Angewandte
Communications
Chemie
Table 1: Palladium-catalyzed methoxycarbonylation of ethylene: Inves-
[
a]
tigation of various ligands.
[
b]
[b]
[b]
L
Yield [%]
L
Yield [%]
(t)
L
Yield [%]
(t)
[
c]
[c]
[c]
(t)
L5
L7
99 (30 min)
99 (10 min)
50 (20 h)
99 (10 h)
99 (10 h)
L16
L17
L18
L19
L20
L21
L22
99 (10 min)
92 (20 h)
64 (20 h)
72 (20 h)
95 (20 h)
91 (20 h)
4 (20 h)
L23
L24
L25
L26
L27
L28
L29
99 (10 min)
34 (20 h)
3 (20 h)
92 (20 h)
91 (20 h)
0 (20 h)
dppf
L12
L13
L14
L15
99 (10 h)
99 (10 min)
14 (20 h)
[
a] Reaction conditions: Pd(acac) (6.5 mg, 0.04 mol%), bidentate
2
(
0.16 mol%) or monodentate ligand (0.32 mol%), PTSA (61 mg,
0
.6 mol%), CO (30 bar), MeOH (20 mL), ethylene (1.5 g, 53.6 mmol).
[
[
b] Determined by GC analysis using isooctane as the internal standard.
c] Reaction time. acac=acetylacetonate, PTSA=para-toluenesulfonic
acid.
Similarly, the related ligands L18–L21 afforded MeP in
4–95% yield after 20 h. However, low yields or only traces of
6
the desired product were detected when L22, L24, or L25
were used, demonstrating the importance of both sterically
hindered and basic groups on the P atoms. L23 with a
1
-adamantyl group was also demonstrated to be a highly
active ligand. When L26 or L27 was used, the desired product
was afforded in high yield after 20 h. On the other hand,
xanthene-based ligand L28 showed no activity at all. Finally,
with monodentate ligand L29, MeP was observed in only 14%
yield.
Figure 4. Palladium-catalyzed methoxycarbonylation of ethylene with
various ligands a) at 238C and b) at 808C without additional acid.
To compare the efficiency of L5 and the ferrocenyl ligands
more clearly, the methoxycarbonylation of ethylene was
carried out at room temperature under otherwise identical
conditions. As shown in Figure 4a, ligands L15, L16, and L23
notably displayed unprecedented activity and afforded the
desired product in quantitative yield within 3 h (activity
order: L16 > L15 > L23). L16 performed particularly well and
gave almost full conversion within 1 h. Ligand L5 showed
significantly lower activity, and the yield of MeP was < 10%
in this case. L12–L14, which were active at 808C, did not give
the desired product under these milder conditions, which
revealed that both sterically hindered and amphoteric groups
on the P atoms are indispensable.
the catalyst (see Figure S1). These results suggest that the
crucial PdÀH complex in the catalytic cycle can be generated
efficiently under much milder conditions.
Next, we applied our ferrocenyl ligands in the
Pd-catalyzed methoxycarbonylation of propylene under the
optimized reaction conditions (see Figure S2). As expected,
with ligands L15 and L16, propylene was carbonylated at
408C into the desired ester in quantitative yield and high
n-selectivity within 2 h. Again, only trace amounts of the
product were detected with L5 as the ligand. In addition, with
L15, the reaction still worked well at 238C and gave the
desired ester in high yield after 20 h. To the best of our
knowledge, this is the first example of propylene alkoxycar-
bonylation at such low temperatures.
A general drawback of current Pd-catalyzed alkoxycar-
bonylation processes is the requirement for acid cocatalysts,
which might lead to corrosion of the equipment. Indeed, the
C olefins are mainly obtained as coproducts of naphtha
4
combination of PdCl and our ferrocenyl ligands leads to an
cracking to produce ethylene. Hence, the Pd-catalyzed
methoxycarbonylation of Raffinate-1 was performed with
2
[23]
active catalyst system that allows for efficient ethylene
methoxycarbonylation without additional co-acids. As
shown in Figure 4b, L15 performed best in terms of activity
and gave MeP in almost quantitative yield within 1 h. With
ligands L16 and L23, the desired product was also obtained in
high yield after slightly longer reaction times (5 h). To
compare the activity of the corresponding catalysts, L5 was
also investigated, and MeP was detected in < 5% yield.
Encouraged by these results, we next carried out the reaction
under acid- and halide-free reaction conditions. Interestingly,
ligands L5 and L15 at 1208C for 20 h. High yields of C esters
5
were obtained with L15 within 10 h (see Figure S3). On the
other hand, with L5, a long induction period of 10 h was
observed, which was explained by inhibition by 1,3-buta-
[
24]
diene. This result demonstrates the durability of the new
catalyst, which is crucial in industry when dealing with bulk
chemicals with small amounts of impurities.
Finally, the effect of different alcohols was explored in the
alkoxycarbonylation of ethylene. With ligand L15, the
reaction was carried out at 808C in the corresponding alcohol.
a high yield of MeP was detected using Pd(OAc) and L15 as
2
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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