Photosensitized electron transfer oxidation of sulfides: A steady-state study

Article abstract of DOI:10.1002/ejoc.200800048

The photosensitized electron-transfer oxidation of a series of ethyl sulfides RSEt (1, R = C₁₂H₂₅; 2, PhCH₂CH ₂; 3, PhCH₂; 4, PhCMe₂; 5, Ph₂CH) has been examined in acetonitrile and the product distribution discussed on the basis of the mechanisms proposed. In nitrogen-flushed solutions, cleaved alcohols and alkenes are formed, whereas under oxygen, in reactions that are 10-70 times faster, sulfoxides and cleaved aldehydes and ketones are formed in addition to the afore-mentioned products. Two sensitizers are compared, 9,10-dicyanoanthracene (DCA) and 2,4,6-triphenylpyrylium tetrafluoroborate (TPP⁺BF₄^-), the former giving a higher proportion of the sulfoxide, the latter of cleaved carbonyls. The sulfoxidation is due to the contribution of the singlet oxygen path with DCA. Oxidative cleavage, on the other hand, occurs both with DCA and with TPP⁺ which is known to produce neither singlet oxygen nor the superoxide anion. This process involves deprotonation from the α position of the sulfide radical cation, but the TPP⁺ results suggest that O₂^.- is not necessarily involved and non-activated oxygen forms a weak adduct with the radical cation promoting α-hydrogen transfer, particularly with benzylic derivatives. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800048

FULL PAPER
DOI: 10.1002/ejoc.200800048
Photosensitized Electron Transfer Oxidation of Sulfides: A Steady-State Study
Sergio M. Bonesi,[a,b] Maurizio Fagnoni,[b] and Angelo Albini*[b]
Keywords: Electron transfer / Oxidation / Photochemistry / Reaction mechanisms / Sulfides
The photosensitized electron-transfer oxidation of a series of
ethyl sulfides RSEt (1, R = C12H25; 2, PhCH2CH2; 3, PhCH2;
4, PhCMe2; 5, Ph2CH) has been examined in acetonitrile and
the product distribution discussed on the basis of the mecha-
nisms proposed. In nitrogen-flushed solutions, cleaved
alcohols and alkenes are formed, whereas under oxygen, in
reactions that are 10–70 times faster, sulfoxides and cleaved
aldehydes and ketones are formed in addition to the afore-
mentioned products. Two sensitizers are compared, 9,10-di-
cyanoanthracene (DCA) and 2,4,6-triphenylpyrylium tetra-
sulfoxidation is due to the contribution of the singlet oxygen
path with DCA. Oxidative cleavage, on the other hand, oc-
curs both with DCA and with TPP+ which is known to pro-
duce neither singlet oxygen nor the superoxide anion. This
process involves deprotonation from the α position of the sul-
·–
fide radical cation, but the TPP+ results suggest that O2 is
not necessarily involved and non-activated oxygen forms a
weak adduct with the radical cation promoting α-hydrogen
transfer, particularly with benzylic derivatives.
fluoroborate (TPP+BF4–), the former giving a higher pro- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
portion of the sulfoxide, the latter of cleaved carbonyls. The
Germany, 2008)
nism that has been found viable by computational analysis
suggests that addition of superoxide gives a thiadioxirane
rather than the open-chain persulfoxide as the first interme-
diate (path iii).[5] An alternative path, likewise computation-
ally supported, explains the difference by the reaction of
the sulfide radical cation with molecular oxygen rather than
with superoxide (path iv).[6]
Introduction
Organic sulfides have an accessible oxidation potential
and single-electron transfer (SET) oxidation plays an im-
portant role in the chemistry and biochemistry of these
compounds.[1] Among the reactions involving such a
mechanism, photosensitized ET (PET) oxygenation has an
important place. PET has been the subject of a number of
studies[2] and has been found to be an alternative to the
well-known addition of singlet oxygen to sulfides, charac-
terized by a larger range of applications (see below).[3] Such
a difference in the scope of the reactions contrasts with the
earlier proposal[4] that the same intermediate is involved in
the two reactions. Thus, after the initial redox step, second-
ary electron transfer to oxygen from the sensitizer radical
anion and combination of the sulfide radical cation with
the resulting superoxide anion could generate a persulfoxide
1
that is also the intermediate formed by the addition of O2
to the sulfide (compare paths i and ii in Scheme 1). Recent
in depth investigations with the support of experimental
and computational evidence[5,6] ascertained that ET oxi-
dations involve the sulfide radical cation, but demonstrated
that at least in some cases the reaction does not exactly
duplicate the chemistry observed with singlet oxygen, which
suggests that a different intermediate is involved. A mecha-
Scheme 1. Alternative paths for the photosensitized oxidation of
sulfides.
Investigations are complicated by the fact that the same
sensitizers may act both by generating singlet oxygen and
by a PET path,[5–7] as well as by the dependence of the
reactivity of the sulfides on the structure and conditions,
such that it is possible that a shift to a different mechanism
occurs under slightly different conditions. Furthermore, sul-
fide radical cations are known to undergo fragmentation
processes that may be fast enough to compete with the reac-
tion with oxygen.
[a] CHIDECAR-CONICET, Dep. Quim. Org., Fac. Cien. Ex.
Nat., Universidad de Buenos Aires,
3er Piso, Pabellon II, Ciudad Universitaria, 1428 Buenos Aires,
Argentina
[b] Dep. Organic Chemistry, University of Pavia,
v. Taramelli 10, 27100 Pavia, Italy
Fax: +39-0382-987232
E-mail: angelo.albini@unipv.it
2612
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Photosensitized Electron Transfer Oxidation of Sulfides
As a contribution to the current debate, our results with fide, ethyl dodecyl disulfide and dodecanethiol (see
a number of representative sulfides are presented herein. Scheme 2 and Table 1). In oxygen-equilibrated solutions,
These have been obtained by using two sensitizers, both in however, the sulfoxide was the main product (see Table 2).
the presence and in the absence of oxygen. The chosen sen-
sitizers, 9,10-dicyanoanthracene (DCA) and 2,4,6-tri-
The reaction of 2 was even slower under nitrogen, with
the formation of styrene (and 2-phenylethanol) as well as
three disulfides [diethyl, bis(phenethyl), phenethyl ethyl].
Under oxygen flushing, both cleaved products, phenylace-
taldehyde and styrene (observed at low conversion, later
itself oxidized), and the sulfoxide were formed in compar-
able yields.
Benzyl sulfide 3 gave 1,2-diphenyl-1,2-bis(ethylthio)eth-
ane, 1,2-diphenylethyl ethyl sulfide and benzaldehyde with
DCA as the sensitizer under nitrogen. This reaction was
somewhat more closely examined and it was found that ad-
dition of methanol (10% volume) increased the yield of the
bis(ethylthio)ethane, whereas addition of biphenyl (BP, 1 )
increased that of the diphenylethyl sulfide. Benzaldehyde
was the only product formed with TPP+ under nitrogen. On
the other hand, benzaldehyde and the sulfoxide were
formed with both sensitizers under oxygen, although in dif-
ferent yields.
–
phenylpyrylium tetrafluoroborate (TPP+ BF4 ), were de-
emed to be suitable probes as both of them are powerful
oxidants in the excited state, making ET from the sulfides
efficient. However, in the ground state, DCA is reduced at
a potential (–0.89 V vs. SCE in MeCN)[8] comparable to
that of oxygen (–0.87 V),[9] with TPP+ above it (–0.37 V),[10]
so that reduction of oxygen according to Scheme 1 is feas-
ible only with the former sensitizer.
Results
We wished to explore chemical paths that may be in-
formative about the mechanism and chose a series of ethyl
sulfides, RSEt, in which the R group was thought to be
susceptible to some diagnostic reaction. Their sensitized
photochemistry, both in the absence and in the presence of
oxygen, was compared. The ethyl sulfides studied included
The cumyl sulfide 4 was more reactive than 3 when pho-
the dodecyl (1), phenethyl (2), benzyl (3), cumyl (4) and tosensitized by either DCA or TPP+ under nitrogen. The
benzhydryl (5) derivatives. Solutions in acetonitrile were ir- sensitization gave diethyl disulfide, but all of the phenyl-
radiated in the presence of the above sensitizers under con- containing products were sulfur-free, namely, α-methylsty-
ditions in which only the latter absorbed. The results refer rene, cumyl alcohol, acetophenone and bicumyl.[11] The re-
to a 20–30% conversion, for which further reactions of the action in the presence of oxygen remained slow with DCA
primary products could be considered as less important.
and the product distribution did not include, in contrast to
Photosensitization of 1 with both DCA and TPP+ re- the previous cases, the sulfoxide. The main products with
quired several hours in nitrogen-flushed solutions. The both sensitizers were α-methylstyrene and acetophenone,
products formed were 1-dodecene, dodecanal, diethyl disul- along with some dicumyl disulfide in the case of DCA.
Scheme 2. Products formed upon photosensitization of sulfides 1–5 in nitrogen- and oxygen-equilibrated solutions.
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S. M. Bonesi, M. Fagnoni, A. Albini
Table 1. Products from the photosensitization of sulfides 1–5 in nitrogen-equilibrated acetonitrile solution.
FULL PAPER
Sulfide
Sensitizer
Products, % yield
From R+
Carbonyl derivatives
Radical coupling products Products involving α-
deprotonation
C12H25SEt
PhC2H4SEt
DCA
TPP+
C10H21CH=CH2, 13
C11H23CHO, 6
C11H23CHO, 44
C12H25S2Et, 6
C12H25S2Et, 31
C12H25SH, 6
[PhC2H4S]2, 3
PhC2H4S2Et, 4
EtS2Et, 2
[PhC2H4S]2, 16
PhC2H4S2Et, 25
EtS2Et, 20
DCA
TPP+
PhCH=CH2, 36[a]
PhCH=CH2, 38[a]
PhCH2SEt
DCA
PhCHO, 35
PhCHO, 23
[PhCH(SEt)]2, 17
PhCH(SEt)CH2Ph, 2
[PhCH(SEt)]2, 5
DCA[b]
PhCH(SEt)CH2Ph, 24
[PhCH2]2, 1
[PhCH(SEt)]2, 29
PhCH(SEt)CH2Ph, 2
DCA[c]
PhCHO, 9
TPP+
DCA
PhCHO, 54
PhCOMe, 3
PhCMe2SEt
Ph2CHSEt
PhC(=CH2)Me, 11
PhCMe2OH, 20
PhC(=CH2)Me, 5
PhCMe2OH, 40
Ph2CHNHAc, 21[d]
Ph2CHOH, 35
[PhCMe2]2, 4
EtS2Et, 9
[PhCMe2]2, 2
EtS2Et, 18
[Ph2CH]2, 15
[Ph2C=]2, 6
Ph2CH2, 12
EtS2Et, 10
[Ph2CH]2, 15
[Ph2C=]2, 6
Ph2CH2, 12
EtS2Et, 9
TPP+
DCA
PhCOMe, 18
Ph2CO, 39
TPP+
Ph2CHNHAc, 8
Ph2CHOH, 26
Ph2CO, 22
[a] Including some 2-phenylethanol. [b] In the presence of BP (M). [c] In the presence of 10% methanol (v/v). [d] 10% Ph2CHNHAc +
11% Ph2CHNH2.
Table 2. Products from the photosensitization of sulfides 1–5 in (and the corresponding acetamide) as well as diphenylmeth-
oxygen-equilibrated acetonitrile.
ane, tetraphenylethane and tetraphenylethylene. Under oxy-
gen, the main products were again the acetamide and
benzophenone, with a 2% yield of the sulfone and 12% of
the sulfoxide in the case of DCA, but only 1% of the sulfox-
ide with TPP+.
The efficiency of the photosensitization of sulfides 1–5 is
documented in Table 3 in which the rates of consumption
[µmolmin–1] of the sulfides are compared, which are pro-
portional to the rate constants. It appears that the reaction
is 10–70 times more efficient under oxygen than under ni-
Sulfide
Sensitizer
Products, % yield
Fragmentation Conserving the
skeleton
C12H25SEt
DCA
TPP+
C11H23CHO, 8
C11H23CHO, 21
PhCH=CH2, 30[a]
PhCH2CHO, 23
PhCH=CH2, 50[a]
PhCH2CHO, 19
PhCHO, 26
C12H25SOEt, 40
C12H25SOEt, 50
PhC2H4SOEt, 44
PhC2H4SEt DCA
TPP+
PhC2H4SOEt, 30
PhCH2SEt
DCA
TPP+
PhCH2SOEt, 47
PhCH2SOEt, 53
PhCHO, 29
PhCMe2SEt DCA
PhC(=CH2)Me, 5[a]
PhCOMe, 20
Table 3. Amount of photoreacted sulfides [µmolmin–1] in aceto-
nitrile under nitrogen and under oxygen.[a]
[PhCMe2S]2, 5
PhC(=CH2)Me, 5[a]
PhCOMe, 28
TPP+
Sulfide
Sensitizer Under N2
Under O2
C–S bond
[PhCMe2S]2, tr
Ph2CHNHAc, 22
Ph2CHOH, 10
Ph2CO, 53
Ph2CHNHAc, 11
Ph2CHOH, 3
cleavage [%]
Ph2CHSEt
DCA
TPP+
Ph2CHSOEt, 12
Ph2CHSO2Et, 2
C12H25SEt
DCA
TPP+
0.036
0.011
0.047
0.045
0.052
0.040
0.10
0.084
0.031
0.056
2.21
0.23
1.43
0.97
2.38
0.49
0.06
0.73
0.85
0.36
16
35
55
69
35
Ph2CHSOEt, 1
PhC2H4SEt DCA
TPP+
DCA
TPP+
Ph2CO, 87
PhCH2SEt
37
[a] Further oxidized during the course of the reaction.
PhCMe2SEt DCA
100
100
85
TPP+
DCA
TPP+
Ph2CHSEt
As for benzhydryl sulfide 5, this again gave diethyl disul-
fide as the only sulfur-containing product under nitrogen,
100
along with benzhydrol, benzophenone, benzhydrylamine [a] Normalized at equal adsorbed light flux.
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Photosensitized Electron Transfer Oxidation of Sulfides
R2S·+ Ǟ R+ + RS·
(4)
(5)
trogen (the only exception is 4 with DCA) and that, at equal
absorbed flux, DCA is more effective than TPP+ (except
with 4). Further important evidence is the quenching of the
sensitizers’ fluorescence by the above sulfides: all these com-
pounds were effective quenchers, with rate constants kf Ͼ
5ϫ109 –1 s–1.
RCH2SR·+ Ǟ RCH·SR + H+
The likelihood of such fragmentations can be evaluated
by means of a thermochemical cycle. Abstraction of an elec-
tron weakens a σ bond and the free energy change for radi-
cal cation cleavage [RX·+ Ǟ R+ + X·; Equation (6)] differs
from that of the neutral species by the difference between
the oxidation potential of the molecule and that of the radi-
cal corresponding to the cation formed.
Discussion
Sulfide Radical Cations: Primary Processes
The sulfides quench the fluorescence of both the aro-
matic nitrile DCA and the phenylium salt TPP+ at rates
close to diffusion control and the electron transfer from
these donors, both aliphatic and aromatic, to the singlet
excited state of both DCA and TPP+ is thermoneutral or
exoergonic.[12] Thus, photosensitization generates a radical-
ion pair (with DCA) or a radical-ion/radical pair in the case
of TPP+, for which the ground state of the sensitizer is a
cation; see Equations (1) and (2).
∆G(RX·+) = ∆G(RX) – [E°(RX) – E°(R·)]
(6)
In the case of sulfide radical cations, if cleavage of the
C–S bond occurs, it will give an alkyl cation [Equation (4)]
and not an alkyl radical [Equation (7)] as the latter interme-
diates are oxidized at a less positive potential than alkylthiyl
radicals.[13]
R2S·+ Ǟ R· + RS+
(7)
In this work, asymmetric sulfides of the general structure
RSEt have been considered. With compounds 1 and 2, in
which R is a non-stabilized alkyl radical, both alkyl cations
may be formed, whereas with derivatives 3–5, in which R is
1DCA + R2S Ǟ DCA·– + R2S·+
(1)
1TPP+ + R2S Ǟ TPP· + R2S·+
(2) a (α-substituted) benzyl group, a regioselective process
should take place [Equation (8)].
In many of the known photoinduced electron-transfer
processes, the largely exothermic back electron transfer
(BET) competes with the chemical reaction of the radical
ions. The present case is no exception and, despite efficient
initial ET, the chemically unproductive back electron trans-
fer shown in Equation (3) greatly diminishes the reaction
efficiency.
RSEt·+ Ǟ R+ + EtS·
(8)
Evaluation of the thermochemistry of the process shown
in Equation (8) according to Equation (6) shows that it is a
strongly endoergonic process for dialkyl sulfides 1 and 2
(bond dissociation free energy, BDFE = 45 kcalmol–1,
Table 4), whereas the lowering of the C–S bond energy and
the easier oxidation of the radicals make fragmentation of
the benzyl sulfide radical cations less endoergonic (BDFE
of 28 kcalmol–1 for 3·+ and 13.5 kcalmol for both 4·+ and
DCA·– (or TPP·) + R2S·+ Ǟ DCA (or TPP+) + R2S
(3)
However, some chemical reaction does occur, more sig- 5·+).[13]
nificantly in the case of the benzyl sulfides 3–5. Such reac-
Deprotonation [Equation (5)] from benzyl sulfides has
tions can be rationalized as involving a fragmentation of actually been detected[14] and thus is expected to occur in
the sulfide radical cation, namely, either cleavage of the car- compounds 3 and 5 [see Equation (9)], for which ∆GC–H is
bon–sulfur bond [Equation (4)] or deprotonation from the zero or slightly negative.[15] Deprotonation following oxi-
α position [Equation (5)].
dation is a common process both for benzylic derivatives
Table 4. Thermodynamic parameters for the fragmentation of the radical cations of sulfides 1–5.
Sulfide (RSEt)
EOX [V] (vs. SCE)
BDE (RSEt) [kcalmol–1] E° (R·) [V] (vs. SCE) BDE (RSEt·+) [kcalmol–1] BDFE [kcalmol–1][a]
C12H25SEt
PhC2H4SEt
PhCH2SEt
PhCMe2SEt
Ph2CHSEt
1.65[b]
1.65[b]
1.60[f]
1.60[d]
1.34[d]
70[c]
1.00[d]
1.00[d]
0.73[g]
0.16[g]
0.53[g]
54 (69.2)
53.5 (72.2)
38 (51.4)
23.5 (39.6)
25.5 (39.0)
45
45
28
13.5
13.5
69.5[e]
56[c]
57[h]
53.5[i]
[a] Calculated by subtracting an entropic correction (T∆S°) of 13 kcalmol–1, corresponding to the entropy change associated with the
C–S homolysis in methyl ethyl sulfide, ∆S°(MeS–Et) = S°(MeSEt) – S°(MeS·) – S°(Et·), as calculated from literature values, see ref.[32]
[b] Value assumed to be equal with that of diethyl sulfide. [c] See ref.[33] [d] Value estimated from the IP, see ref.[13] [e] Based on the
hypothesis that β-phenyl substitution has a minor effect (ca. 0.5 kcalmol–1) on the BDE, as is the case, for example, with the iodide, see
ref.[34] [f] Assumed to be equal to that of the corresponding methyl benzyl sulfide, see ref.[35] [g] See ref.[36] [h] Based upon the hypothesis
that α,α-dimethyl substitution lowers the BDE by about 1.3 kcalmol–1, as it does with benzyl chlorides, see ref.[37] [i] Based on the
hypothesis that α-phenyl substitution lowers the BDE by 4.5 kcalmol–1, as it does with alkanes, see ref.[38]
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S. M. Bonesi, M. Fagnoni, A. Albini
FULL PAPER
and sulfides, and this is a particular instance of such general observed with sulfides 3–5 (although not with 3 and TPP+),
cases. Reference to previous work[16] suggests, however, or disproportionate, as observed in the case of 5, with the
that, although thermodynamically favoured, deprotonation formation of diphenylmethane and diphenylethylene.
may be a slow reaction and that anyway its rate is largely
Deprotonation [Equation (9), path b in Scheme 3] com-
dependent on the structure of the radical cation (stereoelec- petes with C–S bond cleavage in the case of sulfide 5 and
tronic factor) and on the protic acceptance by the solvent is practically the only process available to sulfide 3, for
or by additives present.
which path a is strongly endoergonic. The resulting (α-
thio)benzyl radicals are rather persistent species and un-
dergo coupling (path bЈ, accompanied by cross coupling
with benzyl radicals arising from path aЈЈЈ in the case of 3)
or are oxidized by the ground-state sensitizer (path bЈЈ).[17b]
This is a thermodynamically favoured process, which is im-
portant particularly with TPP+, a better ground-state oxi-
dant than DCA, and leads to the observed ketones as the
final products by hydrolysis of the thioacetals. As demon-
strated in the case of 3, prolonging the lifetime of the radi-
cal cation (through secondary electron transfer to biphenyl)
favours C–S bond cleavage (higher yield of 1,2-diphenyl-
ethyl ethyl sulfide, see Table 1), whereas methanol acts as a
base, favouring deprotonation [higher yield of the bis(me-
thylthio)diphenylethane].
With compounds 1 and 2, both paths a and c are endoer-
gonic to such a degree that fragmentation of either one of
the C–S bonds is insignificant, and to the small extent to
which it does occur, it is unselective, forming both the ethyl
cation (path c in Scheme 3) or the other alkyl cation (paths
a), as well as the thiyl radicals. In nitrogen-flushed solutions
the products are the alkenes from the alkyl cations and the
disulfides from the thiyl radicals. The small amount of do-
decanaldehyde formed from 1 is probably due to residual
traces of oxygen.
PhCHRSEt·+ Ǟ PhC·RSEt + H+
(9)
Photosensitized Reaction in the Absence of Oxygen
The experiments in the absence of oxygen were aimed at
ascertaining whether fragmentation of sulfide radical cat-
ions was a viable path under the conditions used. At least
with benzyl sulfides 3–5, the extent of the reaction is suf-
ficient to identify the paths followed. C–S bond fragmenta-
tion; (path a, Scheme 3) leads to benzyl cations that are
stabilized by weakly nucleophilic acetonitrile. This allows
subsequent water addition, leading to the products actually
isolated, a benzyl alcohol from 4 and N-benzylacetamide
from 5 (Ritter addition, path aЈ). In the case of 4, α-methyl-
styrene is also formed and may result from deprotonation
of the benzyl cation (aЈЈ) or by dehydration of the
alcohol.[11]
Furthermore, since the redox equilibrium between benzyl
radicals and cations lies at a moderately positive potential
and a strongly reducing agent is present, the sensitizer radi-
cal anion,[17a] yet another process may occur, namely, the
reduction of benzyl cations to radicals [Equation (10), path
aЈЈЈ in Scheme 2].
PhCRRЈ+ + A·– Ǟ PhCRRЈ· + A
(10)
Reactions in the Presence of Oxygen
This process is thermodynamically allowed for the three
Equilibration with oxygen has a varied effect, in particu-
benzyl cations considered with both DCA·– and with TPP·. lar in the case of DCA, for which it ranges from efficient
Thus, benzyl radicals are formed by this path, but not by sulfoxidation versus virtually no reaction in the case of do-
direct fragmentation, as seen above [Equation (7)]. These decyl sulfide 1 to almost no difference between reactions
radicals either couple to give bibenzyls, which are indeed under O2 and N2 in the case of cumyl sulfide 4 (Table 2
Scheme 3. Photosensitized reactions of sulfides.
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Photosensitized Electron Transfer Oxidation of Sulfides
and Table 3). The sulfoxidation of 1 clearly involves singlet
Addition of O2·– to a sulfide radical cation has been dem-
oxygen with persulfoxide 6 as the key intermediate, as pre- onstrated to occur (in the case of thianthrene),[21a] but how
viously demonstrated for diethyl and dibutyl sulfide with efficient it is is doubtful as the highly exoergonic back elec-
DCA.[18,19] In fact, with sulfides in the 0.01–0.05  concen- tron transfer (path eЈЈ, ∆H = –64 kcalmol–1 for Me2S·+)
1
tration range, both direct oxygen sensitization with DCA should compete favourably.[5,6] Furthermore, the fact that
(with a limiting quantum yield of 2) and the formation of oxidative C–S bond cleavage is also observed, although
3DCA after ET quenching by sulfides occur and lead to with a lower efficiency, with TPP+, which does not form
singlet oxygen (path f, Scheme 4).[5,6] Consistent with this O2·–, leaves room for an alternative hypothesis that non-
1
idea, TPP+ that does not form O2 remains an inefficient activated O2 is involved. One possibility is that deproton-
sensitizer of 1. DCA-sensitized oxidation under O2 is also ation is a reversible process under these conditions and only
efficient with phenethyl and benzyl sulfides 2 and 3, for trapping by oxygen of the benzyl radical leads to an
which sulfoxidation is accompanied by C–S bond cleavage. irreversible reaction (path bЈЈЈ). Another one is that oxygen
With these compounds, the latter process also occurs in forms a complex with the radical cation; it was previously
dye-sensitized oxygenation, although to a lesser extent.[18] calculated that the dimethyl sulfide radical cation formed a
When TPP+ is the sensitizer, the reaction is less efficient loose dipole complex with oxygen (Me2S+·OO·, dS–O
=
(but more than with 1) and gives a larger proportion of the 2.6 Å, path d).[6,21b] Reasonably, when a benzylic rather
aldehyde. Oxidative cleavage of benzhydryl sulfide 5 is the than a methyl hydrogen is present, the complex R2S+·OO·
predominating process with DCA (6:1 with respect to sulfo- may undergo intramolecular hydrogen abstraction, a pro-
xidation) and virtually the exclusive reaction with TPP+.
cess that finally leads to C–S bond cleavage (path dЈ). Thus,
Thus, in the DCA reaction part of the process occurs by it may be that reaction via 7 becomes significant when a
the singlet oxygen path and part by the PET mechanism. convenient chemical path from it is available, such as the
The quite efficient reaction of benzyl sulfide 3 compared abstraction of benzylic hydrogen. Some interaction with the
with the absence of O2-induced oxidation with the cumyl benzene ring may also be involved as benzylic deproton-
sulfide 4 (see below) and the importance of the oxidative ation has also been invoked for the TPP+-sensitized oxidat-
C–S bond cleavage with the former compound suggest that ive cleavage of benzyl ethers[22] and alcohols[23] in the pres-
the presence of an easily abstracted (benzylic) hydrogen is ence of oxygen.
determining. It may be recalled that the reaction of 3 with
The case of cumyl sulfide 4 is contrasting. With DCA,
1O2 had been shown to involve two intermediates, namely introducing oxygen changes little the product distribution
a first formed “loose” complex (giving benzaldehyde) that (see below) and halves the efficiency. This is in accord with
evolved to a “tight” complex and then to the sulfox- the fact that this hindered sulfide does not react signifi-
ide.[18a,20] It may be that an intermediate similar to the cantly either with O2 [as confirmed by the inefficient Rose
1
“loose” complex is involved in the non-1O2 path. This may Bengal (RB) sensitization][18a] or with O2·–. More precisely,
be the previously proposed[5] thiadioxirane 6Ј, resulting an indication of a minor reaction with singlet oxygen is
from superoxide addition (path eЈ), but it is different from given by the formation of 5% dicumyl disulfide (only with
the persulfoxide.
DCA), obtained also in the RB oxidation,[18a] but not with
Scheme 4. Photosensitized oxidation of sulfides.
Eur. J. Org. Chem. 2008, 2612–2620
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2617

S. M. Bonesi, M. Fagnoni, A. Albini
FULL PAPER
TPP+. As discussed above, this does not result from the is available. Having a longer S···O distance, 7 is less affected
fragmentation of the radical cation, which occurs in the op- by the steric hindrance of substituents on the sulfide moiety,
posite direction, but rather from persulfoxide 6, as intramo- which in contrast strongly decreases both the dye-sensitized
lecular abstraction in this case necessarily involves the sulfoxidation and the multi-path DCA oxidation, ex-
methylene of the ethyl group (path dЈЈ). The conversion of plaining the restricted range of the rates with TPP+ in com-
these hydroperoxides to thiols, oxidized under the reaction parison with the large range with DCA (see Table 3).
conditions to the observed disulfide, has been previously
Speculating further, and justified by the fact that calcula-
tions are at the moment available only for low-molecular-
commented.[20]
The main process is fragmentation to give the cumyl cat- weight sulfides and by the similar product distribution with
ion, and styrene from this (and by further oxidation, aceto- DCA and TPP+, it can be suggested that the complexes
·–
phenone). In contrast to what was observed with the other formed with O2 and O2, neither of which is significantly
sulfides, for which TPP+ is a less efficient sensitizer than stabilized, as far as is known for simple cases such as
DCA, the oxidation under O2 of 4, which has no α-hydro- Me2S+·OO·(–) [21b]
are barely significant minima on the path
,
gen atoms, is markedly more efficient with TPP+. This sug- leading to the products. When an activated α-hydrogen
gests a generalization about the relative rates of reaction. atom is present, interaction with it is probably as important
Thus, BET with DCA (involving oppositely charged spe- as that with the sulfur atom (formulae 7 and 8: R = H, RЈ
cies) is more efficient than with TPP+ (for which neutral = Ph) and partition between decay to the components, C–S
TPP· is involved)[24,25] so that with the latter sensitizer the bond cleavage to yield an alkyl cation and hydrogen transfer
sulfide radical cation is longer lived and C–S bond fragmen- finally leading to carbonyl derivatives depends on the sul-
tation is expected to be more efficient, as indeed is observed fide structure in the same way.
with 4. However, with 3 and 5, for which hydrogen abstrac-
tion is the key step, the fast BET with DCA is compensated
for by a more efficient reaction, reasonably, due to the for-
mation of superoxide under these conditions. With 1 and 2,
1
on the other hand, it is the contribution of O2 that makes
DCA more efficient.
Conclusions
Experimental Section
The sulfides examined in this work undergo both sulf-
oxidation and oxidative cleavage under oxygen. The results
Materials: The sulfides were prepared according to literature meth-
ods[29] or were commercial products. The sensitizers were of com-
confirm that in DCA-sensitized oxidation, the singlet oxy-
mercial origin, but TPP+ was washed with water and dried before
gen path makes a contribution and is the main path with
use in order to eliminate traces of acids.
simple aliphatic sulfides such as 1. In the other cases, a dif-
Photoreactions: The photoreactions were carried out by using 0.01–
ferent (PET) mechanism operates, which is the only one
0.05  solutions (2 mL) of the sulfide in the presence of DCA or
with TPP+. PET leads to some sulfoxidation (with non-hin-
TPP+ (1ϫ10–3 ) in acetonitrile. The solutions were placed in rub-
dered sulfides), as well as C–S bond fragmentation (forming
ber-stopped Pyrex tubes (л = 1 cm). These were exposed to four
an alkyl cation only with heavily substituted sulfides such
as 4 and 5) and oxidative cleavage to aldehydes and ketones.
The last process involves deprotonation from the α position,
as shown by the increased efficiency when this position is
further activated (benzylic). Comparison with the reactions
carried out under N2 (10–70 times less efficient) supports
the view that it is not the sulfide radical cation that cleaves
directly, but that oxygen must interact in a determining way.
This may involve combination of the sulfide radical cation
with the superoxide anion that is generated when using
DCA[5,28] to form intermediate 6Ј. However, TPP+ sensiti-
zation leads to the same reactions, although with a lower
efficiency (from1/2.5 to 1/5), and, as O2·– is not formed here,
6Ј is not the intermediate. The intermediate may rather be
phosphor-coated 15-W lamps with the centre of emission at 400 nm
while a stream of dry oxygen was passed into the solution (pre-
viously saturated with oxygen) through a needle. The flux imping-
ing on the tubes was around 1ϫ10–6 Einsteinmin–1 cm–2, as deter-
mined by using the DCA-sensitized oxidation of 1,1-diphenylethyl-
ene as the actinometer.[30]
The products were determined by GC on the basis of calibration
curves in the presence of dodecane as the internal standard or by
HPLC with biphenyl as the internal standard. The products were
identified by comparison of their chromatographic characteristics
and mass spectra with those of authentic samples, either of com-
mercial origin or prepared according to published procedures (in
particular, sulfoxides and disulfides),[31] or, in some cases, on the
basis of the mass spectral characteristics.
2-Phenylethyl Ethyl Disulfide: MS: m/z (%) = 198 (19) [M]+, 137
(4), 105 (100), 91 (16), 77 (12), 65 (7).
1
a dipole complex formed with O2 (7). Recalling that O2
and benzyl sulfides first form a “loose” complex (precursor
of benzaldehyde) and then a “tight” complex (precursor of
the sulfoxide), one may suggest that interaction with O2 via
7 would be similar to the “loose” complex and play a role
1,2-Bis(ethylthio)-1,2-diphenylethane: MS: m/z (%) = 151 (100)
[M/2]+, 122 (21), 90 (11), 77 (9).
1,2-Diphenylethyl Ethyl Sulfide: MS: m/z (%) = 242 (5) [M]+, 182
when an easily abstracted hydrogen that is easily abstracted (14), 151 (100), 91 (6), 77 (4).
2618
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2612–2620

Photosensitized Electron Transfer Oxidation of Sulfides
Fluorescence: The fluorescence of DCA and TPP+ was measured
in a Perkin–Elmer spectrofluorimeter in 1 cm cuvettes with four
optical faces and quenching by sulfides 1–5 was determined by the
usual Stern–Volmer treatment. In addition to the known singlet
ing irradiation, reasonably, owing to dehydration promoted by
the acidity formed during the irradiation (see ref.[5b]).
a) ∆GET for electron transfer from the sulfides considered to
singlet-excited DCA and TPP+, calculated by the Weller equa-
tion, ∆G°ET = 23.06[E°(D/D·+) – E°(A·–/A)] – Eexc – e2/εa
(ref.[9b]), ranges between +11 and –25 kcalmol–1, based on the
reported redox potentials (see ref.[10] and the notes to Table 4).
In the case of TPP+ the –e2/εa term is disregarded as no charge
separation is involved. b) D. Rehm, A. Weller, Isr. J. Chem.
1970, 8, 259.
As an example of the ionization potential of the ethyl radical,
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Ruscic, J. Berkowitz, L. A. Curtiss, J. A. Pople, J. Chem. Phys.
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X. Shan, L. Sheng, Y. Zhang, F. Qi, C. Yao, Q. Li, Q. Ji, Chem.
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is 9.08 eff, see: c) M. Ge, J. Wang, X. Zhu, Z. Sun, D. Wang, J.
Chem. Phys. 2000, 113, 1866; the oxidation potentials of these
radicals, E°(EtS·) = 1.85 V and E°(Et·) ≈ 1.00 V vs. SCE were
estimated from the measured IP by the Miller equation, not
taking into account the solvation energy which may heavily
affect the result.
[12]
[13]
lifetime of these compound, this allowed values of
k Ͼ
5ϫ109 –1 s–1 to be assigned to all of the sulfides tested.
[1]
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[3]
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·–
a) In the case of the benzyl cation, ∆G = 23.06[E°(A ) –
E°(PhCH2+)]
=
–37.3 kcalmol–1
with
DCA
and
[4]
[5]
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–28.8 kcalmol–1, and with Ph2CH+ 28.8 and 16.1 kcalmol–1. b)
The oxidation potential of α-thioalkyl radicals should not be
significantly different to those of α-hydroxyalkyl radicals (e.g.,
Me2C·OH = –0.61 V vs. SCE, a value affected by a large uncer-
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[6]
[7]
[18]
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·
In a previous study of the N-methylquinolinium photosensiti-
zation of sulfide 4, α-methylstyrene and 1-phenylethanol were
also found. The percentage of the two compounds varied dur-
lations show that the Me2S+·O2 complex is somewhat destabi-
[11]
lized with respect to the reagents (∆H = +6 kcalmol–1), but
similarly intermediates 6 and 6Ј are also destabilized with re-
Eur. J. Org. Chem. 2008, 2612–2620
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2619

S. M. Bonesi, M. Fagnoni, A. Albini
FULL PAPER
spect to R2S + 1O2 by about the same amount,[5,6] although in
every case subsequent paths from such intermediates to the
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[24] Flash photolysis experiments with the Ph2S/TPP+ system
showed no oxygen quenching of either the sulfide radical cation
or of TPP·[6] (whereas the lifetimes of both R2S·+ and DCA·–
was shortened in the corresponding experiments with DCA be-
cause of ET to O2 from the latter and reaction with O2·– of the
former, path e). This proves that the rate constant for the reac-
tion of both intermediates is kO2 Ͻ 1ϫ106 –1 s–1, but does not
preclude this process having a role in steady-state experiments.
Indeed, the reaction rate of R2S·+ with oxygen depends linearly
on the intermediate concentration and thus on the intensity of
the absorbed light flux, k[Intermediate][O2], rather than on the
square of the flux as the reaction with superoxide, k[Intermedi-
ate]2. Thus, interaction with O2 is more competitive at the low
flux of steady-state irradiation than in flash experiments.
[25] There is some literature evidence that a further transient is in-
[29]
[30]
[31]
[32]
[33]
[34]
volved in the reoxidation of the TPP· radical and it may well
·–
be that this is less efficient in BET than TPP·, DCA·– or O2
.
[35]
Actually, it is known that TPP· is reoxidized to TPP+ by reac-
tion with oxygen,[26] the reoxidation probably involves dimer-
ization to TPP-TPP and SET to TPP-TPP·+ which cleaves to
TPP· + TPP+.[10d] That TPP· ultimately gives back reoxidized
TPP+ is also indicated by the fact that this sensitizer is only
partially consumed when used in solution and not at all when
using the silica-absorbed or zeolited-encapsulated materials.[27]
[26] D. Jacobi, W. Abraham, U. Pischel, L. Grubert, R. Stösser, W.
Schnabbel, J. Chem. Soc. Perkin Trans. 2 1999, 1695.
[36]
[37]
[38]
Received: January 15, 2008
Published Online: April 4, 2008
2620
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2612–2620