First gold( ) complex-catalyzed oxidative carbonylation of amines for the
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syntheses of carbamates
Feng Shi and Youquan Deng*
State Key Laboratory for Oxo Synthesis and Selective Oxidation, and Laboratory of Environmental and Applied
Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China.
E-mail: ydeng@ns.lzb.ac.cn; Fax: +86-931-8277088
Received (in Cambridge, UK) 29th November 2000, Accepted 25th January 2001
First published as an Advance Article on the web 14th February 2001
At 200 °C and 5 MPa of initial total pressure, the oxidative
carbonylation of amines for the synthesis of the correspond-
catalyst 1 was employed, decomposition of the catalyst, i.e.
to Au0, occurred. A very thin film, light-
yellow in color, was observed to have been deposited on the
wall of the glass tube inside the reactor after the reaction; no
such phenomenon was observed with catalysts 2–4.
reduction of Au3+
ing carbamates by Au( ) complexes as catalysts was con-
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ducted with excellent conversion and selectivity.
Gold catalysts have been receiving growing attention and many
reactions catalyzed over Au catalysts,1 such as CO oxidation,2
selective oxidation,3 water-gas shift reactions,4 asymmetric
aldol reactions,5 carbonylation of olefins6 and dehydrogenative
dimerization of trialkylstannane,7 have been reported to be high
performance. These reactions indicate that using gold com-
plexes as catalysts for those synthetic reactions traditionally
catalyzed with Pd, Rh etc. complexes should be not only
possible but also practical and even highly efficient.
The conversion and selectivity for the desired products were
greatly enhanced if extra PPh3, (0.1 g) was further added into
the reaction, entries 6–9. It is noteworthy that 97.2% conversion
and 89% selectivity were achieved over catalyst 2 + PPh3. The
TOF reached 36, which is comparable to the result of
Pd(PPh3)2Cl2 (6) + PPh3 used as the catalyst, where 98.5%
conversion and 86% selectivity was obtained, entry 8. Such
enhancement in catalytic activity may partially be attributed to
the stabilization of organic Au( ) complexes by the additional
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Produced either by oxidative carbonylation of amines or by
reductive carbonylation of nitro compounds in the presence of
an alcohol, carbamates as intermediates for the synthesis of
isocyanates by non-phosgene routes have been extensively
studied over the past two decades.8 Pd, Ru and Rh complexes
and other transition metals were employed as the catalysts,9 and
complexes of Pd coordinated with N-containing compounds
were among the most effective of these catalyst systems.10
Although reductive carbonylation of nitro compounds should be
the more attractive route for indirect production of isocyanates,
the occurrence of catalyst deactivation due to the reduction of
noble metal ions, e.g. Pd2+ to Pd0, under the strong reductive
conditions may be difficult to be overcome,11 and the catalyst
systems were generally more complicated than those for
oxidative carbonylation. Furthermore, no studies concerning
the oxidative carbonylation of amines or reductive carbonyl-
ation of nitro compounds for carbamate formation by Au
complexes have been reported yet.
PPh3 which may replace the oxidized ligand, since it is possible
that small amounts of PPh3 could be oxidized under oxidative
conditions during the reaction. The same main by-product,
quinazoline, was found over catalysts 2 + PPh3 and 6 + PPh3,
indicating that a similar reaction mechanism occurred over
catalysts 2 and 6.
Using alcohol as one of the reaction substrates and solvent
had a strong impact on the reaction. Much higher selectivity
could be achieved when methanol was used, entry 6, although
slightly higher conversion was obtained with ethanol under the
same reaction conditions, entry 7. This may be due to the easier
formation of quinoline in the simultaneous presence of aniline
and ethanol. Treatment of 2,4-diaminotoluene and 4,4A-diami-
nodiphenylmethane with carbon monoxide and methanol in the
presence of catalyst 2 + PPh3 afforded excellent conversion and
selectivity, entries 11 and 12. The main by-product from
2,4-diaminotoluene was the mono-carbonylated product, while
the main by-product from 4,4A-diaminodiphenylmethane was
4,4A-diaminobenzophenone.
Herein, the first example of the oxidative carbonylation of
amines for the synthesis of the corresponding carbamates by Au
complexes, i.e. HAuCl4 1, Au(PPh3)Cl 2, Au(PPh3)2Cl 3,
Au(PPh3)NO3 4 and [Au(PPh3)]2S 5 as the catalysts with
excellent performance, is reported:
An attempt was also made to test the oxidative carbonylation
of aliphatic amines using catalyst 2 + PPh3, the best catalyst
system found for the carbonylation of aromatic amines.
Although the conversion of the corresponding amines was
almost complete for n-hexylamine (entry 13) and cyclohex-
ylamine (entry 14), poor selectivities for the desired product
were obtained, but relatively high selectivities for the corre-
sponding alkylureas were achieved. For 1,6-hexanediamine
(entry 15) almost no desired product could be observed, but it is
noteworthy that 63% selectivity for N,NA-hexylmethylene
[Au(PPh3)x ]y Z
R(NH2)n + CO + O2 + R¢OH æææææÆ
PPh
3
R(NHCO2R¢)n + H2O
Ph, Alkyl
; R¢ = CH3, CH3CH2; n = 1 or 2;
R =
x,y = 1 or 2; Z = Cl, NO3, S.
diformamide could be achieved. This may imply that the Au( )
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complexes could be a promising catalyst for the synthesis of
other N-containing compounds from aliphatic amines under
suitable reaction conditions.
The oxidative carbonylation of a series of aromatic and
aliphatic amines over the above mentioned Au( ) complexes
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was examined (Table 1). When aniline as substrate was
subjected to carbonylation, different catalytic performances
were observed over catalysts 1–5. The highest selectivity was
70% over 2, and the highest conversion 62.2% over 3. The
catalyst 5 was even less effective than catalyst 1. The main by-
product resulting from catalyst 1 was azobenzene, while N-
methylaniline and quinazoline were detected over the catalysts
2–5. Other by-products such as azoxybenzene over catalyst 1
and quinazoline over catalyst 2 were also identified. When
In summary, the experimental results suggest that organic
Au( ) complexes show an excellent performance towards the
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oxidative carbonylation of aromatic amines to form correspond-
ing carbamates, and also exhibit a promising catalytic perform-
ance towards the carbonylation of aliphatic amines to produce
either alkylureas or formamides. To the best of our knowledge
this is the first reported study of Au(I) complexes for this kind
of reaction.
DOI: 10.1039/b009575n
Chem. Commun., 2001, 443–444
This journal is © The Royal Society of Chemistry 2001
443