Substituent effects on the TiO2 photosensitized oxidation reaction of benzyl thioethers and thiols in deaerated acetonitrile

Article abstract of DOI:10.1016/j.jphotochem.2016.03.025

The TiO₂ photosensitized oxidation of benzyl methyl sulfides (X-C₆H₄CH₂SCH₃) and benzyl thiols (X-C₆H₄CH₂SH) has been investigated in Ar-saturated CH₃CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO₂ photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO₂ surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium hexafluorophosphate (MeOP⁺PF₆^-). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C₆H₄CH₂SH^+? are characterized by having the C-S bond almost collinear with the π system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom.

Full text of DOI:10.1016/j.jphotochem.2016.03.025

Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Substituent effects on the TiO photosensitized oxidation reaction of
2
benzyl thioethers and thiols in deaerated acetonitrile
Marta Bettoni , Tiziana Del Giacco *, Fausto Eliseib,c, Cesare Rolb,
a
b,c,
a
b
Giovanni Vittorio Sebastiani , Marina Stradiotto
Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy
Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, 06123 Perugia, Italy
CEMIN, Centro di Eccellenza Materiali Innovativi Nanostrutturati, Università di Perugia, 06123 Perugia, Italy
a
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
The TiO2 photosensitized oxidation of benzyl methyl sulfides (X-C H CH SCH ) and benzyl thiols
6
4
2
3
Received 3 November 2015
Received in revised form 7 March 2016
Accepted 12 March 2016
6 4 2 3
(X-C H CH SH) has been investigated in Ar-saturated CH CN. Steady-state irradiation produced
benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The
results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect
on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable
Available online 26 March 2016
electron transfer from the substrate to the TiO
give the final products. This process was poorly affected by the adsorption of the substrate at the TiO
surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the
2
photogenerated hole, which reasonably deprotonate to
Keywords:
Sulfide
Thiol
Photooxidation
Adsorption
Electron transfer
2
homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium
+
ꢀ
6
). This behavior is likely due to the low hydrogen-bond acceptor ability
hexafluorophosphate (MeOP PF
of divalent sulfur systems. Density functional theory calculations pointed out that the most stable
+ꢁ
6 4
conformations of X-C H CH
2
SH are characterized by having the C ꢀꢀ S bond almost collinear with the
p
system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl
ring and the sulfur atom.
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
photochemical oxidation of benzyl phenyl sulfides sensitized by
TiO
benzaldehyde, through
alcohol, with the involvement of C ꢀꢀ S fragmentation, and (iii)
sulfoxide through oxygen nucleophilic attack coming from the TiO
surface. The competition among these three paths allowed us to
gather information about the chemical properties of such radical
cations, a deduction not able to be obtained in the presence of
oxygen as electron trap, whereby sulfoxide was the principal
product, owing to the efficient triplet oxygen trapping of sulfur
centered radical cations [9–12].
Given the importance that the knowledge of the chemistry of
sulfur radical cations may have both for the practical and
theoretical aspects as well as for important biological implications
[13], in this paper we have extended the study to radical cations
produced by heterogeneous electron transfer oxidation of benzyl
2
in the absence of oxygen, have been found to give: (i)
Our research group has been involved for a long time in the
mechanistic investigation of TiO (as powder, colloid or photo-
anode) sensitized photooxidation of organic compounds. In
particular, with TiO as powder dispersed in deaerated CH CN
and in the presence of a sacrificial electron acceptor (Ag ), detailed
studies have been performed, by product analysis and/or kinetic
approach, on the photooxidation of several organic substrates,
especially benzylic derivatives [1–8]. This investigation allowed us
to collect information about the chemical properties of the radical
cation intermediates in heterogeneous medium, produced through
a
deprotonation process, (ii) benzyl
2
2
2
3
+
the electron transfer from the substrate to the photogenerated
+
hole, (TiO
2
)
h
.
In this context, we investigated some aspects concerning the
reactivity of benzyl phenyl sulfide radical cations (having general
0
+ꢁ
formula Ar SCH
2
Ar ) [3]. These intermediates, generated by
methyl sulfides (X-C
CF ) and analogous benzyl thiols (X-C
X = H; 6, X = 3-NO ) photosensitized by TiO
6
H
4
CH
2
SCH
3
: 1, X = 4-OCH
CH SH: 4, X = 4-OCH
in deaerated CH CN. In
3
; 2, X = H; 3, X = 4-
3
6
H
4
2
3
; 5,
2
2
3
*
Corresponding author at: Università di Perugia, Dipartimento di Chimica,
particular, the oxidation of the last class of compounds has
important applications, especially in synthetic field, and has
Biologia e Biotecnologie, Perugia 06123, Italy.
E-mail address: tiziana.delgiacco@unipg.it (T. Del Giacco).
http://dx.doi.org/10.1016/j.jphotochem.2016.03.025
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

160
M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
recently received remarkable attention [13], but so far only few
papers are known about the oxidation of thiols occurring through
an electron transfer mechanism [14,15].
2.3. Competitive experiments
For the heterogeneous TiO
kinetic experiments were performed at 15 C by irradiating (as
2
sensitized photooxidation, the
ꢂ
The aim of this work is to assess the role of the substrate
adsorption on TiO
2
surface on the reactivity of benzyl tioethers and
3 2
above) the mixtures in 10 ml of CH CN containing TiO and the
thiols, in relation to the nature of the substituent on the aromatic
ring, taking into account the high incidence of the adsorption
found on the behavior of analogous benzyl methyl ethers and
scavenger (in the same amounts used before) and the two
ꢀ3
substrates (sulfides or thiols) both 5.0 ꢃ10 M. For the homoge-
+
ꢀ
neous MeOP PF
6
sensitized photooxidation, the two substrates
ꢀ
3
ꢀ3
benzyl alcohols derivatives (X- C
respectively, with various substituents) in the photooxidation by
TiO [4,5]. Accordingly, the results collected by product analysis
H
6 4
2
CH OCH
3
and X- C
6
H CH
4 2
OH,
(5.0 ꢃ10 M each) and the sensitizer (5.0 ꢃ10 M) were dis-
3
solved in 10 ml of CH CN. The resulting solution was irradiated as
2
above. The krel values, determined by a suitable kinetic equation
[20] were the average of at least three determinations. The average
error was estimated to be ca. 10%.
and competitive experiments were compared with those obtained
in homogeneous medium for the electron transfer oxidation of the
same benzyl methyl sulfides (recently reported [16]) and benzyl
+
ꢀ
thiols studied in this work photosensitized by MeOP PF
deaerated CH CN.
DFT calculations at the B3LYP/6-311G(d,p) level of theory have
6
in
2.4. Reaction product analysis
3
In both homogeneous and heterogeneous experiments the
+ꢁ
+ꢁ
+ꢁ
been also carried out for 4 , 5 and 6 , in order to get information
on the geometry of the radical cations and more importantly on
their charge and spin distribution.
reaction mixture was analyzed after irradiation by GC, GC–MS, and
1
H NMR, adding an internal standard (bibenzyl) in the case of
quantitative analysis. All products formed were identified by
comparison with authentic specimens. Benzaldehydes 1a–3a
(
4
Scheme 1) were commercially available. Dibenzyldisulfides
a–6a (Scheme 3) and 4,4 -dimethoxydibenzyl sulfide were
2. Experimental
0
synthetized and characterized as already reported in the literature
21,22].
GC analyses were carried out on an Agilent 6850 Series II gas-
[
chromatograph using a HP-1 capillary column. 1H NMR spectra
(
(
CDCl
3
with TMS as internal standard) were run on a Bruker AC 200
2
.5. Cyclic voltammetry
200 MHz) spectrometer. GC–MS analyses were performed on a
Hewlett Packard 6890A gas-chromatograph (HP-Innovax capillary
column) coupled with a MSD-HP 5973 mass selective detector
E
p
values were obtained by cyclic voltammetry experiments in
CH
a
1
3
CN–LiClO (0.1 M), conducted with a potentiostat controlled by
4
(
70 eV). E
p
values were obtained with an Amel 552 potentiostat
programmable function generator (cyclic voltammetry at
00 mV s , 1 mm diameter platinum disc anode and SCE as
controlled by a programmable Amel 568 function generator.
ꢀ1
reference).
2.1. Materials
2
.6. Computational methodology
Benzyl thiols 4–5 (purity > 97%), TiO
and P-25, Degussa-99,5%, both dried at 110 C) and CH
2
(anatase, Aldrich-99.9%
ꢂ
Quantum mechanical calculations were carried out by using the
3
CN (HPLC
grade) were commercial samples and were used as received.
Benzyl methyl sulfides 1-3 were prepared by reaction of the
corresponding benzyl thiols with CH I in ethyl alcohol according to
3
Gaussian 09 package [23]. Charge and spin density distribution of
the radical cations were obtained by using the B3LYP functional,
after geometrical optimization performed with the same DFT
model. All calculations were performed with a 6-311G(d,p) basis
set [24].
a literature procedure [17]. The sulfides were characterized as
already described [16]. N-methoxyphenanthridinium hexafluor-
+
ꢀ
6
) was prepared according to a literature
ophosphate (MeOP PF
3
. Results and discussion
procedure [18,19].
3
.1. Photooxidation of benzyl methyl sulfides
2.2. Photochemical oxidation
The TiO
–3 in argon deaerated CH
electron acceptor, produced the corresponding benzaldehydes
1a–3a) as the only products coming from the benzyl moiety
Scheme 1). The percentage yields of the remaining reactants and
products formed after 1 h of irradiation, referred to the initial
amount of substrate, are reported in Table 1.
The material recovery was in the range 83–87%. The sulfur-
containing product, supposedly CH SH (see below), was not
3
2
photosensitized oxidation of benzyl methyl sulfides
CN, in the presence of Ag SO as an
For the heterogeneous sensitized photooxidation, 10 ml of
1
3
2
4
ꢀ2
CH
3
CN containing the substrate (sulfide or thiol 1.0 ꢃ10 M),
ꢀ2
TiO
2
and the scavenger (75 mg of anatase/Ag
2
SO
4
1.0 ꢃ10 M or
(
(
ꢀ2
1
7 mg of P25/Na
2
S
2
O
8
1.0 ꢃ10
M with sulfide and thiol,
respectively) were stirred for 30 min at room temperature in
the dark. The mixture was then externally irradiated in an Applied
Photophysics multilamp apparatus with six phosphor-coated
fluorescent lamps (15 W each) emitting at 355 nm (
D
l
1/2 = 20 nm),
ꢂ
at running water temperature (15 C), with argon bubbling
through the solution. Before analysis of products, the semicon-
ductor powder was left to decant for ca. 20 min. For the
detected in the reaction mixture because of its high volatility.
+
ꢀ
homogeneous MeOP PF
6
sensitized photooxidation, the sub-
ꢀ
2
ꢀ3
strate (1.0 ꢃ10 M) and the sensitizer (5.0 ꢃ10 M) were
3
dissolved in 10 ml of CH CN and the resulting solution was
irradiated with the same equipment used in heterogeneous
conditions. Blank experiments, carried out by irradiating the
+
ꢀ
solutions in the absence of the sensitizer (TiO
not show product formation in any cases.
2 6
or MeOP PF ), did
Scheme 1. Products formed in the photooxidation of sulfides 1–3 by TiO2.

M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
161
Table 1
Chemical yields, relative rates (k
X
/k
H p
) and substrates oxidation potentials (E ) in the
2
TiO sensitized photooxidation reaction of benzyl methyl sulfides 1–3 in
3
Ar-saturated CH CN.
Unreacted sulfide a (%)
4X-C
H
4
CHOa (%)
k
/k
b
P
E V
,c
Sulfide
6
X
H
1
2
3
(X = OCH
(X = H)
3
)
53
63
66
30
25
21
3.6
1.0
0.41
1.55
1.63
1.73
(X = CF
3
)
a
With respect to the initial amount of substrate, after 1 h of irradiation;
ꢀ
2
ꢀ2
[
sulfide] = 1.0 ꢃ10 M, [anatase TiO
2
] = 7.5 g/l, [Ag
2
SO
4
] = 1.0 ꢃ10 M.
b
Determined by kinetic competitive experiments (GC analysis of unreacted
ꢀ3
substrate); [sulfide] = 5.0 ꢃ10 M; the average error is ca. 10%.
c
3
Oxidation peak potential vs SCE in air-equilibrated CH CN [16].
The ratio of the photooxidation rate constant of
-X-C CH SCH (k ) on that of the unsubstituted sulfide (k
was determined by the kinetic competitive method (data in
Table 1). As shown in Fig. 1, log(k /k ) values decrease linearly as
the sulfide oxidation potential (E , Table 1) increases with good
4
6
H
4
2
3
X
H
)
X
H
p
correlation coefficient (R = 0.986). In other words, the reaction rate
depends on the nature of the ring substituent, being increased by
an electron donating group and depressed by an electron
withdrawing substituent.
This behavior and the significantly high value of the slope
ꢀ
1
(
ꢀ5.2 V ) are in line with a single electron transfer (SET) from the
+
substrate to the positive hole, (TiO
2 h
) , as rate determining step, to
give the corresponding radical cation (Scheme 2). This reaction is
+
thermodynamically feasible as the reduction potential of (TiO
2.4 V vs SCE in CH CN [25]) is much higher than that of 1–3 (1.55–
.73 V, Table 1). Actually, 5.2 V
observed for other functional groups (being in the range ꢀ2 to
2 h
)
Scheme 2. Mechanism of the photooxidation of sulfides 1–3 by TiO2.
(
1
3
ꢀ
1
is a value higher than that
formed by deprotonation of the intermediate radical cation, is the
only observed product [16]; this was endorsed by competitive
ꢀ1
ꢀ
3 V [4,5]) for which the slope values had been related to a
kinetic and flash photolysis experiments. Also in that case the plot
substrate like transition state as expected for a slightly exergonic
electron transfer step [26]. Therefore, in the case of benzylic
thioethers, an electron transfer with a more radical cation ꢀlike
transition state should even more hold.
As reported in Scheme 2, it is reasonable that the radical cation
undergoes a deprotonation at the benzylic carbon, as observed for
other benzylic cation radicals (produced from alcohols, ethers,
silanes and diols [1,2,4–6,8]). The formed benzylic radical is
oxidized to carbocation by a second positive hole, as showed in the
photoelectrochemical oxidation of benzylic alcohols and ethers at
ꢀ1
log(k /k
X H
) vs. E
p
was linear, with slope of ꢀ3.8 V , a value close to
that obtained in heterogeneous phase. Therefore, such similar
results support the electron transfer as primary step in the
2
photooxidation of benzyl thioethers with TiO .
On the basis of the similar pattern observed in both
heterogeneous and homogeneous environment the adsorption
phenomena does not seem to play a significant role on the
reactivity of benzyl methyl sulfides, contrary to what reported for
the TiO photosensitized oxidation of various benzyl alcohols and
2
ethers [4,5]. Indeed, with both classes of compounds not all the
substituted derivatives were linearly correlated in the plot log
2
TiO photoanodes [27,28], which yields benzaldehyde.
Our recent mechanistic study on the homogeneous electron
transfer oxidation of the same benzyl thioethers photosensitized
x H p
k /k vs. E ; interestingly, all the substrates with strong electron-
withdrawing substituents showed a similar reactivity, higher than
that expected. This phenomenon was ascribed to the involvement
of a single electron transfer from the lone pair orbital of the OR
by
N-methoxyphenanthridinium
hexafluorophosphate
+
ꢀ
(
MeOP PF
6
3
) in deaerated CH CN showed that the benzaldehyde,
group (the preferential site of adsorption on TiO
2
surface), a
process kinetically little affected by the substituent effect, rather
than from the aromatic ring, whose oxidizability is depressed by
strong electron-withdrawing substituents. Taking into account
that the adsorption of the OR group occurs through the interaction
0
0
0
0
.6
.4
.2
.0
2
between the oxygen lone pair and the hydroxyl group on TiO via
hydrogen bonding [5], it is obvious that such interaction is
minimized with sulfides. This is in line with the known poor ability
of dialkylthioethers (C ꢀꢀ S ꢀꢀ C) as acceptor of hydrogen bonds,
given the considerable polarizability of S and the negligible
electronegativity difference between C and S atoms [29].
-
-
0.2
0.4
1
.55
1.60
1.65
E , V
1.70
1.75
P
Fig. 1. Plot of log(k
sulfides 1-3 in CH
X
/k
CN.
H
) vs. E
p
for TiO
2
photosensitized oxidation of benzyl methyl
3
Scheme 3. Products in the photooxidation of thiols 4–6 by TiO2.

162
M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
3
.2. Photooxidation of benzyl thiols
0
0
0
.4
.2
.0
Upon irradiation of an Ar-saturated CH
3
CN mixture of TiO
as electron
scavenger, the corresponding X-substituted dibenzyldisulfides
a–6a were produced (Scheme 3). Because of the low reactivity
observed for thiols with anatase TiO , these substrates were
oxidized by using the more efficient P25 TiO . The data obtained by
2
in
2 2 8
the presence of benzyl thiols (4–6) and Na S O
4
2
2
quantitative analysis after 1 h of irradiation are shown in Table 2.
The material recovery, considering the moles of unreacted
substrate plus those of disulfide (after doubling), was in the range
-
-
0.2
0.4
91–96% (Table 2).
The redox reaction of persulfate as electron scavenger [30] is
the following (Eq. (1)).
1
.55
1.60
1.65
1.70
E , V
Þeꢀ þ 1=2S
2ꢀ
2ꢀ
P
ðTiO
2
2
O
8
! TiO
2
þ SO
4
ð1Þ
Fig. 2. Plot of log(k
oxidation of benzylic thiols 4–6 in CH
X
/k
H
) vs. E
p
2
for TiO (
6ꢀ
) and MeOP+ PF (&) photosensitized
The electron scavenger Ag
2
SO
4
was not used as in the case of TiO -
2
3
CN.
photooxidation of benzyl methyl sulfides, because in the presence
+
of Ag a scarce recovery of material was detected, probably due to
+
6ꢀ
in deaerated CH
MeOP PF
3
CN. After 30 min of irradiation,
the adsorption of thiol on the particles of the silver salt. It has to be
noted that the same photoproducts were obtained by use of both
electron scavengers.
disulfides were formed with all thiols in high yields (35–50%,
see Table 3), considering that the oxidant moles are half of those of
the thiol. Only in the case of
4 a significant amount of
The kinetics of TiO
investigated. In particular, competitive experiments gave the
relative rate constants, expressed as ratio of k (rate constant of
X-C CH SH) to k (rate constant of C CH SH). An interesting
2
photooxidation of thiols was also
0
4
,4 - dimethoxydibenzyl sulfide (9%) was also detected in the
reaction mixture. Other detected products were those derived
X
+
from the N ꢀꢀ O fragmentation of MeOP (phenanthridine, P, and
6
H
4
2
H
H
6 5
2
methanol) The material recovery was nearly quantitative
observation is that, as pointed out for thioethers, the reaction rate
is affected by the nature of the ring substituent; in fact, it increases
with the electron donating efficiency of the group. A good linear
(
92–99%).
The relative reaction rates (k
good linear correlation in the plot log k
/k
X H
) are reported in Table 3. The
/k vs. E shows a slope of
3.9 V similar to that observed in heterogeneous phase (Fig. 2,
X
H
p
X H p
correlation between log k /k and E , the peak oxidation potentials
ꢀ1
ꢀ
measured in this work (Table 2, see Section 2), was observed (Fig. 2,
black line). This evidence endorses that divalent sulfur of thiols is
not a good hydrogen-bond acceptor [29], contrary to what is
observed with the corresponding ring substituted benzyl alcohols
ꢀ
1
red line), with a slope of ꢀ3.3 V , close to values obtained for TiO
2
-
sensitized oxidation of other organic substrates [4,5] and similar to
that obtained with thioethers 1–3 (ꢀ5.2, see above). Therefore,
taking into account of the favorable thermodynamics of the SET
process, also for the oxidation of benzyl thiols photocatalyzed by
which showed a clear implication of the adsorption in the TiO
photooxidation through the hydrogen-bond formation between
the oxygen lone pair and the hydroxyl group on TiO [4]. In the light
of these results, the operative mechanism for the disulfide
formation from thiols 4–6 in homogeneous phase can be assumed
to be the one shown in Scheme 4. Unfortunately, as reported for
thioethers [16], the intermediate radical cation was not detected by
flash photolytic experiments because unstable, due to the high
acidity, under the reported experimental conditions.
2
2
TiO
2
is plausible to assume the formation of a thiol radical cation as
first step of the process (Scheme 4). The final products (disulfides
4
a–6a) come from the dimerization of benzyl thiyl radicals
ꢁ
(
X-C
6
H CH
4 2
S ) [14], formed by the deprotonation of the SH group
of the thiol radical cation, being supposedly the hydrogen bound to
the sulfur more acid than the benzylic one. Actually, it is reported
that the hydrogen sulfide radical cation, the intermediate
identified by ESR spectroscopy starting from hydrogen sulfide, is
0
The formation of 4,4 -dimethoxydibenzyl sulfide observed only
+
in the MeOP photooxidation of 4 implicates a competitive reaction
+ꢁ
a strongly acid species, being pK
(H
a 2
S
) equal to ꢀ23 (calculated at
ꢂ
2
5 C on the basis of a thermochemical cycle [15]) much smaller
than that of deprotonation of benzylic derivatives (for example,
pK of 4-methoxytoluene radical cation is 0.45 [31]).
The adsorption phenomena were also evaluated for thiols by
a
comparing the results in heterogeneous phase with those
obtained, by product analysis and kinetic competitive experi-
ments, for the homogeneous oxidation of 4–6 photosensitized by
Table 2
Chemical yields, relative rates (k
sensitized photooxidation reaction of benzyl thiols 4–6 in Ar-saturated CH
X
/k
H
) and peak oxidation potentials (E
p
) in the TiO
CN.
2
3
Unreacted thiola (%) (X-C
S)
2
a
(%)
k
/k
b
P
E V
,c
Thiol
6
H
4
CH
2
X
H
4
5
6
(X = 4-OCH
(X = H)
3
)
80
83
90
12
8
6
1.7
1.0
0.69
1.55
1.61
1.67
(X = 3-NO
2
)
a
With respect to the initial amount of substrate after 1 h of irradiation; the
ꢀ
2
disulfide yields were doubled; [thiol] = 1.0 ꢃ10 M, [P25 TiO
2
2 2 8
] = 1.7 g/l, [Na S O ] =
ꢀ
2
1
.0 ꢃ10 M.
b
Determined by kinetic competitive experiments (GC analysis of unreacted
ꢀ3
substrate); [thiol] = 5.0 ꢃ10 M; the average error is ca. 10%.
c
Oxidation peak potential vs SCE in air-equilibrated CH
3
CN.
Scheme 4. Mechanism of the photooxidation of sulfides 4–6 by TiO2.

M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
163
Table 3
+
6ꢀ
Chemical yields, relative rates (k
X
/k
H
) and substrates oxidation s (E
p
) in the MeOP PF photoinduced photooxidation reaction of benzyl thiols 4–6 in Ar-saturated CH
3
CN.
Unreacted thiola (%)
(X-C
H
4
CH
a
(%)
(X-C
H
4
CH
)
2
Sa (%)
b
Thiol
6
2
S)
2
6
2
X H
k /k
4
5
6
(X = 4-OCH
(X = H)
3
)
35
51
44
53
48
48
9
–
–
1.5
1.0
0.51
(X = 3-NO
3
)
a
ꢀ2
+
ꢀ] = 5.0 ꢃ10ꢀ3 M.
With respect to the initial amount of substrate after 30 min of irradiation; the disulfide and sulfide yields were doubled; [thiol] = 1.0 ꢃ10 M, [MeOP PF
Determined by kinetic competitive experiments (GC analysis of unreacted substrate); [thiol] = 5.0 ꢃ10ꢀ3 M; the average error is ca. 10%.
6
b
path with the dimerization of the thiyl radical. Likely, a reaction
mechanism could be that shown in Scheme 5, where
4
-CH
3
O ꢀꢀ C
6 4 2
H CH S is firstly oxidized to cation by phenanthridine
+
ꢁ
radical cation P , which is the effective oxidant (Ered = 2.0 V vs SCE
+
in CH
18]). Afterwards the thiyl cation loses sulfur giving a benzyl cation
3
CN, produced by dissociation of MeOP singlet excited state
[
stabilized by the electron donating effect of 4-methoxy substitu-
ent, which attacks the nucleophilic sulfur of another thiol molecule
0
giving the final product 4,4 -dimethoxydibenzylsulfide [15].
2
Likely, this product is not formed by 4 in the TiO sensitized
oxidation because of the more favorable dimerization of benzyl
thiyl radicals due to the relatively higher concentration of these
intermediates close to the TiO
2
surface [1].
Fig. 3. Definition of geometrical features of radical cations 4+ꢁ and 5+ꢁ (left) and 6+ꢁ
An interesting observation regards the smaller difference of the
oxidation potentials between 6 and 4 (corresponding to 0.12 V, see
Table 2) with respect to that between the corresponding benzyl
(right) reported in Table 4.
Table 4
alcohols (3-NO
calculated by oxidation potentials of 2.80 V [32] and 1.55 V [5] vs
SCE in CH CN, respectively. This weaker substituent effect is in
2
ꢀꢀ C H
6 4
2 6 4 2
CH OH and 4-MeO ꢀꢀ C H CH O) of 1.25 V,
Most significant dihedral angle (deg) and bond lengths (Å) of radical cations 4+ –6
ꢁ
+ꢁ
3
in CH CN (CPCM model) optimised by B3LYP/6-311G(d,p).
3
F
d1
d2
d3
d4
d5
d6
d7
agreement with the presence of two possible electron extraction
sites in the thiol structure: the aromatic ring, as for benzyl alcohols,
and the sulfur atom. In order to deepen this aspect, DFT
calculations were performed; the geometrical features, such as
dihedral angles and bond lengths (defined in Fig. 3) of radical
4+
5
6
ꢁ
95.1
95.3
94.8
1.349
1.351
1.352
1.857
1.847
1.841
1.488
1.492
1.498
1.422
1.419
1.414
1.370
1.377
1.377
1.424
1.409
1.399
1.313
1.083
1.482
+
ꢁ
ꢁ
+
cations 4+ ꢀ6 are reported in Table 4.
ꢁ
+ꢁ
From these data, it can be observed that the conformations of
the radical cations have very similar geometries. In all optimized
Table 5
NPA Charges (qNPA) and Mulliken Spin Densities for the Most Stable Conformers of
Radical Cations 4+ ꢀ 6
ꢁ
+ꢁ
.
conformations, the CH
phenyl ring (the dihedral angle
2
ꢀꢀ S bond is almost perpendicular to the
ꢂ
F
is ca. 95 ). The atomic charges
X
C
6
H
4
CH
2
S
H
were obtained by natural population analysis (NPA) [23], while the
unpaired electron spin densities were calculated using the
Mulliken population analysis. From the NPA charges and spin
density data obtained for the thiol radical cations (Table 5), it can
+ꢁ
4
q
spin
NPA
0.080
0.175
0.629
0.625
0.073
0.098
0.226
0.124
ꢀ0.005
ꢀ0.021
5+
ꢁ
q
NPA
0.420
0.521
0.130
0.300
0.507
0.150
ꢀ0.014
spin
ꢀ0.014
+ꢁ
+ꢁ
+ꢁ
be noted that the charge and spin density of 4 , 5 and 6 are
mainly delocalized between the ring and sulfur, ie. the two
electron extraction sites of the thiol. In other words, such
6+
ꢁ
qNPA
spin
ꢀ0.353
0.644
0.400
0.149
0.395
0.632
0.163
ꢀ0.018
0.000
ꢀ0.014
distribution in the radical cation involves
a through-space
interaction between the aromatic ring and the sulfur p orbital,
favored from the conformational geometry, as already observed for
significantly on the sulfur atom (from 0.098 to 0.395 and from
.226 to 0.632, respectively) at the expense of the substituted ring
X-C (from 0.709 to 0.291 and from 0.800 to 0.400, respectively)
0
6 4 2 3
thioethers of analogous structure (X-C H CH SCH ) [16]. In
6 4
H
particular, the positive charge and spin density increase
on going from electron donor to electron withdrawing ring
Scheme 5. Oxidation mechanism of 4-methoxybenzyl thiyl radical by P+
ꢁ
.

164
M. Bettoni et al. / Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 159–164
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6
H
4
2
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(
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Benzyl methyl sulfides (1–3) and thiols (4–6) produced
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2
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Acknowledgments
The authors thank MIUR (Ministero dell’Universitàe della
Ricerca, Rome, Italy) and University of Perugia (PRIN 2010–2011,
no. 2010FM738P) for funding.
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