.35 (4); 0.66 (5).
Ϫ
Ϫ
Formation of RC(O)S2 species and oxidation of RC(O)S ions
by sulfur
The shift in the oxidation wave of RC(O)S ions towards less
Sulfur, which is regenerated by the fast reaction (15), can be
proposed as a mediator in order to facilitate the slow electrode
process [reaction (10b)] (homogeneous redox catalysis ):
Ϫ
1
1
oxidizing potentials is typical of the electrocatalytic mechanism
1
in reactions (12) and (13), as observed with alkyl and aro-
Ϫ
Ϫ
[
RC(O)] S + 2e → 2RC(O)S
(10b)
1
,10
2 2
matic thiolates.
Ϫ
So, [RC(O)] S /RC(O)S redox systems are much less
f
2 2
Ϫ
Ϫ
RC(O)S + ¹ S
RC(O)S2
(12)
(13)
irreversible in the presence of small amounts of sulfur because
of the ‘simultaneous’ catalytic processes, respectively [reactions
(14) and (15)], for reduction reaction (10b), and reactions (12)
and (13) for oxidation reaction (10). This noteworthy effect is
illustrated by comparison (Table 2) of ∆E1/2 = E1/2(O) Ϫ E1/2(R)
in the absence or presence of sulfur. In the last case, the over-
2
¯
²
b
Ϫ
Ϫ
RC(O)S2 Ϫ e → ¹ [RC(O)] S + ¹ S
2
2
2
¯
²
¯
²
This phenomenon implies the fast formation [reaction (12)]
of RC(O)S2 ions 1c–3c and their electrooxidation [reaction
13)] at a greater rate than that of RC(O)S into the same
species i.e. [RC(O)] S , with recovery of sulfur. With R = C H ,
sulfur appears as unreactive towards RC(O)S ; the formation
of RC(O)S2 1c is nevertheless evidenced by the presence of its
electrocatalytic current (∆E1/2 = Ϫ0.37 V). In fact, equilibrium
reaction (12) is only displaced by the consumption [reaction
13)] of C H C(O)S ions at the electrode surface. Their oxi-
Ϫ
Ϫ
potential reduces by more than 1 V.
(
Ϫ
Simultaneously with the formation of RC(O)S ions, the
2
2
2
6
5
Ϫ
Ϫ
partial oxidation of RC(O)S ions 2a–3a to [RC(O)]
2
S
2
2b–3b
Ϫ
3
Ϫ
occurs with the appearance of S
tion (16)].
3᝽
ions (λmax = 617 nm) [reac-
Ϫ
Ϫ
Ϫ
2RC(O)S + 3S
2
[RC(O)]
2
S
2
+ 2S
3
᝽
(16)
(
6
5
2
dation wave current is limited by the rate of reaction (12f) as
shown by its increase with concentration in sulfur (Fig. 1,
curves 2–6).
When the preceding RC(O)S /RC(O)S2 solutions are
progressively oxidized at controlled potential, the limiting
Ϫ
S3
ions are also detected on voltammograms by their
1,4
᝽
monoelectronic oxidation and reduction waves
[E1/2(O) =
Ϫ0.20 V, E1/2(R)) = Ϫ1.10 V]. However, the reduction current is
markedly enhanced (see Fig. 2, curves 2,3) because of the
homogeneous catalytic reduction of [RC(O)] S in the diffusion
layer by S3
reactions (17) and (18).
Ϫ
Ϫ
2
2
current i(R ) increases up to a greater value than that of
2Ϫ
Ϫ 4
1
ions which are better reducing agents than S3᝽ ,
the initially added sulfur. The considerable enhancements
i(R1)exp/i(R1)calc‡ which are displayed in Fig. 5 were studied by
direct addition of sulfur to diacyl disulfides 1b–3b, obtained by
Ϫ
Ϫ
2Ϫ
2S3᝽
+ 2e → 2S
(17)
(18)
3
Ϫ
exhaustive oxidation of RC(O)S solutions. These observations
are consistent with the indirect reduction [reactions (14) and
2Ϫ
Ϫ
Ϫ
[
RC(O)] S + 2S → 2RC(O)S + 2S
2
2
3
3
᝽
Ϫ
2Ϫ
(
15)] of [RC(O)] S species by S
or S8 polysulfide ions.
2
2
3
᝽
Ϫ
Ϫ
Whatever the oxidation mode of RC(O)S /RC(O)S
2
Ϫ
2Ϫ
S + 2e → S
(14)
(15)
8
8
solutions, electrochemical [reactions (10) and (13)], or chem-
ical by sulfur [reaction (16)], only diacyl disulfides are
obtained. Consequently, if the electrochemical reduction reac-
tion (19) of [RC(O)] S can be conceived, a backward oxidation
2
Ϫ
Ϫ
[
RC(O)] S + S
8
→ 2RC(O)S + S8
2
2
2
4
is impossible.
‡
i(R1)calc values for sulfur in the absence of diacyl disulfides were
obtained by use of the experimental coefficient i(R )/[S ] = (34.0 ± 0.5)
1
8 0
Ϫ1
3
Ϫ
Ϫ
µA mmol dm , depending on our working gold-disc electrode.
[RC(O)]
2
S
4
+ 2e → 2RC(O)S
2
(19)
J. Chem. Soc., Perkin Trans. 2, 1997
475

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Tetrasulfides were only obtained during reactions between
Table 3 Composition at equilibrium (mol R%) after addition of sulfur
(y = 8[S ] /C = 1) to C solutions of thiocarboxylate ions 2a (R = CH ),
Ϫ
12
RC(O)S and S Cl .
Because of concurrent formations of RC(O)S2 ions [reac-
tion (12)] and [RC(O)] S species [reaction (16)] in the course of
8
0
3
2
2
t
Ϫ
3a (R = Bu )
2
2
Ϫ3
C/10 mol
Ϫ
the addition of sulfur to RC(O)S ions 2a–3a the following
equilibria, together with their corresponding constants, have to
be taken into account.
Ϫ3
Species dm
a (%) b (%) c (%)
2a
2.85
3.60
81
68
2
15
17
17
3
a
S8
4S2
2RC(O)S2
[RC(O)] S + 2S
(20)
(12)
(16)
(11)
(21)
(22)
(23)
(24)
Ϫ
Ϫ
2
RC(O)S + S2
Ϫ
Ϫ
2
RC(O)S + 3S2
2
2
3
᝽
Ϫ
2Ϫ
2S3᝽
+ S2
S8
4
Ϫ1
Ϫ7
3
Ϫ9
K = [S ] [S ] = 1.0 × 10 mol dm
1
2
8
Ϫ 2
Ϫ Ϫ2
Ϫ1
K = [RC(O)S ] [RC(O)S ] [S ]
2
2
2
Ϫ 2
Ϫ Ϫ2
Ϫ3
K = [(RCOS) ][S ] [RC(O)S ] [S ]
3
2
3
᝽
2
2
Ϫ
Ϫ Ϫ2
Ϫ1
6
6
Ϫ2
K = [S8
][S3᝽
] [S ] = 0.59 × 10 dm mol
4
2
K and K were determined previously in DMA at 297 K. For
1
4
Ϫ
known initial concentrations C = [RC(O)S ] at definite values
0
Fig. 6 Evolution of voltammograms during the addition of benzoyl
of y = 8[S ] /C, all concentrations of species in equilibrium can
8
0
Ϫ
Ϫ3
Ϫ3
chloride to a solution [C H C(O)S ] = 2.80 × 10 mol dm + 8[S ] =
6
5
0
8 0
be obtained from constants K , K and conservation eqns. (25)–
1
4
Ϫ3
Ϫ3
2
.64 × 10 mol dm . [RC(O)Cl]/C = 0 (1); 0.27 (2); 0.54 (3); 0.98 (4).
(27).
Ϫ
Ϫ
Ϫ
of RX (C H I, C H I, C H Br). The reactivity of RC(O)S
C = [RC(O)S ] + [RC(O)S ] + 2[(RCOS) ] (25)
2
5
3
7
3
7
2
2
ions, alone or in the presence of sulfur, has been tested with
regard to some acyl chlorides.
Ϫ
Ϫ
2Ϫ
8
[S ] = 8[S ] + 2[S ] + [RC(O)S ] + 3[S ] + 8[S8
]
(26)
(27)
8
0
8
2
2
3
᝽
Ϫ
2Ϫ
Ϫ
Ϫ
2
[(RCOS) ] = [S3 ] + 2[S8
]
Reactivity of RC(O)S /RC(O)S ions towards acyl chlorides
2
2
᝽
Alkyl halides are known for undergoing rapid nucleophilic sub-
stitutions with RC(O)S ions to produce thioesters, whose
methods of preparation have been reviewed. When benzyl
bromide is added to CH C(O)S ions in DMA, their anodic
wave [E1/2(O) = +0.31 V] decreases at the expense of the oxid-
ation current of Br ions [3Br → Br + 2e ,E1/2(O) ≈
Ϫ
2Ϫ
Ϫ
[
S3 ] and [S8 ] were evaluated from A617 and A515 measure-
ments on spectra. For y values <0.14 (experimental conditions
of Figs. 2–4), sulfur is totally consumed by reactions (12) and
16), and [RC(O)S ] is easily deduced from eqn. (26); ε
RC(O)S2 ions can thus be calculated firstly after substraction
of S3 (2a, 3a) and S8 (3a) specific absorbances at Amax
values on Fig. 1, ref. 1).
᝽
1
4
Ϫ
3
Ϫ
(
of
2
max
Ϫ
Ϫ
15
Ϫ
Ϫ
Ϫ
3
Ϫ
2Ϫ
+0.70 V] according to the stoichiometry in reaction (28).
᝽
(
Ϫ
Ϫ
3
Ϫ1
Ϫ1
C
6
H
5
CH
2
Br + CH
3
C(O)S →
ε336(CH COS2 ) = (4800 ± 300) dm mol cm
3
Ϫ
CH C(O)SCH C H + Br (28)
3
2
6
5
Ϫ
3
Ϫ1
Ϫ1
ε467(CH COS2 ) = (800 ± 100) dm mol cm
3
In the same way, acyl chlorides RC(O)Cl readily react [reac-
tion (29)] with RC(O)S ions, with the appearance of Cl ions
Ϫ
Ϫ
t
Ϫ
3
Ϫ1
Ϫ1
ε330(Bu COS2 ) = (4400 ± 300) dm mol cm
Ϫ
Ϫ
For each value of y > 0.14, S and S concentrations could not
RC(O)Cl + RC(O)S → [RC(O)] S + Cl
(29)
8
2
2
be estimated by i(R ) measurements because of the catalytic
1
Ϫ
2Ϫ
currents which were described previously (Fig. 5). [S ], [S8
and [RCOS ] were, respectively, deduced from A , A and
]
Ϫ
3
᝽
[E1/2(O) ≈ +0.65 V] at the expense of RC(O)S , and the growth
of a reduction wave only for [C H C(O)] S [E1/2(R) = Ϫ1.29 V].
On a preparative scale (see Experimental), the addition
Ϫ
2
617
515
6
5
2
corrected Amax values. These concentrations led to [S ] and [S ]
8
2
with application of eqns. (26) and (21), thus giving access to K2
of C H CH Br or C H C(O)Cl at 40 ЊC to electrogenerated
6
5
Ϫ
2
6
5
and K at 293 K.
Ϫ
Ϫ
3
RC(O)S ions [CH C(O)S and C H C(O)S , respectively]
3 6 5
afforded the expected products: CH C(O)SCH C H (yield
3
2
6
5
3
Ϫ1
K (CH ) = (48 ± 4) dm mol
;
1
2
3
78%) and [C H C(O)] S (67%) were identified by GC–MS, H
6 5 2
13
6
Ϫ2
Ϫ2
K (CH ) = (12 ± 2) dm mol
and C NMR spectroscopies.
3
3
The progress of the reaction between benzoyl chloride and a
t
3
Ϫ1
Ϫ
Ϫ3
Ϫ3
K (Bu ) = (81 ± 8) dm mol
;
2
solution [C H C(O)S ] = 2.80 × 10
mol dm + 8[S ] =
6
5
0
8 0
t
4
6
Ϫ3
Ϫ3
K (Bu ) = (1.7 ± 0.3) × 10 dm mol
2.64 × 10 mol dm (y = 0.94) is shown by the voltammetric
recordings in Fig. 6: during consumption of RC(O)S ions
[decrease of their anodic current at the expense of Cl ,
3
Ϫ
Ϫ
The compositions of solutions at equilibrium reported in
Table 3, with initial 2a,3a concentrations (C) from Figs. 2–4,
and y = 1 were calculated by the use of K and K .
ArS2 species are better nucleophiles than the corresponding
thiolates: when Ar = 4-NO C H , the reaction rates (SN
mechanism) of ArS2 with alkyl halides increased by a factor of
E1/2(O) ≈ +0.65 V], the formation of [C H C(O)] S entails at
6
5
2 x
first (curve 2) a catalytic growth in the reduction wave of sulfur
2
3
Ϫ
[E1/2(R) ≈ Ϫ0.40 V], then a considerable drop in intensity (curves
1
3
3,4) as if S were completely consumed. Dibenzoyl disulfide
2
6
4
2
8
Ϫ
[E1/2(R) = Ϫ1.14 V] can be proposed as the major product of
Ϫ
Ϫ
1
0 in comparison with those of ArS , regardless of the nature
the overall reaction (30), even if only traces of RC(O)S2 ions
4
76
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
Ϫ
Ϫ
RC(O)Cl + RC(O)S + ¹S → [RC(O)] S + Cl
(30)
d, J 7.4); δC 127.4 (4 C), 128.7 (4 C), 133.8 (2 C), 135 (2 C)
and 186 (2 C); m/z 242 (M , <2%), 105 (100), 77 (55) and 51
2
2 2
¯
²
+
were previously evidenced during the reaction of sulfur with
(25).
Ϫ
RC(O)S species (R = C H ).
Reaction of 1a and benzoyl chloride in the presence of sulfur.
6
5
This assumption was confirmed by two synthesis [R = C H ,
Thiobenzoic acid: 0.827 g (6.0 mmol); S 0.391 g (12.2 mmol S);
benzoyl chloride: 0.757 g (5.38 mmol). Products: 1.18 g (~80%);
6
5
8
Ϫ
(
CH ) C] based on reaction (30) [electrogenerated RC(O)S
3
3
ions 1a,3a; addition of sulfur in excess (y ≈ 2); addition of
RC(O)Cl]: [(CH ) CC(O)] S was the only product isolated
from δ (4 H, d), dibenzoyl disulfide (~93%) and dibenzoyl sul-
H
1
9
3
3
2
2
fide (~7%). Dibenzoyl disulfide: mp 135–136 ЊC (lit., 136–
136.5 ЊC): δH 7.53–7.75 (6 H, m) and 8.13 (4 H, d, J 7.4); δC
128.1 (4 C), 129 (4 C), 133.8, 134 (2 C) and 186 (2 C); direct
(
yield 72%) whereas [C H C(O)] S greatly predominated over
6 5 2 2
[
C H C(O)] S (~93/7). And yet mixtures [RC(O)] S/[RC(O)] S
6 5 2 2 2 2
+
could be expected from reactions between acyl chlorides and
thiocarboxylate ions in the presence of sulfur according to
moderately displaced equilibrium reaction (12) in favour of
introduction mode m/z 274 (M , 4%), 105 (50), 77 (100) and 51
(25).
Reaction of 3a and trimethylacetyl chloride in the presence of
Ϫ
RC(O)S2 ions. The electron delocalization induced by the car-
sulfur. Trimethyl thioacetic acid: 0.944 g (7.98 mmol); S : 0.506 g
8
Ϫ
Ϫ
bonyl group in RC(O)S is very much lowered in RC(O)S2
anions, probably because the S᎐S bond does not transmit the
(15.8 mmol S); trimethylacetyl chloride: 0.865 g (7.17 mmol).
Product: di(trimethylacetyl) disulfide (1.21 g, 72%), δH 1.37 (18
1
6
Ϫ
+
conjugation. The great increase in reactivity of RC(O)S ions
in comparison with that of RC(O)S , as was the case for ArS
ArS species, has been rationalized in terms of the ‘α-effect’;
H, s); δ 27 (6 C), 46.8 (2 C) and 199 (2 C); m/z 234 (M , <2%),
2
C
Ϫ
Ϫ
/
2
85 (53), 57 (100) and 41 (11).
Ϫ
17
this effect is generally observed when unshared electron pairs lie
Ϫ
Ϫ
Conclusions
on an atom adjacent to a nucleophilic centre (ClO ,RO ؒ ؒ ؒ ).
2
Alkylthiocarboxylate ions behave towards sulfur like aryl-
thiolates bearing electron-withdrawing groups. In both cases,
the competing formation of disulfide ions and oxidation into
disulfide moleclules were low; for example, from known equi-
Experimental
Materials and equipment
All thiocarboxylic acids RC(O)SH (R = Me, Bu , Ph) and acyl
t
10
librium constants K and K in DMA, the addition of sulfur
2
3
Ϫ
Ϫ3
Ϫ3
chlorides, obtained from Aldrich (as well as DMA), were dis-
tilled under dry nitrogen just before use. Solvent purification
and storage after addition of N(Et) ClO (Fluka) 0.1 mol
(y = 1) to [4-NO C H S ] = 3.0 × 10 mol dm led us to cal-
culate (Ar%) 80% ArS , 15% ArS , 5% Ar S at equilibrium.
2
6
4
0
Ϫ Ϫ
2
2 2
As shown with thiolate ions, the reaction between thiocarboxy-
late ions and sulfur is consistent with an initial monoelectronic
transfer reaction (31).
4
4
Ϫ3
9
dm as supporting electrolyte have been reported elsewhere.
Spectroelectrochemical equipments, electrodes, the thermo-
statted flow-through cell and the two-compartment preparative
cell were the same as previously described.
identification of the synthesized products were performed by
4
,15
Ϫ
ؒ
Ϫ
Analysis and
RC(O)S + S
RC(O)S + S
(31)
2
2
᝽
1
GC–MS spectrometry (Hewlett-Packard 5989A, EI 70 eV), H
Concurrent couplings of intermediate radicals again yield,
on the one hand, diacyl disulfides and polysulfide ions, and
on the other hand, acyldisulfide ions. All species are in equi-
librium, thiocarboxylates being of weaker reducing power than
thiolates.
1
3
(
(
200.132 MHz) and C (50.323 MHz) NMR spectroscopy
Bruker AC 200 spectrometer, CDCl as solvent, J values in
3
Hz).
Preparative electrolysis
In the presence of sulfur the enhanced reactivity of thio-
carboxylate ions towards acyl chlorides leads to the noteworthy
formation of diacyl disulfides.
Ϫ
The reactions of RC(O)S ions, alone or in the presence of
sulfur, with acyl chlorides (or benzyl bromide) were carried out
in the same way on a preparative scale: 0.8–1 g of thiocarboxy-
The present results will be invoked to explain the nucleophilic
3
Ϫ
lic acid was dissolved in 120 cm of the catholyte N(Et) ClO
substitution of S3᝽ ions on acyl chlorides.
4
4
Ϫ3
0
.5 mol dm in DMA and electrolysed at controlled potential
(
Ϫ1.5 V < E < Ϫ1.2 V) on a large gold grid electrode, within 2
h. When used, solid sulfur in powder form was then poured in
the cathodic compartment. Acyl chloride or benzyl bromide
dissolved in 20 cm of DMA was added in deficit (mol. 90%)
with respect to initial RC(O)SH. After heating at 40 ЊC during
1
Acknowledgements
Grateful thanks are due to Service d’Analyse Chimique du
Vivant de Tours (SAVIT) for recordings of NMR and mass
spectra.
3
0 min, the reaction medium was diluted with 4 vol. of 3%
aqueous NaHCO3 before extraction with diethyl ether. The
organic phase was thoroughly washed with water in order to
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8
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Paper 6/06701H
Received 30th September 1996
Accepted 4th December 1996
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4
78
J. Chem. Soc., Perkin Trans. 2, 1997