Efficient palladium/1,10-phenanthroline-catalyzed reductive carbonylation of mono- and dinitroarenes to urethanes in phosphonium salt ionic liquids

Article abstract of DOI:10.1021/ol802096x

(Chemical Equation Presented) The highly reactive and selective reductive carbonylation of mono- and dinitroarenes to the corresponding mono- and diurethanes was carried out in the presence of a PdCl₂/Phen catalytic system, using PSIL110[C₁₄H₂₉(C₆H ₁₃)₃P⁺PF₆^-] as the ionic liquid under mild reaction conditions. No cocatalyst is required.

Full text of DOI:10.1021/ol802096x

ORGANIC
LETTERS
2008
Vol. 10, No. 21
5079-5082
Efficient Palladium/1,10-Phenanthroline-
Catalyzed Reductive Carbonylation of
Mono- and Dinitroarenes to Urethanes
in Phosphonium Salt Ionic Liquids
Qian Yang,† Al Robertson,‡ and Howard Alper*,†
Centre for Catalysis Research and InnoVation, Department of Chemistry, UniVersity of
Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada, and Cytec Canada, Inc.,
9061 Garner Road, Niagara Falls, Ontario L2E 6S5, Canada
howard.alper@uottawa.ca
Received September 8, 2008
ABSTRACT
The highly reactive and selective reductive carbonylation of mono- and dinitroarenes to the corresponding mono- and diurethanes was carried
out in the presence of a PdCl2/Phen catalytic system, using PSIL110[C14H29(C6H13)3P+PF6-] as the ionic liquid under mild reaction conditions.
No cocatalyst is required.
Some mono- and diurethanes are useful agriculture chemicals
and pesticides. They are also valuable precursors for isocy-
anates, which are important intermediates for the manufacture
of commercially important materials, such as high-perfor-
mance plastics, polyurethane foams, elastomers, coatings, and
adhesives. The conventional production of isocyanates
involves the reduction of nitro compounds to amines,
followed by reaction with phosgene. Since highly toxic
phosgene is used and large amounts of corrosive HCl are
produced during this procedure, efforts have been made to
develop alternative, phosgene-free processes.1,2 A promising
approach is the direct preparation of urethanes by the
reductive carbonylation of nitro compounds with carbon
monoxide in the presence of transition-metal catalysts,
followed by conversion to isocyanates.2,3 This type of
reaction has been studied extensively during the last three
decades. In many cases, large amounts of corrosive Lewis
and Bro¨nsted acids and/or a base were needed as cocatalysts
to achieve good results. The volatility and toxicity of these
acids may cause serious environmental problems. Further-
more, most of the reactions usually require high carbon
monoxide pressures. Therefore, there is a need to develop
new methods involving greener reaction conditions such as
solvents.
Ionic liquids (ILs) have recently attracted considerable
attention as environmentally friendly reaction media because
† University of Ottawa.
(2) Reductive carbonylation: (a) Paul, F. Coord. Chem. ReV. 2000, 203,
269, and references cited therein. (b) Cenini, S; Ragaini, F. Catalytic
ReductiVe Carbonylation of Organic Nitro Compounds; Kluwer Academic:
Dordrecht, 1997, and references cited therein. (c) Tafesh, A. M.; Weiguny,
J. Chem. ReV. 1996, 96, 2035, and references cited therein.
‡ Cytec Canada, Inc.
(1) Oxidative Carbonylation: (a) Zhang, X.; Lu, S. Synth. Commun. 2007,
37, 3291. (b) Orejon, A.; Castellanos, A.; Salagre, P.; Castillon, S.; Claver,
C. Can. J. Chem. 2005, 83, 764. (c) Shi, F.; Peng, J.; Deng, Y. J. Catal.
2003, 219, 372. (d) Kim, H. S.; Kim, Y. J.; Lee, H.; Park, K. Y.; Lee, C.;
Chim, C. S. Angew. Chem., Int. Ed. 2002, 41, 4300. (e) Shi, F.; Deng, Y.
Chem. Commun. 2001, 443. (f) Valli, V. L. K.; Alper, H. Organometallics
1995, 14, 80. (g) Leung, T. W.; Dombek, B. D. J. Chem. Soc., Chem.
Commun. 1992, 205. (h) Halligudi, S. B.; Bhatt, K. N.; Khan, M. M. T. J.
Mol. Catal. 1991, 68, 261. (h) Alper, H.; Hartstock, F. W. J. Chem. Soc.,
(3) (a) Wang, X.; Li, P.; Yuan, X.; Lu, S. J. Mol. Catal. A: Chem. 2006,
253, 261. (b) Wang, X.; Ling, G.; Xue, Y.; Lu, S. Eur. J. Org. Chem. 2005,
1675. (c) Skupinska, J.; Karpinska, M.; Ołowek, M. Appl. Catal. A: Gen.
2005, 284, 147. (d) Skupinska, J.; Karpinska, M.; Ołowek, M.; Kasprzycka-
Guttman, T. Cent. Eur. J. Chem. 2005, 3, 28. (e) Skupinska, J.; Karpinska,
M. Appl. Catal. A: Gen. 2004, 267, 59. (f) Skupinska, J.; Karpinska, M.;
Chem. Commun. 1985, 1141
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Wachowski, L.; Hofman, M. React. Kinet. Catal. Lett. 2004, 82, 311.
10.1021/ol802096x CCC: $40.75
Published on Web 10/15/2008
2008 American Chemical Society

of their low volatility, nonflammability, capability to dissolve
various organic and inorganic compounds, and potentially
recyclable properties. Imidazolium-based ionic liquids are
most commonly used as these alternative solvents.4 In recent
years, phosphonium salt ionic liquids (PSILs) have been the
subject of some publications. They have higher thermal
stability than the nitrogen-based ILs. McNulty and co-
workers and others recently described several different
reactions in PSILs, and good to excellent results were
obtained.5 To our knowledge, there is only one publication
on the reductive carbonylation of nitrobenzene that was
carried out with imidazolium-based ionic liquids as cocata-
lysts and ethanol as the solvent.6
Table 1. Reductive Carbonylation of 2-Nitrotoluene with
Different Catalytic Systems in Various PSILsa
sel (%)
PSILb yieldc (%) (2a:2b)
entry
catalytic systems
1
2
PdCl2/Ru3(CO)12/2,2′-bipy
PdCl2/Ru3(CO)12/2,2′-bipy
PdCl2/Ru3(CO)12/2,2′-bipy
PdCl2/Ru3(CO)12/2,2′-bipy
Ru3(CO)12/2,2′-bipy
PdCl2/2,2′-bipy
PdCl2/Phen
PdCl2/Phen
Ru3(CO)12/Phen
Ru3(CO)12
109
101
101
110
109
109
109
101
109
109
110
51
31
48
72
63
16
83
ND
69
36
92
82:18
47:53
81:19
58:42
84:16
100:0
96:4
3d
4d
5
Herein, we report our results on the highly reactive and
selective reductive carbonylation of mono- and dinitroarenes
to the corresponding mono- and diurethanes in good yields
and selectivities, using PdCl2/Phen (Phen: 1,10-phenanthro-
line) as the catalytic system and PSIL110 [C14H29
6e
7
8f
9
ND
87:13
68:32
100:0
(C6H13)3P+PF6 ] as the reaction medium.
-
10
11
We investigated the reductive carbonylation of nitroarenes
in PSILs with ruthenium and/or palladium transition-metal
catalysts. The reductive carbonylation of 2-nitrotoluene in
the presence of methanol was chosen as the model reaction,
and the reaction was effected under different conditions. The
results are presented in Table 1.
PdCl2/Phen
a Reaction conditions: 1 (2 mmol), cat. (0.1 mmol), ligand (0.2 mmol),
b
MeOH (30 mmol), 135 °C, 24 h. PSIL101: C14H29(C6H13)3P+Cl-.
PSIL109: C14H29(C6H13)3P+NTf2-. PSIL110: C14H29(C6H13)3P+ PF6
.
-
c Isolated yield. d CO (1000 psi), 150 °C. e Significant starting material
remained after 24 h. f ND ) Not determined, many products.
We first examined PdCl2/Ru3(CO)12/2,2′-bipy as the cata-
lytic system and ran the reactions in different PSILs in order
to determine the optimal phosphonium salt-based ionic liquid
for the reaction (Table 1, entries 1-4).7 We found that with
the PSILs as reaction media, the reaction could occur at lower
carbon monoxide pressure (500 psi) than that reported in a
number of other publications, but yields and selectivities were
moderate.
82:18 to 96:4. When the reaction was carried out in PSIL101
[C14H29(C6H13)3P+Cl-], the desired product was obtained in
trace quantities with nearly all of the starting material
recovered after 24 h (Table 1, entry 8). When Ru3(CO)12/
Phen or Ru3(CO)12 was used as the catalytic system, both
the yield and selectivity decreased (Table 1, entries 9 and
10). We were pleased to learn that when the reaction was
run in PSIL110 as the ionic liquid 2a was isolated in 92%
yield and no amine byproduct was detected (Table 1, entry
11).
According to the literature, palladium(II) with Phen is also
a very effective catalytic system for the reductive carbony-
lation of nitroarenes.8 Therefore, we ran the reaction in
In order to try to improve the reaction efficiency, the
reductive carbonylation of p-nitrotoluene 3 was carried out
PSIL109[C14H29(C6H13)3P+NTf2 ] with PdCl2/Phen as the
-
catalytic system (Table 1, entry 7). The product yield
increased from 51% to 83%, and selectivity increased from
Table 2. Reductive Carbonylation of p-Nitrotoluene Catalyzed
by PdCl2/Phen with Different Catalyst Loadings and CO
Pressuresa
(4) (a) Welton, T. Coord. Chem. ReV. 2004, 248, 2459. (b) Wilkes, J. S.
Green. Chem. 2002, 4, 73. (c) Dupont, J.; De Souza, R. F.; Suarez, P. A. Z.
Chem. ReV. 2002, 102, 3667. (d) Sheldon, R. A. Chem. Commun. 2001,
2399. (e) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39,
3772. (f) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391. (g)
Welton, T. Chem. ReV. 1999, 99, 2071.
(5) (a) Cao, H.; McNamee, L.; Alper, H. J. Org. Chem. 2008, 73, 3530.
(b) McNulty, J.; Nair, J. J.; Robertson, A. J. Org. Lett. 2007, 9, 4575. (c)
McNulty, J.; Cheekoori, S.; Bender, T. P.; Coggan, J. A. Eur. J. Org. Chem.
2007, 9, 1423. (d) McNulty, J.; Nair, J. J.; Cheekoori, S.; Larichev, V.;
Capretta, A.; Robertson, A. J. Chem.sEur. J. 2006, 12, 9314. (e) Vygodskii,
Y. S.; Shaplov, A. S.; Lozinskaya, E. I.; Filippov, O. A.; Shubina, E. S.;
Bandari, R.; Buchmeiser, M. R. Macromolecules 2006, 39, 7821. (f)
McNulty, J.; Cheekoori, S.; Nair, J. J.; Larichev, V.; Capretta, A.; Robertson,
A. J. Tetrahedron Lett. 2005, 46, 3641. (g) McNulty, J.; Capretta, A.;
Cheekoori, S.; Clyburne, J. A. C.; Robertson, A. J. Chim. Oggi 2004, 22,
13. (h) Gerritsma, D. A.; Robertson, A. J.; McNulty, J.; Capretta, A.
Tetrahedron Lett. 2004, 45, 7629. (i) Ramani, A.; Earle, M. J.; Robertson,
A. J. PCT Int. Appl. 2003, 24. (j) McNulty, J.; Capretta, A.; Wilson, J.;
Dyck, J.; Adjabeng, G.; Robertson, A. J. Chem. Commun. 2002, 1986. (k)
Karodia, N.; Guise, S.; Newlands, C.; Andersen, J.-A. Chem. Commun.
1998, 2341. (l) Kaufmann, D. E.; Nouroozian, M.; Henze, H. Synlett 1996,
1091.
cat. loading
(mol %)
pressure time
entry
solb
(psi)
(h)
yieldc (%)
1
2
3
4
5
6
5
5
2
2
1
5
IL110
IL110
IL110
IL110
IL110
benzene
500
200
200
200
200
200
24
24
48
24
24
48
96
93
91
92
89
9
a Reaction conditions: 3 (2 mmol), MeOH (30 mmol), 24 h. b PSIL110:
(6) Shi, F.; He, Y.; Li, D.; Ma, Y.; Zhang, Q.; Deng, Y. J. Mol. Catal.
A: Chem. 2006, 244, 64.
C14H29 (C6H13)3P+ PF6-, 2.0 g. c Isolated yield.
(7) Valli, V. L. K.; Alper, H. J. Am. Chem. Soc. 1993, 115, 3778.
5080
Org. Lett., Vol. 10, No. 21, 2008

Table 3. Reductive Carbonylation of Various Mononitroarenes
Table 4. Reductive Carbonylation of Dinitroarenes Catalyzed by
Catalyzed by PdCl2/Phen in PSIL110a
PdCl2/Phen in PSIL110a
a Reaction conditions: dinitroarene (2 mmol), PdCl2 (2 or 5 mol %),
Phen (4 or 10 mol %), MeOH (60 mmol), CO (200 psi), 135 °C, PSIL
110: 2.0 g, 24 h. b CO (500 psi). c Isolated yield.
a Reaction conditions: nitroarene (2 mmol), PdCl2 (0.02 mmol), Phen
(0.04 mmol), MeOH (30 mmol), CO (200 psi), 135 °C, 24 h, PSIL110:
2.0 g. b Isolated yield.
substituents on the aromatic ring, and the results are
summarized in Table 3.
using reduced catalyst loadings and lower carbon monoxide
pressures. The results are listed in Table 2.
The results in Table 2 indicate that decreasing the pressure
of carbon monoxide from 500 psi to 200 psi has almost no
effect on the yield of 4 (Table 2, entries 1-2).
When the catalyst loading was reduced to 2 or 1 mol %,
the product was still formed in 89-92% yield (Table 2,
entries 3-5). However, when the reaction was carried out
in 12 mL of dry benzene with 5 mol % of PdCl2/Phen as
the catalyst, the desired product was obtained in only 9%
isolated yield after 48 h reaction time (Table 2, entry 6).
To investigate the scope of this reaction, we carried out
the reductive carbonylation reaction for other mononitroare-
nes with various electron-donating and electron-withdrawing
The results in Table 3 show that by the use of PdCl2/Phen
as the catalytic system and PSIL110 as the ionic liquid, the
reductive carbonylation of mononitroarenes proceeded well
with both electron-donating and electron-withdrawing sub-
stituents on the aromatic ring, affording urethanes in 79-96%
yields. No amine byproduct was detected in these reactions.
The reductive carbonylation of dinitro aromatics to com-
mercially more important diisocyanates or diurethanes is
more challenging because of their reduced reactivity com-
pared to the mononitroarenes.9 To investigate the applicabil-
ity of the above reaction system for the reductive carbony-
lation of dinitroarenes, several dinitro substrates were
subjected to the reaction with PdCl2/Phen as the catalytic
system and PSIL110 as the reaction medium (Table 4).
(8) (a) Gasperini, M.; Ragaini, F.; Cenini, S.; Gallo, E.; Fantauzzi, S.
Appl. Organomet. Chem. 2007, 21, 782. (b) Gasperini, M.; Ragaini, F.;
Remondini, C.; Caselli, A.; Cenini, S. J. Organomet. Chem. 2005, 690,
4517. (c) Ragaini, F.; Gasperini, M.; Cenini, S. AdV. Synth. Catal. 2004,
346, 63. (d) Ragaini, F.; Cognolato, C.; Gasperini, M.; Cenini, S. Angew.
Chem., Int. Ed. 2003, 42, 2886. (e) Gasperini, M.; Ragaini, F.; Cenini, S.;
Gallo, E. J. Mol. Catal. A: Chem. 2003, 204-205, 107. (f) Paul, F.; Fischer,
J.; Ochsenbein, P.; Osborn, J. A. C. R. Chimie 2002, 5, 267. (g) Gallo, E.;
Ragaini, F.; Cenini, S.; Demartin, F. J. Organomet. Chem. 1999, 586, 190.
(g) Wehman, P.; Kaasjager, V. E.; de Lange, W. G. J.; Hartl, F.; Kamer,
P.C. J.; van Leeuwen, P. W. N. M. Organometallics 1995, 14, 3751.
First, 2, 4-dinitrotoluene reacted with carbon monoxide
and methanol in the presence of 5 mol % of PdCl2 and 10
mol % of Phen at 135 °C and 200-500 psi in 2.0 g of
PSIL110 for 24 h. The product, dimethyl 4-methyl-1,
3-phenylenediurethane, was obtained in 83-85% isolated
yield (Table 4, entries 1-2). Reduction of the catalyst loading
Org. Lett., Vol. 10, No. 21, 2008
5081

to 2 mol % lowered the urethane yield from 85% to 78%
(Table 4, entry 3). Several other dinitroarenes were subjected
to these reaction conditions, and the diurethanes were isolated
in 70-88% yields (Table 4, entries 4-12).
For the reductive carbonylation of dinitroarenes, we used
1, 3-dinitrobenzene under the same reaction conditions in
(C4H9)4P+Br-, (C4H9)4P+Cl-, (C8H17)4P+Br-, and C14H29
(C4H9)3P+Cl-, but the reactions were much less efficient and
only traces of product could be detected. When
C2H5(C4H9)3P+-OP(O)(OC2H5)2 was used as the reaction
medium, the desired product was formed in 71% isolated
yield.
In summary, the reductive carbonylation of mono- and
dinitroarenes was carried out in the presence of the PdCl2/
Phen catalytic system, using PSIL110 as the ionic liquid.
Important advantages of this reaction system include high
yields, excellent selectivities for the urethanes, and the
possibility of carrying out the reaction at considerably low
carbon monoxide pressures. In addition, no cocatalyst is
required for the reaction.
Acknowledgment. We are grateful to Cytec Canada, Inc.
and to the Natural Sciences and Engineering Research
Council for support of this work.
Supporting Information Available: Detailed experimen-
tal procedures and spectral data for all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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