PdCl2/ZrO2–SO24 FOR SYNTHESIS OF CARBAMATES
527
When the reaction was complete, the reactor system was n-hexylamine; and N,N0-dicyclohexylethane diamide
cooled to ambient temperature and the resulting liquid (24% ), cyclohexanone (9% ), and ethyl esters of C4 to C6
mixture was directly analyzed with a Hewlett–Packard carboxylic acids (2% ) for cyclohexylamine. Under the
6890/5793 GC–MS instrument equipped with a HP-5MS same reaction conditions, the 1,6-hexyldiamine was almost
column (30 m in length). The qualitative and quantitative completely oxidized to C4 to C6 aliphatic acid esters, prob-
data of the substrate and products were directly given by ably because the aliphatic amines possess relatively strong
the system of GC/MS chemstation according to the area of reducibility and could be easily overoxidized. Therefore,
each chromatographic peak.
oxidative carbonylation of cyclohexylamine (entry 11) and
The experimental results for the carbonylation of dif- 1,6-hexyl diamine (entry 13) with lower oxygen concentra-
ferent aliphatic and aromatic amines are listed in Table 1. tion, i.e., the CO/O2 pressure ratio = 3.3/0.7, was further
For aromatic amines, it can be seen that ZrO2–SO24 had conducted. The results showed that conversion decreased
a strong impact on both conversion and selectivity; e.g., with decreasing oxygen concentration. The selectivity for
77% of aniline conversion and 70% of selectivity for the the desired product was also decreased to 32% for cyclo-
corresponding carbamate were obtained if PdCl2 was used hexylamine, and the main products obtained were amide
as catalyst only. The main by-products formed were 22% (monoamide 44% , diamide 40% ) for 1,6-dihexylamine.
quinoline and 7% benzamide (entry 1, Table 1). The con-
Under the same reaction conditions, RuCl3/ZrO2–SO42
version and selectivity to the desired product could be catalyst was investigated for the oxidative carbonyla-
much enhanced if PdCl2/ZrO2–SO24 was used as catalyst tion of amines (entries 14–16). The experimental re-
(entries 2 and 3), and almost 100% conversion and 99% se- sults indicate that the catalytic performance was less ef-
lectivity were achieved when methanol was used as solvent fective in comparison with PdCl2/ZrO2–SO24 catalyst.
(entry 2). The corresponding turnover frequency (TOF) The trend of conversion toward aromatic or aliphatic
reached 380, which is much higher than that reported in amines and the distribution of by-products obtained from
previous literature (7). This may imply that there exists RuCl3/ZrO2–SO24 catalyst were also similar to those of
some kind of synergism between PdCl2 and ZrO2–SO42
the PdCl2/ZrO2–SO24 catalyst system, suggesting that the
for the highly selective formation of corresponding car- mechanisms of oxidative carbonylation was similar on both
bamates. In comparison with ethanol, slightly higher se- catalysts.
lectivity for the desired product could be achieved when
From further analysis of the experimental results of
methanol was used as solvent; reaction results with bet- entries 1, 2, and 6–8, it can be seen that ZrO2–SO42
ter repeatability, however, were obtained with ethanol as possessed strong oxidizing capability and played an impor-
solvent. When -napthylamine was oxidatively carbony- tant role in further promoting the oxidation and esterifi-
lated, almost 100% conversion and 95% selectivity could be cation of carbonylated intermediates into corresponding
achieved as the reaction proceeded in only 15 min. It is note- carbamates during the reaction.
worthythat reasonablyhigh conversion and selectivitywere
In summary, the synthesis of carbamates by oxidative
achieved when the substrate used was 2,4-diaminetoluene, carbonylation of amines over a PdCl2(RuCl3)/ZrO2–SO42
which is more difficult to carbonylate and is an im- catalyst system can proceed with high conversion and selec-
portant precursor for more useful toluene diisocyanate tivity, especially for aromatic amines. This catalyst system
formation. The main by-product was monocarbamate is relatively simple and can be further optimized. To our
(entry 5).
knowledge, it is the first time that such a catalyst system
The oxidative carbonylation of aliphatic amines was also has been used for the oxidative carbonylation of amines.
investigated under similar reaction conditions. They could The role of ZrO2–SO24 and the synergism between homo-
be almost completely converted either in the presence of geneous PdCl2(RuCl3) and heterogeneous ZrO2–SO24 is
PdCl2/ZrO2–SO24 catalyst or in the presence of PdCl2 now under further investigation.
(entry 6) or ZrO2–SO24 (entry 7) as catalyst only. The sele-
ctivities for the desired products, however, were generally
lower in comparison with the aromatic amines, and varied
REFERENCES
greatly with different aliphatic amines. For the aliphatic n-
1. Cenini, S., and Ragaini, F., “Catalytic Reductive Carbonylation of
butyl-, n-hexyl-, and cyclohexylamine (entries 8, 9, and 10),
the selectivities for the corresponding carbamates were 80,
81, and 65% , respectively. The main by-products during ox-
idative carbonylation were oxidized aliphatic amines, i.e.,
ethyl butyrate (15% ), N-butylformamide (3% ), n-butyric
aldehyde (2% ) for n-butyl amine; hexaldehyde (11% )
and ethyl esters of C4 to C6 carboxylic acids (8% ) for
Organic Nitro Compounds.” Kluwer Academic, Dordrecht, 1997.
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(1993).
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(1994).
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Soc. Chem. Commun. 217 (1996).