Nucleophilic substitution of S-phenyl thiol esters by electrogenerated polysulfide ions in N,N-dimethylacetamide

Article abstract of DOI:10.1039/a707562f

The reactivity of electrogenerated S^1/3- polysulfide ions S^.-₃ (?^2-₆) towards S-phenyl thiol esters RC(O)SPh [R = Me (1), Et (2), Prⁱ (3)] has been followed by spectroelectrochemistry in N,N-dimethylacetamide at 24°C. With 1 and 2 reactions readily lead to thiocarboxylate ions and phenyltetrasulfanide ions, PhS^-₄, from the nucleofugic benzenethiolate ions in presence of sulfur. With 3 kinetic studies imply that S^2-₆ species are the nucleophilic S^1/3- agents rather than S^.-₃ radical anions in a second order rate process (k = 30 ± 3 dm³ mor^-1 s^-1).

Full text of DOI:10.1039/a707562f

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Nucleophilic substitution of S-phenyl thiol esters by electrogenerated
polysulfide ions in N,N-dimethylacetamide
Abdelkader Ahrika, Meriem Anouti, Julie Robert and Jacky Paris*
Laboratoire de Physicochimie des Interfaces et des Milieux Réactionnels,
UFR Sciences et Techniques, Parc de Grandmont, 37200 Tours, France
1
3
–
The reactivity of electrogenerated S ؊ polysulfide ions S3
(
S62؊) towards S-phenyl thiol esters
Ϫ
᝽
RC(O)SPh [R ؍ Me (1), Et (2), Pri (3)] has been followed by spectroelectrochemistry in N,N-dimethyl-
acetamide at 24 ЊC. With 1 and 2 reactions readily lead to thiocarboxylate ions and phenyltetrasulfanide
ions, PhS4؊, from the nucleofugic benzenethiolate ions in presence of sulfur. With 3 kinetic studies imply
1
–
that S62؊ species are the nucleophilic S ؊ agents rather than S3 Ϫ radical anions in a second order rate process
3
᝽
(k ؍ 30 3 dm3 mol؊1 s؊1).
S-alkyl and S-aryl carbothioates (‘thiol esters’) are useful
intermediates in organic synthesis, as they have a higher reactiv-
ity towards nucleophiles than their oxygen homologues. Their
acylating properties have been notably used in lactonisations,
leading to macrocyclic natural products1 and the preparation
of acylstannanes,2 whereas superoxide ions substitute on
dithioic S,SЈ-diesters.3
disc electrode). In the course of its electrolysis at controlled
potential on R1 [reaction (6), probably through the initial elec-
S8 ϩ 2eϪ → S82Ϫ
(6)
Ϫ
tron transfer6 S2 ϩ eϪ→S2 followed by the reactions of S2 on
᝽
the dimeric form S42Ϫ, then on S62Ϫ up to S82Ϫ], S82Ϫ ions [λmax1 = 515
nm, εmax1 = 3800 dm3 molϪ1 cmϪ1; λmax2 = 360 nm, εmax2 = 9000
We recently reported on the reactions between S3 Ϫ polysulfide
᝽
ions and RC(O)X species [X = Cl;4 SC(O)R and OC(O)R5] in
dipolar aprotic medium. Two successive and fast steps were
evidenced with acyl chlorides and thioanhydrides: initial substi-
tution [reaction (1)] followed by reaction (2) of thiocarboxylate
dm3 molϪ1 cmϪ1; ε617 = 300 dm3 molϪ1 cmϪ1; E (R) = Ϫ1.10 V,
₁
₂
E (O) = Ϫ0.20 V] disproportionate into S3 Ϫ ions [λmax = 617 nm,
₁
₂
᝽
εmax = 4390 dm3 molϪ1 cmϪ1; E (R) = Ϫ1.10 V, E (O) = Ϫ0.20 V]
₁
₁
₂
₂
and sulfur [equilibrium (7), eqn. (8)]. The total consumption of
Ϫ
Ϫ
Ϫ
5
2
f
–
RC(O)X ϩ 2S3 → RC(O)S ϩ S2 ϩ X
(1)
S82Ϫ
2S3 Ϫ ϩ S2
᝽
(7)
᝽
b
RC(O)SϪ ϩ ¹S2 ϩ RC(O)X → [RC(O)]2S2 ϩ XϪ (2)
¯
²
K2 (297 K) = [S3Ϫ]2[S2][S82Ϫ Ϫ1
]
= 1.7 × 10Ϫ6 mol2 dmϪ6 (8)
᝽
ions in the presence of sulfur, leading to diacyl disulfides. With
anhydrides, the first step occured at a lower rate.
We examine here the reactivity of electrogenerated S3
ions towards a series of S-phenyl thiol esters RC(O)SPh [R =
Me (1), Et (2), Pri (3)] in N,N-dimethylacetamide (DMA). Both
the known spectroelectrochemical characteristics of sulfur/
polysulfide ions and the behaviour of the nucleofugic benzene-
thiolate ions in the presence of sulfur enabled the reactions to
be followed by UV–VIS absorption spectrophotometry coupled
with stationary voltammetry.
1
–
sulfur leads to S Ϫ polysulfide ions, S3 Ϫ and S62Ϫ (λmax = 465 nm,
3
᝽
εmax = 3100 dm3 molϪ1 cmϪ1) in equilibrium [equilibrium (9),
Ϫ
᝽
Ϫ
S62Ϫ
2S3
᝽
(9)
eqn. (10)]. In dilute solutions, [S62Ϫ] remains low in comparison
K3(297 K) = [S3 Ϫ]2[S62Ϫ Ϫ1 = 0.043 mol dmϪ3 (10)
]
᝽
to [S3 Ϫ] (e.g. [S62Ϫ] = 0.42 × 10Ϫ3 mol dmϪ3 at total concentration
᝽
[S3 Ϫ]0T = [S3 Ϫ] ϩ 2[S62Ϫ] = 5.0 × 10Ϫ3 mol dmϪ3).
᝽
᝽
Results
In DMA we showed that thiolate ions react with sulfur in two
Characteristics of sulfur/polysulfides and benzenethiolate–sulfur
solutions in DMA
The partial dissociation of cyclooctasulfur S8 into S2 molecules
(~50% at [S8]T = 1.5 × 10Ϫ3 mol dmϪ3) in dimethylacetamide
was recently proposed by our group6 [equilibrium (3), eqns. (4)
parallel and fast ways:7 (i) weak oxidation [reaction (11)] giving
2RSϪ ϩ 3S2 → RS2R ϩ 2S3
(11)
Ϫ
᝽
disulfides and polysulfide ions and (ii) dominating reaction (12)
2RSϪx Ϫ 1 ϩ S2 2RSxϪ
(12)
S8
4S2
(3)
(4)
K1(297 K) = [S2]4[S8]Ϫ1 = 10Ϫ7 mol3 dmϪ9
yielding successive formation of RSxϪ ions (R = alkyl, x = 2–5;
R = aryl, x = 2–4):
and (5)]. As in other aprotic media, sulfur reduces in DMA in
K(x) = [RSxϪ]2[RSxϪϪ 1
]
Ϫ2[S2]Ϫ1
(13)
1
–
[S8]T = [S8] ϩ 4[S2]
(5)
The reactivity of RSϪ ions towards sulfur is the same as that
of ‘thionucleophilic’ Nu species such as CNϪ, SO32Ϫ and Ar3P,
leading to SNu products, with the hypothetical opening of the
S8 ring as the rate-determining step.8 By analogy with RSϪ/O2
processes,9 we suggested7a a very different mechanism involving
initial monoelectronic transfer [reaction (14)] followed by coup-
6
two bielectronic steps with respect to total S8 (waves R1,
₁
₁
E = Ϫ0.40 V vs. ref.† and R2, E = Ϫ1.10 V on a rotating gold-
₂
₂
† All potentials are expressed in comparison to the reference electrode
(ref. 6) Ag/AgCl(s), KCl sat. in DMA/N(Et)4ClO4 0.1 mol dmϪ3
.
J. Chem. Soc., Perkin Trans. 2, 1998
607

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Table 1 Spectroelectrochemical characteristics and successive form-
ation constants of PhSϪx ions in N,N-dimethylacetamide
PhSϪx
λmax/nm
εmax/dm3 mol cmϪ1
E (O)/V K(x)a/mol dmϪ3
₁
₂
PhSϪ
PhS2Ϫ
PhS3Ϫ
PhS4Ϫ
309
310b
460
460
18200
3200b
400
ϩ0.16
—
Ϫ0.03
ϩ0.10
ϩ0.10
3.4 × 109
1.2 × 107
5.8 × 104
900
a K(x) at 298 K and ionic strength = 0.1 mol dmϪ3
.
b From ref. 7b.
Fig. 2 Evolution of voltammograms during the reaction of S-phenyl
1
–
thioacetate with S Ϫ ions. Same conditions as for Fig. 1. Rotating gold-
3
disc electrode, Ω = 1000 rev minϪ1, diameter = 2mm. E vs. reference Ag/
AgCl(s), KCl sat. in DMA/N(Et)4ClO4 0.1 mol dmϪ3. Scan rate: 10 mV
sϪ1
.
Reactivity of S3 Ϫ ions with S-phenyl thiol esters
᝽
With S-phenyl thioacetate (1) and S-phenyl thiopropionate (2)
esters the fast reactions were followed at 24 ЊC by the same
evolutions of A = f(λ) and i = f(E) recordings as shown in Figs.
1(a), 1(b) and 2 when CH3C(O)SPh was added to a solution
[S3 Ϫ]0T = 4.30 × 10Ϫ3 mol dmϪ3. (i) As long as the ratio y =
᝽
[RC(O)SPh]ad./[S3 Ϫ]0T remained less than 0.25 [Figs. 1(a) and 2,
᝽
Ϫ
curves (2)–(5)], the consumption of S3 ions was evidenced by
᝽
₁
the fall in anodic and cathodic currents [E (O) = Ϫ0.20,
₂
₁
E (R) = Ϫ1.10 V], and A617 decreased to the benefit of A490
₂
through an approximate isosbestic point at 525 nm without
any appearance of sulfur (no wave R1 on voltammograms), in
agreement with reactions (7b) and (17) and (18), summarized
by the overall reaction (19).
Ϫ
RC(O)SPh ϩ 2S3 → RC(O)SϪ ϩ PhSϪ ϩ S2
(17)
(18)
(7b)
(19)
5
2
–
᝽
Ϫ
Ϫ
3
–
PhS ϩ 2S2 → PhS4
Fig. 1 Evolution of UV–VIS spectra during the addition of S-phenyl
1
3
–
Ϫ
᝽
Ϫ
thioacetate to a S solution [S3 ]0T = 4.30 × 10Ϫ3 mol dmϪ3. (a) y =
2S3 Ϫ ϩ S2 → S82Ϫ
᝽
Ϫ
[RC(O)SPh]ad./[S3 ]0T = 0 (1); 0.04 (2); 0.08 (3); 0.13 (4); 0.21 (5). (b)
᝽
y = 0.30 (6); 0.38 (7); 0.46 (8); 0.72 (9); 0.89 (10); 1.36 (11). Scan rate:
RC(O)SPh ϩ 4S3 Ϫ → RC(O)SϪ ϩ PhS4Ϫ ϩ S82Ϫ
1000 nm minϪ1
.
᝽
Sulfur coming from substitution (17) reacts preferentially
Ϫ
Ϫ
ؒ
Ϫ
᝽
with PhSϪ rather than S3 ions, up to PhS4Ϫ: as an example,
RS ϩ S2 → RS ϩ S2
(14)
᝽
for [S3 Ϫ]0T = 4.30 × 10Ϫ3 mol dmϪ3 and [RC(O)SPh]ad.
=
᝽
0.5375 × 10Ϫ3 mol dmϪ3 (y = ), K1, K2, K3 and K(x) constants for
1
lings between RS and S2 Ϫ radicals into RS3Ϫ ions (reducible into
–
ؒ
8
᝽
RS2Ϫ by RSϪ in excess), RS2R and polysulfide ions. With less
reductive aromatic thiolates ArSϪ (Ar = C6H5, 4-CH3C6H4),
oxidation (11) was practically unobserved. For values of x
greater than 2, RSxϪ ions were characterized7a by a visible band
(λmax = 460–470 nm) and their oxidation wave into RS2R. Table
1 summarizes spectroelectrochemical parameters for C6H5SxϪ
ions, and their successive formation constants K(x). Whatever
the nature of R, RS4Ϫ ions partly disproportionate according to
the key equilibrium (15) [with eqn. (16)], at the junction of
concurrent eqns. (11) and (12).7a
reaction (17) give calculated concentrations at equilibrium:
[S3 Ϫ]T = 2.175 × 10Ϫ3, [S2] = 8.1 × 10Ϫ6, [S82Ϫ] = 0.525 × 10Ϫ3 and
᝽
[PhS4Ϫ] = 0.537 × 10Ϫ3 mol dmϪ3, with [PhS4Ϫ] ≈ [S82Ϫ] bearing out
reaction (19). The maximal absorption of S82Ϫ ions at 515 nm
was slightly shifted to shorter wavelengths (≈490 nm) by the
presence of PhS4Ϫ ions. From7 ε490(S82Ϫ) ≈ 3500 and ε490
-
(PhS4Ϫ) ≈ 800 dm3 molϪ1 cmϪ1, variations ∆([S3 Ϫ]T)/∆[S82Ϫ] =
᝽
Ϫ4.0 0.2 (0 < y < 0.2) satisfied reaction (19). (ii) For 0.25 <
Ϫ
y < 0.50 [curves (6)–(8)], the consumption of S3 and S82Ϫ ions
᝽
[probably through the shift (7f) because S82Ϫ ions are less nucleo-
philic species than S1–3Ϫ ones10] entailed both decreases of A617
and A490, and growths of the oxidation current of PhS4Ϫ ions
2RS4Ϫ
RS2R ϩ 2S3
(15)
(16)
Ϫ
᝽
₁
[E (O) = ϩ0.10 V] and of the reduction wave R1 of sulfur
₂
₁
[E (R) = Ϫ0.40 V], according to reaction (20).
₂
K4 = [RS2R][S3 Ϫ]2[RS4Ϫ]Ϫ2
᝽
RC(O)SPh ϩ 2S3 Ϫ → RC(O)SϪ ϩ S2 ϩ PhS4Ϫ (20)
᝽
When RS2R and S3 Ϫ were mixed in the ratio 1:2, spectra and
᝽
Ϫ
voltammograms were the same as those obtained by mixing
sulfur and thiolate ions in the proportion [S]ad./[RSϪ]0 = 3. In the
present study, PhS4Ϫ ions will be directly generated by electro-
reduction of diphenyl disulfide into benzenethiolate ions, then
stoichiometric addition of sulfur as previously described,7a and
the value K4 (298 K) = 2.0 × 10Ϫ4 mol dmϪ3 (R = Ph) will be
taken again.
However for y = 0.5, S3 and S82Ϫ ions were not eliminated
᝽
because of their partial regeneration by the extensive dispro-
portionation [reaction (15)] of PhS4Ϫ ions followed by reaction
(7b): e.g. with initial conditions [PhS4Ϫ]0 = [S2]0 = 2.15 × 10Ϫ3 mol
dmϪ3, at equilibrium [PhS4Ϫ] = 0.55 × 10Ϫ3, [S2] = 1.49 × 10Ϫ3
,
[S3 Ϫ]T = 0.276 × 10Ϫ3 and [S82Ϫ] = 0.66 × 10Ϫ3 mol dmϪ3 from K2
᝽
and K4 constants; (iii) beyond y = 0.5 [curves (9)–(11)] the
608
J. Chem. Soc., Perkin Trans. 2, 1998

View Article Online
evolution of VIS spectra was strictly the same as that observed
with the direct addition of RC(O)SPh to a solution [PhS4Ϫ]0 =
2.15 × 10Ϫ3 mol dmϪ3: the substrate in excess displaces
equilibrium (15) by the reaction (20) with polysulfide ions.
In the presence of sulfur, RC(O)SϪ ions weakly lead to the
formation [reaction (21), with eqn. (22)] of RC(O)S2Ϫ species as
f
2RC(O)SϪ ϩ S2
2RC(O)S2Ϫ
(21)
(22)
b
K5 = [RC(O)S2Ϫ]2[RC(O)SϪ]Ϫ2[S2]Ϫ1
previously reported.11 For example, with R = CH3 and
[CH3C(O)SϪ]0 = 2[S2]0 = 2.15 × 10Ϫ3 mol dm3 RC(O)S2Ϫ reaches
17% at equilibrium from11 K5 (293 K) = 48 dm3 molϪ1. As
observed with thiolate ions,7 RC(O)SϪ species directly oxidize
Ϫ
Fig. 3 Calculated curves of concentrations [S3
]
(1), S82Ϫ (2), S8 (3),
T
᝽
Ϫ
RC(O)SϪ (4), PhS4Ϫ (5) as a function of y = [RC(O)SPh]ad./[S3
]
0
T
᝽
compared with experimental values during the addition of S-phenyl
1
3
–
Ϫ
thioproprionate to a S Ϫ solution [S3 ]0T = 4.30 × 10Ϫ3 mol dmϪ3
₁
᝽
into disulfides [reaction (23), R = CH3, E (O) = ϩ0.31 V], or at
₂
2RC(O)SϪ Ϫ 2eϪ → [RC(O)]2S2
(23)
the lower potentials of RC(O)S2Ϫ ions [E (O) ≈ ϩ0.09 V] in the
₁
₂
presence of traces of sulfur, by the electrocatalytic process11
given by reactions (21) ϩ (24).
2RC(O)S2Ϫ Ϫ 2eϪ → [RC(O)]2S2 ϩ S2
(24)
We verified that CH3C(O)SϪ/CH3C(O)S2Ϫ ions, which were
generated by initial electroreduction of thiolacetic acid,11 are
unreactive towards the substrate CH3C(O)SPh {(i = f(E) and
A = f(λ) recordings of an initial solution [CH3C(O)SϪ]0 =
1.45 × 10Ϫ3 mol dmϪ3
,
[S]0 = 7.85 × 10Ϫ3 mol dmϪ3 were
Fig. 4 Kinetic studies of the reaction of S-phenyl thioisobutyrate with
unmodified by the addition of [CH3C(O)SPh] = 3.25 × 10Ϫ3
1
–
Ϫ at 24 ЊC. Variation of the initial rate as a function of [S3 Ϫ]0.
3
S
᝽
mol dmϪ3}; so in the course of the overall reaction (20)
Ϫ
₁
At617/l = ε3[S3 Ϫ]t ϩ ε8[S82Ϫ
]
(26)
(0 < y < 0.5, Fig. 2) the anodic currents of PhS4 (E = ϩ0.10 V)
₂
t
᝽
and CH3C(O)SϪ/CH3C(O)S2Ϫ ions (E ≈ ϩ0.09 V) progressively
₁
₂
took the place of S3 Ϫ/S82Ϫ one (E = Ϫ0.20 V).
₁
The order n of the reaction concerning S3 Ϫ was obtained by
᝽
₂
᝽
During the progress y of the fast reaction between S-phenyl
Ϫ 0
the initial-rate method. As long as the ratio ∆[RC(O)SPh]/[S3
]
T
᝽
thiopropionate ester (2) and an initial solution [S3 Ϫ]0T =
1
᝽
–
remains less than 4, the overall reaction (19) can be considered
4.30 × 10Ϫ3 mol dmϪ3, [S3
was accurately determined
Ϫ
]
T
᝽
(0 < y < 1.5) with A675 measurements, a wavelength at which
RC(O)SPh ϩ 4S3 Ϫ → RC(O)SϪ ϩ PhS4Ϫ ϩ S82Ϫ (19)
᝽
the radical anion alone absorbs [ε675 (S3 Ϫ) = 1825 dm3 molϪ1
᝽
cmϪ1]; [S8]T was estimated (y > 0.3) from i(R1) values by use of
the coefficient i(R1)/[S8]T = 34.0 0.5 µA mmolϪ1 dm3 depend-
ing on our working electrode; [S82Ϫ] was deducted from spectro-
photometric parameters at 490 and 617 nm with a significant
precision for 0 < y < 0.5. Fig. 3 shows the changes of these
‘experimental’ concentrations, and of all the calculated values‡
from constants K1–K4 and equations of conservation of sulfur,
thiolester and charges as a function of y. It summarizes the
previously described steps: (i) 0 < y р 0.25 [eqn. (19)] without
any detection of sulfur and maximal formation of S82Ϫ ions for
y ≈ 0.25; (ii) quantitative generation of RC(O)SϪ ions up to
y = 0.5 [eqn. (20)]; (iii) y > 0.5: partial consumption of PhS4Ϫ
ions from the shift of equilibrium (15), with an advancement
for y = 1 of 70% with respect to the balance [reaction (25)] of
eqns. (15) ϩ (20).
alone, without any absorbance of PhS4Ϫ and S62Ϫ ions at 617
nm.7
In nucleophilic processes, the reactivity of the least reducing
polysulfide ions S82Ϫ 12 was negligible in comparison to that of
S3–1Ϫ species;10 here again the rate of reaction (19) greatly
Ϫ
decreased when 3 was added to a solution S3 saturated with
᝽
sulfur. ([S8]T ≈ 9 × 10Ϫ3 mol dmϪ3), so the rate equation can be
expressed as eqns. (27) and (28). Constant K3 and the linked
Ϫ
d[S3
]
T = kobs[RC(O)SPh]t[S3
]
᝽
Ϫ n
1
–
(27)
(28)
vt = Ϫ4
t
᝽
dt
Ϫ
]
ln(ν0/[RC(O)SPh]0) = lnkobs ϩ nln[S3
0
᝽
variations of [S3 Ϫ]t and [S82Ϫ]t [reaction (19)] easily give eqn. (29).
᝽
RC(O)SPh ϩ S3 Ϫ → RC(O)SϪ ϩ ¹PhS2Ph ϩ S2 (25)
᝽
¯
²
Ϫ
1
4[S3
]
1 dA617
0
᝽
1
–
(29)
ν0 = Ϫ
ͩ
1 ϩ
ͪ ͩ ͪ
Kinetic studies of the reaction of S ؊ ions with S-phenyl
thioisobutyrate
Probably owing to steric hindrance, S-phenyl thioisobutyrate
reacted slowly with S31–Ϫ ions. In that case the kinetic study was
carried out at 24 ЊC on the recordings of A617 vs. time after the
addition of substrate 3 to [S3]0T solutions [eqn. (26)].
3
4ε3 Ϫ ε8
K3
l
dt
t → 0
The determination of ν0 for six pairs of values ([S3 Ϫ]0T,
᝽
[RC(O)SPh]0) enabled us to obtain (Fig. 4) n = 1.98 0.08 and
kobs = 710 40 dm6 molϪ2 sϪ1 at an ionic strength equal to 0.1
mol dmϪ3. The hypothesis of a trimolecular process in the rate-
determining step is relatively improbable. This fact, which was
previously noticed in the course of the slow reactions between
S31–Ϫ ions and nitroaromatic halides10a or vicinal dibromides,10b
‡ The minor formation of acyl disulfide ions according to equilibrium
(21) was not taken into account in our iterative calculations.
J. Chem. Soc., Perkin Trans. 2, 1998
609

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led us to propose also S62Ϫ ions as the effective nucleophilic
agents in substitutions (20). It can be attributed to the more
localized charge on terminal sulfur atoms of S62Ϫ compared to
was distilled under dry nitrogen just before handling. The puri-
fication of N,N-dimethylacetamide and its storage after add-
ition of N(Et)4ClO4 0.1 mol dmϪ3, spectroelectrochemical
equipment, electrodes and the thermostatted (24.0 0.5 ЊC)
flow-through cell have been described previously.6
S3 Ϫ ones, as proposed by Meyer et al.13 using calculations by the
᝽
extended Hückel method. The rate is thus characterized by the
constant k = kobs × K3 with k (3) = 30 3 dm3 molϪ1 sϪ1
.
1
–
Generation of S ؊ solutions and procedures
3
1
–
Initial S Ϫ solutions (40 cm3) were obtained by exhaustive elec-
3
Discussion
trolysis [reaction (34)] of sulfur (0.1–0.65 mmol S) at controlled
When S-methyl thiobutanoate and S-ethyl thioacetate, or ethyl
Ϫ
Ϫ
8
3
8
–
Ϫ
–
S8 ϩ e → S3
(34)
and phenyl acetate were added at 24 ЊC to S3 solutions in
᝽
3
᝽
DMA in the ratio [RC(O)XRЈ]/[S3 Ϫ]0T = 5 (X = S, O) reactions
᝽
did not occur to any appreciable extent. The reactivity of S1–3Ϫ
potential of a large gold grid electrode on the plateau of the R2
wave (E = Ϫ1.3 V). S3 ions in equilibrium with S62Ϫ [reaction
Ϫ
ions towards S-phenyl thiolesters in N,N-dimethylacetamide
᝽
Ϫ
(9)] were the only species in solution when A617 reached a
can be compared to that of O2 ions on O-phenyl esters in
᝽
pyridine14 [k(CH3CO2Ph) = 160, k(CH3CO2Et) = 1.1 × 10Ϫ2
maximum.
dm3 molϪ1 Ϫ1; 20 ЊC ?] or DMF15 {k[4-ClC6H4OC(O)Ph] =
s
The stoichiometry of the fast overall (19) and (20) processes
was studied by the progressive addition of concentrated solu-
tions of RC(O)SPh 1 or 2 in DMA (νmax = 4 cm3) to S3–1Ϫ ions
25, k(C6H5CO2Ph) = 3.0 dm3 molϪ1 Ϫ1; 20 ЊC}. On the basis
s
of cyclic voltammetry experiments,14 the implication of an
initial addition–elimination mechanism (30) was proposed for
(4.2 × 10Ϫ3 < [S3 Ϫ]0T < 4.60 × 10Ϫ3 mol dmϪ3).
᝽
Initial rate measurements were based on A617 changes vs. time
when a small volume (νmax = 1 cm3) of S-phenyl thioisobutyrate
in DMA was added to each of six solutions 0.77 × 10Ϫ3
O–
k
•–
RC(O)X + O2
R
C
X
RC(O)OO• + X–
< [S3 Ϫ]0T < 3.18 × 10Ϫ3 mol dmϪ3 (0.37 > y > 0.18) at 24 ЊC; the
᝽
OO•
transfer of the reaction medium to the spectrophotometric cell
(1 mm pathlength) took about 15 s, whereas reaction half-times
with respect to S3 were such as 230 s > t > 70 s.
Ϫ
RC(O)X ϩ O2 reactions evolved with esters, followed by the
᝽
Ϫ
₁
reduction of the acylperoxyradical [reaction (31)]. Owing to the
᝽
₂
RC(O)OO ϩ O2 Ϫ → RC(O)OOϪ ϩ O2
(31)
ؒ
᝽
References
1 (a) S. Masamume, H. Yamamoto, S. Kamata and A. Fukuzawa,
J. Am. Chem. Soc., 1975, 97, 3513; (b) H. Yamamoto, S. Kamata and
W. Schilling, J. Am. Chem. Soc., 1975, 97, 3515; (c) K. C. Nicolaou,
Tetrahedron, 1977, 33, 683.
presumed high nucleophilicity of RC(O)OOϪ anions, the final
formation of carboxylate ions14,15 was ascribed to subsequent
reactions (32) and (33).
2 A. Capperucci, A. Degl’Innocenti, C. Faggi, G. Reginato and
A. Ricci, J. Org. Chem., 1989, 54, 2966.
3 R. D. Webster and A. M. Bond, J. Chem. Soc., Perkin Trans. 2, 1997,
1075.
4 J. Robert, M. Anouti, M. Abarbri and J. Paris, J. Chem. Soc., Perkin
Trans. 2, 1997, 1759.
RC(O)OOϪ ϩ RC(O)X → [RC(O)]2O2 ϩ XϪ (32)
[RC(O)]2O2 ϩ 2O2 Ϫ → 2RC(O)OϪ ϩ 2O2
(33)
᝽
5 J. Robert, M. Anouti and J. Paris, New J. Chem., 1998, in the press.
6 G. Bosser and J. Paris, New J. Chem., 1995, 19, 391 and references
cited therein.
The net rate depended on the stability of the leaving group
XϪ[k(Ar OϪ) ӷ k(RЈOϪ)] as observed in the present study with
thiol esters [k(PhSϪ) ӷ k(RSϪ)]. Reactions of acyl chlorides
1
3
7 (a) G. Bosser, M. Anouti and J. Paris, J. Chem. Soc., Perkin Trans. 2,
1996, 1993; (b) M. Benaïchouche, G. Bosser, J. Paris, J. Auger and
V. Plichon, J. Chem. Soc., Perkin Trans. 2, 1990, 31.
8 S. Oae, Organic Sulfur Chemistry: Structure and Mechanism, CRC
Press, Ann Arbor, 1991, pp. 119–134 and references cited therein.
9 C. Degrand and H. Lund, Acta Chem. Scand., Ser. B, 1979, 33, 512.
10 (a) M. Benaïchouche, G. Bosser, J. Paris and V. Plichon, J. Chem.
Soc., Perkin Trans. 2, 1991, 817; (b) G. Bosser and J. Paris, J. Chem.
Soc., Perkin Trans. 2, 1992, 2057.
11 J. Robert, M. Anouti and J. Paris, J. Chem. Soc., Perkin Trans. 2,
1997, 473.
12 J. Paris and V. Plichon, Electrochim. Acta, 1982, 27, 1501.
13 B. Meyer, L. Peter and K. Spitzer, in Homolytic Rings, Chains and
Macromolecules of Main-group Elements, ed. A. L. Rheingold,
Elsevier, 1977, 477.
14 M. J. Gibian, D. T. Sawyer, T. Ungermann, R. Tangpoonpholvivat
and M. M. Morrison, J. Am. Chem. Soc., 1979, 101, 640.
15 F. Magno and G. Bontempelli, J. Electroanal. Chem., 1976, 68, 337.
16 J. E. Dixon and T. C. Bruice, J. Am. Chem. Soc., 1972, 94, 2052 and
references cited therein.
17 A. R. Forrester and V. Purushotham, J. Chem. Soc., Perkin Trans. 1,
1987, 945.
–
5
and ‘thioanhydrides’ with S Ϫ ions occurred in two overall steps
(1) and (2), respectively analogous to (30) ϩ (31) and (32), but
RC(O)S2Ϫ ions greatly dissociate into RC(O)SϪ and sulfur [eqn.
(21b)].11 The only formation of diacyl disulfides by the addition
11
of RC(O)Cl to RC(O)SϪ ϩ S2 [eqn. (2)] was explained by an
enhanced reactivity of minory RC(O)S2Ϫ ions with respect to
thiocarboxylate ions ‘α-effect’.16 In the same conditions [step
(2)], anhydrides [RC(O)]2O were practically unreactive5 as
shown here with S-phenyl thiol esters. So, in the course of
RC(O)ORЈ ϩ O2 Ϫ processes, ions RCO2Ϫ ions could result from
᝽
the dissociation of RC(O)OOϪ species rather than from eqns.
(32) ϩ (33). This hypothesis agrees with the dispute about an
intermediate diacyl peroxide en route to the acid RCO2H when
superoxide ions reacted with phenylbenzoate esters in
benzene.17
To conclude, in dipolar aprotic medium thiocarboxylate ions
are readily obtained from S62Ϫ ions and the more efficient acyl-
ating agents S-phenyl thiol esters than O-homologues, probably
due to the weak conjugation between sulfur and the carbonyl
group.18
18 M. W. Cronyn, M. Pin chang and R. A. Wall, J. Am. Chem. Soc.,
1955, 77, 3031.
Experimental
Materials and equipment
All the organic compounds of commercial origin (purity
> 98%) were used as received except for thiolacetic acid which
Paper 7/07562F
Received 20th October 1997
Accepted 12th December 1997
610
J. Chem. Soc., Perkin Trans. 2, 1998