Photocatalytic formation of a carbamate through ethanol-assisted carbonylation of p-nitrotoluene

Article abstract of DOI:10.1039/b418571d

The nitroarene p-nitrotoluene is converted with a selectivity higher than 85% to the corresponding carbamate at room temperature and atmospheric pressure, using photoexcited particles of TiO₂ as catalyst and EtOH as carbonylating species. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418571d

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COMMUNICATION
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Photocatalytic formation of a carbamate through ethanol-assisted
carbonylation of p-nitrotoluene{
a
Andrea Maldotti,* Rossano Amadelli, Luca Samiolo, Alessandra Molinari, Andrea Penoni,
a
a
a
b
b
Stefano Tollari and Sergio Cenini
c
Received (in Cambridge, UK) 10th December 2004, Accepted 13th January 2005
First published as an Advance Article on the web 7th February 2005
DOI: 10.1039/b418571d
The nitroarene p-nitrotoluene is converted with a selectivity
higher than 85% to the corresponding carbamate at room
temperature and atmospheric pressure, using photoexcited
powerful oxidizing properties of the photogenerated holes should
lead to the oxidation of the employed alcohol to CO and H
2
2
O
?
with the possible involvement of adsorbed OH radicals.
9,10
1
1
A typical photocatalytic experiment was carried out irradiating
particles of TiO as catalyst and EtOH as carbonylating species.
2
with light of wavelength higher than 380 nm deaerated powder
2
3
Aromatic isocyanates are important intermediates in fine and
industrial chemistry, which are currently prepared by reduction
dispersions of TiO
p-nitrotoluene (3.0 6 10 mol dm ) containing mixtures of
EtOH/ CH CN (3 : 7 v/v). The gas chromatographic analysis
revealed that 7% of 1 disappeared after 8 h of photoexcitation at
25 ¡ 1 uC and 760 Torr of N . The main product was the
2
(3 g dm , Degussa P25) suspended in
22
23
1
and phosgenation of the corresponding nitroarene. A possible
3
synthetic route to produce these compounds in the absence of
phosgene, which is a poisonous and corrosive gas, is the reductive
2
2–5
carbamate 2 with only minor amounts of p-toluidine 3, by
comparison with the retention times of those of authentic samples
and by co-injection with the authentic compounds. Specifically, the
gas chromatographic analysis showed that the carbamate was
obtained with 75% selectivity and that the detected products
represented more than 90% of the converted p-nitrotoluene.
Control experiments demonstrated that 1 did not undergo any
reaction in the absence of TiO2 or without irradiation. The
carbamate 2 was prepared as standard for the gas chromato-
carbonylation of nitroarenes with CO (eqn. (1)). This process
may be also carried out in the presence of an alcohol (ROH) in
order to produce carbamates that can be subsequently cracked
into isocyanates (eqn. (2)).
catalyst
ArNO2z3CO DCCA ArNCOz2CO2
catalyst
(1)
ArNO2z3COzROH DCCA ArNHCOORz2CO2 (2)
In this communication we present, as a preliminary account, a
novel and simple approach for the conversion of p-nitrotoluene to
the corresponding carbamate, using photoexcited particles of
12,13
graphic analyses following a previously reported procedure.
It is reasonable to ascribe the described photocatalytic
carbonylation of the nitroarene 1 to different cross reactions
involving some of the reactive species originated from sequential
monoelectronic redox processes induced by the semiconductor.
Several transient reactive species, including nitroso and nitrene
derivatives, may originate from the conduction-band-assisted
reduction of 1. Other reactive intermediates are expected to be
formed as a consequence of the simultaneous hole-assisted
oxidation of EtOH, as briefly discussed in the following. The
involvement of the initial reagents 1 and EtOH in the overall
process is also plausible.
2
commercial TiO as catalyst and EtOH instead of CO as
carbonylating species (eqn. (3)). This photocatalytic reaction is
carried out in mild temperature and pressure conditions irradiating
with light of the near ultraviolet.
2
p-CH3-C6H4-NO2z3C2H5OH DCCA
TiO2, hn,
0
25 C, 760 Torr
1
(3)
(3)
2
p-CH3-C6H4NHCO2C2H5z3H2O
2
2
The TiO -assisted oxidation of EtOH has been experimentally
Studies focusing on the use of semiconductors such as TiO as
2
followed by infrared spectroscopy. Thin films of TiO deposited on
2
heterogeneous photocatalysts have attracted considerable interest
in the field of ‘‘green chemical processes’’, where one seeks a high
glass supports were irradiated in the bottom of a 10 cm length
infrared gas cell containing air saturated with vapours of EtOH.
Photochemical excitation of this film caused a significant decrease
of some absorptions typical of EtOH in the gas phase: 2800–
efficiency and selectivity under mild and environment-friendly
6
conditions. Photoexcitation of TiO
2
particles at wavelengths
2
1
21
corresponding to the band gap value promotes electrons in the
conduction band leaving positive holes in the valence band. If
electrons and holes can reach the surface of the semiconductor
before recombination takes place, they can induce redox processes
3000 cm , 1050–1100 cm . The simultaneous growing of new
2
1
bands in the 1700–1800 cm range indicated that EtOH was
partially converted to carbonylic derivatives. The formation of a
2
1
new absorption at 2720 cm typical of aldehydic C–H bonds is a
clear indication of the presence of acetaldehyde among these
products. Photochemical excitation was also accompanied by the
of organic substrates. Here, in the absence of O
2
, conduction band
7,8
electrons are expected to reduce the nitroarene 1, while the
formation of CO
2
, as shown by the growth of its typical
21
21
absorptions at 2350 cm and 660 cm
.
{
Electronic Supplementary Information (ESI) available: IR spectra. See
http://www.rsc.org/suppdata/cc/b4/b418571d/
mla@dns.unife.it
Although we do not have infrared evidence of CO formation,
there is reason to expect that this compound is a relevant
*
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intermediate in the photocatalytic system. Indeed, in one oxidation
?
route, photogenerated OH radicals can abstract formyl hydrogens
1 does not decrease significantly during the 8 h of the experiment;
(iii) the carbamate 2 is obtained with about 85% selectivity.
In this work, p-nitrotoluene has been chosen as a substrate
because, in the given experimental conditions, it undergoes neither
direct photochemical reactions, nor facile redox processes of the
para-substituent methyl group. These requirements set the limits to
the choice of other nitroarenes, and appropriate experimental
conditions for further investigations should comply with these
criteria. Thus, for example, p-nitrobenzaldehyde does not appear
to be a suitable substrate since we verified that both functional
14
from acetaldehyde yielding acyl radicals (first step in eqn. (4)).
These, in turn, are well known to undergo very fast decarbonyla-
tion according to the second step in eqn. (4). Gas chromatographic
analysis provided evidence that addition of CO did not affect
significantly yield and selectivity of the photocatalytic conversion
of 1 to 2, suggesting that the photogenerated CO, if it is involved,
is likely to participate in the carbonylation process as an adsorbed
species derived, in turn, from the adsorbed aldehyde.
groups undergo reduction at photoexcited TiO
2
, thus severely
Some additional experiments indicate that nature and reactivity
of the employed hole-scavenger had a pronounced influence on the
selectivity of the photocatalytic process. In fact, with MeOH as
photooxidizable substrate instead of EtOH, the amine 3 was the
only observable photoproduct of 1. Other experiments showed
that also in the presence of 2-PrOH the photoinduced reduction of
complicating the overall process.
In conclusion, this work describes optimal conditions in which
the reactive species generated both from the conduction-band
electrons and from the valence-band positive holes are precursors
of carbamate formation. The described reaction is a new appealing
synthetic route in applied organic chemistry, since it may produce
very important chemicals such as carbamates with good selectivity
and without using harmful reagents such as phosgene and CO. In
particular, we achieve the goal of generating carbonylating species
in situ from EtOH in mild temperature and pressure conditions
using an inexpensive photocatalyst that combines unique char-
acteristics, such as stability and environmental tolerance. In order
to rationalize the drastic influence of alcohol on the activity of
1
proceeded mainly to amine formation: 85% of 3, 15% of 2. A
plausible explanation is that both MeOH and 2-PrOH produce
lower amounts of CO in comparison to EtOH. Indeed, hydrogen
atom abstraction from formaldehyde (from MeOH oxidation) is
comparatively more difficult than from acetaldehyde (eqn. (4), step
1). Moreover, further oxidation of acetone (from 2-PrOH) can
3
2?
COCH radicals which are known to yield substantial
lead to CH
amounts of ketene (CH
15
2
CO) instead of CO .
2
TiO , more experiments are needed.
This completely new photochemical activity of TiO2 is of
general interest in the field of photocatalysis. In fact, we definitely
confirm that suitable experimental conditions can be found in
ð4Þ
2
order to control the well known powerful reactivity of TiO which,
therefore, can be successfully employed also for synthetic purposes
in addition to environmental decontamination. Further studies are
also in progress to establish the possible employment of other
semiconducting compounds able to utilize the visible light for the
exploitation of solar light. Future developments will also have to
take into account the effect of other substituents on the nitroarene.
The peculiar reactivity of EtOH has prompted us to carry on
photocatalytic experiments in neat EtOH. It is seen in Fig. 1 that
the formation of both 2 and 3 follows a zero-order kinetics,
indicating that consecutive reactions can be ruled out. It is
noteworthy that the use of neat ethanol as dispersing medium
enhances both efficiency and selectivity of the photocatalytic
process. In particular, it is seen that: (i) the overall rate of
conversion of 1 (about 13%) almost doubles with respect to the
experiments carried out in mixed solvent; (ii) the conversion rate of
a
Andrea Maldotti,* Rossano Amadelli, Luca Samiolo,
a
a
b
a
b
Alessandra Molinari, Andrea Penoni, Stefano Tollari and
c
Sergio Cenini
Dipartimento di Chimica, Sezione ISOF –CNR, Universit a` di Ferrara,
a
Via L. Borsari 46, 44100, Ferrara, Italy. E-mail: mla@dns.unife.it
Dipartimento di Scienze Chimiche e Ambientali, Universita`
b
dell’Insubria, Via Valleggio 11, 22100, Como, Italy
Dipartimento di Chimica Inorganica Metallorganica e Analitica,
c
I.S.T.M.- C.N.R., Universit a` di Milano, Via G. Venezian 21, 20133,
Milano, Italy
Notes and references
1
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2
S. Cenini and F. Ragaini, Catalytic Reductive Carbonylation of Organic
Nitro Compounds, Kluwer Academic Publishers, Dordrecht, 1997.
F. Paul, Coord. Chem. Rev., 2000, 20, 269.
A. M. Tafesh and J. Weguny, Chem. Rev., 1996, 96, 2035.
F. Ragaini, C. Cognolato, M. Gasperini and S. Cenini, Angew. Chem.
Int. Ed., 2003, 42, 2886.
3
4
5
6
7
A. Maldotti, A. Molinari and R. Amadelli, Chem. Rev., 2002, 102, 3811.
P. Paola, C. Minero, M. Vincenti and E. Pelizzetti, Catal. Today, 1997,
39, 187.
8
9
A. Maldotti, L. Andreotti, A. Molinari, S. Tollari, A. Penoni and
S. Cenini, J. Photochem. Photobiol., A, 2000, 133, 129.
P. Pichat, M. N. Mozzanega and H Courbon, J. Chem. Soc., Farady
Trans. 1, 1987, 83, 697.
Fig. 1 Formation of 2 and 3 upon irradiation (l . 380 nm) of TiO
22
2
23
)
2
3
(
3 g dm ) dispersed in deaerated p-nitrotoluene (3.0 6 10 mol dm
containing EtOH.
1
750 | Chem. Commun., 2005, 1749–1751
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1
0 A. Sclafani, M. N. Mozzanega, M. Naella and P. Pichat, J. Photochem.
Photobiol., A, 1991, 59, 181.
1 12 mg of TiO were suspended in 3 ml of the desired solution
and irradiated under continuous stirring at 25 ¡ 1 uC with a Oriel
291 Xe/Hg lamp. Selection of excitation wavelengths was
12 P. A. S. Smith, Org. React., 1946, 3, 337.
13 All the analyses were performed with a DANI 8521 Gas-
Chromatograph. The capillary column was: Column Restek
Corporation Rtx-5 Amine (Crossbond 5% diphenyl–95% dimethyl
polysiloxane; 30 m 6 0.32 mm id, 1.50 mm df).
1
2
6
performed by using a glass filter which cuts off the light of
wavelength lower than 380 nm. A Pyrex reactor was used as a reaction
vessel.
14 D. C. Nonhebel, J. M. Tedder and J. C. Walton, Radicals, Cambridge
University Press, Cambridge, 1979.
15 F. O. Rice and R. E. Varnerin, J. Am. Chem. Soc., 1955, 77, 221.
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Chem. Commun., 2005, 1749–1751 | 1751