Palladium nitrosoarene complexes in reductive carbonylation of nitroarenes

Article abstract of DOI:10.1023/A:1022151806085

The reactions of carbon monoxide with the palladium nitrosoarene complexes Pd₂(μ-OCOR)₂(-CH₂C₆H ₄NO)₂ (1, R = Me, CF₃, Bu^t, or Ph) and Pd₂(μ-OCOR)₂(PhNC₆H₄NO) ₂ (2, R = Me, CF₃, Bu^t, or Ph) were studied. Complexes 1 contain the o-nitrosotoluene molecule metallated at the methyl group. In complexes 2, the phenyl-o-nitrosophenylamide ligand coordinated via two nitrogen atoms can be considered as a nitrosobenzene derivative bearing the NPh group in the ortho position of the Ph ring. It all cases, carbonylation of the complexes afforded the corresponding aryl isocyanates Ar-N=C=O or the products of their further transformations. The mechanism of reductive carbonylation of nitroarenes catalyzed by palladium compounds and the role of palladium nitrosoarene complexes as possible intermediates in this process are discussed.

Full text of DOI:10.1023/A:1022151806085

2286
Russian Chemical Bulletin, International Edition, Vol. 51, No. 12, pp. 2286—2289, December, 2002
Palladium nitrosoarene complexes in reductive carbonylation of nitroarenes
ꢀ
T. A. Stromnova and S. T. Orlova
N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 954 1279. Eꢀmail: strom@igic.ras.ru
The reactions of carbon monoxide with the palladium nitrosoarene complexes Pd2(µꢀ
OCOR)2(—CH2C6H4NO)2 (1, R = Me, CF3, But, or Ph) and Pd2(µꢀOCOR)2(PhNC6H4NO)2
(2, R = Me, CF3, But, or Ph) were studied. Complexes 1 contain the oꢀnitrosotoluene molꢀ
ecule metallated at the methyl group. In complexes 2, the phenylꢀoꢀnitrosophenylamide ligand
coordinated via two nitrogen atoms can be considered as a nitrosobenzene derivative bearing
the NPh group in the ortho position of the Ph ring. It all cases, carbonylation of the complexes
afforded the corresponding aryl isocyanates Ar—N=C=O or the products of their further
transformations. The mechanism of reductive carbonylation of nitroarenes catalyzed by pallaꢀ
dium compounds and the role of palladium nitrosoarene complexes as possible intermediates in
this process are discussed.
Key words: palladium, reduction, reductive carbonylation, nitroarenes, nitrosoarenes, aryl
isocyanates, phenyl isocyanate.
As part of our continuing studies of the reacꢀ
tivities of palladium complexes containing the
nitrosoarene ligands, we examined the reactions of
Pd2(µꢀOCOR)2(—CH2C6H4NO)2 (1) and Pd2(µꢀ
OCOR)2(PhNC6H4NO)2 (2) (R = Me, CF3, But, or Ph),
which we have synthesized previously,1—3 with carbon
monoxide under mild conditions.
The reaction of complex 1 (R = Me) with CO afꢀ
forded acetic acid and tolyl isocyanate along with pallaꢀ
dium black (Scheme 1).
Scheme 1
Complexes 1 bear the oꢀnitrosotoluene molecule
metallated at the Me group as a bidentate chelating ligand.
Complexes 2 contain the phenylꢀoꢀnitrosophenylamide
ligand coordinated via two N atoms, which can also be
considered as a nitrosobenzene derivative bearing the NPh
group in the ortho position of the Ph ring. Such complexes
containing the coordinated nitrosoarene molecule were
postulated4—7 as active intermediates in the palladiumꢀ
catalyzed reductive carbonylation of nitroarenes, which
has promise as an industrial procedure for the preparation
of aromatic isocyanates, carbamates, and urea derivatives.
Results and Discussion
Previously,8 it has been found that complexes 1 and 2
are readily reduced with different reducing agents (H2,
NaBH4, or alcoholic solutions of alkali) under mild conꢀ
ditions. Our experiments demonstrated that these comꢀ
plexes react with CO in a solution in benzene or toluꢀ
ene under mild conditions (∼20 °C, рCO = 1 atm, τ =
10—15 h). Under the reaction conditions, the palladium
atom in these complexes is reduced to metal and the
nitrosoarene ligands are transformed into the expected
reductive carbonylation products.
Tolyl isocyanate was obtained in 8—12% yields with
respect to the amount of the complex used (∼0.2 mol of
tolyl isocyanate per mole of the complex), whereas aceꢀ
tic acid was produced in a nearly stoichiometric yield
(∼2 mmol of acetic acid per mole of the complex). Apparꢀ
ently, tolyl isocyanate was obtained in such a low yield
because of high reactivities of aryl isocyanates, particuꢀ
larly, in the presence of palladium compounds. It is beꢀ
lieved that under the reaction conditions, tolyl isocyanate
either undergoes dimerization, trimerization, and polyꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2123—2126, December, 2002.
1066ꢀ5285/02/5112ꢀ2286 $27.00 © 2002 Plenum Publishing Corporation

Reduction of palladium nitrosoarenes complexes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2287
merization or reacts with water that is present in the solꢀ
vent (Scheme 2) to give highꢀboiling products, which are
difficult to identify.9,10
It is believed that Nꢀphenyltrifluoroacetamide is
formed in the reaction of trifluoroacetic acid, which is
generated by reduction of the complex, with phenyl isoꢀ
cyanate (Scheme 5). Such a reaction with strong carboxyꢀ
lic acids is typical of aryl isocyanates.9,10
Scheme 2
Scheme 5
Complexes 2 (R = Me or CF3) react with CO under
the same conditions. The reaction of complex 2 (R = Me)
afforded acetic acid and phenyl isocyanate (Scheme 3). It
should be noted that the yield of phenyl isocyanate generꢀ
ated in this reaction, like that obtained in the aboveꢀ
considered process, was much lower than the stoichioꢀ
metric value (10—15% from the starting complex).
Hence, our study showed that the reactions of CO
with palladium complexes containing the coordinated
nitrosoarene molecules afford aryl isocyanates ArN=C=O,
i.e., the products of reductive carbonylation of nitroarenes,
which occurs according to the overall equation
ArNO2 + 3 CO
2 CO2 + ArNCO.
The mechanism of reductive carbonylation of nitroꢀ
arenes catalyzed by noble metal (Ru, Rh, Pd) compounds
has been the subject of prolonged discussion. Most reꢀ
searchers assumed11,12 that this reaction involves a seꢀ
ries of successive carbonylation—decarbonylation steps
(Scheme 6). The carbonylation step produces fiveꢀ or sixꢀ
membered metallocycles whose decarboxylation affords
coordinated nitrosoarenes. The coordination of the
nitrosoarene ligand in the intermediate B can give rise to
both the threeꢀmembered Pd—N—O— metallocycle and
a complex with the donorꢀacceptor Pd←N bond. The
subsequent steps afford the coordinated arylnitrene ligands
[ArN:]. Depending on the substrate, the subsequent steps
yield either aryl isocyanate or carbamate.
Scheme 3
Intermediates A, B, and C were isolated and structurꢀ
ally characterized (see Scheme 6) in the studies of rutheꢀ
niumꢀ13,14 or rhodiumꢀbased catalysts for reductive
carbonylation.15—17 In some cases, subsequent transforꢀ
mations of these intermediates gave rise to aryl isocyanꢀ
ates or carbamates. In the investigations of palladiumꢀ
based catalytic systems, a possible intermediate (D, see
Scheme 6) was isolated and structurally characterized only
once.12,18 However, the products of subsequent transforꢀ
mations of this complex provide only indirect evidence
that this complex can be involved in carbonylation of
nitroarenes.
The reaction of CO with complex 2 containing the
trifluoroacetic ligands gave Nꢀphenyltrifluoroacetamide
PhNHC(=O)CF3 along with palladium black (Scheme 4).
Scheme 4
Data on reactions of palladium complexes containing
the nitrosoarene ligands (intermediates or models of inꢀ
termediates of type B in Scheme 6) with carbon monoxꢀ
ide are lacking in the literature. Apparently, the formaꢀ
tion of phenyl isocyanate and tolyl isocyanate in the reacꢀ
tions of palladium nitrosoarene complexes with carbon
monoxide is evidence that reductive carbonylation of
nitroarenes in the presence of palladium compounds proꢀ
ceeds through the formation of coordinated nitrosoarenes.

2288
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002
Stromnova and Orlova
Scheme 6
highꢀboiling reaction products and Reoplex for analyses of lowꢀ
boiling organic products).
The GLCꢀMS analyses of organic products were carried out
on an Automass 150 instrument (Delsi Nermag, France). The
recording conditions were as follows: EI, 70 eV, capillary colꢀ
umns with PEGꢀ20M phases for analyses of lowꢀboiling organic
products and Silicon SEꢀ30 for analyses of highꢀboiling reaction
products.
Reactions of complexes 1 and 2 with carbon monoxide were
carried out at 20 °C and CO pressure of 1 atm. in a twoꢀneck
flask equipped with a magnetic stirrer, a sampler with a rubber
diaphragm, and a system for evacuation and gas supply. A
weighed sample of the complex (0.25 mmol; 0.14 g of comꢀ
plex 1; 0.18 or 0.21 g of complex 2 for R = Me and CF3,
respectively) was placed in a flask and the solvent (benzene or
toluene, 2 mL) was added. The system was evacuated, CO was
supplied, and the reaction mixture was stirred. The liquid reacꢀ
tion products were analyzed by GLC and GLCꢀmass specꢀ
trometry.
Carbonylation of complex 1 (R = Me) afforded tolyl isocyꢀ
anate as the major product; the yield was 8—12% with respect to
the amount of the complex used. MS, m/z (Irel (%)): 133 [M]+
(92), 132 [M – H]+ (25), 105 [M – CO]+ (42), 104 [M – HCO]+
(100), 91 [M – CH2CO]+ or [M – CO – N] (13), 78
[M – CO – NCH]+ (38), 77 [M – HCO – NCH]+ (21), 63
[M – NCO – C2H4]+ (8). The yield of AcOH was ca. 100%
from the complex used.
Carbonylation of complex 2 (R = Me) produced phenyl
isocyanate as the major product; the yield was 10—15% from the
complex used. MS, m/z (Irel (%)): 119 [M]+ (100), 91 [M – CO]
(39), 64 [M – CO – NCH]+ (87), 63 [M – HCO – NCH]+
(39). The yield of AcOH was ca. 100% from the complex used.
Carbonylation of complex 2 (R = CF3) afforded Nꢀphenylꢀ
trifluoroacetamide. MS, m/z (Irel (%)): 189 [M]+ (84), 120
[M – CF3]+ (58), 92 [M – CF3 – CO] (61), 77 [M – CF3 –
NHCO]+ (100), 65 [M – CF3 – CO – NCH]+ (54).
Experimental
Nitrosoaromatic compounds oꢀR´C6H4NO (R´ = H or Me)
were prepared according to a general procedure19 by the oxidaꢀ
tion of phenylhydroxylamine with sodium bichromate in aqueꢀ
ous H2SO4 (R´ = H) or by the reduction of the corresponding
nitro compound (R´ = Me). The resulting nitrosoarenes were
isolated from the reaction mixture by azeotropic steam distillaꢀ
tion under atmospheric or reduced pressure. The purities of the
compounds were checked by the TLC on Silufol and melting
points. Because the nitroso group in nitrosoaromatic compounds
is readily oxidized by atmospheric oxygen, nitrosoarenes were
stored over a short period of time in Schlenk vessels under argon
in a refrigerator, and all reactions with these compounds were
carried out under an inert atmosphere (N2 or Ar).
We acknowledge A. E. Gekhman for performing
GLCꢀmass spectrometric analysis and helpful discussion.
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 99ꢀ03ꢀ
32519).
References
1. T. A. Stromnova, S. T. Orlova, I. P. Stolyarov, S. B. Katser,
and I. I. Moiseev, Dokl. Akad. Nauk SSSR, 1997, 352, 68
[Dokl. Chem., 1997 (Engl. Transl.)].
Palladium
complexes
with
nitrosoarene
(1) and
2. T. A. Stromnova, S. T. Orlova, D. N. Kazyul´kin, I. P.
Stolyarov, and I. L. Eremenko, Izv. Akad. Nauk, Ser. Khim.,
2000, 147 [Russ. Chem. Bull., Int. Ed., 2000, 49, 150].
3. S. T. Orlova, D. N. Kazyul´kin, D. V. Pashchenko, I. P.
Stolyarov, and T. A. Stromnova, Zh. Neorg. Khim., 2000, 45,
472 [Russ. J. Inorg. Chem., 2000, 45 (Engl. Transl.)].
4. Yu. B. Yan and B. K. Nefedov, in Sintezy na osnove oksidov
ugleroda [Syntheses Based on Carbon Oxides], Khimiya, Mosꢀ
cow, 1987, 125 (in Russian).
ligands Pd2(OCOR)2(CH2C6H4NO)2
Pd2(OCOR)2(PhNC6H4NO)2 (2) (R = Me, CF3, But, or Ph)
were prepared according to procedures described previously1,2
from tetranuclear palladium carbonyl carboxylates
Pd4(µꢀCO)4(µꢀOCOR)4 (R = Me, CF3, But, or Ph) and the
corresponding nitrosoaromatic compounds.
The organic solvents were purified according to standard
procedures.20
The GLC analysis of the liquid products was carried out on a
3700 chromatograph (OVꢀ101 on Chromaton for analyses of
5. G. W. Parshal and S. D. Ittel, in Homogeneous Catalysis,
J. Wiley and Sons, New York, 1992, 114.

Reduction of palladium nitrosoarenes complexes
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2289
6. F. Ragaini and S. Cenini, J. Mol. Catal., A: Chem., 1996,
109, 1; F. Ragaini, M. Macchi, and S. Cenini, J. Mol.
Catal., A: Chem., 1997, 127, 33.
7. S. Cenini and F. Ragaini, Reductive Carbonylation of Organic
Nitroaromatic Compounds, Kluwer Acad. Publishers,
Dordrecht, The Netherlands, 1997.
8. S. T. Orlova, D. N. Kazyul´kin, L. K. Shubochkin, D. I.
Shishkin, and T. A. Stromnova, Izv. Akad. Nauk, Ser. Khim.,
2001, 1609 [Russ. Chem. Bull., Int. Ed., 2001, 50, 1689].
9. KirkꢀOthmer Encyclopedia of Chemical Technology,
Ed. M. Grayson, A WileyꢀInterscience—J. Wiley and
Sons, New York—Chichester—Brisbane—Toronto, 1981, 13,
798 pp.
10. L. F. Fieser and M. Fieser, Organic Chemistry, D. C. Heath,
Boston, 1944.
11. P. Wehman, L. Borst, P. C. J. Kamer, and P. W. N. M.
van Leeuwen, J. Mol. Catal., A: Chem., 1996, 112, 23.
12. F. Paul, J. Fischer, P. Ochsenbein, and J. A. Osborn, Orgaꢀ
nometallics, 1998, 17, 2199.
13. J. D. Gargulak and W. L. Gladfelter, J. Am. Chem. Soc.,
1991, 113, 1054.
14. S. J. Skoog, J. P. Campbell, and W. L. Gladfelter, Organoꢀ
metallics, 1994, 13, 4137.
15. F. Ragaini and S. Cenini, Organometallics, 1994, 13, 1178.
16. S. Cenini, F. Porta, M. Pizzotti, and C. Crotti, J. Chem.
Soc., Dalton Trans., 1985, 163.
17. L. Dahlenburg and C. Prengel, Inorg. Chim. Acta, 1986,
122, 55.
18. P. Leconte, F. Metz, A. Mortreux, J. A. Osborn, F. Paul,
F. Petit, and A. Pillot, J. Chem. Soc., Chem. Commun.,
1990, 1616.
19. Organic Syntheses, Ed. C. F. H. Allen, New York, 1948, 20.
20. A. Weissberger, E. S. Proskauer, J. A. Riddick, and E. E.
Toops, Organic Solvents, Interscience Publishers, New
York, 1955.
Received December 18, 2001;
in revised form July 24, 2002