Direct Irradiaton of Aryl Sulfides: Homolytic Fragmentation and Sensitized S-Oxidation

Article abstract of DOI:10.1021/acs.joc.7b01518

The direct irradiation of diphenyl sulfide and p-substituted thioanisoles in the presence of oxygen was investigated by means of both steady state and laser flash photolysis experiments. Two competitive pathways took place from the triplet excited state o

Full text of DOI:10.1021/acs.joc.7b01518

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Article
Direct irradiaton of aryl sulfides. Homolytic
fragmentation and sensitized S-oxidation.
Sergio Mauricio Bonesi, Stefano Crespi, Daniele Merli, Ilse Manet, and Angelo Albini
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01518 • Publication Date (Web): 04 Aug 2017
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Direct irradiaton of aryl sulfides. Homolytic fragmentation and sensitized S-oxidation.
a,b,*
b
c
d
b
Sergio M. Bonesi , Stefano Crespi , Daniele Merli , Ilse Manet and Angelo Albini
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a
er
Departamento de Química Orgánica, CIHIDECAR – CONICET, , 3 Piso, Pabellón 2, Ciudad
Universitaria, FCEyN, University of Buenos Aires, Buenos Aires, 1428, Argentina.
Phone/FAX:+541145763346.
b
PhotoGreen Lab, Department of Chemistry, V.le Taramelli 12, 27100 Pavia, Italy.
c
Department of Chemistry, V.le Taramelli 12, 27100 Pavia, Italy.
d
ISOF-CNR, via Gobetti 101, 40129, Bologna, Italy.
Abstract The direct irradiation of diphenyl sulfide and pꢀsubstituted thioanisoles in the presence of
oxygen was investigated by means of both steady state and laser flash photolysis experiments. Two
competitive pathways took place from the triplet excited state of thioanisoles, CꢀS bond cleavage,
finally leading to aryl sulfinic acid and sensitized oxidation leading to Sꢀoxidation. Coꢀoxidation of
dodecyl methyl sulfide occurred efficiently implying that an Sꢀpersulfoxide intermediate is involved
during the sensitized oxidation. On the other hand, triplet state of diphenyl sulfide also showed
competitive CꢀS bond cleavage giving phenyl sulfinic acid and ionization to diphenyl sulfide radical
cation that in turn led to diphenyl sulfoxide. The rate constants of the above reactions were
determined by timeꢀresolved experiments.
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Table of Content
O2
*
Singlet oxygen
sensitization
1O2 + ArSR
3
10 nm
3
Sensitized
oxidation
ArSR
+
O
2
ArSR
ArS(O)R
Solvent
φISC
O
2
ArS(O)OH
C-S Cleavage
0
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0
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0
S
S
CH
3
ArSR
=
X = OMe; Me; H; CN; NO
2
X
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Introduction
During the last years, our research group has been involved in the study of the photo oxidation of
sulfides providing new insight into both the oxygen sensitization and electron transfer oxidation
1
reactions of alkyl and aryl sulfides , through steadyꢀstate and laser flash photolysis experiments. In
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the following, a rather unexpected observation is reported, that is that the direct irradiation (310 nm,
with no added energy or electron transfer photosensitizer) of thioanisoles and diphenyl sulfide in
oxygen saturated solutions caused CꢀS bond cleavage and formation of sulfinic acids, along with Sꢀ
oxidation of sulfides giving sulfoxides as photoproducts (eq. 1).
.
Two processes appear to occur competitively via aryl sulfenyl radicals (ArS ) are the reasonable
intermediates and further reaction with molecular oxygen gives the observed aryl sulfinic acids. In
the literature, two mechanisms have been reported for the fragmentation of CꢀS bond. One involves
the reaction of oxygen singlet of aryl methyl sulfides (benzyl, benzhydryl and cumyl aryl sulfides)
and occurs efficiently from the Sꢀpersulfoxide intermediate through a Pummerer rearrangement
affording carbonyl compounds (aldehydes and ketones) as photoproducts in aprotic solvents along
2
,1a,1b,1c
with the corresponding sulfoxides and sulfones.
The second one, involves an electron
3
transfer process releasing aryl thiyl radical and alkyl cation as intermediates, as demonstrated by
Baciocchi and coworkers, who observed the CꢀS bond cleavage of aryl 1ꢀmethylꢀ1ꢀarylmethyl
sulfide radical cations under steadyꢀstate conditions and revealed such intermediates by laser flash
photolysis experiments. However, we have earlier demonstrated that diethyl and diphenyl sulfide
radical cations do not fragment the CꢀS bonds and the corresponding sulfoxides were obtained as
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the photoproducts under electron transfer photosensitization of O ꢀsaturated solutions. On the other
2
hand, irradiation (254 nm) of a series of alkyl ethyl sulfides (RSEt) under inert atmosphere, where
the second Sꢀbonded substituent R is an alkyl or benzyl group, underwent photo homolysis
providing end photoproducts that arose from coupling or disproportionation of the alkyl and thiyl
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radicals formed with high quantum yields (0.41 – 0.90). To summarize, these reports show that
C ꢀS bond of sulfides can fragments under different experimental conditions, viz., singlet oxygen
sp3
sensitization via a sulfide radical cation and from the excited state (singlet or triplet) of a sulfide.
Recently, Jiang and coꢀworkers have studied the photochemistry of diphenylquinoxaline derivatives
bearing phenyl thioethers as type I photo initiator in photopolymerization experiments and have
6
shown that Csp2ꢀS bond of the thioether moiety fragmented under direct irradiation (365 nm).
Notice that irradiation of diaryl sulfoxides is another interesting example of the easy fragmentation
of a C ꢀS(O) bond. In fact, Jenks and coꢀworkers have studied the photochemistry of diphenyl and
sp2
7
ditolyl sulfoxides and αꢀcleavage is the preferred fragmentation pathway. Also, the CꢀS bond
ꢀ
1
strength in Ph
2
S=O and related compounds has to be around 65 kcal.mol which is lower than the
ꢀ1
ꢀ1
energies of the singlet (E
S
T
≈ 90 – 100 kcal.mol ) and triplet (E ≈ 75 – 80 kcal.mol ) states.
Therefore, the C ꢀS cleavage of these compounds is markedly exothermic regardless of the excited
sp2
state involved.
On the other hand, photoinduced Sꢀoxidation is commonly carried out by reaction with singlet
8
oxygen generated by energy transfer from triplet sensitizers. However, in some cases, it has been
9
demonstrated that such interaction does not occur with unitary efficiency. In other words, the
“
“
chemical” combination of the organic molecule (A) with oxygen is accompanied by purely
physical” decay (eq. 2) and further processes, such as the decay of the triplet sensitizer not
accompanied by oxygen sensitization and electron transfer.
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3
1
(eq. 2)
Sens + hν
1O2
Sens* + O
2
kd
kr
3O2
kET
A
3
Sens*
AO2
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Sens
+
O2
In general, aryl alkyl (as well as diaryl) sulfides appear to be less reactive towards singlet oxidation
and have received less attention than dialkyl sulfides, though the singlet oxygenation of thioanisoles
1
0
has been considered in mechanistic papers. Addition of protic additives (alcohol and acids),
1
1
zeolites and clays provides photo oxidation of sulfides more efficiently. Likewise, Sꢀoxidation of
alkyl and aryl sulfides could be achieved under electron transfer sensitization in the presence of
12
molecular oxygen and involved sulfide radical cation as the key intermediate. Sulfoxides are
generally the main products, but variable amounts of C–S bond cleavage products can also be
obtained, depending on the substrate structure.
As it was reported above, we have studied the photochemistry of alkyl sulfides by direct irradiation
5
with wavelength of 254 nm under Ar atmosphere as well as photosensitized oxidation reactions
1
4
under energy transfer and electron transfer conditions of dialkyl, arylalkyl and diaryl sulfides in
different solvents and with different photosensitizers. However, according to our knowledge, the
possibility of oxidation of sulfides by direct irradiation (310 nm) in the presence of oxygen was not
previously investigated making this one of our reason to develop the study. Therefore, based on the
preliminary reports mentioned above and considering that the direct irradiation of sulfides was not
reported yet encouraged us to do more extensive work for which we chose a series of pꢀsubstituted
thioanisoles and diphenyl sulfide as substrates, as reported below, and examined in terms of product
distribution, chemical quenching and laser flash photolysis experiments.
Results
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Photo oxidation of diphenyl sulfide. Irradiation of diphenyl sulfide (Ph S) in the presence of dyes,
2
such as Rose Bengal (RB), under oxygen flushed solutions causes no chemical change. However,
direct irradiation of diphenyl sulfide in acetonitrile with λexc = 310 nm under oxygen atmosphere
2
caused the generation of diphenyl sulfoxide (12.8 %) and phenyl sulfinic acid (32.0 %) when Ph S
0
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is consumed in up to 55% yield. The course of the reaction in MeCN is detailed in Fig. 1, where it
is apparent that CꢀS cleavage to form phenyl sulfinic acid is the main process.
0.030
Ph S
2
0.025
Ph2SO
PhS(O)OH
0
.020
.015
0
0.010
0
.005
.000
0
0
20
40
60
80
100
120
140
t / min
Figure 1. Consumption of diphenylsulfide (black circles) and formation of phenylsulfinic acid (red
triangles) and diphenylsulfoxide (blue triangles) with reaction time.
The UVꢀvisible spectra of diphenyl sulfide show two absorption bands located at 296 and 320 nm.
1
The first weak absorption band was assigned to the L
b
state while strong absorption band was
1
13
assigned to the L
a
state by comparison with reported data. Diphenyl sulfide is suitable compound
1
that can absorb the incident light (310 nm) because a maximal overlapping between the L band
b
and the incident light takes place as it was verified when the recorded UVꢀvisible absorption spectra
of a solution of thioanisole in MeCN and the emission spectra of the lamps used (310 nm) were
overlaid each other.
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The photo oxidation of diphenyl sulfide also occurs in methanol and dichloromethane, a polar protic
and an aprotic solvents, respectively, providing the same product distribution. Noteworthy, in any
experiment diphenyl sulfone was not formed even at high sulfide conversion. The quantum yields
of formation of diphenyl sulfoxide (Ph SO) (φPh2SO, see Table 1) were also measured and in aprotic
2
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solvents the quantum yields are quite similar while in protic solvents, such as methanol, the value is
halved.
It is known from the literature that dye photo oxidation of dialkyl and alkylaryl sulfides generates
1
0c,11d
sulfoxides at a noticeably enhanced rate in aprotic solvents in the presence of acids.
On the
contrary, addition of cesium carbonate to the reaction mixture suppressed the formation of sulfoxide
(even after a further period in the darkness) as can be seen in Table 1. Likewise, no significant
oxidation was detected when diphenyl sulfide was treated with hydrogen peroxide in acetic acid in
the darkness.
a
Table 1. Direct photo oxidation of diphenylsulfide.
ꢀ1
b
Solvent Sensitizer or Additives
Conv. (%) [Ph SO] / ꢀmol min
φPh2SO
2
1
MeOH RB (visible light)
0
0
ꢀ1
+
MeCN
ꢀꢀꢀꢀꢀ
23.0
0.070 (0.120 ꢀmol.min of H ) 0.048
MeOH ꢀꢀꢀꢀꢀ
34.4
30.7
24.1
11.9
0.039
0.062
0.096
0.073
0.060
0
0.026
0.041
DCM
ꢀꢀꢀꢀꢀ
ꢀ
3
MeCN
MeCN
MeCN
MeCN
MeCN
CH
CH
3
3
COOH (2.5x10 M)
ꢀ3
COOH (7.5x10 M)
ꢀ3
ClCH
2
COOH (5.0x10 M) 20.3
25.0
34.5 mg Cs CO / hν + 20 25.0
34.5 mg Cs CO
2
3
0
2
3
min in the darkness
a
b
Sulfide concentration: 0.010 M. RB = Rose Bengal. Actinometer: KI (0.6 M), KIO
3
(0.1 M) and
ꢀ
14
Na
2
B
2
O
7
.10H
2
O (0.01 M) in water; φ(I
3
) = 0.38; λexc = 310 nm. Error: ± 0.001.
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The role of oxygenated intermediates has been investigated through their oxygenꢀtransfer
1
5
properties. In the present case, dodecyl methyl sulfide was chosen as an adequate oxygen acceptor
because it neither absorbs nor reacts at the excitation wavelength (310 nm). Thus, coꢀirradiation of
diphenyl sulfide (0.010 M) and dodecyl methyl sulfide (0.010 M) in oxygen saturated MeCN
solution caused oxidation of both sulfides (Scheme 1). Under these conditions, dodecyl methyl
sulfide was coꢀoxidized to the corresponding sulfoxide and sulfone supporting that Foote’s
mechanism via singlet oxygen and persulfoxide is operating.
0
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0
S
O
S
O
S
O
S
O
O
hν
OH
S
+
+
+
+
R
Me
R
Me
MeCN; O2
S
R
Me
2
.6 %
11.1 %
35.9 %
12.7 %
(
R = C12H25)
Conv(Ph S) = 13.8 %
2
Conv(RSMe) = 48.6 %
Scheme 1. Coꢀirradiation of diphenyl sulfide and dodecyl methyl sulfide at 310 nm.
Time-resolved spectroscopy of diphenyl sulfide. Irradiation of diphenyl sulfide in acetonitrile
solution with a 266 nm laser pulse under nitrogen atmosphere provides transient absorption spectra
(see fig. 2(a)) showing five characteristic absorption bands located at 300, 340, 460, 600 and 740
nm. Comparison with literature shows that two transient species are responsible for these bands,
viz. diphenyl sulfide radical cation and phenyl thiyl radical, with the bands located at 340, 600 and
1
6, 4
7
40 nm attributed to the former, and those at 300 and 460 nm to the latter one.
Quenching of the
N):
.+
Ph
2
S radical cation was obtained by addition of triethylamine which is easily oxidized (Eox(Et
3
17a
17b
0
.69 V vs E (Ph S): 1.43 V (vs. SCE)). In fact, in fig. 2(b), the bands belonging to the radical
ox 2
cation disappear completely while those of the phenyl thiyl radical remain. This quenching
experiment demonstrated that both transient intermediates are simultaneously formed after the laser
2
pulse by two different pathways from the triplet state of diphenyl sulfide: (i) ionization of Ph S
.+
.
2
giving the Ph S and the electron and, (ii) CꢀS cleavage providing radical PhS . As reported in the
literature, diphenyl sulfide showed a minimal fluorescence emission but a phosphorescence
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quantum yield of 0.45. The φ
p
f
/φ ratio was also reported and the fluorescence quantum yield was
1
8
easily estimated as 0.004. Then, we suggest that intersystem crossing to the triplet state of
diphenyl sulfide was largely predominating taking into account the reported φ and φ values and
f
p
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hence, the ionization process as well as the CꢀS bond cleavage observed after the laser pulse is more
likely to proceed from the triplet excited state.
.+
.
Further, the transient decay traces of both intermediates, Ph S and PhS , were also recorded at 740
2
and 460 nm, respectively, where overlapping was minimal, under N
shown in fig. 3. Both transients showed nice secondꢀorder decays under N
solutions and from the slopes of the linear regression of the reciprocal of the concentration of the
2
and O
2
atmospheres and are
2
ꢀsaturated acetonitrile
10
ꢀ1 ꢀ1
transients vs. time plots the bimolecular rate constants, kBET and krecomb, of 10 M .s were
obtained (see Table 2).
(a)
(b)
0.04
0.03
0.02
0.01
0.030
3
00 nm
without Et N
3
Et N (0.024 M)
3
0.025
0.020
0.015
740 nm
340 nm
.
+
Ph S
2
4
60 nm
6
00 nm
.
PhS
0.010
0.005
0.000
0.00
300
400
500
λ / nm
600
700
800
3
00
400
500
λ / nm
600
700
800
2
Figure 2. (a) Transient absorption spectra recorded after a laser pulse (9 ꢀs; λexc: 266 nm) of N ꢀ
ꢀ4
saturated acetonitrile solution of diphenyl sulfide (6.03x10 M). (b) Overlay of transient absorption
2
spectra recorded at 9 ꢀs after a laser pulse (λexc: 266 nm) of N ꢀsaturated acetonitrile solution of
ꢀ4
diphenyl sulfide (6.03x10 M) in the absence and in the presence of trimethylamine (0.024 M).
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The rate constant kBET was attributed, for the case of diphenyl sulfide radical cation, to the backꢀ
electron transfer process between this transient and the ejected electron. Likewise, for the case of
phenyl sulfide radical, the rate constant kRecomb represented the recombination process of the radicals
formed after the CꢀS fragmentation in the solvent cage.
0
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0
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0
.
+
.
2
On the other hand, the decay traces of transients Ph S and PhS showed a pseudoꢀfirst order decay
under oxygen atmosphere indicating that both transients reacts efficiently with molecular oxygen.
After fitting analysis of the data the pseudoꢀfirst rate constants were obtained and a secondꢀorder
6
7
ꢀ1 ꢀ1
rate constant of 10 – 10 M s was calculated by considering the concentration of O
2
solubilized
in each of the solvents used (see Table 2).
(a)
(b)
0.02
N
2
O
2
0.04
0.03
0.02
0.01
0.00
-0.01
N
2
O2
0
.01
0.00
0
20
40
t / ꢀs
60
80
100
0
20
40
t / ꢀs
60
80
100
+.
Figure 3. Decay traces of Ph S after a laser pulse (λ : 266 nm) of acetonitrile solutions of Ph S
2
exc
2
ꢀ4
(
2 2
6.03 × 10 M) recorded at: (a) 740 nm and (b) 450 nm under N and O atmospheres.
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.
+
. a
Table 2. Spectroscopic data and bimolecular rate constants of transients Ph
2
S and PhS .
N2
O2
.+
-1 -1
ꢀ1 ꢀ1
ꢀ1 ꢀ1
Ph S (ε = 5600 M .s )
λ
abs / nm
k
BET / M .s
k
ox / M .s
2
740
1
0
0
7
MeCN
MeOH
340; 740 1.01x10
344; 720 1.24x10
3.23x10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
7
1.04x10
N
2
O
2
.
-1 -1
ꢀ1 ꢀ1
ꢀ1 ꢀ1
PhS (ε450 = 1800 M .s )
k
Recomb / M .s
k’ox / M .s
1
0
0
7
MeCN
MeOH
296; 450 1.01x10
1.94x10
1
6
304; 448 1.67x10
8.69x10
a
ꢀ4
Concentration of diphenyl sulfide: 6.03x10 M. The concentration of molecular oxygen was
ꢀ3
ꢀ3
2.42x10 M in MeCN and 6.58x10 M in MeOH.
Direct photo oxidation of para-substituted thioanisoles. The irradiation was extended to thioanisole
and some of its paraꢀsubstituted derivatives. As it was observed for diphenyl sulfide, thioanisole
derivatives also show two typical absorption bands located around 260 and 300 nm. The first weak
1
1
absorption band was assigned to the L state while strong absorption band was assigned to the L
b
a
13
1
state by comparison with reported data. The L
b
band of thioanisoles overlaps with the incident
light (310 nm) and the absorption process occurs efficiently promoting the photoreaction. The
course of the reaction was monitored for thioanisole in MeCN under oxygen atmosphere (see fig. 4)
and showed that acidity generation (formation of phenylsulfinic acid) was even more predominating
over the oxidation (formation of phenyl methyl sulfoxide). A similar photochemical behavior was
observed when the photoreaction of thioanisole was carried out in methanol and in dichloromethane
under oxygen atmosphere. The results are collected in Table 3. The rate of formation of phenyl
methyl sulfoxide in methanol is four times higher than in aprotic solvents (MeCN and
dichloromethane), which showed quite similar values in the last two solvents. Noteworthy, the rate
of formation of sulfoxide under direct irradiation of thioanisole in methanol shows the same order
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
of magnitude of that obtained under Rose Bengal sensitized conditions indicating that proticity of
11d
the solvent enhances the formation of the sulfoxide.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.030
.025
PhSMe
PhS(O)Me
PhS(O)OH
0
0.020
0.015
0.010
0.005
0.000
0
20
40
60
80
t / min
100
120
140
Figure 4. Consumption of thioanisole (black circles) and formation of phenylsulfinic acid (red
triangles) and phenyl methyl sulfoxide (blue triangles) vs irradiation time.
a
Table 3. Direct photo oxidation of thioanisole.
b
Solvent Sensitizer or Additives
Conv. (%) [PhS(O)Me] / ꢀmol φ
1
PhS(O)Me
ꢀ1
min
MeOH
MeCN
MeOH
DCM
RB (visible)
ꢀꢀꢀꢀꢀ
25.5
35.2
30.5
48.5
27.2
0.136
0.033
0.189
0.045
0.052
0.174
0.213
0
0.022
0.124
0.019
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀ
MeCN
MeCN
MeCN
MeCN
Acetone (8:2)
ꢀ3
ClCH COOH (2.5x10 M) 25.7
2
ꢀ3
ClCH
2
COOH (5.0x10 M) 27.2
Cs CO
2
3
(34 mg) 36.0
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a
b
Sulfide concentration: 0.010 M. RB = Rose Bengal. Actinometer: KI (0.6 M), KIO
3
(0.1 M) and
ꢀ
14
Na B O .10H O (0.01 M) in water; φ(I ) = 0.38; λ = 310 nm.
2
2
7
2
3
exc
As it is apparent in Table 3, direct irradiation of thioanisole in MeCN took place also in the
presence of an external triplet energy sensitizer (acetone) and was accelerated by the addition of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ
3
acids such as chloroacetic acid (compare the rate in the presence of acid (5x10 M), 0.213
ꢀ1
ꢀ1
ꢀ
mol.min with that in neat MeCN, 0.033 ꢀmol.min ). On the other hand, no oxidation to
sulfoxide took place in the presence of bases, but the cleavage to the phenyl sulfinate was
maintained.
The direct irradiation of paraꢀsubstituted thioanisoles was carried out under the same conditions
and the results are collected in Table 4. As is apparent, the photo oxidation of these thioanisoles
occurred in protic and aprotic solvents and a marked substituent effect was observed. Thus,
electronꢀdonating substituted thioanisoles made the photo oxidation faster. However, the CꢀS
cleavage of thioanisoles to give the corresponding aryl sulfinic acids remaining competitive also in
the presence of a base such as Cs CO where no photo oxidation took place (see for example in
2
3
2 3
Table 4, pꢀmethoxythioanisole in MeCN in the presence of Cs CO ).
a
Table 4. Direct photo oxidation of pꢀsubstituted thioanisole derivatives.
SMe
ꢀ1
c
Solvent Conv. (%) [SO] / ꢀmol min
φSO
1
R
MeO
MeCN
MeCN
DCM
20.1
22.3
25.2
25.9
26.7
45.3
10.6
0.118
0
0.077
ꢀꢀꢀ
b
0.119
0.055
0.123
0.071
0.131
0.078
0.036
0.080
0.046
0.086
MeOH
MeCN
DCM
Me
MeOH
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
H
MeCN
MeCN
DCM
35.2
36.0
48.5
30.5
21.1
33.2
11.2
8.0
0.033
0
0.022
ꢀꢀꢀ
b
0.045
0.189
0.041
0.077
0.027
0.010
0.023
0.007
0.024
0.124
0.017
0.050
0.018
0.007
0.015
0.005
MeOH
MeCN
DCM
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
CN
NO
MeOH
MeCN
DCM
2
14.0
7.0
MeOH
a
b
Sulfides concentration: 0.010 M. SO = sulfoxides. Carried out in the presence of Cs
2
CO
3
(34
c
ꢀ
mg). Actinometer: KI (0.6 M), KIO
3
(0.1 M) and Na
2
B
2
O
7
.10H
2
O (0.01 M) in water; φ(I
3
) = 0.38;
1
4
λ
exc = 310 nm.
The coꢀphotooxidation of thioanisole and pꢀmethoxythioanisole in the presence of dodecyl methyl
sulfide was likewise explored and gave results similar to those obtained for Ph S (see Scheme 2).
2
Thus, a common intermediate is formed that oxidizes the oxygen acceptor dodecyl methyl sulfide.
O
S
O
S
SMe
O
S
O
O
hν
S
Me
OH
S
R
Me
+
+
+
+
R
Me
R
Me
X
(R = C12H25
)
MeCN; O2
X
X
X = H
(Conv. = 29.7 %) (Conv. = 17.9 %)
16.4 %
7.3 %
13.3 %
16.7 %
9.8 %
1.4 %
1.2 %
X = OMe
Conv. = 11.9 %) (Conv. = 11 %)
(
4.6 %
Scheme 2. Coꢀirradiation of thioanisoles and dodecyl methyl sulfide.
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On the basis of the above data collected for thioanisole and pꢀsubstituted thioanisole, steadyꢀstate
and timeꢀresolved spectroscopies were further studied to shed some light on the reaction mechanism
and, particularly, on the Sꢀoxidation and CꢀS fragmentation pathways of these substrates.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Steady-state and time-resolved spectroscopy of thioanisoles. The oxidation pathway. The
fluorescence emission spectra of thioanisoles were recorded in MeCN using a solution of 4ꢀ
chloroanisole as fluorescence actinometer. All the substrates showed a minimal fluorescence
emission and the fluorescence quantum yields are collected in Table 5. With these molecules,
intersystem crossing to the triplet excited state was in all case largely dominating. Timeꢀresolved
experiments were then carried out. In every case an emission spectrum identical to that obtained by
using perinaphthenone, and attributed to the singlet oxygen phosphorescence, was obtained in
MeCN. Comparison of the zero time intensity of the signal allowed measuring the production of
ꢁ ꢁ
singlet oxygen (φ ) compiled in Table 5, which turned out to be around 0.15. Because the φ values
obtained for the thioanisole derivatives are less than unitary, a number of mechanisms have been
considered for rationalizing this behavior that include chemical combination, energy transfer and
fragmentation.
ꢁ
Table 5. Fluorescence quantum yield and singlet oxygen quantum yield (φ ) of thioanisole and pꢀ
substituted thioanisole derivatives measured in MeCN.
SMe
a
b
Atmosphere
Ar
φ
f
φ
ꢁ
R
MeO
0.008
0.007
0.006
0.005
0.004
0.10
0.17
0.15
O
2
Me
H
Ar
O2
Ar
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
O2
Ar
0.003
ꢀ
4
CN
NO
4x10
3x10
0.16
0.15
ꢀ4
O
2
2
Ar
Not detected
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
19
Actinometer: acetonitrile solution of 4ꢀchloroanisole under Ar atmosphere; φ = 0.019 ; Error:
f
b
20
±
0.002. Values relative to acetonitrile solution of perinaphthenone; φ
ꢁ
= 0.95 ; Error: ±0.002.
Additionally, timeꢀresolved experiments have been carried out using laser flash photolysis and the
decay of the phosphorescence emission of singlet oxygen in the presence of the thioanisole
derivatives was further analyzed in MeCN. Figure 5 shows typical decay traces of the emission of
singlet oxygen in the presence of pꢀmethoxythioanisole and pꢀcyanothioanisole, respectively. The
decay of the singlet oxygen phosphorescence emission at 1230 nm in the presence of the
thioanisoles was well fitted by a biꢀexponential decay by using a laser pulse with an energy of ca 4
mJ and a λexc = 266 nm, thus giving two observed lifetime values, τ and τ , with the former much
d
ox
larger than the latter one.
(a)
(b)
0.032
0.056
0
.052
.048
.044
0
.028
.024
0
0
0
0.020
0.040
0
1e-4
2e-4
t / s
3e-4
4e-4
0
1e-4
2e-4
t / s
3e-4
4e-4
Figure 5. Singlet oxygen phosphorescence emission (λmax: 1230 nm) decay traces in the presence
of: (a) pꢀmethoxythioanisole and (b) pꢀcyanothioanisole.
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Two competitive pathways of deactivation of the singlet oxygen produced are attributed to the
d
physical deactivation of the singlet oxygen (k ) and to the reaction between singlet oxygen and the
nucleophilic sulfide (k ) to give the corresponding sulfoxide, respectively. The spectroscopic data
ox
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
are collected in Table 6. The large values obtained were assigned to the physical deactivation of the
singlet oxygen (τ
which is 86.9 ꢀs in MeCN. Then, the small values collected in the table were attributed to the
reaction between thioanisoles and singlet oxygen. In the same table the τ /τox ratio which is equal to
the ratio of kox.[ArSMe]/k was also included. The thus calculated values showed the expected trend
d
) by comparison of the singlet oxygen decay value in the absence of sulfides,
d
d
of the substituent effect, electronꢀdonating substituents enhance the oxidation rate constant while
electronꢀwithdrawing substituents lower it and rather favor the physical deactivation of singlet
oxygen.
Table 6. Lifetimes (τ and τox) of singlet oxygen and τd/τox ratio in the presence of thioanisole and pꢀ
d
substituted thioanisole derivatives measured in MeCN at room temperature.
SMe
a
a
b
τ
d
/ ꢀs
τ
ox / ꢀs
τ τ
d/ ox
R
OMe
Me
H
134.0
115.0
84.2
12.7
18.2
12.5
16.1
23.0
10.6
6.3
6.7
4.9
3.1
CN
79.2
NO
2
71.8
a
b
d
Errors = ±0.1. The ratio τd/τox is equal to the ratio kox.[ArSMe]/k .
2 2
The C-S fragmentation pathway of thioanisoles. Laser flash photolysis of N ꢀ and O ꢀsaturated
acetonitrile solutions of thioanisoles were carried out with a laser pulse of λexc: 266 nm and
transient absorption spectra were obtained. The absorption spectra showed two characteristic bands
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
located around 295 – 310 nm and 450 – 530 nm as can be seen in fig. 6 for pꢀmethoxy and pꢀmethyl
thioanisoles in MeCN under N atmosphere.
2
(a)
(b)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.08
0.08
0.06
0.06
0
0
0
.04
.02
.00
0
.04
0.02
0.00
300
350
400
450
500
550
600
3
00
350
400
450
500
550
600
λ / nm
λ / nm
Figure 6. Transient absorption spectra recorded after a laser pulse (9 ꢀs; λ : 266 nm) of N ꢀ
exc
2
ꢀ4
saturated acetonitrile solution of (a) pꢀmethylthioanisole and (b) pꢀmethoxythioanisole (6.03x10
M).
Addition of radical cation scavengers such as triethylamine or NaN
3
caused no change of the
.
transient absorption spectra and thus, the spectra can be assigned to aryl thiyl radicals (ArS ).
21
Similar spectra have been obtained by laser flash photolysis and pulse radiolysis. Therefore, CꢀS
cleavage process is a competitive pathway for thioanisoles.
The transient decay traces of such radical were recorded under N
fig. 7 for pꢀmethoxythioanisole and pꢀcyanothioanisole, respectively. Under N
traces showed nice secondꢀorder decays from which, after linear regression fittings, bimolecular
2
and O
2
atmospheres as shown in
2
the transient decay
10
ꢀ1 ꢀ1
rate constants (krecomb) were obtained (rather independent of the substituent, 1 to 2x10 M .s ) and
are collected in Table 7. Such rates (kRecomb) were attributed to the recombination rate constants of
the radicals formed after the CꢀS fragmentation from the triplet state of thioanisoles.
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(a)
(b)
0
.12
.10
N2
0
0
.05
.04
O
N
2
2
0
O
2
λ = 325 nm
0.08
λ = 525 nm
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.03
0
.06
.04
0
0
.02
.01
0
0.02
0.00
0.01
0.00
-
0
20
40
t / ꢀs
60
80
100
0
20
40
t / ꢀs
60
80
100
.
Figure 7. Decay traces of aryl thiyl radical (ArS ) after a laser pulse (λ : 266 nm) of acetonitrile
exc
ꢀ
4
solutions of thioanisoles (6.03 × 10 M) recorded at: (a) 325 nm for pꢀmethoxythioanisole and (b)
525 nm for pꢀcyanothioanisole under N and O atmospheres.
2
2
a
Table 7. Bimolecular rate constants of transient aryl thiyl radicals in MeCN.
N
2
O
2
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
Thiyl radicals
k
recomb / M .s
k’ox / M .s
.
9
7
pꢀMeOPhS
9.82x10
2.06x10
1.39x10
9.31x10
1.43x10
3.06x10
.
10
10
9
7
pꢀMePhS
4.17x10
.
7
PhS
4.05x10
.
7
pꢀCNPhS
3.29x10
.
10
6
pꢀNO
2
PhS
1.36x10
a
ꢀ4
ꢀ3
Concentration of thioanisoles: 6.03x10 M. The concentration of molecular oxygen was 2.42x10
M in MeCN.
On the other hand, O
2
ꢀsaturated acetonitrile solutions of thioanisoles afforded pseudoꢀfirst order
7
ꢀ1 ꢀ1
decay traces that, after fitting analysis, secondꢀorder rate constants (k’ox) of 10 M .s were
ꢀ3
calculated taking into account the concentration of O
2
dissolved in MeCN (2.42x10 M). These
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
data are also collected in Table 7. The change in the kinetic profile of the decay traces from N to
2
O atmospheres supports that aryl thiyl radicals react smoothly with molecular oxygen to give aryl
2
sulfenyl peroxyl radicals in competition with recombination. Another feature observed when
2 2
comparing the decay trace profiles obtained in N ꢀ and O ꢀsaturated solutions is that molecular
0
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oxygen diminishes the optical density at timeꢀzero after the laser pulse from one fifth to one sixth
fold indicating that molecular oxygen quenches efficiently the triplet excited state of the
thioanisoles.
Discussion
Direct irradiation of diphenyl sulfide. As hinted above diphenyl sulfide reacted efficiently upon
direct irradiation (310 nm) in MeCN, MeOH and DCM under oxygen atmosphere. Steadyꢀstate and
laser flash photolysis experiments reported above give credence to the mechanism shown in
Scheme 3. Upon irradiation of Ph
2
S at 310 nm efficient population of the triplet state is achieved.
Two main pathways compete with the physical deactivation (k
d
; Scheme 3) of the triplet state, viz.
.
i) homolytic CꢀS bond fragmentation (path (a)) giving intermediate phenyl thiyl radical (PhS ) that
finally evolves to phenyl sulfinic acid (main photoproduct under O ) and ii) ionization of diphenyl
2
.+
sulfide to diphenyl sulfide radical cation (Ph S ) (path (b)) evolving in the presence of molecular
2
oxygen to Ph
2
SO. Laser flash photolysis experiments of N
2
ꢀ and O
2
ꢀsaturated acetonitrile solution
.
of Ph S with λ = 266 nm gave the transient absorption spectra of both intermediates, viz. PhS
2
exc
.+
and Ph
2
S , which were formed simultaneously at 9 ꢀs after the laser pulse (see fig. 2).
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O2.-
RSMe
Ph S
Ph S
2
RS(O)Me + Ph S=O
2
2
path (c)
O2.-
3O2
3O2
kBET
kd
e
kox
Ph S
2
hν
φISC
3
*
.+
Ph S
Ph S
Ph S
Ph S-OO
2
Ph S=O
2
2
2
2
2
path (b)
O2.-
3O2
path (d)
3O2
10
11
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60
path (a)
kRecomb
path (e)
path (g)
3O2
H+
Ph S
PhS + Ph
PhS-OO
PhS(O)OH
2
k'ox
path (f)
H O
2
Scheme 3. Reaction mechanism proposed for direct irradiation of diphenyl sulfide.
Under N
2
, recombination of the radical gave back Ph
2
S (path (e)) with a second order rate constant
1
0
ꢀ1 ꢀ1
10
ꢀ1 ꢀ1
(
kRecomb) of 1.01x10 M .s in MeCN and 1.67x10 M .s in MeOH. Likewise, diphenyl sulfide
radical cation formed by ionization of triplet Ph
transfer process (path (c)) in both solvents, with a second order rate constant (kBET) of 10 M .s
see Table 2).
On the other hand, laser experiments carried out under O
2 2
S (path (b)) under N decays by backꢀelectron
1
0
ꢀ1 ꢀ1
(
2
atmosphere showed that both transient
species, PhS and Ph S , react smoothly with molecular oxygen with bimolecular rate constants (k
.
.+
2
ox
6
7
ꢀ1 ꢀ1
and k’ox) ranging from 10 to 10 M .s (see path (f) and path (d) in Scheme 3). In the case of
.
phenyl thiyl radical (PhS ) reaction with molecular oxygen gives intermediate phenyl sulfenyl
.
peroxyl radical (PhSOO ) that, in the presence of adventitious water in the solvent, evolves finally
to the phenyl sulfinic acid which is the main photoproduct detected under steadyꢀstate experiments.
.+
Likewise, diphenyl sulfide radical cation (Ph S ) reacts with molecular oxygen to provide the
2
persulfoxide radical cation intermediate as shown in path (d) that finally evolves to diphenyl
2
sulfoxide (Ph S=O), the other photoproduct detected under steadyꢀstate experiments. Dialkyl and
diaryl sulfides radical cation are prone to react with molecular oxygen as has been early
4
demonstrated by laser flash photolysis forming the corresponding sulfoxides as photoproducts.
Other related Reactive Oxygen Species (ROS) such as singlet oxygen and superoxide ion, can be
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2
2
2
2
2
2
2
2
2
2
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involved in the overall processes, the superoxide ion may be formed by trapping the electron ejected
from Ph S. The fate of this anion has been the subject of some controversy in the case of sulfides,
2
.ꢀ
.+
.ꢀ
.+
and both back electron transfer between O
2
and R
2 2 2
S as well as addition reaction of O with R S
4,22
have been proposed. The role of superoxide ion is more precisely indicated by the decay traces of
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
.
+
Ph
2
S measured at 740 nm under oxygen atmosphere which shows a pseudoꢀfirst order kinetics in
.
ꢀ
.+
all the solvents studied. If a direct reaction of O2 with Ph S was the primary oxidation pathway,
2
.
ꢀ
secondꢀorder kinetics would be observed which is not the case in our experiments. Therefore, O2
has a role, but does not add to the diphenyl sulfide radical cation. A reasonable alternative is that
.
ꢀ
.+
2 2
O operates as electron carrier in the reduction of Ph SOꢀO (path (g), Scheme 3) in the presence
of a molecule of diphenyl sulfide to give diphenyl sulfoxide.
An active role of singlet oxygen has been previously excluded by Baciocchi for Nꢀ
22
methylquinoliniumꢀmediated electronꢀtransfer oxidation of sulfides, because the reaction is faster
than expected. In our case, a comparison between the bimolecular rate constant (k
r
) between Ph
2
S
4
ꢀ1 ꢀ1 23
and singlet oxygen (3.9×10 M .s ) and the values shown in Table 2, highlights that this process
.
+
does not compete efficiently with the reaction of Ph
2
S and molecular oxygen. Furthermore, in our
experimental conditions no Ph SO was formed, a photoproduct that was usually observed under
2
2
1
singlet oxygen sensitization of sulfides . Therefore, we excluded reaction of singlet oxygen with
Ph S from the reaction mechanism as a competitive oxidative pathway.
2
As far as we know, the homolytic fragmentation of the Csp2ꢀS bond from the triplet state of diphenyl
sulfide is reported here for the first time. However, this kind of fragmentation has been recently
6
7
reported by Jiang and coꢀworkers and has been also observed with aryl alkyl sulfoxides. In the
case of aryl alkyl sulfide radical cations the homolytic fragmentation of CꢀS bond occurs efficiently
8
releasing aryl thiyl radicals and the corresponding carbocations but the CꢀS bond of diphenyl
4
sulfide radical cation does not fragment as it was early suggested. In fact, our results confirms
.
+
2 2 2
Ph S behavior. Upon laser flash photolysis excitation of a N ꢀsaturated solution of Ph S in MeCN
.
.+
both PhS and Ph S transients are formed simultaneously but, addition of triethylamine as a radical
2
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.
+
.
cation quencher to the solution, inhibits the formation of Ph S but not the formation of PhS .
2
.
+
.
Therefore, we can conclude that these results demonstrate that i) Ph S does not fragment to PhS
2
+
and Ph and ii) Csp2ꢀS homolytic fragmentation occurs competitively from the triplet state of Ph
2
S
.
+
2
and not from the Ph S .
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Direct irradiation of thioanisoles. As in the case of diphenyl sulfide direct irradiation (310 nm) of
thioanisoles provided aryl sulfinic acid as the main photoproduct along with the corresponding
sulfoxides under oxygen atmosphere in MeOH, MeCN and DCM (see Table 4 and fig. 4). Based on
steadyꢀstate and laser flash photolysis experiments described above we propose the reaction
mechanism for the direct irradiation of the thioanisoles shown in Scheme 4. All the thioanisoles
f
showed very low fluorescence quantum yields (φ < 0.008; see Table 5) and intersystem crossing to
the triplet state in all cases predominated. Thus, direct excitation of these substrates at 310 nm
populated efficiently the triplet state which is the photo reactive state involved in the photoreaction.
In the case of thioanisoles, two distinctly and competitive pathways occurred: homolytic CꢀS
fragmentation (path (a); Scheme 4) and singlet oxygen sensitization (path (b); Scheme 4).
2
Irradiation of N ꢀsaturated acetonitrile solutions of thioanisoles with a laser pulse (266 nm) afforded
transients absorption spectra which were assigned to the aryl thiyl radical demonstrating that CꢀS
homolytic fragmentation occurred efficiently (see, for example, fig. 6 for the cases of pꢀmethylꢀ and
pꢀmethoxythioanisoles). Noteworthy, no radical cation transients of thioanisoles were detected in
laser experiments indicating that no ionization occurred after the laser pulse as it was observed for
the case of diphenyl sulfide. This means that Csp3ꢀS cleavage in thioanisoles competes efficiently
with the ionization process despite the fact that thioanisoles have oxidation potentials ranging
1
0c
between 1.13 to 1.70 V vs SCE and are susceptible to oxidation under laser flash photolysis
conditions.
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2
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3O2
ArSMe
kd
k'd
O
3O2
kox
ArSMe
hν
φISC
O
S
3ArSMe*
1O2 + ArSMe
ArSMe
2 ArS(O)Me
Ar
Me
path (b)
path (e)
path (f)
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0
path (a)
path (h) PhS(O)OH
path (i)
OOH
kRecomb
S
ArSMe
PhS(O)OH
RS(O)Me + ArS(O)Me
ArS + Me
Ar
Me
path (c)
O(O)SPh
H+
k'ox
PhS-OO
PhS(O)OH
RSMe
path (d)
H O
2
path (g)
3O2
Scheme 4. Proposed reaction mechanism for direct irradiation of thioanisoles.
The absorption decay traces of the aryl thiyl radicals were also recorded in MeCN under N
2
9
10
ꢀ1 ꢀ1
atmosphere and showed clean secondꢀorder kinetics (kRecomb) of 10 to 10 M .s (see Table 7 and
fig. 7) which were assigned to the recombination process of the radical species formed inꢀcage after
2
the laser pulse (path (c); Scheme 4). When the same experiments were carried out under O ꢀ
saturated acetonitrile solutions a noticeable change in the decay traces of the transients were
observed (see fig. 7). In fact, pseudoꢀfirst order kinetics was obtained for all the thioanisoles studied
7
ꢀ1 ꢀ1
and bimolecular rate constants (k’ox) of 10 M .s were calculated using the concentration of
ꢀ3
dissolved molecular oxygen in MeCN of 2.42x10 M (see Table 7). This behavior was easily
attributed to the reaction of molecular oxygen with aryl thiyl radical (path (d); Scheme 4) giving
.
phenyl sulfenyl peroxyl intermediate PhSOO that evolve to the phenyl sulfinic acid in the presence
of adventitious water in the solution. Noteworthy, pathways (a) and (d) and subsequently formation
of phenyl sulfinic acid clearly describes the results obtained under steadyꢀstate and timeꢀresolved
experiments for the thioanisoles studied.
On the other hand, thioanisoles behaved as smooth singlet oxygen sensitizers when irradiated under
oxygen atmosphere (path (b); Scheme 4) with a production of singlet oxygen (φ
ꢁ
) of ca 0.10 ꢀ 0.15
(
see Table 5). Two competitive pathways take place: i) physical deactivation of the singlet oxygen
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(k ) to the ground state and, ii) reaction of O with nucleophilic thioanisoles to give the
d
2
corresponding Sꢀpersulfoxide intermediates (path (e) in Scheme 4), as supported by the analysis of
the decay curves of the phosphorescence emission of singlet oxygen with two distinct lifetime
values, τ and τ (Table 6). Thus, τ described the physical decay of singlet oxygen while τ refers
d
ox
d
ox
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to the reaction between singlet oxygen and the nucleophilic thioanisoles (kox in path (e); Scheme 4).
Sꢀpersulfoxide intermediates were formed with thioanisoles. Reaction of the intermediate with a
molecule of thioanisole afforded the corresponding thioanisole sulfoxides (path (f)). Formation of
Sꢀpersulfoxide intermediate occurs during singlet oxygen photo sensitization of sulfides as have
1
0, 14a, 24
been reported in the literature.
However, protonation of Sꢀpersulfoxide intermediate during
the photoreaction takes place and the oxidant intermediate is more likely to be the Sꢀ
hydropersulfoxide intermediate (path (h) in Scheme 4). The catalytic effect of acids on the
photosensitized oxidation of sulfides is well documented in the literature and the intermediate Sꢀ
10c,11d
hydropersulfoxide has been proposed.
This intermediate is also a good oxidant species that
evolves to the corresponding sulfoxide releasing the acid catalyst, in our case, the phenyl sulfinic
acid (path (i); Scheme 4). It is clearly that the acidity in all the solvents studied arises from the
formation of phenylsulfinic acid during the irradiation. Thus, efficient protonation of the Sꢀ
persulfoxide intermediate accelerates the oxidation process and promotes high regio selectivity in
favor of the sulfoxide over the sulfone (compare the data in Table 3 of the rate of formation of
2
sulfoxide in MeCN with the increasing amounts of ClCH COOH). In fact, no sulfones were
detected during direct irradiation of thioanisoles, a typical side product formed under singlet oxygen
1, 23a
sensitization of sulfides.
Sulfone derivatives are formed from a Sꢀhydroxypersulfonium ylide as
10b
it was early reported by Sawaki and later by us in the photosensitized oxidation of 4ꢀsubstituted
10c
thioanisoles. However, this ylide that is formed during the irradiation is inhibited under the acidic
condition of the photoreaction and sulfones are not formed which is in agreement with our
observations. Therefore, we suggest that the formation of sulfoxides is more likely to proceed by
the sequence of path (h) and path (i) (see Scheme 4) under acid catalysis than through path (f).
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Further, the Sꢀpersulfoxide intermediate is a good oxidant species and is able to coꢀoxidize dodecyl
methyl sulfide when is added to the reaction mixture. In fact, direct irradiation of a solution of the
thioanisole in the presence of dodecyl methyl sulfide (RSMe in Scheme 4), which does not absorb
at 310 nm, coꢀoxidation take place (path (g)) and both sulfoxides are formed in good yields (see
Scheme 2).
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0
Conclusions
The photochemical reaction of diphenyl sulfide and pꢀsubstituted thioanisoles examined in the
present paper take place, at least, through two competitive reaction mechanisms, SꢀC bond cleavage
and Sꢀoxidation occurring in polar and nonꢀpolar solvents. Photo homolytic fragmentation of Csp2ꢀS
.
(
for diphenyl sulfide) and C ꢀS (for thioanisoles) give the corresponding aryl thiyl radicals (ArS )
sp3
which are characterized by laser flash photolysis spectroscopy (see path (a) in Schemes 3 and 4).
Under inert atmosphere the radicals recombine to the starting sulfides (path (e) in Scheme 3 and
9
10
ꢀ1 ꢀ1
path (c) in Scheme 4) with rate constants of 10 – 10 M .s . On the other hand, aryl thiyl radicals
react with oxygen affording aryl sulfinic acids (paths (d) and (f) in Schemes 3 and 4, respectively)
6
7
ꢀ1 ꢀ1
with rate constants of 10 – 10 M .s and these compounds are the main photoproducts formed
during irradiation.
.
+
In the case of diphenyl sulfide ionization from the triplet state (path (b) in Scheme 3) gives Ph
2
S
that reacts smoothly with molecular oxygen (path (d)). This reaction occurs with bimolecular rate
7
ꢀ1 ꢀ1
.+
constants (kox) of 10 M .s affording intermediate Ph
2 2
SOO that finally evolves to Ph SO in the
presence of Ph S and an electron donor, e.g. superoxide ion (path (g)). Besides, under inert
2
.
+
atmosphere, Ph S turns back to Ph S by an electron transfer process (path (c)) with bimolecular
2
2
10
ꢀ1 ꢀ1
rate constants of 10 M .s . Noteworthy, superoxide ion is formed during the irradiation of Ph
2
S
and operates as an electron carrier (see Scheme 3).
Oxidation of thioanisoles takes place by singlet oxygen sensitization (path (b) in Scheme 4) that
arise from the triplet excited state of thioanisoles with φ of 0.10 – 0.15. Because thioanisoles are
ꢁ
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reactive with singlet oxygen, formation of the Sꢀpersulfoxide intermediates is the next reaction step
(path (e)) that converts efficiently to the corresponding sulfoxides (path (f)). However, paths (h)
and (i) (see Scheme 4) are the main pathways for the oxidation process under acid catalysis because
phenyl sulfinic acid is the catalyst that is formed during the irradiation of thioanisoles. In fact, acid
catalysis accelerates the sensitized oxidation reaction and favors the regioselectivity of the oxidation
reaction giving exclusively the corresponding sulfoxides.
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Finally, the finding that CꢀS fragmentation and Sꢀsulfoxidation of diaryl and aryl alkyl sulfides are
competitive pathways under direct irradiation (310 nm) sheds some light on the photochemistry of
thioethers.
Experimental details
Materials Thioanisoles, diphenyl sulfide, diphenyl sulfoxide, dodecyl methyl sulfide and benzene
sulphynic acid, benzene sulphinic acid and perinaphthenone were commercial products. Sulfoxides
25
and sulfones used as reference compounds were prepared via published procedures. The dodecyl
26
methyl sulfoxide was prepared via published procedures.
Photoreactions The photoreactions were carried out by using 0.01 M solutions of the sulfides in
different solvents (methanol, acetonitrile and dichloromethane). The solutions were contained in
rubber stoppered, 1 cm diameter quartz tubes and a steam of dry oxygen saturated with the
appropriate solvent was passed into the solution through a needle during 10 minutes. The quartz
tubes were exposed to ten phosphorꢀcoated 15 W lamps (Rayonet) emitting at 310 nm. The
photooxidation reaction of diphenyl sulfides and thioanisole in the presence of Rose Bengal (in
methanol) were carried out using 0.01 M solutions and were contained in rubber stoppered, 1 cm
diameter Pyrex tubes. A steam of dry oxygen saturated with methanol was passed into the solution
through a needle during 10 minutes. The solutions were irradiated with four visible 15 W lamps
emitting at 528 nm. Coꢀoxidation experiments were carried out by using 0.01 M solutions of both
sulfides according to the procedure above described.
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6
The products were determined by GC on the basis of calibration curves in the presence of undecane
as the internal standard. The identification of benzene sulphonic acid was based on the comparison
of the retention time of an authentic commercial sample by ionic chromatography. Potentiometric
acid titrations (NaOH 0.1M) have been followed by using an Orion mod. 250 potentiometer
equipped with an Orion pH glass combined electrode mod 91ꢀ56. The titrations by using 0.1M
NaOH have been performed on 10 mL photolyzed solutions in MeOH diluted with 100 mL water.
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Quenching Measurements Rate constants for the quenching of singlet oxygen were obtained from
1
the shorthening of the (O
sulfides in aerated CDCl
2
) ꢁg emission lifetime at 1.27
ꢀ
m in the presence of known amounts of
3
. Singlet oxygen was generated by energy transfer to O
2
from the triplet
state of perinaphthenone, populated by laser excitation (Nd:YAG laser, 532 nm). The nearꢀIR
luminescence of molecular oxygen was observed at 90° geometry through a 5 mm thick ARꢀcoated
silicon metal filter with wavelength pass >1.1 ꢀm and an interference filter at 1.27 ꢀm by means of
a preꢀamplified (lowꢀimpedance) Ge photodiode cooled at 77 K (time resolution 300 ns). Secondꢀ
order analysis of the emission decay was performed with the exclusion of the initial part of the
signal, affected by scattered light, sensitizer fluorescence, and the formation profile of the emission
signal itself. Comparison of the zero time intensity of the signal of the sulfides with that of the
ꢁ
perinaphthone allowed measuring the production of singlet oxygen (φ ).
Fluorescence quantum yield UV and fluorescence spectra were recorded on a Jasco Vꢀ550 UV
spectrophotometer and a PerkinElmer LS55 luminescence spectrometer, respectively. Fluorescence
f
quantum yield (φ ) were measured in acetonitrile and calculated by using 4ꢀchloroanisole as
1
9
fluorescence reference.
Chemical quantum yield The light flux was determined by irradiation of an aqueous solution of KI
ꢀ
(0.6 M), KIO
3
(0.1 M) and Na
2
B
2
O
7
.10H
2
O (0.01 M) and measuring the formation of I
3
by UVꢀ
visible spectroscopy after irradiation with ten phosphorꢀcoated 15 W lamps (Rayonet) emitting at
ꢀ
14
3
310 nm (φ(I ) = 0.38).
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The Journal of Organic Chemistry
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Acknowledgement
S.M.B is a research member of CONICET (Argentinean National Council of Research).
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