TiO2-sensitised photo-oxidation mechanism of indane and some of its hetero-analogues in deaerated CH3CN

Article abstract of DOI:10.1002/poc.1086

A mechanistic study, principally based on product analysis, relative to the TiO₂-photosensitized oxidation of indane and some of its hetero-analogues, in deaerated CH₃CN and in the presence of Ag ₂SO₄, was performed. In particular: (i) 1-acetamidoindan (principal product), indene, 1-indanol and 1-indanone were obtained from indan; (ii) 5-methoxyindan gave 6-methoxyindene (principal product) and 5-methoxy-1-indanone; (iii) 2,3-dihydrobenzofuran, 2,3-dihydroindole and 2,3-dihydrobenzothiophene produced benzofuran, indole and benzothiophen (the last one accompanied by minor amounts of 2,3-dihydrobenzothiophene-1-oxide), respectively. Considering the previous studies on photo-oxidation of analogous substrates as benzylic derivatives (arenes, alcohols and ethers) and from reaction product profiles, an electron-transfer mechanism (from the substrate to the photogenerated hole) is suggested, where the radical cation intermediate should deprotonate to a benzylic radical. The carbocation obtained from the oxidation of this radical should competitively evolve to alkene, alcohol and acetamide. H₂¹⁸O labelling photo-oxidation experiments suggest that the ketone, when present, should derive from the substrate, through the alcohol as intermediate. Copyright

Full text of DOI:10.1002/poc.1086

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 359–364
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1086
TiO2-sensitised photo-oxidation mechanism of indane
and some of its hetero-analogues in deaerated CH3CN
Marta Bettoni,1 Tiziana Del Giacco,1,2 Cesare Rol1 and Giovanni V. Sebastiani
*
3**
1
`
Dipartimento di Chimica, Universita di Perugia, Via Elce di Sotto, 06123 Perugia, Italy
2
3
`
Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Universita di Perugia,Via Elce di Sotto, 06123 Perugia, Italy
Dipartimento di Ingegneria Civile ed Ambientale, Universita di Perugia, Via G. Duranti, 06125 Perugia, Italy
`
Received 8 December 2005; revised 25 March 2006; accepted 5 April 2006
ABSTRACT: A mechanistic study, principally based on product analysis, relative to the TiO2-photosensitized
oxidation of indane and some of its hetero-analogues, in deaerated CH3CN and in the presence of Ag2SO4, was
performed. In particular: (i) 1-acetamidoindan (principal product), indene, 1-indanol and 1-indanone were obtained
from indan; (ii) 5-methoxyindan gave 6-methoxyindene (principal product) and 5-methoxy-1-indanone; (iii) 2,3-
dihydrobenzofuran, 2,3-dihydroindole and 2,3-dihydrobenzothiophene produced benzofuran, indole and benzothio-
phen (the last one accompanied by minor amounts of 2,3-dihydrobenzothiophene-1-oxide), respectively. Considering
the previous studies on photo-oxidation of analogous substrates as benzylic derivatives (arenes, alcohols and ethers)
and from reaction product profiles, an electron-transfer mechanism (from the substrate to the photogenerated hole) is
suggested, where the radical cation intermediate should deprotonate to a benzylic radical. The carbocation obtained
from the oxidation of this radical should competitively evolve to alkene, alcohol and acetamide. H218O labelling
photo-oxidation experiments suggest that the ketone, when present, should derive from the substrate, through the
alcohol as intermediate. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: titanium dioxide; photo-oxidation; indane; radical cation; benzylic radical; deprotonation
INTRODUCTION
dimers,2c acetamido derivatives2d and ring functionalized
products.2d
The TiO2 (as powder) sensitised photo-oxidation reaction
in CH3CN can be a useful tool, not only to obtain some
mechanistic information about the first (kinetically
significant) steps of the photodegradation of organic
compounds as pollutants in water,1 but also to perform a
variety of substrate transformations.2 In any case, it is
known that the hole, (TiO2)hþ, captures an electron from
the organic substrate while the photogenerated electron is
transferred to a suitable acceptor (usually oxygen).2a
Regarding the second aim (functional group modifi-
cation), oxygenated products (carbonyl compounds,
carboxylic acids, sulphoxides, etc.) are generally obtained
in aerated medium;2a moreover, the reaction becomes
more efficient when Ag2SO4 (Agþ is the sacrificial
electron acceptor) is present in this medium.2b
This oxidative process also has the following synthetic
advantages: the semiconductor is inexpensive, non-toxic,
chemically stable and can be reused after filtration or
centrifugation. Moreover, a large number of organic
compounds can be oxidised due to the high reduction
potential of (TiO2)h þ (2.4 V vs. SCE).3
On the basis of the mechanistic information collected
for the photo-oxidation of benzylic derivatives, in this
paper we report on a mechanistic study, principally based
on product analysis, relative to the TiO2-photosensitized
oxidation of indane and its hetero-analogues (all with
benzylic structure) in deaerated CH3CN and in the
presence of Ag2SO4 (Scheme 1).
It must be observed that, if this silver salt is used in
deaerated medium starting from benzylic derivatives,
the process usually maintains its efficiency and less
conventional oxidation products are obtained such as
RESULTS AND DISCUSSION
The yield and distribution of the products obtained from
the photo-oxidation of compounds 1–5 are reported in the
Table 1.
In particular, in deaerated medium, 1 gives 1-
acetamidoindan, indene, 1-indanol and 1-indanone (entries
1 and 2). Taking into account the previously mechanistic
hypotheses regarding the TiO2-photosensitized oxidation
of some classes of benzyl derivatives in this medium,2d
the steps in Scheme 2 are suggested; in particular, the
*Correspondence to: C. Rol, Dipartimento di Chimica, Laboratorio di
Chimica Organica, Via Elce di Sotto 8, 06123 Perugia, Italy.
E-mail: rol@unipg.it
**Correspondence to: G. V. Sebastiani, Dipartimento di Ingegneria
`
Civile ed Ambientale, Facolta di Ingegneria, Universita di Perugia, Via
G. Duranti 93, 06125 Perugia, Italy.
`
E-mail: gseb@tech.ing.unipg.it
Contract/grant sponsor: Ministero dell’Istruzione, dell’Universita e
della Ricerca; contract/grant number: COFIN 2003.
`
Copyright # 2006 John Wiley & Sons, Ltd.
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360
M. BETTONI ET AL.
compound should derive from the nucleophilic attack of
the solvent on the carbocation through a known pathway
(path a in Scheme 3).2d
A small amount of indene is also present. The reaction
profile of this product shows a maximum in the Figure
and indene does not react after an hour when submitted to
the same reaction conditions at the same starting
concentrations as indane. These results suggest that this
cycloalkene is an intermediate obtained through a side
equilibrium from the deprotonation of the cation (path b).
The profile of indanol, another minor reaction product
(Table 1), shows a maximum (Fig. 1); this alcohol, when
submitted to the same reaction conditions as indan and at
the same initial concentration, gives, as expected,5 the
corresponding carbonyl product (1-indanone). Therefore,
indanol is a detectable intermediate that derives from
the reaction of the carbocation with H2O6 (path c in
Scheme 3), which, in any case, is present in trace amounts
on the TiO2 surface.7
Indanone is obtained from indanol through the same
mechanism previously studied for benzylic alcohols in
deaerated CH3CN.4,5 This reaction should be faster than
the photo-oxidation of indane as the alcohol is more
efficiently adsorbed at the TiO2 surface; in fact, the
adsorption constant of indanol (K ¼ 380 ꢀ 100 Mꢁ1),
determined through a Langmuir-type adsorption iso-
therm,8 is higher than that of indane, not evaluable by this
method (K < 100 Mꢁ1).
Scheme 1.
cation radical, formed from the substrate through electron
abstraction by the photogenerated hole, loses a proton
(a process promoted by oxygenated basic sites of TiO2
from the a-carbon (benzylic site) to give the free radical
10.
2e
)
This intermediate is then oxidised to the corresponding
carbocation; a second hole should be responsible for
this reaction, as observed in the photoelectrochemical
oxidation of benzylic alcohols in deaerated CH3CN
(two electrons, that is two holes, per molecule are
involved).4
The fate of the carbocation (Scheme 3) can be
suggested considering both the product analysis from 1
and the corresponding profiles of product percent versus
reaction time (Fig. 1).
The principal reaction product (Table 1) is the
acetamido derivative. The absence of a maximum in
the reaction profile (Fig. 1) gives further information: in
particular, the acetamido derivative is a reaction product
that becomes the principal one after nearly 2 h. This
It can be observed from the profile that the amide
predominates over the ketone only after 2 h; during
this time the molar amount of adsorbed water
Table 1. Product yields in the TiO2-sensitised photo-oxidation of indane and some of its hetero-analoger in deaerated CH3CN
and in the presence of Ag2SO4.
Product yield, %a
Entry
Substrate
X
Y
t, h
Unreacted substrate, %
6
7
8
9
1
2
3
4
5
6
7
8
9
1
CH2
H
1.0
3.0
2.0b
2.0c
1.0
3.0
2.0
1.0
2.0
66
42
70
56
52
44
71
69
40
4
5
22
2
3
3
6
8
11
26
11
2
3
4
5
CH2
O
NH
S
OCH3
19
37
H
H
H
10
13d
27e
a With respect to the starting material.
b In aerated medium without Ag2SO4.
c In aerated medium.
d 2,3-dihydrobenzothiophene-1-oxide, 5%, is also present.
e 2,3-dihydrobenzothiophene-1-oxide, 13%, is also present.
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J. Phys. Org. Chem. 2006; 19: 359–364

TiO2-SENSITISED PHOTO-OXIDATION OF CYCLIC COMPOUNDS
361
Scheme 2.
(a better nucleofile than CH3CN) is probably sufficient
to compete with the solvent for the capture of the
cation (that is, path c is able to compete with path a in
Scheme 3).
Y ¼ H) formed as in Scheme 2, in aerated medium reacts
very rapidly with oxygen10 instead of undergoing
oxidation to cation; the corresponding peroxy radical
then evolves to indanone (Scheme 4).
A final observation about the profile is that, if the time
is longer than 3 h, the amount of alcohol diminishes, the
ketone concentration remains practically constant and the
amide percent increases. These findings are inline with
the reversibility of the alcohol formation that, while the
reaction is going on, favours the irreversible process to
acetamide.
To acquire further mechanistic information, we have
also carried out the reaction starting from 1 in aerated
medium; the carbonyl compound (1-indanone, entry 3) is
the only product, which is expected based on the results
previously obtained from similar cyclic benzylic hydro-
carbons.9 In this medium and in the presence of Agþ, as
electron acceptor, the same product is obtained, but more
efficiently (entry 4).
In line with an electron transfer to the hole as a
kinetically significant step (Scheme 2), the reaction from
5-methoxyindan (2) is more efficient than that from 1
(compare entries 1 and 5), according to the lower
oxidation potential of
2
(Ep ¼ 1.40 V vs. SCE)
with respect to that of 1 (Ep ¼ 1.97 V). In contrast to
the reaction from 1, only two products are obtained
from 2, 5-methoxy-1-indanone and 6-methoxyindene.
6-Methoxyindene is the principal product as it is more
stable than indene (in the case of 2, the equilibrium b in
Scheme 3 from the carbocation is shifted toward the
cycloalkene). A possible explanation for the absence of
the 5-methoxy-substituted alcohol (never detected from
0.25 to 3 h) should be that: (i) the alcohol should be
oxidized more quickly than 1-indanol to the correspond-
ing indanone (compare the previously measured relative
reactivity of 4-methoxybenzyl and benzyl alcohol,
krel ¼ 245) and (ii) the alcohol amount formed should
be less than that from 1, because of competition with
alkene.
According to the previously suggested mechanistic
hypotheses on the photo-oxidation of alkylaromatic
compounds in aerated medium,2a,b the exclusive for-
mation of the ketone represents an evidence in favour of
the intermediate 10. In fact, the radical 10 (X ¼ CH2,
Scheme 3.
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J. Phys. Org. Chem. 2006; 19: 359–364

362
M. BETTONI ET AL.
Figure 1. The reaction profile (product percent vs. reaction time) of the TiO2 sensitised photo-oxidation of indane in deaerated
CH3CN and in the presence of Ag2SO4.
To confirm path c in Scheme 3, that considers
the alcohol as an intermediate even if it was never
detected, a labelling experiment was carried out in the
presence of H218O. In particular, we observed that 18O
(after correction considering the 18O of the labelled
water) is completely (100 ꢀ 7%) incorporated by
5-methoxyindanone.
It must be observed that in the 2þ. structure, two types
of non-equivalent benzylic hydrogens are present (at 1-
and 3-C) but, in this reaction, the only one removed from
the cation radical is that in the para-position (at 1-C) with
respect to the methoxy group (only 6-methoxyindene
and 5-methoxy-1-indanone are present as products,
Scheme 5).
The absence of the acetamido derivative from 2 could
be ascribed to the fact that the carbocation in Scheme 3
is more efficiently adsorbed at the semiconductor
surface (because of the more effective electronic
interaction between the p aromatic system of the
substrate, due to the presence of the electron donor
OCH3 group in the ring11) than the unsubstituted one;
therefore, the first intermediate should preferentially
interact with the adsorbed water rather than with
CH3CN (path c to indanone is favoured with respect to
path a to acetamide).
Accordingly the positive charge density in the indan
radical cation should be higher at 1-C than at 3-C, as
previously reported for 4- and 3-methoxytoluene.12
Finally, the only product obtained from 2,3-dihy-
drobenzofuran (3) and 2,3-dihydroindole (4) is benzo-
furan and indole, respectively (entries 6 and 7). Inline
with the suggested mechanism in Scheme 3, path b
should be much more favoured than a and c in both
cases because it yields a bicyclic hetero-aromatic
compound that is more stable than the alkenes obtained
from 1 and 2. Also, the aromatization product
(benzothiophene) is obtained from 2,3-dihydroben-
zothiophene (5) but it is accompanied by a minor
amount of the corresponding sulphoxide. The product
molar ratio from the reaction of 5 is nearly independent
of the reaction time (entries 8 and 9). Two parallel
reactions, benzylic deprotonation and sulphoxidation2e
from the radical cation (both induced by the basic
centres of TiO2), should compete (Scheme 6) because a
high positive charge density should be localized at the
sulphur atom.13
Scheme 4.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 359–364

TiO2-SENSITISED PHOTO-OXIDATION OF CYCLIC COMPOUNDS
363
Scheme 5.
Scheme 6.
It must be observed that this reaction can represent a
suitable method to aromatize some hetero-cyclic com-
pounds by dehydrogenation in mild experimental
conditions (Eq. 1).
(HP-Innovax capillary column, 15 m) coupled with a
MSD-HP 5973 mass selective detector (70 eV).
GC analyses were carried out on a HP 5890 gas-
chromatograph (HP-Innovax capillary column, 15 m).
ð1Þ
Materials
TiO2 (anatase, Aldrich, 99.9%, dried at 110 8C), CH3CN
(HPLC grade, water content 0.02% from Karl–Fischer
coulometry), Ag2SO4, indan, 5-methoxyindan, 2,3-
dihydrobenzofuran, 2,3-dihydrobenzothiophene, 2,3-
dihydroindole, 1-indanone, 5-methoxy -1-indanone,
1-indanol, indene, benzofuran, benzothiophene and
indole were commercial samples.
CONCLUSION
From the product distribution and reaction product
profiles, the reaction mechanism of TiO2 sensitized
photo-oxidation of indane and its hetero-analogues in
deaerated CH3CN has been proposed. This has been
possible as: (i) besides the carbonyl compound (typical of
the reaction in the presence of oxygen) peculiar reaction
products (acetamido derivative, alcohol and alkene) are
formed; (ii) some mechanistic informations had been
previously obtained in the study of the photo-oxidation of
analogous compounds as benzylic derivatives in the same
medium.
Photochemical oxidation
A solution of the substrate (1.0 ꢂ 10ꢁ2M) in N2- or
O2-purged CH3CN (20 ml, HPLC grade), in the presence
of 130 mg of TiO2 and of 0.30 mmol of Ag2SO4, was
externally irradiated using a Helios Italquartz 500 W high
pressure mercury lamp (through Pyrex filter), under
stirring at room temperature; the oxidation was also
performed in O2-purged CH3CN without Ag2SO4. The
TiO2 powder was then filtered through double paper and
repeatedly washed with CH3CN and diethyl ether; the
reaction mixture was poured into NaCl saturated water
and extracted with ether. The quantitative analysis of the
crude material (substrate þ product) was performed by
1H-NMR and by GC with suitable internal standard; the
material recovery was generally ꢃ80%. The reaction
profile from indan (in deaerated medium and in the
EXPERIMENTAL
1H-NMR spectra were run on a Bruker AC 200
(200 MHz) spectrometer, from solutions in CDCl3 with
TMS as internal standard. GC-MS analyses were
performed on a Hewlett Packard 6890A gas-chromatograph
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 359–364

364
M. BETTONI ET AL.
presence of Ag2SO4) was obtained by GC analysis of
reaction samples at different times, after above workup.
filters) as reported17. Each K value corresponds to the
average of two determinations.
Photochemical oxidation of 2 in
the presence of H218O
Acknowledgements
This work was carried out with the financial support of the
`
Ministero dell’Istruzione, dell’Universita e della Ricerca
(COFIN 2003).
The photo-oxidation of 2, sensitised by TiO2, was also
carried out in CH3CN/H218O (99.5/0.5 v/v). The reaction
mixture was analysed by GC-MS. The percent of
incorporated 18O in 5-methoxy-1-indanone was deter-
mined by the relative abundances of the peaks at m/z 162
and 164 [Mþ]. The percentage of incorporated 18O
(corrected considering the 18O content of the labelled
water) was 100 ꢀ 7%.
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Adsorption equilibrium constants
Equilibrium constants (K) of indanol and indane onto
TiO2 (Degussa P-25, particle concentration 20 g dmꢁ3) in
CH3CN (HPLC grade) were evaluated using different
initial substrate concentrations (Co) at room temperature
(temperature was not controlled because it is reported9
that K value is not significantly influenced in the range
22–40 8C). The equilibrium concentrations values (C)
were obtained (by HPLC) after overnight equilibration of
the shaken suspensions and filtration (through Millipore
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 359–364