Angewandte
Chemie
DOI: 10.1002/anie.200603751
Carbonylation
Borate Esters as Alternative Acid Promoters in the Palladium-
Catalyzed Methoxycarbonylation of Ethylene**
Alta C. Ferreira,* Renier Crous, Linette Bennie, Anna M. M. Meij, Kevin Blann,
Barend C. B. Bezuidenhoudt, Desmond A. Young, Mike J. Green, and Andreas Roodt*
Since the early 1990s there has been considerable interest in
the alkoxycarbonylation of olefins, a potentially important
[
1–5]
reaction for the production of commodity chemicals.
The
attention devoted to this chemistry resulted in the develop-
[
6]
ment by Lucite International of a two-step process for the
production of methyl methacrylate (MMA) in which the
initial step, the carbonylation of ethylene, is catalyzed by a
palladium/bidentate phosphine/acid system. The choice of
acid in this step is important, as it determines the type of
counterion available for the cationic palladium species. A
strongly coordinating anion will reduce the rate of the
kinetically important addition of CO to C H , whereas
[
12]
In the early 1990s British Petroleum
described the
application of borosalicylic acid as a proton source for the
palladium-catalyzed polymerization of ethylene and carbon
monoxide, again in the presence of a chelating phosphine
ligand. BSA forms during the condensation reaction between
salicylic acid and boric acid (B(OH) ) to yield the 1:1 or 1:2
3
[
13]
borate complexes.
The formation of the 1:2 complex
2
4
weakly coordinating or noncoordinating anions allow the
facile coordination of these reagents.
liberates one proton and three water molecules. X-ray crystal
structure analyses of borate complexes with salicylic acid
[
7,8]
[14,15]
Strong acids, such as methanesulfonic acid (MSA) or p-
toluenesulfonic acid, which contain weakly coordinating
anions, are typically used to achieve the required reaction
rates; however, one consequence when using monodentate
confirm the existence of these species.
The performance of BSA (formed in situ and preformed)
in the palladium/triphenylphosphine-catalyzed carbonylation
of ethylene was compared with that of MSA and TFA as
benchmarks. The reaction rates in the presence of different
[
9]
phosphine ligands is the rapid alkylation thereof. This loss of
phosphine inevitably leads to unstable palladium species and
subsequent metal plating. Although the utilization of a weak
acid, such as trifluoroacetic acid (TFA), can partially decrease
the formation of phosphonium salts, significant loss of
phosphine still occurs, and hence complex and expensive
chelating ligand systems had to be developed for this type of
acids, as well as the amount of PPh remaining after a TON of
3
1000 had been reached, are reported in Table 1 (TON = mol
Table 1: Palladium-catalyzed methoxycarbonylation of ethylene with
[a]
various acid promoters.
À1 [b,c]
[d]
[
10,11]
Entry
Acid
T [8C]
TOF [h
]
STY
PPh
3
remaining [%]
reaction.
[
e]
Our aim was to identify alternative acid promoters to
enable the effective use of simple monodentate ligands. We
report herein the use of bis(salicylato)boric acid (borosalicylic
acid, BSA) as an attractive acid promoter for the palladium-
catalyzed methoxycarbonylation of ethylene with triphenyl-
phosphine as the ligand [Eq (1)].
1
2
3
4
5
6
7
MSA
BSA
BSA
TFA
MSA
BSA
TFA
110
110
110
110
120
120
120
2130
1020
886
4.50
2.15
2.02
1.14
10.64
3.77
2.45
28
>99
[f]
[
g]
>99
72
9
77
10
572
3528
1249
812
[
(
[
(
a] pfinal =20 bar (CO/C H 1:1), MeOH (120 mL); entries 1–4: Pd(OAc)
[
*] Dr. A. C. Ferreira, Dr. R. Crous, Dr. L. Bennie, Dr. A. M. M. Meij,
Dr. K. Blann, Dr. D. A. Young, Dr. M. J. Green
SASOL Technology
2
4
2
2 mm), PPh3 (100 mm), acid (200 mm; [B(OH) ]=200 mm for BSA,
3
B(OH) ]/[salicylic acid] 1:2); entries 5–7: Pd(OAc) (3 mm), PPh3
3
2
150 mm), acid (450 mm; for BSA: B(OH)3 (450 mm), salicylic acid
1350 mm)). [b] Calculated after 10 min. [c] Turnover frequency [mol 1
1
Klasie Havenga Road, Sasolburg, 1947 (South Africa)
(
Fax: (+27)11-522-3856
E-mail: alta.ferreira@sasol.com
formed per mol Pd and h] calculated according to the gas-uptake curve.
d] Site–time yield [mol 1 consumed per mol active sites and h at low
conversion] calculated according to the gas-uptake curve. [e] Calculated
[
Dr. B. C. B. Bezuidenhoudt, Prof. A. Roodt
The Department of Chemistry
University of the Free State
Bloemfontein, 9300 (South Africa)
Fax: (+27)55-444-6384
3
after TON=1000. [f] After 10 h, 94% of PPh remained. [g] Preformed
BSA was used.
E-mail: roodta.sci@mail.uovs.ac.za
methyl propionate (1) formed per mol catalyst). MSA at
108C showed the highest activity and TFA at 1208C the
lowest activity when the total concentration of acid was
identical. The results of a typical reaction promoted by
BSA are shown in Figure 1a, and in Figure 1b those of
[
**] The authors thank SASOLTechnology for financial support, and A.R.
thanks the research fund of the UFS. The South African NRF is
gratefully acknowledged (GUN 2068915, to A.R.).
1
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 2273 –2275
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2273