Oxidation of Aromatic Sulfides Photosensitized by TiO2 in CH3CN in the Presence of Ag2SO4. The Role of TiO2 in the Chemistry of Sulfide Radical Cations

Article abstract of DOI:10.1021/jo9615559

A product study of the photooxidation of the aromatic sulfides 1 - 5 sensitized by TiO₂ in MeCN, in the presence of Ag₂SO₄, has been carried out. With benzyl phenyl sulfide (1), 4-methoxybenzyl phenyl sulfide (2), and phenethyl phenyl sulfide (4), the photooxidation led mainly to aldehydic products, benzaldehyde, 4-methoxybenzaldehyde, and phenylacetaldehyde, respectively. With benzyl 4-methoxyphenyl sulfide (3), the major product was the corresponding sulfoxide. Diphenyl sulfoxide was the only product observed in the oxidation of diphenyl sulfide (5). These results suggest α-C-H deprotonation as the major reaction path for the radical cations of 1, 2, and 4, and an active role in this respect of the oxygenated basic sites present on the TiO₂ surface is envisaged. An interaction of these oxygenated centers with the sulfur atom of the adsorbed radical cation is also suggested to explain the remarkable finding that substantial amounts of sulfoxide are observed with 3 and 5, in spite of the fact that the photooxidations were carried under nitrogen. Different than with the sulfides, the photooxidation of 4-methoxybenzyl phenyl sulfoxide leads exclusively to products of C - S bond cleavage, which is attributed to the much greater stability of sulfinyl than thiyl radicals.

Full text of DOI:10.1021/jo9615559

J . Org. Chem. 1997, 62, 4015-4017
4015
2 3
Oxid a tion of Ar om a tic Su lfid es P h otosen sitized by TiO in CH CN
in th e P r esen ce of Ag SO . Th e Role of TiO in th e Ch em istr y of
2 4 2
Su lfid e Ra d ica l Ca tion s
,
†
‡
‡
,‡
Enrico Baciocchi,* Tiziana Del Giacco, Marina I. Ferrero, Cesare Rol,* and
Giovanni V. Sebastiani*,
§
Dipartimento di Chimica, Universit a` “La Sapienza”, Piazzale A. Moro, 00185 Roma, Italy,
Dipartimento di Chimica, Universit a` di Perugia, Via Elce di Sotto, 06100 Perugia, Italy, and Istituto per
le Tecnologie Chimiche, Facolt a` di Ingegneria, Universit a` di Perugia, 06100 Perugia, Italy
Received August 9, 1996X
A product study of the photooxidation of the aromatic sulfides 1-5 sensitized by TiO
2
in MeCN, in
the presence of Ag SO , has been carried out. With benzyl phenyl sulfide (1), 4-methoxybenzyl
2
4
phenyl sulfide (2), and phenethyl phenyl sulfide (4), the photooxidation led mainly to aldehydic
products, benzaldehyde, 4-methoxybenzaldehyde, and phenylacetaldehyde, respectively. With
benzyl 4-methoxyphenyl sulfide (3), the major product was the corresponding sulfoxide. Diphenyl
sulfoxide was the only product observed in the oxidation of diphenyl sulfide (5). These results
suggest R-C-H deprotonation as the major reaction path for the radical cations of 1, 2, and 4, and
2
an active role in this respect of the oxygenated basic sites present on the TiO surface is envisaged.
An interaction of these oxygenated centers with the sulfur atom of the adsorbed radical cation is
also suggested to explain the remarkable finding that substantial amounts of sulfoxide are observed
with 3 and 5, in spite of the fact that the photooxidations were carried under nitrogen. Different
than with the sulfides, the photooxidation of 4-methoxybenzyl phenyl sulfoxide leads exclusively
to products of C-S bond cleavage, which is attributed to the much greater stability of sulfinyl
than thiyl radicals.
In tr od u ction
oxybenzyl phenyl sulfoxide (6), was also investigated. The
photochemical oxidation of sulfides induced by TiO has
been previously studied, but always with oxygen as the
2
The heterogeneous photochemical oxidations of organic
compounds sensitized by TiO , as a powder suspended
2
electron trap.5
-8
Under these conditions, sulfoxides are
often the exclusive products owing to the efficient triplet
in MeCN, involve the transfer of one electron from the
substrate to the hole photogenerated into the semicon-
ductor; a radical cation forms, and at the same time the
electron in the conduction band is captured by a suitable
acceptor, generally molecular oxygen.1
9
oxygen trapping of sulfur centered radical cation. More-
•-
over, O
cation.
2
is also formed, which can react with the radical
Recently, we found that oxygen can conveniently be
Resu lts a n d Discu ssion
+
replaced by Ag as electron trap in the photooxidation
of alkylaromatic compounds.2,3 Under these conditions,
In a typical experiment, a deaerated solution of the
substrate in MeCN was subjected to external irradiation
by a 500 W high-pressure mercury lamp, with a Pyrex
the nature of the products could be unambiguously
related to the various decomposition paths followed by
filter, in the presence of TiO
bubbling of N . During the irradiation, Ag was formed
presumably deposited on TiO ) but without any conse-
2 2 4
and Ag SO , under gentle
the radical cations, which is not possible when working
in the presence of oxygen.1,4
2
(
2
It has been considered of interest to extend this
approach to aromatic sulfides, in order to get insight on
the 3-fold reactivity of aromatic sulfide radical cations
3
quence for the photochemical process. The quantitative
analysis of the reaction products and unreacted sub-
1
strates was performed by H-NMR and/or by GC. In all
R
(C -H deprotonation, C-S bond cleavage, and attack at
cases, in order to avoid overoxidation, the conversion into
products was kept below 40%. The material balance is
satisfactory, ranging from 85 to 95% with respect to the
substrate.
The product distributions observed in the reaction of
-6 are displayed in Table 1. Even though the efficiency
of the process is generally quite modest, the results
provide us with interesting information about the proper-
ties of sulfides radical cations when generated under
sulfur) under the heterogeneous reaction conditions of
this photochemical process. Thus, we now report on the
photooxidation of benzyl phenyl sulfide (1), 4-methoxy-
benzyl phenyl sulfide (2), benzyl 4-methoxyphenyl sulfide
(
3), phenethyl phenyl sulfide (4), and diphenyl sulfide (5),
1
catalyzed by TiO in the presence of Ag SO in deaerated
2
2
4
MeCN. The photooxidation of a sulfoxide, namely 4-meth-
†
Universit a` “La Sapienza”.
Dipartimento di Chimica, Universit a` di Perugia.
Facolt a` di Ingegneria, Universit a` di Perugia.
Abstract published in Advance ACS Abstracts, May 1, 1997.
‡
§
(5) Davidson, R. S.; Pratt, J . E. Tetrahedron Lett. 1983, 24, 5903-
X
5906.
(
1) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341-357.
(6) Fox, M. A.; Abdel-Wahab, A. Tetrahedron Lett. 1990, 31, 4533-
4536.
(2) Baciocchi, E.; Rol, C.; Rosato, G. C.; Sebastiani, G. V. J . Chem.
Soc., Chem. Commun. 1992, 59-60.
3) Baciocchi, E.; Rol, C.; Sebastiani, G. V.; Taglieri, L. J . Org. Chem.
994, 59, 5272-5276.
4) Baciocchi, E.; Rol, C.; Rosato, G. C.; Sebastiani, G. V. Tetrahedron
Lett. 1992, 33, 5437-5440.
(7) Fox, M. A.; Abdel-Wahab, A. J . Catal. 1990, 126, 693-696.
(8) Fox, M. A.; Kim, Y.; Abdel-Wahab, A.; Dulay, M. Catal. Lett.
1990, 5, 369-376.
(9) Riley, D. P.; Smith, M. R.; Correa, P. E. J . Am. Chem. Soc. 1988,
110, 177.
(
1
(
S0022-3263(96)01555-1 CCC: $14.00 © 1997 American Chemical Society

4
016 J . Org. Chem., Vol. 62, No. 12, 1997
Baciocchi et al.
Ta ble 1. P r od u ct Yield s in TiO2-P h otosen sitized Oxid a tion of Ar om a tic Su lfid es a n d 4-MeOP h CH2SOP h in Dea er a ted
MeCN in th e P r esen ce of Ag2SO4a
b
products (yields, %)
b
entry
substrate
unreacted substrate, %
time, h
1
2
3
4
5
6
7
8
9
PhCH2SPh (1)
61
74
85
84
70
67
72
85
68
4
4
0.5
1
2
4
8
4
0.5
PhCHO (27)
(19)
4-MeOPhCHO (8)
PhCH2SOPh (3)
c
(2)
d
d
d
4-MeOPhCH2SPh (2)
4-MeOPhCH2OH (2)
(12)
(17)
(3)
(4)
e
4-MeOPhSCH2Ph (3)
PhCH2CH2SPh (4)
PhSPh (5)
PhCHO (5)
4-MeOPhSOCH2Ph (17)
PhCH2CH2SOPh (8)
PhCH2CHO (13)
PhSOPh (10)f
4-MeOPhCH2SOPh (6)
4-MeOPhCH2X
g
h
(X ) OH, 19; X ) NHCOMe, 8)
a
No reaction takes place in the absence of TiO2. b With respect to starting material. The average error is (1. c In the presence of 2%
H2O. d PhSSPh is also present, but it was not quantitatively determined. When MeCN was deoxygenated by the freeze-thaw technique,
PhCHO (4%) and 4-MeOPhSOCH2Ph (16%) were obtained. In MeCN deoxygenated as in footnote e PhSOPh (10%) was produced.
e
f
g
+
Acetamide comes from reaction of the carbocation (4-MeOPhCH2 ) with the solvent followed by addition of adventitious water to the
nitrilium ion. PhSSO2Ph is present in an equivalent amount.16
3 h
Sch em e 1
4-methoxybenzyl alcohol and diphenyl disulfide. The
latter compounds should derive, as suggested above, from
C-S bond cleavage in the radical cation (path b in
Scheme 1, n ) 1, Ar ) 4-MeOPh, Ar′ ) Ph). The
hypothesis that some oxidation of the alcohol to aldehyde
takes place can be ruled out since the ratio between the
two products is independent of the reaction time (entries
3
-5 in Table 1). In the photooxidation of phenethyl
phenyl sulfide (4), phenylacetaldehyde is the major
product accompanied by phenethyl phenyl sulfoxide, thus
indicating that, in this case too, path a in Scheme 1 (n )
2
, Ar ) Ar′ ) Ph) is the main reaction route of the radical
cation.
Sulfoxidation is the main reaction of 3, and of course,
the only product in that of 5. The fact that deprotonation
•
+
is only a minor reaction path of 3 has already been
observed in anodic oxidation studies, even under basic
conditions.10 A plausible explanation is that in 3 there
can be a substantial delocalization of the positive charge
in the methoxy group. This lowers the charge density
•+
at the sulfur atom and consequently decreases the C
bond acidity.
R
-H
these conditions. In the photolysis of benzyl phenyl
sulfide (1), benzaldehyde is the major reaction product
accompanied by very small amounts of benzyl phenyl
sulfoxide. Reasonably, the formation of benzaldehyde
The above results indicate that, contrary to previous
conclusions,5 deprotonation is a much more important
reaction route than C-S bond cleavage for sulfide radical
cations, when these species are generated by sensitized
-7
R
can be ascribed to the C -H deprotonation of the sulfide
radical cation10 (path a in Scheme 1, n ) 1, Ar ) Ar′ )
Ph). Accordingly, in this process a benzyl radical is
formed that can be converted into an R-hydroxy sulfide
by oxidation to carbocation and reaction of the latter with
adventitious water present in the medium. A facile
conversion of the R-hydroxy sulfide into benzaldehyde in
photolysis in the presence of TiO . This finding is very
2
surprising since, when generated electrochemically in the
•
+
•+
same solvent, 1 and 2 did not undergo any deproto-
1
0
nation, but C-S bond cleavage was the major pathway
for 1 and the exclusive pathway for 2 . This drastic
•
+
•+
difference is very interesting, as it suggests an active role
+
11
the presence of Ag is expected. No formation of benzyl
alcohol (or acetamide) is observed, which leads us to
exclude the occurrence of path b (C-S bond cleavage).12
In line with this conclusion, no diphenyl disulfide, which
should also form in path b, was found among the reaction
products.
of TiO not only in the generation of the radical cation
2
but also in its chemistry. The most reasonable hypoth-
esis is that the sulfide radical cations are generated at
the surface of TiO where deprotonation can be promoted
2
by the oxygenated basic sites (containing hydroxyl
groups).13 In other words, TiO appears to exert the role
2
Deprotonation is also the predominant path for the
intermediate radical cation formed in the photooxidation
of 2. Accordingly, 4-methoxybenzaldehyde is the main
reaction product, accompanied by small amounts of
of a rather strong base. Such an active role of TiO
2
is
confirmed by experiments carried out in the presence of
1
8
14
18
H
2
O (0.5%) showing that only ca. 60% of O is
(13) (a) Primet, M.; Pichat, P.; Mathieu, M. V. J . Phys. Chem. 1971,
7
5, 1216-1220. (b) Bredow, T.; J ug, K. J . J . Phys. Chem. 1995, 99,
(
10) Baciocchi, E.; Rol, C.; Scamosci, E.; Sebastiani, G. V. J . Org.
Chem. 1991, 56, 5498-5502.
11) Satchell, D. P. N.; Satchell, R. S. In Supplement S: The
285-291. (c) Goldstein, S.; Czapski, G.; Rabani, J . J . Phys. Chem. 1994,
98, 6586-6591. (d) Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac,
A. D.; Thurnauer, M. C. J . Phys. Chem. 1993, 97, 7277-7283. (e)
Umapathy, S.; McQillan, A. J .; Hester, R. E. Chem. Phys. Lett. 1990,
170, 128-132. (f) Boxall, C.; Kelsall, G. H. J . Chem. Soc., Faraday
Trans. 1991, 87, 3537-3545.
(
Chemistry of Sulfur Containing Groups; Patai, S., Rappoport, Z., Eds.;
J ohn Wiley and Sons: New York, 1993; pp 626-628.
(
excluded as the alcohol is stable in the medium (see reaction from 2
and 6).
12) Complete oxidation of benzyl alcohol to benzaldehyde can be
(14) The content of non- 18O-enriched H
experiments is nearly 0.02%.
2
O in MeCN used in these

Oxidation of Aromatic Sulfides
J . Org. Chem., Vol. 62, No. 12, 1997 4017
ously described.10 Phenethyl phenyl sulfide (4) was prepared
2
incorporated into the benzaldehyde. Clearly, TiO basic
from the reaction of phenethyl bromide with thiophenol as
sites are not only involved in the deprotonation process
but also in the subsequent steps leading to the R-hydroxy
sulfide.
It seems reasonable to suggest that the oxygenated
2
sites of TiO also react with the sulfur atom of the radical
cation, thus explaining the remarkable finding that
sulfoxides are formed in substantial amounts in the
reactions of 3-5, either when the irradiations were
carried out under nitrogen or when (photooxidation of 3
and 5) the solutions were previously deoxygenated by the
still more efficient freeze-thaw technique, which makes
very unlikely a reaction of the sulfide radical cation with
adventitious oxygen. Moreover, the possibility can also
9
1
described before [ H-NMR δ 7.35-7.10 (m, 10H, ArH), 3.15
(
m, 2H, CH ), 2.90 (m, 2H, CH ); MS m/ z (rel intensity) 214
2
2
+
M , 123 (100), 105, 91, 77, 65, 51, 45]. 4-Methoxybenzyl
phenyl sulfoxide came from a previous study.
17
P h otoch em ica l Oxid a tion . Reactions have been carried
out, at room temperature, by external irradiation (500 W high-
pressure mercury lamp, Pyrex filter), under magnetic stirring
2
and gentle N bubbling, of an acetonitrile solution (25 mL) of
substrate (0.3 mmol), in the presence of TiO
Ag SO (0.3 mmol); the reactor was a cylindrical flask (i.d. )
.6 cm, h ) 16 cm) equipped with a water cooling jacket and
2
(130 mg) and
2
4
1
intensive condenser. The photooxidation of 3 and 5 was also
performed in a water-cooled Schlenk tube, freezing, degassing
in the vacuum, and thawing the reaction mixture (for three
times) before the irradiation. After double paper filtration of
2
be excluded that traces of H O present in the solvent play
a significant role in the sulfoxides formation since only
negligible amounts of 18O were found in the produced
sulfoxide when the photooxidation of 5 was carried out
2
TiO , the reaction mixture was poured into water and ex-
tracted with ether. The reaction product analysis was per-
formed by 1H-NMR in the presence of an internal standard
(1,4-dimethoxybenzene or 1,2-diphenylethane). The amount
1
8
15
in MeCN to which 0.5% of H
2
O was added.
1
of unreacted substrate was determined, when possible, by H-
Finally, we have also studied the photooxidation of
NMR of the crude product or by GC analysis of reaction
mixture before workup, in the presence of an internal standard
4
-methoxybenzyl phenyl sulfoxide (6), which led to the
formation of 4-methoxybenzyl alcohol, N-(4-methoxyben-
(
9
1,2-diphenylethane). Material balance was always ca. 85-
zyl)acetamide, and diphenylthiosulfonate (Table 1, entry
5% vs the amount of starting substrate.
•
+
9
). Clearly, 6 exclusively undergoes C-S bond cleavage
Rea ction P r od u cts. The crude reaction product was
to form a benzyl carbocation and the sulfinyl radical,
chromatographed on silica gel eluting with n-hexane, n-
hexane:ethyl ether (from 9:1 to 1:1 v/v), ethyl ether, and
chloroform. The structure of isolated products (benzaldehyde,
4-methoxybenzaldehyde, phenylacetaldehyde, benzyl phenyl
sulfoxide, benzil 4-methoxyphenyl sulfoxide, 4-methoxybenzyl
phenyl sulfoxide, diphenyl sulfoxide, phenethyl phenyl sulfox-
ide, benzyl alcohol, 4-methoxybenzyl alcohol, N-(4-methoxy-
benzyl)acetamide, diphenyl disulfide, and phenyl benzeneth-
iosulfonate) was attributed by comparison with authentic
•
PhSO ; the latter, by dimerization followed by rearrange-
ment, is converted in PhSSO
2
Ph.16 A similar result has
recently been observed also in the homogeneous oxidation
of 6 by Co(III).17 The predominance of the C-S bond
cleavage path with respect to deprotonation in the
sulfoxide radical cation, an outcome opposite to that
observed with sulfides radical cations, is certainly due
•
to the much greater stability of PhSO with respect to
10,17,18
specimens, commercial or available from previous works.
•
16
PhS .
18O.
2
P h ot och em ica l Oxid a t ion in t h e P r esen ce of H
The photooxidations of 1 and 5 were also carried out in MeCN:
1
8
Exp er im en ta l Section
1H-NMR spectra were obtained on a 200 MHz spectrometer
2
H O (99.5:0.5), and the reaction mixture was analyzed by
GC-MS. From the comparison of the MS spectrum of ben-
zaldehyde obtained from 1 [MS m/ z (rel intensity) 105 (60),
06 (58), 107 (100), 108 (87)] and that of commercial benzal-
dehyde [105 (100), 106 (85), 107 (7)] a molar ratio PhCH O/
PhCH O ) 1.5 was determined. The comparison for diphenyl
sulfoxide produced from 6 [MS m/ z (rel intensity) 202 (100),
03 (14), 204 (12), 205 (2)] and that of commercial diphenyl
sulfoxide [MS m/ z (rel intensity) 202 (100), 203 (15), 204 (6),
in CDCl . GC-MS analyses were performed on an instrument
3
1
equipped with a 20 m × 0.2 mm silica capillary column coated
18
with 5% diphenyl- and 95% dimethylpolysiloxane from 45 to
16
3
00 °C, connected with a mass selective detector at 70 eV. GC
analyses were carried on an identical capillary column from
5 to 280 °C. The water amount in MeCN was determined
with a Karl Fischer coulometer.
Ma ter ia ls. TiO (anatase, dried at 110 °C), Ag
2
4
18
2
05 (1)] showed that only 5% of the original H
incorporated in the former compound.
2
O was
2
2
SO
O (91 atom % O), and diphenyl sulfide
were commercially available. Benzyl phenyl sulfide (1),
-methoxybenzyl phenyl sulfide (2), and benzyl 4-methoxy-
phenyl sulfide (3) were prepared and characterized as previ-
4
, MeCN
1
8
18
(
HPLC grade), H
2
Ack n ow led gm en t. This work has been carried out
with the contribution of the Ministry of University and
Technological Research (MURST) and the National
Council of Research (CNR).
4
(
15) After this addition, ca. 96% of the water present in the solvent
O since our MeCN already contained 0.02% H O (Karl Fischer
2 2
18
is H
coulometric titration).
16) Kice, J . L. In Free radicals; Kochi, J . K., Ed.; J ohn Wiley and
Sons: New York, 1973; Vol. 2, pp 715-718.
17) Baciocchi, E.; Lanzalunga, O.; Marconi, F. Tetrahedron Lett.
994, 35, 9771-9774.
J O9615559
(
(
(18) Baciocchi, E.; Piermattei, A.; Ruzziconi, R. Synth. Commun.
1988, 18, 2167-2170.
1