Table 1 Influence of the choice of the co-catalyst on the catalyst
productivity in the oxidative carbonylation of 1aa
Table 2 Solvent effect and substrate scope in the oxidative carbonyla-
tion of 1a–ca
Entry
Co-catalyst
Additive
TON
Entry
Substrate
Solvent
TON
1
2
CuCl2
Mn(acac)3
—
CuCl2
Mn(acac)3
—
NaOAc
NaOAc
NaOAc
KBr
KBr
KBr
12
7
5
20
255
30
1
2
3
4
5
6
7
8
1a
1b
1c
1a
1a
1a
1b
1c
DME
DME
DME
THF
Me-THF
Ethyl acetate
Ethyl acetate
Ethyl acetate
255
342
91
784
697
775
615
231
3
4b
5b
6b
a Reaction conditions: [1a]0
=
0.45 mM, 1a : Pd(OAc)2 : co-
catalyst : NaOAc = 1250 : 1 : 20 : 1000, solvent = DME, T = 60 ◦C, 20 bar
CO : O2 : N2 (6 : 3 : 91), time = 20 h. b 1a : Pd(OAc)2 : co-catalyst : KBr =
1250 : 1 : 100 : 1000.
a Reaction conditions: [1]0 = 0.45 mM, 1 : Pd(OAc)2 : Mn(acac)3 : KBr =
1250 : 1 : 20 : 100, solvent (2 mL), T = 60 ◦C, 20 bar CO : O2 : N2 (6 : 3 : 91),
time = 20 h.
We started our investigations by carrying out the oxidative
carbonylation experiments under the conditions developed by
Tam,7 with the variant that molecular oxygen was used as
the oxidant, while CuCl2 was employed in sub-stoichiometric
amounts, in a Pd : Cu ratio of 1 : 20 (Table 1, entry 1). Compared
to the reference system, the loading of the palladium precursor
was decreased to reduce Pd(0) aggregation. By following this new
procedure, the reaction proceeded although with a low catalyst
productivity (TON = 12 with respect to palladium). Under the
same conditions, the use of Mn(acac)3 instead of CuCl2 gave
an even lower TON of 7 (Table 1, entry 2). Interestingly, when
the additive NaOAc was replaced by KBr, Mn(acac)3 greatly
outperformed CuCl2 resulting in a TON of 255 (Table 1, entries
4–5), which means that every Mn ion was utilized on average
11 times. In the absence of co-catalyst much lower catalyst
productivities were obtained (Table 1, entries 3 and 6).
This series of experiments clearly shows that in consistency
with the concept (Scheme 2) the combination of a palladium
catalyst with an appropriate co-catalyst provides a highly
efficient system for the oxidative carbonylation of ethylene glycol
1a with molecular oxygen as the oxidation agent. The higher
TON obtained with Mn(acac)3 compared to CuCl2 indicates that
it provides the right redox potential within the redox cascade. A
low efficiency of CuCl2 for the oxidative carbonylation of 1a was
also reported by Tam,7 who suggested that the substrate binds
strongly to the copper ions and thus makes itself inaccessible to
the Pd catalyst.
We noted that in experiments carried out in 1,2-
dimethoxyethane (DME) with Mn(acac)3 as the co-catalyst a
brown solid was present at the end of the reaction. Given that the
mass balance of reactants and products was close to quantitative
(≥98%), the formation of insoluble polycarbonates is excluded.
Also, no side products were observed by gas chromatography.
We speculate that a coordination compound containing Mn-
DME units was formed. To test this hypothesis, a series of
non-chelating aprotic polar solvents, namely THF, Me-THF
and ethyl acetate were evaluated under the reaction conditions
reported above. The transformation benefited largely from the
change of reaction media, since TONs close to or higher than 700
were attained (Table 2, entries 4–6). This corresponds to catalyst
activities of 35–39 molsubstrate/molpalladium/h over a longer period
of time (20 h). Clearly, non-chelating solvents are beneficial.
Substituted ethylene glycols such as methyl ethylene glycol 1b
and phenyl ethylene glycol 1c were also evaluated. While in ethyl
acetate the productivity decreased in the sequence 1a > 1b >
1c (Table 2, entries 6–8), the trends were not as clear in DME
(Table 2, entries 1–3). The steric requirement of the substituent
R apparently plays an important role.
Several Pd salts were evaluated in the absence of additives
(Table 3). Also in this case, PdBr2 was most efficient (Table
3, entry 3) indicating that the presence of bromide ions is
essential to achieve a good catalyst performance. Note that the
combination of Pd(OAc)2 and KBr (Table 3, entry 4) clearly
outperformed PdBr2. A similar, PdI2-KI catalyst system had
been reported by Gabriele et al. for the oxidative carbonylation
of various aliphatic diols with oxygen as the sole oxidant.8
However, when we used KI as additive under our conditions,
the results remained inconclusive.
The use of KBr as an additive also seems to be essential.
Even in the absence of co-catalyst, higher catalyst produc-
tivities were achieved with KBr as the additive compared to
NaOAc. As the effectiveness of the two co-catalysts under
study varied significantly in relation to the additive, it prompted
us to evaluate several other additives such as KCl, LiCl and
tetrabutylammonium bromide.13,14 None of these additives used
in combination with Mn(acac)3, was superior to KBr (see
supplementary information†).
Table 3 Influence of the palladium precursor on the conversion
obtained in the oxidative carbonylation of 1aa
Entry
Catalyst
Additive
TON
In order to assess the role of KBr in more detail, a series of tests
at different Pd : KBr ratios was carried out. The amount of KBr
barely affected the catalyst productivity in the Pd : KBr range
of 1 : 5 to 1 : 500 and a constant TON between 105 and 123 was
observed at a reduced reaction time of 2 h (see supplementary
information†). A Pd : KBr ratio of 1 : 5 was adequate to achieve
a TON of 117. This is equivalent to a catalyst activity (TOF)
of 59 molsubstrate/molpalladium/h. When KBr was omitted, a much
lower TON of 14 was obtained.
1
2
Pd(OAc)2
PdCl2
PdBr2
—
—
—
KBr
13
5
91
123
3
4b
Pd(OAc)2
a Reaction conditions: [1a]0 = 0.45 mM, 1a : Pd(OAc)2 : Mn(acac)3
=
1250 : 1 : 20, solvent
= DME (2 mL), T =
60 ◦C, 20 bar
CO : O2 : N2 (6 : 3 : 91), time = 2 h. b 1a : Pd(OAc)2 : Mn(acac)3 : KBr =
1250 : 1 : 20 : 100.
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The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 292–295 | 293
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