Angewandte
Research Articles
Chemie
Dupont, Shell, Dow, Kuraray and Sinopec investigated the
synthesis of adipate esters from 1,3-butadiene.[10] Neverthe-
less, no industrially viable process has been developed yet,
although some pilot facilities involving at least two-step
reactions were realized (Scheme 1b and c).[12] Most notable,
Drent and co-workers described a single-pot dimethoxycar-
bonylation of 1,3-butadiene in the presence of a single
catalytic system.[10k] A drawback of these processes is the
low regioselectivity for the desired linear diester, although
high selectivity has been obtained in related hydroxycarbo-
nylations.
Recently, we discovered that a particular bidentate
phosphine ligand (HeMaRaPhos) L1 allowed the palladi-
um-catalyzed dicarbonylation of 1,3-butadiene towards dia-
lkyl adipates in ꢀ 95% yield and ꢀ 97% selectivity
(Scheme 1d).[12] Crucial for the success of this ligand is the
intrinsic basic pyridyl substituent combined with a tert-butyl
group on one of the phosphorous atoms, which proved to be
essential for high activity in other palladium-catalyzed
alkoxycarbonylation reactions too.[13]
In order to understand the different reactivity of He-
MaRaPhos and 1,2-bis-di-tert-butylphosphinoxylene (L2,
dtbpx), we re-investigated the reactivity of the latter system
in more detail. Notably, L2 is commercially used in the
alkoxycarbonylation of ethylene (Lucite a-process)[4] and can
be considered as a benchmark ligand for such reactions. In our
study, we discovered a peculiar solvent effect, which laid the
basis for a new protocol for direct dicarbonylation of 1,3-
butadiene and related dienes in the presence of several
palladium catalyst systems.
HeMaRaPhos L1 or dtbpx L2 under previously optimized
conditions (1208C, 40 bar CO with p-toluenesulfonic acid as
a co-catalyst; Table 1). As expected in the presence of L1 both
reactions worked well and the desired esters were obtained
after 24 h in 86/85% yield and 97% selectivity (Table 1
entries 1–2). Surprisingly, applying L2 at similar conditions
a significantly different performance was observed depending
on the alcohol used (Table 1, entries 3–4). In case of
methanol, no desired dicarbonylation was detected and no
conversion took place at all. A similar behavior was observed
using ethanol and n-propanol (Figure S4). In contrast, using
n-butanol the corresponding di-n-butyl diester was obtained
in 80% yield and 98% selectivity. For the performance of
other alcohols see Figure S4. To understand this unusual
behavior, the reaction of 1,3-butadiene with methanol was
performed in different solvents (Table 1, entries 5–11). All
these experiments were completed in the presence of 4 equiv
of methanol. We first tested toluene, which is frequently used
in carbonylation reactions and were surprised to observe the
desired product in substantial amount. In fact, simply the
addition of toluene led to 80% of 4aa with 97% selectivity
compared to the originally non-reactive system.
Similarly, in the presence of tetrahydrofuran, heptane, and
dichloromethane, product 4aa is formed, albeit in lower yield.
Other co-solvents such as ethyl acetate and acetonitrile only
led to traces of 4aa. As toluene gave the best result among the
tested solvents, the influence of the toluene concentration on
the mono- and dicarbonylation process was investigated in
more detail. As shown in Figure 1, our recently developed
palladium catalyst system with L1 is very robust and not
affected by the concentration of methanol. Consequently,
product yields between 80 and 85% were obtained. On the
other hand, the catalyst with L2 is very sensitive with respect
to the methanol concentration and the product yield of 4aa is
increased with decreasing methanol concentration. Compar-
ing the kinetic behavior of both catalyst systems, some
peculiar differences became apparent (Figure 2). The one
containing dtbpx L2 shows a prolonged induction period
(6–8 h), while in the presence of L1 in the first two hours 3aa
Results and Discussion
At the beginning of this work, we compared the dicarbo-
nylation of 1,3-butadiene with industrially most relevant
methanol and n-butanol in the presence of Pd(TFA)2 and
Table 1: Pd-catalyzed alkoxycarbonylation of 1,3-butadiene with metha-
nol and butanol: Variation of solvents.
Entry
Ligand
Solvent
Yield [%]
Selectivity
(n-/iso-)
1
2
3
4
5
6
7
8
9
L1
L1
L2
L2
L2
L2
L2
L2
L2
L2
L2
MeOH
nBuOH
86
85
0
80
80
62
25
0
97/3
97/3
–
98/2
97/3
96/4
97/3
–
MeOH
nBuOH
toluene
THF
heptane
MeCN
CH2Cl2
31
trace
18
96/4
–
97/3
10
11
EtOAc
toluene:MeCN (1:1)
For reaction conditions and more details, see Supporting Information,
Section 3.8, Table S7.
Figure 1. Pd-catalyzed alkoxycarbonylation of 1,3-butadiene with meth-
anol in toluene.
9528
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9527 –9533