Carbonylation of o-phenylenediamine and o-aminophenol with dimethyl carbonate using lead compounds as catalysts

Article abstract of DOI:10.1006/jcat.2000.3075

Lead compounds are active catalysts for carbonylation and carbonylation/methylation of o-phenylenediamine and o-aminophenol with dimethyl carbonate (DMC). 2-Benzimidazolone was obtained in 84% yield by the reaction of o-phenylenediamine with DMC for 1 h at 443 K in the presence of Pb(NO₃)₂. In the presence of Pb(OAc)₂, the reaction quantitatively gave 1,3-dimethyl-2-benzimidazolone, which was formed by methylation of the primary product, 2-benzimidazolone, at 473 K. The effects of reaction variables on the yields of 2-benzimidazolone and 1,3-dimethyl-2-benzimidazolone were examined. The reaction of o-aminophenol with DMC selectively gave a carbonylation product, 2-benzoxazolone, or a carbonylation/methylation product, 3-methyl-2-benzoxazolone, depending on the reaction conditions in the presence of Pb(OAc)₂.

Full text of DOI:10.1006/jcat.2000.3075

Journal of Catalysis 197, 91–97 (2001)
doi:10.1006/jcat.2000.3075, available online at http://www.idealibrary.com on
Carbonylation of o-Phenylenediamine and o-Aminophenol with Dimethyl
Carbonate Using Lead Compounds as Catalysts
Yue Fu, Toshihide Baba,1 and Yoshio Ono
Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Received May 19, 2000; revised September 1, 2000; accepted September 1, 2000
Dimethyl carbonate (DMC) has been used as a substi-
tute of phosgene for a variety of carbonylation reactions
(8, 9). For example, aniline undergoes methoxycarbonyla-
tion to afford methyl phenylcarbamate in the presence of
lead compounds (10). Since DMC can be industrially pro-
duced by the reaction of methanol and carbon monoxide,
carbonylation reactions with DMC are becoming more at-
tractive in many cases.
In this work, we attempted the carbonylation of 1 and
2 with DMC to synthesize 3 and 4, respectively. We found
that lead compounds were very good catalysts for these
carbonylations.
Lead compounds are active catalysts for carbonylation
and carbonylation/methylation of o-phenylenediamine and o-
aminophenol with dimethyl carbonate (DMC). 2-Benzimidazolone
was obtained in 84% yield by the reaction of o-phenylenediamine
with DMC for 1 h at 443 K in the presence of Pb(NO3)2. In the
presence of Pb(OAc)2, the reaction quantitatively gave 1,3-
dimethyl-2-benzimidazolone, which was formed by methylation of
the primary product, 2-benzimidazolone, at 473 K. The effects
of reaction variables on the yields of 2-benzimidazolone and 1,3-
dimethyl-2-benzimidazolone were examined. The reaction of o-
aminophenol with DMC selectively gave a carbonylation product,
2-benzoxazolone, or
a carbonylation/methylation product, 3-
methyl-2-benzoxazolone, depending on the reaction conditions in
c
the presence of Pb(OAc)2.
2001 Academic Press
Key Words: o-phenylenediamine; o-aminophenol; dimethyl car-
bonate; lead compounds; 2-benzimidazolone; 2-benzoxazolone.
INTRODUCTION
2-Benzimidazolone 3 and 2-benzoxazolone 4 and their
derivatives are useful chemicals mainly as the intermedi-
ates in production of pharmaceuticals, pesticides, and her-
bicides. In practice, these compounds are produced by the
reactions of phosgene with o-phenylenediamine 1 and o-
aminophenol 2 and their derivatives (1–3). Because phos-
gene is a very toxic substance, it is important to develop a
nonphosgene reaction.
Several other synthetic methods have also been reported.
The direct carbonylation of 1 and 2 with carbon monoxide
catalytically affords 3 and 4, respectively, in the presence
of selenium and a base (4, 5). High yields of 3 (99% ) and
of 4 (84% ) were attained at 373 K. 3 and 4 can be also
prepared in high yields by reductive carbonylation of o-
nitroaniline and o-nitrophenol, respectively, with carbon
monoxide, sulfur, and water in the presence of Et3N (6, 7).
3 and 4 are also synthesized by the reaction of urea with 1
and 2, respectively, in the presence of H2SO4 or HCl (1–3).
1,3-Dimethylbenzimidazolone
5
and 3-methyl-2-
benzoxazolone 6, respectively, are also important inter-
mediates for various synthetic purposes. These compounds
are prepared by methylation of 3 and 4, respectively, with
use of methyl iodide or dimethyl sulfate as a methylating
agent. We found that 5 and 6 were easily obtained directly
from 1 and 2 by their reactions with DMC in the pres-
ence of lead compounds. Here, these reactions give rare
examples in which DMC serves as both carbonylation and
methylating agents at the same time.
EXPERIMENTAL
Metal salts of Pb, Sn, and Zn and metal oxides of Pb,
Sn, and Ti obtained from Wako Pure Chemicals Ind., were
used without treatment, unless otherwise note. Pb(OAc)2
was obtained by heating Pb(OAc)2 3H2O at 353 K for 2 h.
DMC of guaranteed grade was dehydrated with molecular
sieve 3A. Other reagents were of guaranteed grade and
used without further purification.
1 To whom correspondence should be addressed. Fax: +81-3-5734-2878.
E-mail: tbaba@o.cc.titech.ac.jp.
91
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

92
FU, BABA, AND ONO
The reactions were carried out in a 50 cm3 autoclave with- uct), 1-methyl-2-benzimidazolone 8, and 5 (carbonylation/
out using a solvent. DMC, 1 (or 2), and a catalyst were methylation products) were the predominant products,
charged in the autoclave under a nitrogen atmosphere. The though N-methyl-o-phenylenediamine was also produced.
autoclave was then set in an oil bath heated at the reaction This indicates that carbonylation was promoted selectively
temperature. At the end of the reaction time, the autoclave over these catalysts.
was cooled to room temperature and the catalyst was sep-
The product distribution strongly depended on the
arated by a centrifuge. The products in the reaction of 1 catalyst. Pb(NO3)2 and PbI2 catalysts selectively gave
with DMC were analyzed by HPLC equipped with a col- 3, the highest selectivity of 77% for 3 being obtained
umn of Crest Pak C-185 (4.6 mm i.d. 150 mm) and an UV with Pb(NO3)2. The selectivities for 3 in the presence
detector. The mobile phase was a 35 : 65 (v/v) methanol– of PbO and Ti(OCH3)4 were 65 and 66% , respectively.
water solution of NaH2PO4. The products 3 and 5 in the Pb(OAc)2 3H2O and Pb(OAc)2 Pb(OH)2 were selective
reaction of 1 with DMC were identified with GC–MS and for the production of 5. Zn(OAc)2 2H2O and Zn(OAc)2
1H NMR. The authentic sample of 5 was prepared by the also showed high activities and selectivities for 5. Sn(OAc)2
reaction of 3 with dimethyl sulfate. The products in the re- showed a high activity with a moderate selectivity for 5.
action of 2 with DMC were identified with GC–MS and PbCO3 and 2PbCO3 Pb(OH)2 showed very low activities,
1H NMR and determined with a gas chromatograph with a probably because these compounds hardly dissolved in the
capillary column of CT-1.
reaction medium. The other compounds dissolved in the
reaction medium. However, solids came out of the reaction
medium while the reaction proceeded. For example, the
formation of PbCO3 was observed by XRD measurements
after the reaction was carried out by using Pb(NO3)2.
In the presence of TiO2, SnO2, and MgO, the conversion
of 1 was improved, but the yields of 3 were 4, 1, and 7% ,
respectively.
The results in Table 1 show that the selectivities for car-
bonylation and methylation strongly depend on the nature
of catalysts. An effort was made to maximize the yield of 3
or 5 with use of lead compounds as catalysts.
RESULTS AND DISCUSSION
Carbonylation of Phenylenediamine with DMC
(a) Catalytic activities of metal compounds. The reac-
tion of 1 with DMC was carried out without using a sol-
vent. Without a catalyst, the conversion of 1 was 30%
at 433 K for 20 h and the product mainly consisted of
N-methylphenylenediamine 7 (selectivity, 75% ) and N,N0-
dimethylphenylenediamine (selectivity, 20% ). The selectiv-
ity for 3 was 5% . The predominant reaction is therefore
N-methylation of 1 rather than carbonylation.
Table 1 shows the results of the reactions of 1 with DMC
in the presence of lead, zinc, tin(II), and titanium(IV)
compounds at 433 K or 443 K for 1 h. In the presence
of these compounds as catalysts, 3 (carbonylation prod-
(b) Change in the yield of 2-benzimidazolone 3 with reac-
tion time. Asshown in Table 1, Pb(NO3)2 isthe most active
and selective catalyst for the production of 3 in the reaction
of 1 and DMC. Figure 1 shows the effect of reaction time
on the conversion of 1 and the yields of the products in the
TABLE 1
Catalytic Activities of Various Compounds for the Reaction
of 1 with DMC
Selectivity/%
Conversion
Catalyst
Pb(NO3)2
PbO yellow
PbI2
Pb(OAc)2 3H2O
Pb(OAc)2 Pb(OH)2
Zn(OAc)2 2H2O
Zn(OAc)2
of 1/%
3
8
5
7
89
60
53
51
48
35
38
13
79
63
30
77
65
72
29
31
11
19
69
66
4
10
3
3
2
2
5
3
2
3
2
4
1
54
56
63
59
5
13
28
24
15
12
23
20
24
30
44
75
PbCO3
Ti(OCH3)4
Sn(OAc)2
1
39
0
13
0
Nonea
5
FIG. 1. Effect of reaction time on the product distribution in the re-
action of 1 with DMC at 433 K; 1 = 3 mmol, DMC = 6 mmol, PbNO3 =
0.3 mmol. Conversions of 1 (᭡) and DMC (᭺); yields of 3 (3), 8 (᭿), 5
(4), and 7 (᭹).
Conditions: reaction temperature = 433 K (*443 K); reaction time =
1 h; 1 = 3 mmol; DMC/1 = 2; catalyst = 0.3 mmol.
a Other product was N,N0-dimethyl-o-phenylenediamine.

LEAD CATALYSTS FOR CARBONYLATION WITH DMC
93
presence of Pb(NO3)2 at 433 K. The conversion of 1 rapidly
increased to 90% in 1 h and then stayed constant at longer
reaction times. The yield of 3 went through a maximum at a
reaction time of 30 min, the maximum yield being 67% . The
yields of 8 and 5 increased concomitantly with a decrease
in the yield of 3. The yield of N-methylphenylenediamine
7 reached 6% in 1 h and stayed constant. The yields of 8
and 5 were not increased by increasing the reaction time
from 14 h to 35 h. This was simply due to almost complete
consumption of DMC in the first 3 h.
These results clearly indicate that 8 and 5 were formed
by the methylation of 3, not by the carbonylation of 7. This
does not mean that the carbonylation of 7 with DMC does
not proceed. By raising the reaction temperature from 433
to 473 K, the carbonylation of 7, however, proceeded. Thus,
8 was obtained with 100% selectivity at a complete con-
version of 7 in 20 h, when the reaction of 7 (3 mmol) with
FIG. 2. Effect of reaction temperature on the product distribution in
DMC (9 mmol) was carried out in the presence of Pb(NO3)2 the reaction of 1 with DMC. Reaction time = 0.5 h; 1 = 3 mmol, DMC =
6 mmol, Pb(NO3)2 = 0.3 mmol. Conversions of 1 (᭡) and DMC (᭺); yields
of 3 (3), 8 (᭿), 5 (4), and 7 (᭹).
(0.3 mmol). This result shows that the formation of 3 by the
reaction of 1 with DMC proceeds more easily than that of
7. The carbonylation of 7 with DMC for the formation of
8 proceeded at 473 K but did not proceed at 433 K, since
7 is presumably more sterically hindered than 1 though the
basisity of 3 is lower than that of 7.
the conversion of 1 and the yields of products at 443 K in the
reaction of 1 with DMC in the presence of Pb(NO3)2. At a
DMC/1 ratio of 1.0, the yield of 3 was 55% . With increasing
the DMC/1 ratio, both the conversion of 1 and the yield of 3
increased. The yield of 3 attained the highest value of 88%
with a 92% conversion of 1 at the ratio of 2.5. Increasing
the ratio to 3 was not favorable for selective production of
3 because of the methylation of 3 to 5.
(e) Effect of the ratio of Pb(NO3)2/1. The effect of the
molar ratio of Pb(NO3)2/1 on the yield of 3 at 443 K is shown
in Fig. 4. Both the conversion of 1 and the yield of 3 at 30 min
increased with the ratio up to 0.1 and were unchanged with
(c) Effect of reaction temperature on the yield of 2-
benzimidazolone 3. The reaction of 1 with DMC in the
presence of Pb(NO3)2 was carried out at various tempera-
tures. The reaction time was shortened, being 30 min. The
changes in the conversion of the reactants and the yields of
the products are shown in Fig. 2. The conversions of both 1
and DMC increased with reaction temperature. The yield
of 3 monotonically increased from 21% to 80% by increas-
ing the reaction temperature from 413 to 443 K and stayed
constant in the range from 443 to 473 K. 8 and 5 were not
formed below 443 K, since the reaction time was not enough
long. These compounds were formed at 473 K, the yields of
8 and 5 being 10% and 5% , respectively. 7 was formed in
about 4% yield in the temperature range studied.
At 443 K, 3 was obtained in about 80% yield with a 95%
selectivity based on 1. The selectivity for 3 based on DMC
was about 80% in the temperature range of 413–443 K, but
decreased to 42% at 473 K.
FIG. 3. Effect of the ratio of DMC/1 on the product distribution in
the reaction of 1 with DMC at 443 K. Reaction time = 0.5 h; 1 = 3 mmol,
Pb(NO3)2 = 0.3 mmol. Conversions of 1 (᭡); yields of 3 (3), 5 (4), and 7
(d) Effect of the ratio of DMC/1 on the product distribu-
tion. Figure 3 shows the effect of the ratio of DMC/1 on (᭹).

94
FU, BABA, AND ONO
(g) Methylation of 2-benzimidazolone 3 and 1-methyl-2-
benzimidazolone 8. In order to show that DMC serves as
a methylating agent, the reaction of 3 and 8 with DMC was
carried out at 473 K in the presence of Pb(NO3)2. The re-
action conditions were as follows: reactant, 3 mmol; DMC,
15 mmol; Pb(NO3)2, 0.3 mmol; reaction time, 20 h. In both
cases, 5 was obtained in a 100% yield.
FIG. 4. Effect of the ratio of Pb(NO3)2/1 on the product distribution
in the reaction of 1 with DMC at 443 K. Reaction time = 0.5 h; 1 = 3 mmol,
DMC = 7.5 mmol. Conversions of 1 (᭡); yields of 3 (3), 5 (4), and 7 (᭹).
The reactions of 3 and 8 with methanol, one of the prod-
ucts, was also carried out under the same reaction con-
ditions. The conversions were low (4–6% ) in both cases.
Therefore, methanol was not reactive for the methylation
involved. This indicates that the active reagent for methy-
lating the intermediates 3 and 8 is almost exclusively DMC,
but not methanol.
further increasing the ratio of Pb(NO3)2/1. The selectivity
for 3 was always high at this temperature.
(f) Synthesis of 1,3-dimethylbenzimidazolone 5. The
general synthesis method for 5 is N-methylation of 3 with
methyl iodide in the presence of sodium hydroxide. As de-
scribed earlier, an appreciable amount of 5 was formed in
the reaction of 1 with DMC depending on the catalyst used.
Here, we attempted the direct synthesis of 5 from 1 by its
reaction with DMC.
To enhance the selectivity for 5 the higher DMC/1 ra-
tio of 5 and longer reaction time of 20 h were employed.
The catalysts used were acetates of Pb, Sn, and Zn and
Pb(NO3)2. The results are shown in Table 2. Under these
conditions, the conversion of 1 was 100% in every case. The
reaction was very selective in the presence of Pb(OAc)2
and Sn(OAc)2 , the selectivities for 5 being 100% and 98% ,
respectively. The selectivity was moderate in the case of
Zn(OAc)2 2H2O. However, Pb(NO3)2 was not an effec-
tive catalyst for selective production of 5, this catalyst being
more suitable for selective production of 3.
Carbonylation of o-Aminophenol with DMC
(a) Effect of reaction temperature. The reaction of 2
with DMC (DMC/2 = 2) was carried out in the presence of
Pb(OAc)2 in the temperature range between 413 and 473 K.
The results are shown in Fig. 5. The conversion of 2
TABLE 2
Synthesis of 6 by the Reaction of 1 with DMC
Yield/%
Conversion
Catalyst
Pb(OAc)2
Sn(OAc)2
Zn(OAc)2 2H2O
Pb(NO3)2
of 1/%
3
8
5
7
100
100
100
100
0
0
0
0
100
98
83
0
2
4
9
0
13
39
FIG. 5. Effect of reaction temperature on the product distribution in
the reaction of 2 with DMC. Reaction time = 3 h; 2 = 3 mmol, DMC =
6 mmol, Pb(OAc)2 = 0.3 mmol. Conversion of 2 (᭺); yields of 4 (᭹), 6 (᭡),
N,N-dimethylanisidine (3), and N,N-dimethyl-o-aminophenol + N-
methyl-o-anisidine products (4).
23
29
Conditions: reaction temperature = 473 K; reaction time = 20 h; 1 =
3 mmol; DMC/1 = 5; catalyst = 0.6 mmol.

LEAD CATALYSTS FOR CARBONYLATION WITH DMC
95
The results described above show that the rates of car-
bonylation and methylation depend greatly on the ratio of
DMC and 2. The lower DMC/2 or higher concentration
of 2 facilitates methylation. At low DMC/2 ratio, methy-
lation of 2 occurs to give N,N-dimethylanisidine together
with a carbonylation/methylation product 6. At a DMC/2
ratio of 5, direct methylation of 2 becomes slower and 2
undergoes carbonylation to give 4, which is readily methy-
lated to give 6 as a main product. At a DMC/2 ratio of
20, methylation becomes slower and the methylation of the
primary carbonylation product 4 does not occur extensively
and the main product at this stage becomes 4. The slight in-
crease in the yield of 6 at higher DMC/2 ratios may be sim-
ply due to higher concentration of the methylating agent,
DMC.
There is a possibility that the methylation at lower
DMC/2 values is catalyzed by the reactant itself, namely,
FIG. 6. Effect of the ratio of DMC/2 on the product distribution in the
reaction of 2 with DMC. Reaction temperature = 473 K; reaction time = o-aminophenol, which might serve as a base catalyst. It
3 h; 2 = 3 mmol, Pb(OAc)2 = 0.3 mmol. Conversion of 2 (᭺); yields of
4 (᭹), 6 (᭡), and methylated products (3).
has been shown that methylation of phenol proceeds se-
lectively in the presence of base catalysts such as K2CO3
and CsOH/Al2O3 (11).
(c) Effect of reaction time on the product distribution.
Figure 7 shows the effect of the reaction time on the yield
of 4. The reaction was carried out at 273 K with a DMC/2
ratio of 15 using Pb(OAc)2 as a catalyst. Under these con-
ditions, the product was mainly 4. At a reaction time of
0.25 h, the yield was 83% and it increased to 89% at
0.5 h, and did not change at longer reaction times. The
greatest part of the other products was 6, the yield being
below 3% .
increased from 59% to 97% with increasing reaction
temperature. The main products were 4, 6, and N,N-
dimethyl-o-anisidine. Doubly methylated products (N,N-
dimethyl-o-aminophenol and N-methyl-o-anisidine) were
also formed in a small amount at lower temperatures.
The yield of 4 was 14% and did not change with tem-
perature, while the yield of 6 increased from 13% to
54% with increasing reaction temperature. The yield of
N,N-dimethyl-o-anisidine was slightly increased from 22%
to 25% .
(d) Effect of the catalyst amount. Figure 8 shows the
effect of the ratio of Pb(OAc)2/2 on the yield of 4 at 473 K
(b) Effect of ratio of DMC/2 on the product distribution.
The effect of the DMC/2 molar ratio was examined in the
range between 1 and 50 at 473 K, using Pb(OAc)2 as a cata-
lyst. The amount of 2 was kept constant, 3 mmol. The ra-
tio greatly affected the product composition as shown in
Fig. 6. The conversion of 2 was 74% at a DMC/2 ratio of 1
and it increased with the ratio and reached 100% at a ratio
of 7.5. The products were mostly 4 and 6. At lower DMC/2
values, the methylation products of 2 such as N,N-dimethyl-
o-anisidine was formed as described in the previous
section.
The yield of 4 was about 40% at a DMC/2 ratio of 1. It
decreased with increasing the ratio up to 5, where the yield
became zero. The yield of 4 again increased as the ratio was
increased and became a constant value of 89% in the range
between 5 and 20. The yield of 4 again decreased slightly
with further increasing the DMC/2 ratio.
The yield of 6 was just opposite to that of 2. It first in-
creased and reached a maximum value of 92% at a DMC/2
ratio of 5, where the conversion of 2 was 97% . The yield of
6 decreased at higher ratios, to 9% at a ratio of 20. It then
gradually increased at higher ratios.
FIG. 7. Effect of the reaction time on the yield of 4 in the reaction
of 2 with DMC. Reaction temperature = 473 K; 2 = 3 mmol, DMC =
45 mmol, Pb(OAc)2 = 0.3 mmol. Conversion of 2 (᭺); yields of 4 (᭹),
6 (᭡), and methylated products (3).

96
FU, BABA, AND ONO
scheme:
The activities for carbonylation and methylation of a
catalyst probably depend on the ligand and/or counteran-
ion of Pb2+ ions. Reactant or intermediate amines can
be the ligand to possibly modify the catalytic nature of
metal cations. The balance of the two functions, carbony-
lation and methylation, decides the distribution of the
products. As described above, o-aminophenol seems to
serve as a base catalyst for its own methylation with
DMC.
FIG. 8. Effect of the ratio of Pb(OAc)2/2 on the yield of 4 in the
reaction of 2 with DMC. Reaction temperature = 473 K; reaction time =
0.5 h; 2 = 3 mmol, DMC = 45 mmol. Conversion of 2 (᭺), yields of 4 (᭹),
6 (᭡), and methylated products (3).
at a reaction time of 0.5 h. The Pb(OAc)2/2 ratio was var-
ied between 0.025 and 0.1 by changing the amount of the
catalyst. The yield of 4 increased from 33% to 89% by in-
creasing from 0.025 to 0.1. The product distribution hardly
changed with the Pb(OAc)2/2 ratio.
CONCLUSION
Lead compounds are very effective agents for carbony-
lation of 1 and 2 with DMC. The reaction of 1 with DMC
afforded 3 in an 88% yield at 443 K, using Pb(NO3)2 as
a catalyst. When the DMC/1 ratio was high enough, the
reaction of 1 with DMC quantitatively gave 5 in the pres-
ence of Pb(OAc)2 at 473 K. The reaction proceeded succes-
sively. Thus, 3 is the primary product formed by carbonyla-
tion of 1, while 5 was the secondary product formed by the
methylation of 3. In this case, DMC serves as a reagent for
methylation as well as carbonylation.
The yield of 4 could be increased to 89% even at the
lowest Pb(OAc)2/2 ratio, 0.025, by prolonging the reaction
time to 3 h.
Reaction Mechanism
The plausible reaction mechanism for the carbonylation
of 1 is expressed as follows:
The reaction of 2 with DMC at a DMC/2 ratio of 15 gave 4
in an 89% yield at 473 K, while 6 was obtained in a 92% yield
when the DMC/2 ratio was 5 in the presence of Pb(OAc)2.
In the latter case, the reactant itself, 2 serves as a base cata-
lyst for the methylation, while the carbonylation of 2 is cata-
lyzed by Pb(OAc)2.
REFERENCES
1. Hofmann, K., “The Chemistry of Imidazole and its Derivatives
(Part I),” p. 285. Interscience Publishers, London, 1953.
2. Smith, D. M., in “The Chemistry of Heterocyclic Compounds, Vol. 40,
Benzimidazoles and Congeneric Tricyclic Compounds” (P. N. Preston,
Ed.), p. 311. Wiley, New York, 1981.
3. Boyd, G. V., Katritzky, A. R., and Rees, C. W., “Comprehensive
Heterocyclic Chemistry,” Vol. 8, p. 178. Pergamon Press, Oxford,
1984.
Here, the Pb species serves as a Lewis acid to enhance the
polarization of the carbonyl group of DMC, which interacts
with the amino group of 1. The carbonylation of 2 plausibly
proceeds in a similar mechanism.
The methylation of 3 with DMC in the presence of a
Pb compound is considered to proceed in the following

LEAD CATALYSTS FOR CARBONYLATION WITH DMC
97
4. Sonoda, N., Yamamoto, G., Natsukawa, K., Kondo, K., and Murai, S.,
Tetrahedron Lett. 1969 (1975).
7. Macho, V., Kra´lik, M., and Bojsova´, E., Collect. Czech. Chem. Com-
mun. 62, 325 (1997).
5. Yoshida, T., Kanbe, N., Murai, S., and Sonoda, N., Bull. Chem. Soc.
Jpn. 60, 1793 (1987).
8. Tund, P., and Selva, M., Chem. Tech. 25, 31 (1995).
9. Ono, Y., Appl. Catal. 155, 133 (1997).
6. Miyata, T., Kanbe, N., Murai, S., and Sonoda, N., Nihon Kagaku Kaishi
1332 (1987).
10. Fu, Z.-H., and Ono, Y., J. Mol. Catal. 91, 399 (1994).
11. Fu, Z.-H., and Ono, Y., J. Mol. Catal. A 118, 293 (1997).