Anodic oxidation of disulfides: Detection and reactions of disulfide radical cations

Article abstract of DOI:10.1021/jo401263z

The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/[NBu₄][B(C₆F₅)₄] electrolyte. Cyclic voltammetry scans of (p-RC₆H₄) ₂S₂ (R = Me, 1

Full text of DOI:10.1021/jo401263z

Article
Anodic Oxidation of Disulfides: Detection
and Reactions of Disulfide Radical Cations
Kevin Lam, and William E. Geiger
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo401263z • Publication Date (Web): 15 Jul 2013
Downloaded from http://pubs.acs.org on July 16, 2013
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Anodic Oxidation of Disulfides: Detection and Reactions of Disulfide Radical Cations
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Kevin Lama and William E. Geiger*
Department of Chemistry, University of Vermont, Burlington, VT 05405 USA
william.geiger@uvm.edu
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a Present address: Department of Chemistry, School of Science and Engineering, Nazarbayev
University, Astana, Republic of Kazakhstan 010000
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Abstract
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The anodic oxidation of five diaryldisulfides have been studied in a
dichloromethane/[NBu4][B(C6F5)4] electrolyte. Cyclic voltammetry (CV) scans of (pꢀRC6H4)2S2
(R = Me, 1a; R = F, 1b; R = OMe, 1c) show modest chemical reversibility for the 10/+ couple
(E1/2 values vs ferrocene: 1.04 V for 1a, 1.21 V for 1b, 0.92 V for 1c), providing the first
voltammetric evidence for the radical cation [Ar2S2]+. A dimer dication, [Ar4S4]2+, is proposed as
an intermediate in the formation of the electrolysis product, the trisulfide [Ar3S3]+. The chemical
reversibility of the oneꢀelectron oxidations of Ar2S2 vanishes in [PF6]ꢀ ꢀ containing electrolytes.
The radical cations of the more sterically constrained orthoꢀsubstituted analogues
dimesityldisulfide (2a, E1/2 = 1.01 V) and bis(2,4,6ꢀtriisopropylphenyl)disulfide (2b, E1/2 = 0.98
V) show less tendency to dimerize. In all cases except 2b, the bulk electrolysis product is
[R3S3]+, consistent with earlier literature reports. A mechanism is proposed in which the
trisulfide is formed by reaction of the dimer dication [Ar4S4]2+ with neutral Ar2S2 to afford the
trisulfide in a net 2/3 eꢀ process. Oxidation of Ar2S2, either anodically or by a strong oneꢀelectron
oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2ꢀsubstituted
cyclohexyldisulfides.
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Introduction
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Owing in part to the importance of organic disulfides in structural biology,1 the
electrochemical oxidation of this class of compounds continues to draw study, adding to an
already extensive body of papers.2 For simple RSSR disulfides, the anodic electrolysis products
and the (1 eꢀ vs 2 eꢀ) electronꢀtransfer stoichiometry depend principally on the nature of R and the
temperature and chemical makeup of the solvent/supporting electrolyte system. An important
general finding is that the radical cations of this series, [RSSR]+, have low kinetic stability. A
few radical cations of cyclic disulfides (such as dithionaphthalene shown below) have been
confirmed by optical or ESR spectroscopy.3,4 However, to the best of our knowledge, radical
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cations of the linear disulfides [RSSR]+, R = alkyl or aryl, although frequently invoked as
intermediates, have not been confirmed even as a transient species by spectroscopy or cyclic
voltammetry (CV). Rather, [RSSR]+ has been reported to be kinetically unstable with respect to
sulfurꢀsulfur cleavage,5ꢀ8 thereby initiating routes to formation of more stable in situ products
such as the sulfenium ion [RS]+ 5ꢀ13 or the trisulfide [R3S3]+.5,10,11,14 With the goals of finding
direct electrochemical evidence for [RSSR]+ and adding to mechanistic understanding of the
oxidativelyꢀinduced conversion of disulfides to their cationic products, we have investigated the
anodic oxidation of several diaryl disulfides and dimethyldisulfide in dichloromethane using the
weakly coordinating anion (WCA) [B(C6F5)4]ꢀ as the supporting electrolyte anion. Although the
low nucleophilicity of this anion15 has been extremely valuable in promoting the stability of
highly electrophilic organometallic systems,16,17 it has seen relatively few applications to organic
redox chemistry.14,18 The present paper reports our findings on five diaryldisulfides (1aꢀ1c, 2a
and 2b) and dimethyldisulfide. With the exception of the sterically hindered compound 2b, the
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R
R
S
R
R
S
S
R
S
R
R
R
1a, R = Me
1b, R = F
1c, R = OMe
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2a, R = Me
2b, R = iPr
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longꢀterm (bulk electrolysis time scale) product is, in fact, the expected trisulfide [R3S3]+.
However, some degree of chemical reversibility of the parent 0/+ couple was observed in each
case for CV scans at modest scan rates, giving the first definitive voltammetric evidence for the
radical cations of these compounds, and allowing a more detailed discussion of the mechanism
of anodicallyꢀinduced formation of the trisulfide.
Results and Discussion
The literature paper most relevant to our study is the communication by Matsumoto et al. in
which catalytic amounts of [Ar3S3]+ were shown to initiate stereoselective addition of Ar2S2 to
dienes.14 The authors generated the trisulfide cation in-situ by bulk electrolysis of Ar2S2 in
CH2Cl2/0.1 M [NBu4][B(C6F5)4], essentially the same medium as used in the present work. An
important difference, however, is that the published work was carried out at low temperature
(195 K), a factor that has an important effect on the ability to detect the primary radical cation
[Ar2S2]+, 1+. Although voltammograms of Ar2S2 were not shown or referred to in reference 14,
CV scans of the diaryldisulfides under these lowꢀtemperature conditions are not likely to show
any chemical reversibility for the [Ar2S2]0/+ couple (see below).
The present study focuses on three paraꢀsubstituted arene derivatives, 1aꢀ1c, and the bisꢀ
orthoꢀsubstituted arene derivatives 2a and 2b. Measurements were also made on
dimethyldisulfide. Results on 1aꢀ1c suggested that the overall anodic process proceeds by the
mechanism given in Scheme 1, a key component being the dimer dication, [Ar4S4]2+, 3.
Subsequent preparation and electrochemical testing of the more stericallyꢀencumbered
compounds 2a and 2b confirmed expectations based on this scheme.
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Scheme 1. Proposed mechanism for anodic oxidation of diaryldisulfides
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Oxidation of paraꢀsubstituted arene disulfides, 1aꢀ1c
At room temperature, all three of this group of disulfides displayed some degree of chemical
reversibility with a CV scan rate of 0.2 V sꢀ1 in CH2Cl2/0.05 M [NBu4][B(C6F5)4], permitting
assignment of the E1/2 values of their oneꢀelectron oxidations (Table 1). 1 mM solutions of 1a
and 1c showed scanꢀrate dependent chemical reversibility similar to that shown in Figure 1, ic/ia
ratios going from about 0.5 to 0.75 when scan rates were increased from 0.2 V sꢀ1 to 2 V sꢀ1. At a
given concentration, anodic peak currents increased with the square root of scan rate, confirming
a diffusionꢀcontrolled anodic process. A diffusion coefficient of 1.73 x 10ꢀ5 cm2sꢀ1 was measured
for 1a by chronoamperometry on a 0.025 mM solution at 298 K. No matter what the degree of
chemical reversibility of the 10/+ process, there were never more than two cathodic features:
either that coupled to reduction of the radical cation at the appropriate Epc, or a totally
irreversible peak (lacking a coupled anodic feature) appearing about one volt or more negative of
the E1/2 of the 10/+ couple. The potential of the irreversible wave varies with scan rate (v),
appearing at the Epc values given in Table 1 when v = 0.2 V sꢀ1. The paraꢀsubstituted aryl
derivatives will be discussed somewhat interchangeably owing to their similar overall anodic
behavior.
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Table 1. Potentials in CH2Cl2/0.05 M [NBu4][B(C6F5)4]. Epc was recorded at a scan rate of 0.2 V
sꢀ1.
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Epc (V) of trisulfide
Entry
Compound
E1/2 (V vs FcH)
product 4
ꢀ0.03
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(pꢀMeC6H4)2S2 , 1a
1.04
1.21
0.92
1.01
0.98
1.10
(pꢀFC6H4)2S2 , 1b
0.02
(pꢀOMeC6H4)2S2 , 1c
ꢀ0.05
ꢀ0.14
2a
2b
Not formed
ꢀ0.43
Me2S2
Figure 1. CV of 1a (1.05mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at different scan rates, 25°C, 2 mm
glassy carbon electrode.
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A somewhat increased degree of chemical reversibility was observed for oxidation of the pꢀ
fluoroaryl derivative 1b (solid line in Figure 2). In that case, there is only scant evidence of the
followꢀup product at Epc ≈ 0 V. There are important variations that arise with changes in
temperature and analyte concentration that will be described later. At this point, however, we
bring attention to the complete loss of reversibility for 1b0/+ if the electrolyte anion is [PF6]ꢀ
rather than [B(C6F5)4]ꢀ (dashed line vs solid line in Figure 2). The fact that almost all of the
earlier papers reporting voltammetry of diaryldisulfides5ꢀ12 were carried out using electrolytes
having the traditional smaller anions such as [PF6]ꢀ or [BF4]ꢀ accounts in great part for the
absence of CV detection of [Ar2S2]+ in the previous literature. It is also important to note that the
followꢀup product, identified as the trisulfide [(pꢀFC6H4)3S3]+ (see below), is still observed in the
[PF6]ꢀ ꢀ containing medium (Epc ≈ 0.25 V).
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Figure 2. CV of 1b (1.34mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] (solid line) and in CH2Cl2/0.05M
NBu4][PF6] (dashed line), scan rate 0.2 V/s, 25°C, 2mm glassy carbon electrode.
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That the final oxidation product is the same in both [PF6]ꢀ and [B(C6F5)4]ꢀ electrolytes was
confirmed by bulk anodic electrolysis of 1b in CH2Cl2/[NBu4][B(C6F5)4] at Eappl = 1.38 V. This
reaction passed 1.03 F and gave a single electroactive product with Epc = ꢀ 0.03 V (Figure 3). Reꢀ
electrolysis negative of the product wave (Eappl = ꢀ 0.70 V) consumed only 0.79 F and resulted in
quantitative regeneration of the starting material (see comparative CVs in Figure S1 of
Supplementary Material). In a number of previous papers, including the electrolysis of Ar2S2 in a
[B(C6F5)4]ꢀ electrolyte,14 the main product of the oxidation of Ar2S2 has been shown to be the
trisulfide [Ar3S3]+, 4.5,10,11 This was confirmed in the present case by testing the voltammetric
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Figure 3. CV of 1b (1.91mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] before bulk electrolysis (solid line)
and after bulk electrolysis at 1.38V (dashed line), scan rate 0.2 V/s, 25°C, 2 mm glassy carbon electrode.
behavior of an authentic sample of [Ar3S3]+ generated in situ by the reaction of 1a with an
equivalent of SbCl5.19 The resulting solution had an irreversible reduction wave near the Epc
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value of ꢀ0.03 V observed after the bulk oxidation of 1a. Consistent with this assignment, 1a
itself was formed when [(pꢀMeC6H4)3S3]+ was reduced, either in CV scans or by bulk cathodic
electrolysis. Although Eq 4 therefore describes the gross redox reaction, it is not informative
about the mechanism of this chemically reversible process, especially on the nature of the
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(4)
3 Ar2S2 ꢀ 2 eꢀ
2 [Ar3S3]+
followꢀup reactions beginning with the putative radical cation [Ar2S2]+, 1+. Molecular orbital
theory predicts a relatively strong SꢀS bond in the radical cation [R2S2]+, lowering the likelihood
of simple SꢀS cleavage playing a role in the reaction.20
Earlier work by Jouikov et al. reported a secondꢀorder dependence to the followꢀup reactions
of diaryldisulfide oxidations.21 Our increased ability to observe the disulfide radical cation
allowed a more direct voltammetric study of analyte concentration effects, which are typified for
1aꢀ1c in Figure 4. At higher concentrations of 1a, three important effects are seen: (i) the
oxidation of 1a becomes less chemically reversible (ii) the cathodic wave of the trisulfide
product [(pꢀMeC6H4)3S3]+ increases and (iii) the concentrationꢀnormalized anodic peak current
of 1a decreases . As will be discussed, Scheme 1 is consistent with these experimental findings.
A key element in the scheme is inclusion of the dimer dication, [Ar4S4]2+, 3, formed by radicalꢀ
radical dimerization of 1+ (Eq 2). Nucleophilic attack on 3 by another mole of neutral 1 (Eq 3)
gives the final trisufide 4 in a net 2/3 eꢀ process.
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Figure 4. Background subtracted CV of 1a at different concentrations in CH2Cl2/0.05 M
[NBu4][B(C6F5)4], scan rate 0.2 V/s, 25°C, 2 mm glassy carbon electrode. Note that the ordinate has units
of milliamp/molar.
At low concentrations, the oneꢀelectron oxidation of 1 dominates (Eq 1), accounting for the
closeꢀtoꢀreversible CV behavior at or below approximately 10ꢀ4 M concentrations of 1. If the
only effect of higher concentrations of analyte were to favor the dimer dication in the chemically
reversible process of Eq 2, the measured E1/2 would simply shift negative without changing the
chemical reversibility of the anodic reaction. A further followꢀup reaction must be invoked to
account for the observed loss of reversibility (point (i) in the preceding paragraph) and the
appearance of the cathodic wave for reduction of 4 (point (ii)).
In principle, the additional followꢀup reaction could be of neutral 1 with either the monomer
radical cation [Ar2S2]+ (Eq 5) or the dimer dication [Ar4S4]2+ (Eq 3). Although the radicalꢀ
substrate reaction of Eq 5 cannot be ruled out, since subsequent SꢀS cleavage would give the
observed trisulfide [Ar3S3]+ and a reactive sulfide radical (Eq 6), we consider it a less likely path
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than that of reaction 3. Except in rare cases, radicalꢀsubstrate cations of the type in Eq 5 are
thermodynamically unstable with respect to their oxidation to the corresponding dication.22
Furthermore, in this scenario one would have to account for the fate of the reactive ArSꢁ radical.
Thus, the nucleophilic attack of 1 on 3 rather than on 1+ is the mechanistically simpler route to
the observed product.
Still to be addressed is point (iii) above, namely the decrease of the concentrationꢀnormalized
anodic peak current of 1a as its concentration is increased. This effect is tied to the fact that the
oxidation of 1 is a oneꢀelectron process (Eq 1) in the lowꢀconcentration limit and a 2/3 eꢀ process
(Eq 3) in the highꢀconcentration limit. Furthermore, introduction of the fast secondꢀorder
reaction of Eq 3 causes the anodic wave shape to take on irreversible character, being broader
and having a lower peak current than its reversible counterpart.23 At concentrations of 2ꢀ3 mM,
the width and height of the anodic wave are close to those expected for an irreversible oxidation
process (β transfer coefficient = 0.48) of 2/3 eꢀ stoichiometry.24
It was mentioned above that bulk electrolysis of 1b at room temperature gave an napp value of
1.0. When this electrolysis was repeated at 253 K, a coulometry count of 0.66 F was obtained as
predicted by Scheme 1. It is likely that the somewhat higher electron count for 1b at room
temperature arises from slow catalytic regeneration of 1b by the reaction of 1b+ with
adventitious reductants in the medium. This would not be unusual for strong oxidants such as
1b+. Strengthening this interpretation is the fact that backꢀelectrolysis of [Ar3S3]+ required only
0.78 F while quantitatively regenerating the starting disulfide.
Another experimental finding to be considered is that at reduced temperatures, CV scans of
group 1 compounds lose virtually all of the chemical reversibility for the 10/+ process (see Figure
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5 for 1a). This is explicable in terms of the lower temperature favoring the dimer dication [(pꢀ
MeC6H4)4S4]2+, thereby speeding conversion to the trisulfide [(pꢀMeC6H4)3S3]+ by the reaction
given in Eq 3. The temperature effect also accounts for the fact that Matsumoto et al.14
apparently saw only totally irreversible CVs for Ar2S2 at 195 K.
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Figure 5. CV of 1a (1.05mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 25°C (solid line) and at ꢀ20°C
(dashed line), scan rate 0.2 V/s, 25°C, 2 mm glassy carbon electrode.
The role of the [PF6]ꢀ anion in the reaction is now considered. It is well established that
many cationic organometallics are subject to stronger ionꢀpairing15,16,17,25 and increased
nucleophilic attack16,17,26 when generated in the presence of smaller anions such as [PF6]ꢀ or
[BF4]ꢀ rather than weakly coordinating anions. CV scans of 1aꢀ1c in [PF6]ꢀ ꢀ containing
electrolytes do not diminish the apparent yield of the trisulfide product [Ar3S3]+, as shown in
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Figure 2 for 1b. Thus, nucleophilic attack by [PF6]ꢀ, which often leads to decomposition of
organometallic radical cations,16,27ꢀ29 does not appear to be a factor in the present case, either on
the CV or electrolysis time scale. Furthermore, bulk electrolysis of 1a in CH2Cl2/0.1 M
[NBu4][PF6] followed the same reaction path as seen in solutions of [B(C6F5)4]ꢀ, albeit with
slightly diminished yields. Thus, bulk oxidation of 1a in the [PF6]ꢀ electrolyte passed 0.71 F,
giving a stable solution30 of the trisulfide 4 (Ar = pꢀMeC6H4), bulk reduction of which
regenerated 1a in 80% yield. These findings support those reported earlier for the oxidation of
diaryldisulfides in [BF4]ꢀꢀbased electrolytes.6
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The [PF6]ꢀ electrolyte effect is therefore to diminish the amount of the primary radical cation
1+, while still allowing formation of the major trisulfide product. A reasonable explanation for
this is the occurrence of increased ionꢀpairing between the dimer dication and the [PF6]ꢀ ion. The
accompanying thermodynamic effect would be to shift the monomer/dimer equilibrium in favor
of the dimer, thereby speeding the formation of the trisulfide final product through Eq 3.
Oxidation of bisꢀorthoꢀsubstituted arene disulfides 2a and 2b
The preceding arguments suggests that restricting formation of the dimer dication [Ar4S4]2+
should retard production of the trisulfide oxidation product and, in the absence of new reaction
pathways, increase the lifetime of the primary radical cation [Ar2S2]+. This postulate was tested
by examination of two compounds, namely the dimesityldisulfide complex 2a and the bis(2,4,6ꢀ
triisopropylphenyl)disulfide complex 2b, in which ortho substitution of the aryl groups
introduces steric restraints to the formation of dimer dications,. As will be shown, the results
gave strong support to the mechanistic model of Scheme 1 by bringing the [Ar2S2]0/+ couple
closer to a simple, chemically reversible, oneꢀelectron process.
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Figure 6. Background subtracted CV of 2a at different concentrations in CH2Cl2/0.05 M
[NBu4][B(C6F5)4], scan rate 0.2 V/s, 25°C, 2 mm glassy carbon electrode. Note that the ordinate has units
of milliamp/molar. A higher noise level restricted analysis in the range of ꢀ0.85 V to 0.65 V for 0.1 mM
solution.
Figure 6 shows CVs for different concentrations of 2a at room temperature. Below about 0.5
mM, the 2a0/+ process is very nearly reversible and there is little evidence of the trisulfide
followꢀup product, which only comes into play at higher concentrations. There is a much smaller
change in the concentrationꢀnormalized anodic peak current for 2a compared to the typeꢀ1
disulfides (compare with Figure 4 for 1b). It is also worth noting that, contrary to the behavior of
the electronically similar 1a, some chemical reversibility is seen for the [Ar2S2]0/+ couple even
when [PF6]ꢀ is the electrolyte anion (Suppl Material Fig S4).
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Figure 7. Background subtracted CV of 2b (0.057mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] (solid line)
and DigitalSimulation (dashed line), scan rate 0.2 V/s, 25°C, 2 mm glassy carbon electrode.
Formation of the dimer dication [Ar4S4]2+ appears to be completely cut off when the very
bulky triisopropylꢀsubstituted compound 2b is oxidized. In this case, no evidence for trisulfide
product is observed either in CV scans or after bulk electrolysis, and concentrationꢀnormalized
anodic currents do not change appreciably over the concentration range 0.26 mM to 1.89 mM.
The unusuallyꢀshaped CV scans of 2b (Figure 7) arise from slow heterogeneous electron transfer
(ET) for the 2b0/+ couple rather than from chemical followꢀup reactions. Digital simulations
(dashed line in Figure 7) confirm this, being consistent with a standard heterogeneous rate
constant, ks, of 7.4 x 10ꢀ4 cm sꢀ1. Whereas the reason(s) for the slow ET for this compound
cannot be delineated without further study, we refer the interested reader to more inꢀdepth
studies on how steric effects may influence ks values for other organic redox couples.31
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Oxidation of Dimethyldisulfide. CV scans of 1.5 mM Me2S2 showed only the barest of evidence
for the radical cation [Me2S2]+ at scan rates of 1 V sꢀ1 or less (Figure SM5). This is consistent
with our model in which a secondꢀorder followꢀup reaction occurs more readily when
dimerization of the radical cation is favored. The major product for the oxidation of Me2S2,
responsible for a cathodic wave near ꢀ0.43 V, is almost certainly the trisulfide [Me3S3]+.
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Reactivity of the trisulfide cation
A number of authors have investigated the reactivities of trisulfide cations, [R3S3]+, with
olefins,5,14 alkynes,9 and compounds with other functional groups.6,10,32,33 We thought it useful to
carry out similar reactions between the trisulfides [Ar3S3]+ and a weak nucleophile such as an
olefin in the present [B(C6F5)4]ꢀ ꢀ containing medium. As expected, addition of olefin led to
decrease in the chemical reversibility of the [Ar2S2]0/+ couple and disappearance of the trisulfide
reduction peak (see Figure 8 for 1a in the presence of cyclohexene). Although a reaction between
the radicalꢀcation [Ar2S2]+ and the olefin cannot be ruled out, it seems more likely that the
trisulfide cation is undergoing a rapid reaction with the olefin.
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Figure 8. Cyclic voltammogram of 1a (1.05mM) in CH2Cl2/0.05 M [NBu4][B(C6F5)4], scan rate 0.2 V/s,
2mm diam. glassy carbon electrode, 25°C, without cyclohexene (dashed line) and with 1 equivalent of
cyclohexene (solid line).
7
8
Bulk electrolysis at room temperature in the presence of one equivalent of olefin led to rapid
consumption of the disulfide after only 0.21 F per olefin, indicative of an electrocatalytic
mechanism leading to the formation of the corresponding vicinal disulfide (Eq 7) in high yield,
consistent with previous literature.14 To the best of our knowledge, only aromatic disulfides have
been reported to undergo a catalytic addition to olefin under oxidative conditions.34 In the
present case, however, as denoted in Table 2, electronꢀrich and electronꢀpoor aromatic disulfides
as well as aliphatic disulfides are compatible with the reaction conditions. Moreover, the reaction
could be performed at room temperature rather than at ꢀ78°C. When [NBu4][B(C6F5)4] was
replaced by [NBu4][PF6], only traces of the corresponding 1,2ꢀdisulfide were observed.35 Scheme
2 accounts for the disulfurization reaction.
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Table 2. Electrochemical disulfurization at room temperature in CH2Cl2/0.05M
[NBu4][B(C6F5)4].
Entry
Disulfide
(pꢀMeC6H4)2S2 , 1a
(pꢀFC6H4)2S2 , 1b
(pꢀOMeC6H4)2S2 , 1c
Me2S2
1,2ꢀdisulfide yield (%)
1
2
3
4
97, 7a
91, 7b
78, 7c
94, 8
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Scheme 2. Proposed mechanism for the anodicallyꢀinitiated disulfurization of cyclohexene.
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Finally, the chemical and photochemical oxidation of disulfides, at room temperature, in
presence of one equivalent of olefin and a catalytic amount of either [(2,4ꢀ
Br2C6H3)3N][B(C6F5)4] (5) (Magic Green) or 2,4,6ꢀtriphenylpyrrilium [B(C6F5)4] (6) and UV
light led smoothly, within less than 15 minutes, to the formation of the expected 1,2ꢀdisulfide in
high yield (Eq 8). This would be a versatile alternative to the low temperature electrolysis, and in
some cases might have practical advantages. See Table 3 for yields.
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Table 3. Chemical and photochemical disulfurization at room temperature
Entry
Disulfide
(pꢀMeC6H4)2S2 , 1a
(pꢀFC6H4)2S2 , 1b
(pꢀOMeC6H4)2S2 , 1c
Me2S2
[Cat][B(C6F5)4]
1,2ꢀdisulfide yield (%) (a)
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8
99, 7a
92, 7b
75, 7c
95, 8
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(pꢀMeC6H4)2S2 , 1a
(pꢀFC6H4)2S2 , 1b
(pꢀOMeC6H4)2S2 , 1c
Me2S2
91, 7a
79, 7b
61, 7c
90, 8
(a) Yields are reported for isolated pure compounds
Summary and Conclusions
The anodic oxidation of five different diaryldisulfides has been studied in a
dichloromethane/[NBu4][B(C6F5)4] electrolyte. With one exception, namely the stericallyꢀ
encumbered bisꢀortho substituted triisopropyl compound 2b, bulk oxidation furnishes a
quantitative yield of the triaryltrisulfide [Ar3S3]+, in agreement with previous literature.5,10,11,14
By controlling the concentration of the disulfide, new cyclic voltammetry evidence has emerged,
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providing the first verification of the radical cation [Ar2S2]+ and allowing a more detailed
mechanistic analysis than previously possible.
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9
If the aryl groups contain only a paraꢀsubstituent, the chemical reversibility of the [Ar2S2]0/+
couple is rather modest at CV scan rates of 0.2 to 0.5 V sꢀ1, with only minor changes occurring
with alterations in the paraꢀsubstituent. The reversibility of 10/+ is increasingly disfavored at
higher concentrations of disulfide and at lower temperatures, conditions which favor a secondꢀ
order followꢀup reaction of the radical cation [Ar2S2]+, almost certainly forming the dimer
dication [Ar4S4]2+. A further followꢀup reaction must then occur, most likely between [Ar4S4]2+
and the neutral starting material, to provide the trisulfide final product, [Ar3S3]+. Under
conditions favoring the followꢀup reactions, the anodic electronꢀtransfer stoichiometry is reduced
from the one electron needed for formation of [Ar2S2]+ to the twoꢀthirds electron needed to form
the trisulfide [Ar3S3]+. Replacement of the [PF6]ꢀ supporting electrolyte anion by the weakly
coordinating [B(C6F5)4]ꢀ anion is key to permitting detection of the radical cations. It is likely
that weaker ionꢀpairing between the dimer dication and [B(C6F5)4]ꢀ imparts thermodynamic
stability to the monomeric radical cation, thereby disfavoring the followꢀup reactions required to
obtain the longꢀterm electrolysis product [Ar3S3]+.
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Efficient disulfurization is observed when the oxidation of Ar2S2 is carried out in the presence
of cyclohexene. Initiation of the disulfurization can be carried out either anodically or by use of a
catalytic amount of a sufficiently strong oneꢀelectron oxidizing agent.
Finally, we note that the anodic behavior of diphenyldiselenide, Ph2Se2, was briefly
investigated. CV scans between 0.2 and 1 V sꢀ1 for 0.88 mM Ph2Se2 in CH2Cl2/0.05 M
[NBu4][B(C6F5)4] showed only a chemically irreversible anodic wave at Epa ≈ 1.05 V and a
single cathodic product wave near ꢀ0.1 V.
Experimental
Electrochemical procedures were carried out under dry nitrogen, using either Schlenck
techniques or a drybox maintained at 1ꢀ3 ppm oxygen. Solvents were dried and distilled as
previously described.36 Ditolyldisulfide and dimethyldisulfide were obtained commercially. Diꢀ
pꢀfluorophenyldisulfide, di-p-methoxyphenyldisulfide, dimesityldisulfide and di(2,4,6ꢀ
triisopropylphenyl)disulfide were prepared by reduction of the corresponding sulfonyl chloride
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with triphenylphosphine.37 [NBu4][PF6] (Tokyo Chemical Industry) was recrystallized from
absolute ethanol and vacuum dried for at least 24 h at 100º C. [NBu4][B(C6F5)4] was prepared as
described earlier38 and also vacuum dried at 100º C. A full description of the electrochemical
procedures is available.36 Among the most important features is that a traditional threeꢀelectrode
cell setup was employed for voltammetry, with the reference electrode separated by a fine frit
from the working electrode solution. For bulk electrolyses, a fine frit was also used to separate
the working and auxiliary compartments. The working electrodes were 2 mm glassy carbon disks
for voltammetry and platinum gauze for bulk electrolyses. The experimental reference electrode
was a homemade Ag/AgCl wire, but all potentials in this paper are referred to the
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ferrocene/ferrocenium potential39, obtained using the in-situ method39(b) to track the experimental
potential vs ferrocene. NMR data were obtained using a 500 MHz instrument.
Preparation of [N(2,4ꢀC6H3Br2)3][B(C6F5)4], [5][TFAB]: [5][SbCl6]40 (15g, 14.2 mmol) and
K[TFAB] (Boulder Scientific Co.) (11.3 g, 15.7 mmol) were dissolved in 700 mL of dry
dichloroethane and stirred at room temperature for 30 min. The reaction was cooled at ꢀ20°C for
45 minutes and then filtered. The solvent was removed under reduced pressure and the resulting
green solid was dissolved in 700 mL of dry dichloromethane. The solution was filtered again to
remove residual K[SbCl6]. The filtrate was concentrated to 100 mL and then added to 700 mL of
dry hexane, leaving a green amorphous solid. The liquid was decanted off, another 100 mL of
dichloromethane was added, and the resulting solution was again added to 700 mL of hexanes
(the redissolving process gave a purer final product). After cooling the solution to ꢀ20°C for 30
min, followed by filtration and overnight vacuum drying of the deep green precipitate, 19.9 g
(85%) of the desired product were obtained as a deep green powder. Anal. Calcd for
C42H9BBr6F20N: C, 36.09; H, 0.65; N, 1.00. Found: C, 35.79; H, 0.79; N 1.13
Preparation of [2,4,6ꢀtriphenylpyrrilium][B(C6F5)4], [6][TFAB]: [6][BF4] (1.2g, 3.04 mmol) and
K[TFAB] (Boulder Scientific Co.) (2.18 g, 3.04 mmol) were suspended in 60 mL of dry
diethylether and stirred at room temperature for 12 hours. The solution was then filtered and the
solvent removed under reduced pressure. 2.82g (94%) of the desired product were obtained as a
1
bright yellow powder. H NMR (500 MHz, CDCl ) : 7.70ꢀ7.77 (m, 6H), 7.8 (qt, 3H, J= 7.5Hz,
3
13
1.2Hz), 7.98ꢀ8.00 (m, 2H), 7.20ꢀ7.22 (m, 4H), 8.38 (s, 2H). C NMR (125 MHz, CDCl ) :
3
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114.3, 128.0, 128.2, 128.8, 130.7, 130.9, 132.3, 136.5, 136.6. F NMR (470 MHz, CDCl3) : ꢀ
ꢀ1
6
7
166.7, ꢀ162.8, ꢀ132.6. IR (neat) υ
/cm : 660, 681, 767, 975, 1081, 1458, 1620. Anal. Calcd
max
8
9
for C47H17BF20O: C, 57.11; H, 1.73. Found: C, 57.24; H, 2.01.
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General procedure for oxidativelyꢀcatalyzed desulfurization of olefins. Cyclohexene (58 ꢀL, 0.57
mmol) and the disulphide (0.57 mmol) were dissolved in 0.6mL of dry dichloromethane. Then
[5][TFAB] or [6][TFAB] (0.057 mmol) was added and the solution was stirred (and photolyzed
when [6][TFAB] was used) for 15 minutes at room temperature after what 0.2 mL of
triethylamine were added to quench the reaction. The solvent was removed under reduced
pressure and the crude product was then purified by chromatography over silica gel using diethyl
ether/ hexane (1/9) as eluent.
1
1,2ꢀbis(pꢀtolylthio)cyclohexane 7a. H NMR (500 MHz, CDCl ) : 1.45ꢀ1.48 (m, 2H), 1.67ꢀ1.76
3
(m, 4H), 2.29ꢀ2.34 (m, 2H), 2.41 (s, 6H), 3.26ꢀ3.29 (m, 2H), 7.14 (d, 4H, J=8.09Hz), 7.34 (d,
13
4H, J=8.11Hz). C NMR (125 MHz, CDCl ) : 20.9, 23.4, 29.8, 49.8, 129.4, 130.7, 132.8,
3
ꢀ1
136.9. IR (neat) υ
/cm : 810, 1018, 1090, 1445, 1491, 2856, 2932, 3019. MSEI+ m/z (%) :
max
329.3 (0.4) (MH+), 328.1 (1), 327.2 (5), 205.5 (16), 204.8 (100). HRMSESI+ for C20H25S2 :
calculated = 329.13922, found = 329.13922.
1
1,2ꢀbis((4ꢀfluorophenyl)thio)cyclohexane 7b. H NMR (500 MHz, CDCl ) : 1.35ꢀ1.39 (m, 2H), 1.54ꢀ
3
1.60 (m, 2H), 1.63ꢀ1.68 (m, 2H), 2.16ꢀ2.20 (m, 2H), 3.05ꢀ3.08 (m, 2H), 6.96ꢀ6.99 (m, 4H), 7.34ꢀ7.37 (m,
13
4H). C NMR (125 MHz, CDCl ) : 23.7, 30.4, 50.6, 115.9 (d, J=21.7Hz), 129.2 (d, J=3.4Hz), 135.3 (d,
3
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J=8.1Hz), 162.3 (d, J=247.9). F NMR (470 MHz, CDCl3) : ꢀ113.9. IR (neat) υ
ꢀ1
/cm : 635, 830,
max
1013, 1155, 1225, 1489, 1589, 2858, 2934. MSEI+ m/z (%): 336.2 (1) (MH+), 335.2 (5), 210.5
(5), 209.5 (13), 208.7 (100), 81.1 (10). HRMSESI+ for C18H18F2S2Na : calculated = 359.07102,
found = 359.07102.
1
1,2ꢀbis((4ꢀmethoxy)thio)cyclohexane 7c. H NMR (500 MHz, CDCl ) : 1.31ꢀ1.35 (m, 2H),
3
1.52ꢀ1.55 (m, 2H), 1.63ꢀ1.66 (m, 2H), 3.80 (s, 6H), 6.81 (d, 4H, J=8.75Hz), 7.34 (d, 4H,
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J=8.75Hz). C NMR (125 MHz, CDCl ) : 23.9, 50.7, 55.2, 114.3, 124.5, 135.5, 159.3. IR
3
6
7
8
9
ꢀ1
(neat) υ
/cm : 827, 1031, 1171, 1245, 1284, 1492, 1591, 2836, 2931. MSEI+ m/z (%) :
max
361.2 (1) (MH+), 360.7 (1), 359.1 (5), 223.0 (6), 222.0(15), 221.0 (100),, 139.0 (1). HRMSESI+
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for C20H24O2S2Na: calculated = 383.11099, found = 383.11096.
1
1,2ꢀbis(methylthio)cyclohexane 8. H NMR (500 MHz, CDCl ) : 1.28ꢀ1.32 (m, 2H), 1.44ꢀ1.49
3
13
(m, 2H), 1.66ꢀ1.70 (m, 2H), 2.08 (s, 6H), 2.09ꢀ2.16 (m, 2H), 2.58ꢀ2.61 (m, 2H). C NMR (125
ꢀ1
MHz, CDCl ) : 13.9, 24.5, 31.6, 49.1. IR (neat) υ
/cm : 737, 905, 1008, 1201, 1439, 2834,
3
max
2931, 2974. MSEI+ m/z (%) : 177.5 (5) (MH+), 176.5 (6), 175.5 (58), 174.6 (9), 129.8 (10),
128.8 (100), 127.8 (10), 81.0 (3). HRMSESI+ for C8H16S2Na: calculated = 199.05856, found =
199.0586.
Acknowledgements. The authors are grateful to the National Science Foundation (CHEꢀ
1212339) for support of this research.
Supporting information: 15 pages of cyclic voltammograms, an analysis of the dependence of
anodic peak current as a function of analyte concentration, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
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References
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1. Fass, D. Annu. Rev. Biophys. 2012, 41, 63.
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2. Vertler, H.; Grubner, J.; Pardini, V.L. in Organic Electrochemistry, Lund, H. and Hammerich,
O. (Eds), Marcel Dekker, Inc., New York, 2001, 4th Ed., pp. 623ꢀ638.
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3. (a) Naphthalene disulfide: Zweig, A.; Hoffmann, A.K. J. Org. Chem. 1965, 30, 3997 (b)
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19. The reaction of SbCl5 with Ar2S2 to give in-situ [Ar3S3]+ is well documented. See references
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anodic peak current for a totally irreversible oxidation having a transfer coefficient of 0.48
(measured from the anodic peak width) and an electronꢀtransfer stoichiometry of 0.67 should be
50% of that of a reversible oneꢀelectron process. Values of about 45% are measured from Figure
4.
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