Nucleophilic substitution of acyl chlorides by electrogenerated polysulfide ions in N,N-dimethylacetamide

Article abstract of DOI:10.1039/a700939i

The reactions between acyl chlorides RC(O)Cl (a) [R = Me (1), Prⁱ (2), Buⁱ (3), Ph(4)] and electrogenerated S₃^._(? S₆^2-) ions have been investigated in N,N-dimethylacetamide by spectroelectrochemistry. With R = alkyl, thiocarboxylate ions and sulfur resulting from the fast initial substitutions cause partial formation of both acyl disulfide ions and diacyl disulfides (b) at a y ratio [RC(O)C1]/[S₃^._] of 0.5; the second step stoichiometrically (y = 1) affords diacyl disulfides 1b-4b as the presumed products. The formation of these species only is confirmed on a preparative scale from two sets of experiments: (i) direction addition of acyl chlorides (1a-4a) to chemically generated S^1/3- solutions; (ii) electrolysis of sulfur in the presence of acyl chlorides 2a-4a.

Full text of DOI:10.1039/a700939i

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Nucleophilic substitution of acyl chlorides by electrogenerated
polysulfide ions in N,N-dimethylacetamide
Julie Robert, Meriem Anouti, Mohamed Abarbri and Jacky Paris*
Laboratoire de Physicochimie des Interfaces et des Milieux Réactionnels UFR Sciences et
Techniques, Parc de Grandmont, 37200 Tours, France
i
t
The reactions between acyl chlorides RC(O)Cl (a) [R = M e (1), Pr (2), Bu (3), Ph(4)] and electrogenerated
Ϫ
᝽
2Ϫ
S3
(
S6 ) ions have been investigated in N,N-dimethylacetamide by spectroelectrochemistry. With
R = alkyl, thiocarboxylate ions and sulfur resulting from the fast initial substitutions cause partial
Ϫ
formation of both acyl disulfide ions and diacyl disulfides (b) at a y ratio [RC(O)Cl]/[S3 ] of 0.5; the
second step stoichiometrically (y = 1) affords diacyl disulfides 1b–4b as the presumed products. The
formation of these species only is confirmed on a preparative scale from two sets of experiments: (i)
direction addition of acyl chlorides (1a–4a) to chemically generated S solutions; (ii) electrolysis of sulfur in
the presence of acyl chlorides 2a–4a.
᝽
1
3
–
Ϫ
Ϫ
2Ϫ
The nucleophilic reactivity of polysulfide ions S
(
S6
)
(λmax = 617 nm) through the disproportionation (4) of the red
3
᝽
2
Ϫ
and S8 in N,N-dimethylacetamide (DMA), a dipolar aprotic
solvent, has been the subject of several reports by our group.
With alkyl halides RX, the S 2 reactions led to dialkyl tri- and
1
Ϫ
2Ϫ
S ϩ 2e → S
(3)
(4)
8
8
N
1
a
Ϫ
tetra-sulfides, with probable transient RSx ions (x = 3,4). o-
f
S82
Ϫ
2S3
Ϫ
ϩ S2
Ϫ
and p-nitrophenyl disulfide ions NO C H S were obtained
᝽
2
6
4
2
b
1
b
from S Ar processes on nitroaromatic halides or dinitro-
N
1
d
benzenes. However, we showed that sulfur reacts with thiolate
ions RS in two parallel ways: (i) oxidation (1) into RS R, (ii)
Ϫ 2
2Ϫ Ϫ1
Ϫ6
2
Ϫ6
K (297 K) = [S ] [S ][S
]
= 1.7 × 10 mol dm
(5)
(6)
(7)
1
3
᝽
2
8
Ϫ
2
2
S8
4S2
Ϫ
Ϫ
2
RS ϩ 3S → RS R ϩ 2S
(1)
2
2
3
᝽
4
Ϫ1
Ϫ7
3
Ϫ9
Ϫ
K
2
= [S
2
] [S
8
]
= 10 mol dm
preponderant and successive formation (2) of RSx ions
S82 ions (λ = 515 nm). The elimination of sulfur at ⁸ F mol
Ϫ
Ϫ1
Ϫ
Ϫ
2
RSxϪ1 ϩ S → 2RSx
(2)
max1
–
�₃
2
Ϫ
Ϫ
3
S8 leads to S
species i.e. S3᝽
ions in equilibrium (8) with
2
2,3
3
(
R = alkyl, x = 2–5; R = aryl, x = 1–3; R = 2- or 4-NO C H ,
x = 1–2 in equilibrium). These results were consistent with, at
first, a monoelectronic transfer between RS ions and the
reactive S molecule in equilibrium with cyclic S . The redox
system S /S2 behaves like the O /O
nucleophilic reactions: RX ϩ O (S 2 ), ArX ϩ O (S Ar )
2
6
4
2
Ϫ
Ϫ
S6
2S3᝽ (8)
Ϫ
2
4
Ϫ 2
2Ϫ Ϫ1
Ϫ3
2
8
K (297 K) = [S ] [S6
3
3
]
= 0.043 mol dm
(9)
᝽
Ϫ
᝽
Ϫ
one in the same type of
2
2
2
᝽ 5
Ϫ
Ϫ
6
2
Ϫ
᝽
N
2
᝽
N
their dimer S62Ϫ (λmax = 465 nm). In dilute solutions, [S
2Ϫ
]
7
6
or redox processes such as RS ϩ O , with the difference that
Ϫ
2
remains low in comparison with [S ]; as an example, for
Ϫ
2Ϫ
2,4
3
᝽
S2᝽
can dimerize into S4 ions.
Ϫ
o
Ϫ
2Ϫ
Ϫ3
Ϫ3
Ϫ
[
S3 ]T = [S ] ϩ 2[S ] = 5.0 × 10
mol dm
,
[S ] =
8
᝽
3
᝽
0
6
0
3
᝽
0
2Ϫ
In a recent paper, we reported that electrogenerated thiocarb-
oxylate ions were also able to give S᎐S bonds in the presence
of sulfur, with the same behaviour as arenethiolates bearing
an electron-withdrawing group (e.g. 4-NO C H S ); when
RC(O)Cl (R = Ph, Bu ) were added to [RC(O)S ϩ S ] solu-
tions, only diacyl disulfides were obtained on a preparative
scale. O2 ions react with acyl chlorides and anhydrides in
aprotic solvents, yielding diacyl peroxides. Here we examine the
reactivity of electrogenerated S3 ions towards a series of
RC(O)Cl species (a) [R = Me (1), Pr (2), Bu (3), Ph (4)] by UV–
VIS absorption spectrophotometry coupled with stationary
voltammetry. The results were then confirmed by synthesis and
electrosynthesis.
Ϫ3
Ϫ3
2Ϫ
Ϫ3
Ϫ3
4
.16 × 10 mol dm and [S6 ]0 = 0.42 × 10 mol dm . S
1
8
–Ϫ
3
and S ions oxidize (O ) and reduce (R ) at the same
1
2
potentials.Ϫ
Ϫ
Ϫ
2
6
4
RC(O)S /RC(O)S2 ions will be implicated in the course of
t
Ϫ
1
–Ϫ
2
3
the reactions between RC(O)Cl molecules and S ions. Some
t
of these ions (R = Me, Bu , Ph) have been studied earlier after
Ϫ
9
10
᝽
electroreduction of thiocarboxylic acids and addition of sul-
8
fur: with R = alkyl, the equilibria (10) and (11), analogous to
Ϫ
᝽ i
t
Ϫ
Ϫ
2
RC(O)S ϩ 3S
[RC(O)] S ϩ 2S
(10)
(11)
(12)
(13)
2
2
2
3
᝽
Ϫ
Ϫ
2
RC(O)S ϩ S
2RC(O)S2
2
Ϫ 2
Ϫ Ϫ2
Ϫ3
K = [(RCOS) ][S ] [RC(O)S ] [S ]
4
2
3
᝽
2
Results
Ϫ 2
Ϫ Ϫ2
Ϫ1
K = [RC(O)S ] [RC(O)S ] [S ]
Sulfur–polysulfide ions and thiocarboxylate–acyldisulfide ions
characteristics in DMA
In aprotic media such as DMA, sulfur reduces into two bielec-
tronic steps with respect to S8 (waves R and R ) on a rotating
gold-disc electrode. The stable product of the electrolysis at
5
2
2
6
Ϫ2
3
Ϫ1
K
(Me) = (12 ± 2) dm mol ; K (Me) = (48 ± 4) dm mol
4
5
4
1
2
t
4
6
Ϫ2
K (Bu ) = 1.7 ± 0.3) × 10 dm mol ;
4
Ϫ
t
3
Ϫ1
controlled potential on R1
is the blue anion radical S3᝽
K (Bu ) = (81 ± 8) dm mol
5
J. Chem. Soc., Perkin Trans. 2, 1997
1759

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4
8
Table 1 Spectrophotometric and electrochemical characteristics of sulfur/polysulfide ions and thiocarboxylate-acyl disulfide ions in N,N-
₁
Ϫ3
dimethylacetamide. E₂ at a rotating golds-disc electrode vs. ref. Ag/AgCl(s), KCl in DMA/N(Et) ClO (0.1 mol dm
)
�
sat.
4
4
Ϫ
Ϫ
RC(O)S /R
RC(O)S2 /R
2
Ϫ
Ϫ
Parameter
S8
S8
S3᝽
1
3
4
1
3
4
λmax/nm
262 (8.0)
515 (3.8)
360 (9.0)
617 (4.4)
262 (8.0)
O (0.31)
14
263 (7.0)
O (0.60)
14
312 (5.0)
O(0.72)
16
336 (4.8)
467 (0.8)
O (0.09)
330 (4.4)
O (0.18)
—
O (0.35)
a
e
(εmax
)
₁
Wave (E ₂ /V)
R (Ϫ0.40)
R (Ϫ1.10)
R2
O1
�
1
2
R (Ϫ1.10)
O (Ϫ0.20)
2
1
b
i/c
R (34)
O (26.5)
R (16)
1
1
2
R (35)
O (16)
2
1
a
3
3
Ϫ1
Ϫ1
b
Ϫ1
3
ε/10 dm mol cm
. i = limiting current for each species alone in solution; i/c values (µA mol dm ) at the same rotating gold disc electrode
Ϫ1 c Ϫ
(
diameter = 2 mm; Ω = 1000 rev min ). In the presence of sulfur, the formation of C H C(O)S ions is observed in the lone diffusion layer.
6 5 2
(
1) and (2) were evidenced. Spectrophotometric (λmax, εmax) and
electrochemical [E₂
polysulfide ions and those of RC(O)S /RC(O)S ions are
summarized in Table 1. [S3 ], [S6 ], [S8 ] can be accurately
evaluated from A617, A515 measurements and the values of K ,
₁
(O), (R)] characteristics of sulfur/
E
₁
�
₂�
Ϫ
Ϫ
2
Ϫ
2Ϫ
2Ϫ
᝽
1
K , K ; [S ] could be obtained by the limiting intensity of its R
2
3
8
1
wave.
1
–
Ϫ
3
Reactivity of S ions towards acyl chlorides
Acyl chlorides are very sensitive to hydrolysis into RCO H and
2
HCl acids. The carboxylic acids are practically unreactive with
1
3
–
Ϫ
2Ϫ
S
and S8 polysulfides in dimethylacetamide [pK (CH -
A 3
11 11
CO H) = 12.6;
chloric acid results in the oxidation of S
protonation of these weak basic species [9 < pK (HS ) < 11,
pK (C H CO H) = 11.0 ] whereas hydro-
2
A
6
5
2
Ϫ 2Ϫ
into S8 and the
3
᝽
1
2
Ϫ
A
8
7
< pK (H S ) < 9]. All the substrates RC(O)Cl were carefully
A 2 8
distilled on molecular sieves under dry nitrogen atmosphere just
before use.
The reactions were followed at room temperature by spectro-
electrochemistry during the addition of a concentrated solution
Ϫ2
of acyl chloride (1a–4a) in DMA (3 × 10
mol
1
Ϫ3
Ϫ1
Ϫ3
–Ϫ
3
dm < C < 1.5 × 10 mol dm ) to S ions of total concen-
tration [S3 ]T close to 5.0 × 10 mol dm . The stoichio-
metric processes described below indicated that effective
RC(O)Cl concentrations were ~92 (R = Me, Pr ) to 95%
R = Bu , Ph) of theoretical values. In all cases, the reactions
were fast under our experimental conditions. Figs. 1(a) and (b)
Ϫ
o
Ϫ3
Ϫ3
᝽
i
t
(
and 2 show the general evolutions of UV–VIS spectra and volt-
ammograms when RC(O)Cl (R = alkyl, 1a–3a) were progres-
1
3
–
Ϫ
Fig. 1 Evolution of UV–VIS spectra during addition of tert-butyl-
1–
Ϫ o Ϫ3 Ϫ3
Ϫ
sively added to S solutions, after the example RC(O)-
3
acetyl chloride to a S solution [S3 ]T = 5.17 × 10 mol dm
.
t
Ϫ
o
Ϫ3
Ϫ3
᝽
Cl = Bu C(O)Cl ϩ [S3 ]T = 5.17 × 10 mol dm ; as long as
Ϫ
᝽
o
᝽
(a) y = [RC(O)Cl]/[S3 ]T = 0 (1); 0.02 (2); 0.04 (3); 0.06 (4); 0.08 (5);
Ϫ
o
the ratio y = [RC(O)Cl]ad./[S3 ]T remains below 0.15 (~1/7),
0.11 (6); 0.13 (7); 0.15 (8). (b) y = 0.15 (8); 0.20 (9); 0.31 (10); 0.42 (11);
0.53 (12); 0.75 (13); 0.96 (14); 0.99 (15). Thickness of the cell 0.1 cm.
᝽
Ϫ
2Ϫ
A617 (S3 ) decreases while the bands of S increase (λmax1
=
᝽
8
4
5
15 nm, λ
= 360 nm ) with the occurrence of an isosbestic
max2
8
point at 540.5 nm [Fig. 1(a)]. The calculated variations of
S3 ]T and [S8 ] satisfy ∆[S8 ]/∆[S3 ]T ≈ Ϫ5/14;, and agree
with the overall eqn. (15) = (14) ϩ
by the same oxidation wave according to the electrocatalytic
Ϫ
2Ϫ
2Ϫ
Ϫ
[
process (11) ϩ (16b), as previously observed with thiolate
᝽
᝽
5
–
(4b). The consump-
2
Ϫ
Ϫ
2
RC(O)S Ϫ 2e → [RC(O)] S
(16a)
(11)
2
2
Ϫ
Ϫ
5
–
Ϫ
RC(O)Cl ϩ 2S3᝽
→ RC(O)S ϩ
S2 ϩ Cl
(14)
(4b)
2
Ϫ
Ϫ
2
RC(O)S ϩ S
2RC(O)S2
2
Ϫ
2Ϫ
2S3᝽
ϩ S → S8
2
Ϫ
Ϫ
2
RC(O)S2 Ϫ 2e → [RC(O)] S ϩ S
(16b)
2
2
2
Ϫ
Ϫ
5
–
2Ϫ
Ϫ
RC(O)Cl ϩ 7S3᝽
→ RC(O)S ϩ
S8 ϩ Cl (15)
2
2
2Ϫ
ions. For y = 0.5 [stoichiometry (14)] the polysulfides S
8
Ϫ
tion of sulfur arising from substitution (14) is practically quan-
and S3 are not completely eliminated as shown by the per-
sistence of A515, A617 and i(O ) [Figs. 1(b) and 2, curves 12].
The spectra are the same as those of RC(O)S solutions elec-
᝽
Ϫ
titative according to eqn. (4b) because of the excess in S3 ions
᝽
1
Ϫ
and of the values of K and K . The stoichiometry (15) with
1
2
Ϫ
5
–
2Ϫ
7
S3 →
᝽
S8 (λ = 540 nm) was already observed in the
course of the substitutions of S ions on o- and p-nitro-
NO2 ). For 0.15 <
y < 0.5, the reactions of both S and S8 ions [decreases
trogenerated from RC(O)SH and added to sulfur in a propor-
is
1
3
2
–
Ϫ
Ϫ
tion of 8[S ] /[RC(O)S ] = 5; this was verified between
8
0
0
1
b
1d
Ϫ
o
Ϫ3
Ϫ3
Ϫ3
aromatics NO C H X (X = halides,
[S3 ]T = 4.97 × 10 mol dm ϩ [MeC(O)Cl] = 2.58 × 10
2
6
4
1
3
᝽
0
–
Ϫ
2Ϫ
Ϫ3
Ϫ
Ϫ3
mol dm
(y = 0.52) and [CH C(O)S ] = 2.25 × 10
mol
3
0
Ϫ3
Ϫ3
Ϫ3
of A617, A515; i(O ), E
₁
= Ϫ0.20 V] entail the appearance
dm ϩ 8[S ] = 11.0 × 10
mol dm . The concentrations
1
₂�
8 0
Ϫ
Ϫ
of S8 [i(R ),
ϩ0.17 V] and Cl [E₂
E
₁
= Ϫ0.40 V], RC(O)S /RC(O)S
[i(O) ≈
which were evaluated by the use of K , K , K and K constants
1 2 4 5
t
1
₂�
2
Ϫ
₁
(O) ≈ ϩ0.65 V] ions. In the presence of
sulfur, RC(O)S and RC(O)S2 in equilibrium (11) are detected
(R = Me, Bu , conditions of Table 2) led to values of A and
�
515
Ϫ
Ϫ
A617 in agreement (±10%) with the experimental one. For
1
760
J. Chem. Soc., Perkin Trans. 2, 1997

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Ϫ3
t
Ϫ
Ϫ3
Table 2 Calculated composition (mmol dm ) of solutions of thiocarboxylate ions (R = Me, Bu ) added with sulfur; [RC(O)S ] = 2.60 × 10 mol
0
Ϫ3
Ϫ3
Ϫ3
dm ϩ 8[S ] = 13.0 × 10 mol dm
8
0
Ϫ
Ϫ
Ϫ
2Ϫ
Ϫ
a
b
R
RC(O)S
RC(O)S2
(RCO)2S2
S3᝽
S8
RCOS2 (%)
(RCO) S (%)
2 2
Me
Bu
1.80
0.83
0.63
0.38
0.08
0.70
0.12
0.54
0.02
0.42
24
14.6
6
53.8
t
a
Ϫ
Ϫ
Ϫ
b
Ϫ
RCOS2 (%) = [RC(O)S2 ]/[RC(O)S ] . (RCO) S (%) = 2[(RCO) S ]/[RC(O)S ] .
0
2
2
2
2
0
1
–
Fig. 2 Evolution of voltammograms during the reaction of tert-
1
3
3Ϫ
2Ϫ
Ϫ
–
Ϫ
Fig. 3 Evolution of S (1), S8 (2), apparent S (3), RC(O)S (4)
8
x
butylacetyl chloride with S ions. Same conditions as for Fig. 1. Rotat-
Ϫ1
concentrations during the addition of tert-butylacetyl chloride to a
Ϫ
o Ϫ3 Ϫ3
ing gold-disc electrode, Ω = 1000 rev min , diameter = 2 mm. E vs.
Ϫ3
solution [S
3
᝽
]
Ϫ
T
o= 5.17 × 10
mol dm
as a function of y =
reference Ag/AgCl(s), KCl sat. in DMA–N(Et) ClO (0.1 mol dm ).
4
4
[
RC(O)Cl]/[S3 ]T
᝽
Ϫ
᝽
2Ϫ
Ϫ
Ϫ
y > 0.5, S3 , S8 and RC(O)S /RC(O)S2 concentrations con-
tinue to decrease, as shown by the variations of visible absorb-
ances and oxidation currents [Figs. 1(b) and 2, curves 13, 14].
When y reaches the value 1.0, the solutions (R = alkyl) were
Ϫ
Ϫ
Ϫ
Ϫ
decoloured and the wave of Cl ions (3Cl → Cl ϩ 2e )
was the only anodic one, with an intensity close to that calcu-
3
lated after calibration with a solution of N(CH ) Cl [i(O)/C ≈ 8
3
4
Ϫ1
3
Ϫ
µA mmol dm ]. C H C(O)S ions are not oxidized by sulfur
into diacyl disulfide and polysulfide ions [eqn. (10)], so when
benzoyl chloride was added to S ions, the consumption of
S3 /S8 ions went to completion for y = 0.5. However, in the
presence of sulfur, all the RC(O)S ions (R = alkyl, phenyl)
react with RC(O)Cl yielding [RC(O)] S species (b) [eqn. (17)].
6
5
8
1
3
–
Ϫ
Ϫ
᝽
2Ϫ
Ϫ
8
2
2
Ϫ
Ϫ
RC(O)S ϩ ¹S ϩ RC(O)Cl → [RC(O)] S ϩ Cl (17)
2
2
2
¯
²
1
Fig. 4 Relative increase in the i(R ) reduction current of sulfur vs.
1
o t
–
Ϫ
3
The overall reaction of acyl chlorides with S ions can thus
be summarized by eqn. (18).
[RC(O)Cl/[S8]T ratio. R = Ph (1), Me (2), Bu (3).
‘
mediator’ for the substitution of acyl chlorides by polysulfide
Ϫ
Ϫ
2RC(O)Cl ϩ 2S3᝽
→ [RC(O)]S ϩ ¹S ϩ 2Cl (18)
ions.
2
8
¯
²
Ϫ
2Ϫ
Ϫ
2Ϫ
Fig. 3 sums up the evolution of concentrations [S3 ]T, [S8 ],
S
8
ϩ 2e → S
8
(3)
᝽
Ϫ
[
RC(O)Sx ] (x = 1, 2), apparent [S ] as a function of the
advancement y of the reactions (R = Bu , experimental con-
ditions of Figs. 1 and 2). Whatever the nature of RC(O)Cl
1a–4a), the limiting current of sulfur i(R ) increases beyond
y = 0.5, without noticeable generation of S . This catalytic effect
Ph > Me >> Bu ) was previously explained by the fast reduction
8
t
RC(O)Cl ϩ S82Ϫ → RC(O)S ϩ
Ϫ
7
–
Ϫ
S8 ϩ Cl
(20)
8
The electrochemical reduction of S at controlled potential
(Ϫ1.0 V < E < Ϫ0.8 V) in the presence of
RC(O)Cl is illustrated on Figs. 5 and 6 with the experimental
= 0.83 × 10 mol dm ; [Bu C(O)Cl] = 1.25 ×
mol dm ; [HCl) ≈ 0.2 × 10 mol dm . [The initial con-
centration of hydrochloric acid estimated from i(Cl )/C values
increases with the dilution of RC(O)Cl because of a definite
(
8
1
on the plateau of R
8
1
t
(
(
Ϫ 2Ϫ 8
Ϫ3
Ϫ3
t
19) of [RC(O)] S by S or S8 ions. The same phenomenon
conditions: [S8 0
]
0
2
2
3
᝽
Ϫ3
Ϫ3
Ϫ3 Ϫ3
1
0
0
Ϫ
2Ϫ
Ϫ
S ϩ 2e → S
(3)
8
8
Ϫ3
Ϫ3
fast
water content (~8 × 10 mol dm ) in the solvent].
Ϫ1
2
Ϫ
Ϫ
(19)
[
RC(O)] S ϩ S
8
2RC(O)S ϩ S8
2
2
(
i) For 0 < nF mol RC(O)Cl р 1 (curves 2–4), the over-
all process (21) equivalent to reactions (3) ϩ (20) ϩ (17) is
is observed when an excess of RC(O)Cl is poured into the solu-
observed.
tions at y values greater than 1 (Fig. 3, curve 3); the additions
Ϫ3
Ϫ3
Ϫ
Ϫ
of acyl chlorides to S8 solutions (2.6 × 10 mol dm
<
2RC(O)Cl ϩ S ϩ 2e → [RC(O)] S ϩ 2Cl (21)
2
2 2
Ϫ3
Ϫ3
[
S ] < 3.2 × 10
mol dm ), which were performed with
8
0
t
2Ϫ
R = Ph, Me, Bu , also lead to a great enhancement of i(R )
As soon as polysulfides S8 appear at the electrode surface,
1
t
(
Fig. 4), again in the order Ph > Me ӷ Bu . Sulfur, which is
RC(O)Cl in excess brings about the formation of [RC(O)] S
by reaction (17) on the intermediate RC(O)Sx (x = 1, 2) ions
2
2
Ϫ
quickly regenerated from step (20), can be proposed as a
J. Chem. Soc., Perkin Trans. 2, 1997
1761

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Table 3 Initial conditions and products of the chemical synthesis of diacyl (poly)sulfides in dimethylacetamide, R = 1–4
[
RC(O) S (%)
2 x
2
Ϫ
a
R
(S6 )0/mmol
(RCOCl) /mmol
x = 1
x = 2
x = 3
M/g
Yield (%)
0
b
b
Me
16.3
14.7
12.6
11.6
28
25
22
18.5
—
<2
>99
96
>99
97
trbace
1.31
1.96
1.85
1.75
62
76
72
69
i
b
Pr
Bu
<2
t
—
trace
—
c
Ph
3
a
b
c
1
Yield based upon added RC(O)Cl. From GC–MS. From H NMR.
Table 4 Electrosynthesis of diacyl (poly)sulfides (R = 2–4) in dimethylacetamide
[
RC(O)] S (%)
2 x
a
R
8(S ) /mmol
(RCOCl) /mmol
x = 1
x = 2
x = 3
M/g
Yield (%)
8
0
0
i-C H7
30
35
28
13.8
15.5
14.6
—
—
2
100
>99
98
—
trace
—
0.92
1.12
1.35
64
61
67
3
b
t-C H9
4
c
C H5
6
a
b
c
1
Yield based upon added RC(O)Cl. From GC–MS. From H NMR.
of equilibria (10), (11) and (4) for R = alkyl; the expected species
Ϫ
are detected on i = f(E ) and A = f(λ) recordings, i.e. S
[A617
,
,
3
᝽
2
Ϫ
Ϫ
E
E
₁
(O ) = Ϫ0.20 V], S8
[A515
,
E (O )], R(CO)S2 [A330
(O) ≈ ϩ0.15 V], RC(O)S (A263), with the simultaneous emis-
sion of H S whatever the nature of RC(O)Cl (1a–4a): the pro-
₁
₂
₁
1
₂�
1
Ϫ
�
2
tons are progressively eliminated after the formation of the
H S polysulfane which then disproportionates according to
2
8
1
2
reaction (22).
2
Ϫ
ϩ
7
–
S8 ϩ 2H → H S → H S ϩ
S8
(22)
2
8
2
8
Ϫ1
(
iii) For nF mol RC(O)Cl > 2 (curves 8–9) the electrolysis
of sulfur into polysulfides becomes predominant.
The above spectroelectrochemical results were applied on a
preparative scale by two sets of experiments: direct addition of
1
3
–
Ϫ
Fig. 5 Evolution of voltammograms during the electrolysis of
acyl chlorides to initial S solutions [eqn. (18)]; electrolysis of
Ϫ3
Ϫ3
t
a solution [S ] = 0.83 × 10 mol dm in presence of [Bu C(O)Cl] =
2Ϫ
8
0
0
Ϫ3
Ϫ3
Ϫ1
sulfur in the presence of acyl chlorides [eqn. (21)]. (i) S
6
Ϫ
1
0
.25 × 10 mol dm at E = Ϫ0.9 V. nF mol RC(O)Cl = 0 (1); 0.34 (2);
.68 (3); 1.02 (4); 1.32 (5); 1.70 (6); 2.03 (7); 2.37 (8); 2.71 (9)
(
S3 ) ions were chemically generated from the quanti-
᝽
13
tative oxidation (23) of ‘anhydrous’ Li S by sulfur.
2
2
Ϫ
5
–
2Ϫ
S
ϩ
S8 → S6
(23)
8
2
Ϫ
Orange S6 ions (λ = 465 nm) are the major species on
1
3
max
–
Ϫ
2Ϫ
o
Ϫ2
Ϫ3
concentrated S solutions: for [S
S6 ]0 = 6.0 × 10 mol dm and [S3 ]0 = 5.0 × 10 mol dm
as given by K (297 K) constant. RC(O)Cl (1a–3a) in DMA
]
= 85 × 10 mol dm ,
6
T
Ϫ
2
Ϫ
Ϫ2
Ϫ3
Ϫ2
Ϫ3
[
᝽
3
were added to polysulfides at room temperature up to decolor-
Ϫ
o
ation, which occurred at y = [RC(O)Cl]ad./[S3 ]T values less
᝽
than 1 (~0.85), perhaps because of a composition of Li S
1
3
2
2Ϫ
–
Ϫ
weaker than expected. With PhC(O)Cl 4a, polysulfides S /S
8
were totally consumed [eqn. (14)] for y ≈ 0.40, leading only to
Ϫ
PhC(O)S and sulfur; the addition of PhC(O)Cl was extended
to y = 0.80 in order to carry out the second step (17). The initial
conditions of the syntheses and the nature of their products
have been reported in Table 3; (ii) the electrolysis of solutions
Fig. 6 Evolution of UV–VIS spectra during the electrolysis of a sol-
Ϫ3
Ϫ3
t
Ϫ3
Ϫ3
(S ) ϩ (RCOCl) (R = 2–4) with 8(S ) /(RCOCl) ≈ 2 and (RC-
ution [S ] = 0.83 × 10 mol dm ; Bu C(O)Cl] = 1.25 × 10 mol dm
.
8
0
0
8
0
0
8
0
0
Same conditions as for Fig. 5.
OCl) ≈ 15 mmol were performed in a two-compartment cell
0
Ϫ1
on the basis of eqn. (21), up to 1F mol RC(O)Cl. Table 4
summarizes the conditions and the results of electrosyntheses.
Ϫ
coming from the substitutions (20) and/or (14) with S3 ions
᝽
generated in the diffusion layer by disproportionation (4f). The
cathodic R wave (S ) decreases to the benefit of the only oxid-
1
8
Ϫ
Discussion
ation current of Cl (E₂ ≈ 0.65 V) up to n = 1; the solution
₁
�
1
4
remains colourless and the spectra practically unmodified
Acyl disulfides have been synthesized by chemical or electro-
1
–
3
Ϫ1
Ϫ1
t
3
15
[
ε262(S ) = 2000 dm mol cm ; ε258(Bu COS) = 3000 dm
chemical oxidation of thiocarboxylate ions, treatment of acyl
8
2
4
Ϫ1
Ϫ1
16
mol cm in ref. 8].
chlorides with lithium disulfide or sodium disulfide under
Ϫ1
17
(
ii) For 1 < nF mol RC(O)Cl < 2 (curves 5–7), the electro-
phase-transfer catalysis. Our results establish that these com-
generated polysulfide ions reduce [RC(O)]2S2 species into
thiocarboxylate ions [eqns. (3) ϩ (19)] with the observation
pounds are readily produced by the reactions between stable
1
3
–
Ϫ
S
polysulfide ions and acyl chlorides in dipolar aprotic
1
762
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
media. In a similar way, diacyl peroxides were obtained from
RC(O)Cl before their appreciable dissociation. Nevertheless,
9
superoxide ions and acyl chlorides in two overall steps
this scheme has to be weighed against the formation of
9
b
Ϫ
Ϫ
reported as follows.
(i) nucleophilic substitution (24) of O2᝽
[RC(O)] S by the addition of RC(O)Cl to ‘RC(O)S ϩ nS sol-
2 2
8
utions’. The latter were here achieved in the first step of re-
1
Ϫ
Ϫ
Ϫ
–Ϫ
3
RC(O)Cl ϩ 2O2᝽
→ RC(O)OO ϩ O ϩ Cl
(24)
actions RC(O)Cl ϩ S ions, the stoichiometry of which is
2
described by eqn. (14). Several nucleophilic species can be
on the carbonyl carbon. The implication of an addition–
elimination mechanism rather than a S 2 one was proposed on
the basis of the reactions (25) evolved with esters. The easy
Ϫ
Ϫ
5
–
Ϫ
N
RC(O)Cl ϩ 2S
3᝽
→ RC(O)S ϩ
S2 ϩ Cl (14)
2
1
8
Ϫ
2Ϫ
2Ϫ
Ϫ
viewed: S , S6 , S8
᝽
. The similar reactivities of S3᝽
and
3
᝽
O–
Ϫ
O2 could be consistent with the initial addition–elimination
[eqn. (37)] of the radical trisulfide. With our experimental
k
RC(O)X
+
O2•–
R
C
X
RC(O)OO•
+
X– (25)
OO•
k
Ϫ
ؒ
Ϫ
RC(O)Cl ϩ S3
RC(O)S3 ϩ Cl
(37)
reduction (26) of the neutral acylperoxy radical by homo-
᝽
ؒ
Ϫ
Ϫ
RC(O)OO ϩ O2
→ RC(O)OO ϩ O2
(26)
᝽
device, the rate constants of ‘fast’ reactions are inaccessible
because of the solution transfer to the spectrophotometric cell
Ϫ
geneous electron transfer (26) then leads to RC(O)OO
which takes ca. 10 s. That the reaction is second order with
1
8–20
ions.
‘
The fast and concurrent formation of [RC(O)] O via
self reaction’ (27) of RC(O)OO radicals was not observed in
Ϫ
2
2
respect to S3 was determined in the course of slow substit-
᝽
ؒ
21
1b
utions of nitroaromatic halides or dehalogenations of vic-
1
c
2Ϫ
dibromides. This fact led us to propose S ions as the effect-
1
6
ؒ
–
2
RC(O)OO → [RC(O)] O ϩ O
(27)
2Ϫ
2
2
2
ive nucleophilic S agents. The reactivity of S8 ions, the least
reducing polysulfides, was negligible in comparison with that of
S6 , with a possible advancement of the processes
shift (4f) in the equilibrium between S8 and S ions.
The formation of diacyl disulfides by reactions (17) was
Ϫ
the presence of O2᝽
in excess owing to reduction (28).
2Ϫ
1b,c
by the
1
3
2
Ϫ
–Ϫ
Ϫ
Ϫ
RC(O)] O ϩ 2O
→ 2RC(O)O ϩ 2O
(28)
2
2
2
᝽
2
(
ii) Subsequent nucleophilic substitution (29) of peroxide
Ϫ
Ϫ
RC(O)S ϩ ¹S ϩ RC(O)Cl → [RC(O)] S ϩ Cl
(17)
2
2 2
Ϫ
¯
²
anions on RC(O)Cl. At first sight the reactivities of O
S
when polysulfides are in excess, RC(O)S2 ions are initially
obtained; a second step similar to eqn. (29) affords diacyl
and
1
3
2
᝽
–
Ϫ
ions towards acyl chlorides seem identical: as in eqn. (24),
Ϫ 8
previously explained by an enhanced reactivity of RC(O)S
Ϫ
2
23
Ϫ
2
ions. This ‘α-effect’, also noticed with RS2 ions, is
unquestionable in the example R = Ph since only traces of
Ϫ
PhC(O)S 2Ϫ were detected by voltammetry in the solutions
Ϫ
Ϫ
RC(O)Cl ϩ RC(O)OO → [RC(O)] O ϩ Cl (29)
2
2
PhC(O)S ϩ nS. However with R = alkyl, acyl disulfides could
1
3
instead be generated from the displacement of equilibrium
–Ϫ
1
3
2
Ϫ
–
Ϫ
disulfides [RC(O)] S . These species are reducible by S /S
ions into thiocarboxylates RC(O)S . However, several proposals
can be advanced from the respective experimental results.
2
2
8
(10), by the reaction of RC(O)Cl with SϪ ions beyond the
Ϫ
o
value 0.5 of the ratio y = [RC(O)Cl]/[S ] . The compared
1
3
3
᝽
T
–
Ϫ
Ϫ
Ϫ
nucleophilicities of S , O2
and ArS
ions (Ar = 2-
᝽
x
Ϫ
The substitution of O2
on RC(O)X substrates [reaction
NO C H , 4-NO C H , C H ; x = 1, 2) towards the same sub-
᝽
2
6
4
2
6
4
6
5
1
–
9b,18–20
1
b
24
Ϫ
(
30), X = Cl, OC(O)R, ORЈ] has always been regarded
as
3
strates in S Ar or S 2 processes decrease in the order S
~
N
N
Ϫ
Ϫ
Ϫ
O2 > ArS2 > ArS . From that point of view our present
results indicate that the reactivity of RC(O)S
situated at the level of the ArS2 Ϫone.
The dissociation of RC(O)S2 ions is rather high in dipolar
aprotic media as shown by the values of K (Me) and K (Bu ).
ArO2 ions totally dissociate in the course of S Ar reactions
᝽
k
Ϫ
Ϫ
ؒ
Ϫ
2
species can be
RX ϩ O2
ϩ RO2 ϩ X
(30)
᝽
Ϫ
ؒ
Ϫ
Ϫ
t
RO2
ϩ O2᝽
→ RO2 ϩ O2
(31)
(32)
5
5
Ϫ
N
Ϫ
Ϫ
Ϫ
25
RO2 ϩ RX → RO R ϩ X
2
between O
2
and activated aromatic halides ion DMF or
᝽
6
Ϫ
DMSO yielding ArO ions as products [reactions (38) and
Ϫ
22
Ϫ
analogous to that of O2 on alkyl halides. This mechanism
depends on the evolution of cyclic voltammograms of
O ϩ RX solutions: RO2 ions would quickly react with RX in
excess from their generation [eqns. (30) ϩ (31)]. An identical
interpretation can be proposed to explain the electroreduction
of sulfur in the presence of acyl chlorides since only diacyl
disulfides were obtained for nF mol RC(O)Cl р 1. It would
confirm the involvement of the S /S
(39)]. By analogy with RC(O)S
2
ions, the dissociation of
᝽
Ϫ
Ϫ
Ϫ
Ϫ
2
ArX ϩ O ϩ 2e → ArO ϩ X
2
2
(38)
(39)
Ϫ
Ϫ
ArO2 → ArO ϩ ¹O
2
¯
²
Ϫ1
Ϫ
Ϫ
peroxy anions RC(O)OO (and ROO ?) looks highly probable
when RC(O)X (and RX?) species react to O2
Ϫ
system rather than S /
Ϫ
᝽
2
2
᝽
8
ions [eqns.
2
Ϫ
2,4
S8 one as previously claimed in redox processes. In that case,
reactions (34) and (35) of S2᝽ ions would be faster than their
(
24) ϩ (29)]. On this assumption, diacyl peroxides could be
obtained by the addition of acyl chlorides to solutions of carb-
oxylate ions saturated with oxygen.
Ϫ
Ϫ
Ϫ
S ϩ e → S
(33)
(34)
(35)
2
2
᝽
To conclude, diacyl disulfides are conveniently produced in
1
3
–
Ϫ
almost quantitative yields in the reactions between S ions
and acyl chlorides in dimethylacetamide. The process evolves
via both the partial and concurrent formation of acyl disulfide
ions and oxidation of intermediate thiocarboxylate ions. Fur-
ther investigations of nucleophilic substitutions of polysulfide
ions on other ‘less aggressive’ acylating agents than acyl chlor-
ides, such as anhydrides, thioanhydrides and S-aryl thioesters,
are presently in progress in our group.
Ϫ
→ RC(O)S ؒ ϩ ClϪ
RC(O)Cl ϩ S2
2
᝽
ؒ
Ϫ
Ϫ
RC(O)S2 ϩ S2
→ RC(O)S2 ϩ S2
᝽
Ϫ
Ϫ
RC(O)S2 ϩ RC(O)Cl → [RC(O)] S ϩ Cl (36)
2
2
2
Ϫ
duplication into S4, and RC(O)S2 ions would react with
J. Chem. Soc., Perkin Trans. 2, 1997
1763

View Article Online
ϩ
Experimental
~2% from GC–MS); m/z 174 (M , <2%), 71 (100) and 43 (84);
i
ϩ
(
Pr CO) S (Table 3 ~2% from GC–MS); m/z 238 (M , <2%), 71
2 3
Materials and equipment
(
95) and 43 (100).
Bis(trimethylacetyl) disulfide (3b). δH 1.37 (18 H, s); δ 27.0
All the organic compounds were obtained from Aldrich. Lith-
ium sulfide was purchased from Alfa-Ventron. The purification
of N,N-dimethylacetamide and its storage after addition of
tetraethylammonium perchlorate (Fluka) as supporting electro-
C
ϩ
(6 C), 46.8 (2 C) and 199 (2 C); m/z 234 (M , <2%), 85 (53),
5
7 (100) and 41 (11).
Dibenzoyl disulfide (4b). 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); δ 128.1 (4 C),
27
Ϫ3
26
lyte (0.10 mol dm ) have been reported elsewhere. Spectro-
electrochemical equipments, electrodes, the flow-through cell
and the two-compartment preparative cell were previously
C
1
29 (4 C), 133.8 (2 C), 134 (2 C) and 186 (2 C); direct introduc-
ϩ
tion mode m/z 274 (M , 4%), 105 (50), 77 (100) and 51 (25).
C H CO) S (Tables 3 and 4 ~2–3%) from δ 8.06 (4 H, J 7.4).
1
c,4
1
described.
(
The synthesized products were analysed by H
13
(
6
5
2
H
200.132 MHz) and C (50.323 MHz) NMR (Brucker AC 200
spectrometer) with CDCl as the solvent (internal standard
3
Acknowledgements
Me Si, J values in Hz) and GC–MS (Hewlett-Packard 5989 A,
4
EI 70 eV).
We are grateful to Dr P. Dubois (Service d’Analyse Chimique
du Vivant—Tours) for helpful discussions and recordings of
NMR and mass spectra.
1
3
–
Ϫ
Generation of S solutions
Sulfur (0.08–0.09 mmol S , 40 cm ) was electrolyzed in the flow-
3
8
through cell at controlled potential on the plateau of its second
References
reduction wave R (E = Ϫ1.3 V) on a large gold grid electrode,
2
according to eqn. (40). The residual acidity of the solvent was
1
(a) J. Paris and V. Plichon, Nouv. J. Chim., 1984, 8, 733; (b)
M. Benaïchouche, G. Bosser, J. Paris and V. Plichon, J. Chem. Soc.,
Perkin Trans. 2, 1991, 817; (c) G. Bosser and J. Paris, J. Chem. Soc.,
Perkin Trans. 2, 1992, 2057; (d) G. Bosser, M. Benaïchouche,
R. Coudert and J. Paris, New J. Chem., 1994, 18, 511.
8
–
Ϫ
8
–
Ϫ
S8 ϩ
e →
S3᝽
(40)
3
3
2
Ϫ
2Ϫ
eliminated after protonation of intermediate S3 /S4 ions and
2
3
G. Bosser, M. Anouti and J. Paris, J. Chem. Soc., Perkin Trans. 2,
1
2
disproportionation of H S polysulfanes into H S and sulfur.
2
x
2
1
3
1
996, 1993.
–
Ϫ
When the absorbance reached a maximum of 617 nm, S ions
were the only species in solution. Before spectroelectrochemical
studies their concentrations were accurately calculated from
M. Benaïchouche, G. Bosser, J. Paris, J. Auger and V. Plichon,
J. Chem. Soc., Perkin Trans. 2, 1990, 31.
4 G. Bosser and J. Paris, New J. Chem., 1995, 19, 391 and references
cited therein.
5 D. T. Sawyer and M. J. Gibian, Tetrahedron, 1979, 35, 1471 and
references cited therein.
Ϫ
4
A675 measurements, a wavelength at which S
absorbs alone
3
᝽
3
Ϫ1
Ϫ1
(ε675 = 1825 dm mol cm ).
6
M. Gareil, J. Pinson and J. M. Savéant, Nouv. J. Chim., 1981, 5, 311
and references cited therein.
Chemical and electrochemical syntheses
The chemical syntheses of diacyl disulfides 1b–4b (Table 3) were
7 C. Degrand and H. Lund, Acta Chem. Scand., Ser. B, 1979, 33, 512.
8 J. Robert, M. Anouti and J. Paris, J. Chem. Soc., Perkin Trans. 2,
1997, 473.
carried out according to the same procedure reported on the
2
Ϫ
typical example 1: S6 ions were generated by heating a sol-
3
9 (a) R. Johnson, Tetrahedron Lett., 1976, 5, 331; (b) D. T. Sawyer,
J. J. Stamp and K. A. Menton, J. Org. Chem., 1983, 48, 337.
0 (a) J. P. Stanley, J. Org. Chem., 1980, 45, 1413; (b) A. Le Berre and
Y. Berguer, Bull. Soc. Chim. Fr., 1966, 7, 2368.
ution (200 cm ) of Li S (16.3 mmol) and S (81.5 mmol S) at
2
8
5
0 ЊC for 1 h under N atmosphere. At room temperature, the
2
1
orange–blue polysulfide ions were progressively added to
3
CH C(O)Cl dissolved in 25 cm of DMA, up to decoloration
11 M. Bréant and J. Georges, J. Electroanal. Chem., 1976, 68, 165.
12 J. Paris and V. Plichon, Electrochim. Acta, 1982, 10, 1501.
3
[
~28 mmol in CH C(O)Cl]. After filtration of sulfur, the sol-
3
1
3 M. Delamar and J. C. Marchon, J. Electroanal. Chem., 1975,
3, 351.
ution was diluted with 4 vol. of 3% NaHCO before extraction
with diethyl ether. The organic phase was thoroughly washed
3
6
1
1
4 R. L. Franck and J. R. Blegen, Org. Synth. Coll., vol. III, 1955, 116.
5 Y. Hirabayashi and T. Mazume, Bull. Chem. Soc. Jpn., 1966, 39,
with water and dried (MgSO ). After concentration, traces of
4
solid sulfur were eliminated by filtration at 0 ЊC.
1
971.
The preparative electrolysis (R = 2–4, Table 4) were also per-
16 J. A. Gladysz, V. K. Wong and B. S. Jick, Tetrahedron, 1979, 35,
2329.
17 (a) M. Kodomari, M. Fukuda and S. Yoshitomi, Synthesis, 1981, 8,
637; (b) J. X. Wang, W. Cui, Y. Hu and K. Zhao, Synth. Commun.,
i
formed at room temperature; as an example, Pr C(O)Cl (13.8
3
mmol) was dissolved in 120 cm of the catholyte [N(Et) ClO
4
4
Ϫ3
0
.5 mol dm in DMA] which was then added with solid sulfur
1
995, 25, 889.
(30 mmol S); in this way the solution remained always saturated
1
8 M. T. Gibian, D. T. Sawyer, T. Ungermann, R. Tangpoonpholvivat
with sulfur. The potential of a large gold grid as cathode was
kept between Ϫ0.60 and Ϫ0.80 V. The current retained a great
intensity (220–240 mA) as a result of catalytic effects on the
reduction of sulfur with both reagents RC(O)Cl and products
and M. M. Morrison, J. Am. Chem. Soc., 1979, 101, 640.
19 F. Magno and G. Bontempelli, J. Electroanal. Chem., 1976, 68, 337.
20 J. San Filippo Jr., L. J. Romano, C. I. Chern and J. S. Valentine,
J. Org. Chem., 1976, 41, 586.
2
2
1 K. U. Ingold, Acc. Chem. Res., 1969, 2, 1.
[
RC(O)] S . With R = alkyl, the end of the process was detected
2
2
2 (a) R. Dietz, A. G. Forno, B. E. Larcombe and M. E. Peover,
J. Chem. Soc. (B), 1970, 816; (b) K. Daasbjerg and H. Lund, Acta
Chem. Scand., Ser. B, 1993, 47, 597 and references cited therein.
2Ϫ
by the appearance of the red colour of S8 polysulfides coming
from reduction (19) of diacyldisulfides and equilibriums
(10) ϩ (4). After filtration of excess sulfur and solid N(Et) Cl,
23 J. Dixon and J. Bruice, J. Am. Chem. Soc., 1972, 94, 2052 and refer-
4
the reaction medium was treated in the same way as for chem-
ical syntheses.
ences cited therein.
4 M. Benaïchouche, G. Bosser, J. Paris and V. Plichon, J. Chem. Soc.,
Perkin Trans. 2, 1990, 1421.
5 F. Magno, G. Bontempelli and M. M. Andreuzzi Sedea, J. Electro-
anal. Chem., 1979, 97, 85.
6 J. Paris and V. Plichon, Electrochim. Acta, 1981, 26, 1823.
2
2
2
i
t
The oily products [RC(O)] S (R = Me, Pr , Bu ) were purified
2
x
by column chromatography on silica gel (light petroleum–
diethyl ether as eluent, 80:20); dibenzoyl disulfide was
recrystallized from 1,2-dichloroethane.
27 C. Christophersen and P. Carlsen, Tetrahedron, 1976, 32, 745.
Diacetyldisulfide (1b). δ 2.54 (6 H, s); δ 28.8 (2 C) and 189.5
H
C
ϩ
(
2 C); m/z 150 (M , <2%) and 43 (100).
Paper 7/00939I
Bis(dimethylacetyl) disulfide (2b). δ 1.28 (12 H, d, J 6.9) and
Received 7th February 1997
Accepted 15th April 1997
H
2
.96 (2 H, septet, J 6.9); δ 18.9 (4 C), 41.9 (2 C) and 195 (2 C);
C
ϩ
i
m/z 206 (M , <2%), 71 (77) and 43 (100). (Pr CO) S (Table 3
2
1
764
J. Chem. Soc., Perkin Trans. 2, 1997