Methyl formate as a carbonylating agent for the catalytic conversion of phenol to methyl phenyl carbonate

Article abstract of DOI:10.1039/c3gc36877g

Methyl formate was used as a green and efficient carbonylating agent in the synthesis of methyl phenyl carbonate from phenol. Methyl formate showed better performance compared to toxic CO gas and the ability to produce other useful carbonylated products, e.g., dimethyl carbonate and dimethyl oxalate.

Full text of DOI:10.1039/c3gc36877g

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Methyl formate as a carbonylating agent for the
catalytic conversion of phenol to methyl phenyl
Cite this: Green Chem., 2013, 15, 1146
Received 23rd November 2012,
Accepted 12th March 2013
carbonate†
a
b
b
b
DOI: 10.1039/c3gc36877g
Mohammad S. Yalfani, Giulio Lolli,* Aurel Wolf, Leslaw Mleczko,
a
c,d
www.rsc.org/greenchem
Thomas E. Müller and Walter Leitner
2 3 2
Methyl formate was used as a green and efficient carbonylating against strongly basic methoxide. Using PdCl (PPh ) as a cata-
agent in the synthesis of methyl phenyl carbonate from phenol. lyst under such basic conditions, the oxidative carbonylation
9
Methyl formate showed better performance compared to toxic of alkenes was reported. The key steps in this reaction are (i)
3
CO gas and the ability to produce other useful carbonylated pro- decarbonylation of MF by NaOCH , (ii) formation of a
ducts, e.g., dimethyl carbonate and dimethyl oxalate.
methoxycarbonyl–Pd complex, (iii) insertion of alkene and (iv)
β-hydride elimination giving the olefinic ester.
The oxidative carbonylation of phenol to diphenylcarbonate
1
(DPC) has been studied extensively during recent decades.
The method is one of the most promising alternatives to
1
a
replace the current industrial process of DPC production.
ð1Þ
DPC is a key component for the manufacture of polycarbo-
nates. However, this process needs to be improved due to
hazard problems related to the handling of toxic CO gas.
Thus, a process involving a safer and more sustainable carbo-
nylating agent is preferred for industrial applications.
Methyl formate (MF) has been widely utilized as a C build-
ing block in organic synthesis. This key intermediate can be
2
produced by hydrogenation of CO in the presence of meth-
anol, enabling the effective recycling of carbon into chemical
2
In this study, MF was utilized as a carbonylating agent for
phenol to obtain methyl phenyl carbonate (MPC) as the target
product. MPC can be converted subsequently to DPC by trans-
esterification with phenol. To the best of our knowledge,
there is no report on the application of MF in the carbonyl-
ation of phenol. As phenol is much more acidic than MF, it
reacts faster with methoxide. Therefore, the activation of MF
(Step 1) was separated from the carbonylation of phenol
1
1
0
3
4
5
products. Using an appropriate catalyst, MF can be activated
6
a
to take part in carbonylation reactions. Transition metals
like Ru and Ir are able to activate MF via carbonyl–hydrogen
bond dissociation, which leads to direct methoxycarbonyl-
ation. This reaction usually requires temperatures higher
than 180 °C. Activation of MF by strong bases (e.g., NaOCH3)
results in partial decarbonylation to CO and CH OH at low
temperatures (40–80 °C). Given the weak acidity of MF, the
hydrogen bound to the carbonyl group acts as a proton source
(
Step 2), as summarized in eqn (1) (see Experimental details in
S1, ESI†).
Step 1: Activation of MF was catalysed here by alkali metal
6
b
methoxides MOCH (M = Na or K), which resulted in increas-
3
ing pressure in the reactor. This pressure was generated by the
partial decarbonylation of MF and formation of CO (vide infra).
Step 2: For the carbonylation of phenol by MF, various well-
3
7
8
known Pd(II) catalysts were screened together with Mn(acac)
3
as the co-catalyst. The pressure at the beginning of this step
reached 67–69 bar. As the results in Table 1 show, a high selec-
tivity to MPC along with a low conversion for phenol was
obtained. Another phenolic product was phenyl acetate (PAC).
Among these Pd catalysts, PdBr , Pd(OAc) and Pd(salen)
a
CAT Catalytic Center, RWTH Aachen University, Worringerweg 1, D-52074 Aachen,
Germany
b
Bayer Technology Services GmbH, D-51368 Leverkusen, Germany.
E-mail: giulio.lolli@bayer.com; Fax: +49-214-30-81118; Tel: +49-214-30-48420
c
2
2
Lehrstuhl für Technische Chemie und Petrolchemie, Institut für Technische und
Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, D-52074
Aachen, Germany
showed selectivities to MPC higher than 90% (entries 1, 4 and
5). Exchange of CH Cl by non-halogenated polar-aprotic
solvents, such as dimethyl formamide and tetrahydrofuran,
provided similar results (entries 2 and 3). Also other redox
couples, such as PdBr –CuCl , showed excellent selectivities to
2 2
MPC (97%, entry 9). Homogeneous systems using Pd(OAc)2–
2
2
d
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim
an der Ruhr, Germany
†
Electronic supplementary information (ESI) available: Experimental details,
further data, scheme detailing the formation of DMO. See DOI:
0.1039/c3gc36877g
1
1
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a
Table 1 Carbonylation of phenol by MF using different redox catalysts
selectivity to MPC (60%) was maintained. At amounts lower
than 4.68 mmol resulting in PCO less than 7 bar, phenol con-
version decreased drastically and the selectivity to MPC
dropped to less than 5%. In this case, most of the phenol was
converted to PAC.
b
c
No.
1
Catalyst
Co-catalyst
Conv. (%)
Select. (%)
PdBr
PdBr
PdBr
2
2
2
Mn(acac)
Mn(acac)
Mn(acac)
Mn(acac)
Mn(acac)
Mn(acac)
Mn(acac)
3
3
3
3
3
3
3
1.6
1.4
1.1
1.7
1.7
1.1
1.7
1.8
1.3
3.6
2.3
94.5
93.6
94.4
95.2
91.3
83.7
75.4
85.0
97.0
57.9
59.7
d
2
3
e
The efficiency of MF in the carbonylation of phenol was
evaluated by performing comparative tests, whereby CO/
4
5
6
7
8
9
Pd(OAc)
Pd(salen)
Pd(10%)/C
Pd(5%)/C
2
f
CH OH was used instead of MF under the same reaction con-
3
ditions (see S2, ESI†). Phenol conversion was independent of
Pd(OAc)
PdBr
Pd(OAc)
PdCl
2
Ce(OAc)
CuCl
2
P
CO (Fig. 1), while most importantly phenol conversion in all
2
2
1
1
0
1
2
Cu(OAc)
Cu(OAc)
2
cases was lower with CO/CH OH compared to MF. The selecti-
3
2
(CH
3
CN)
2
2
vity to MPC decreased with decreasing PCO. At low PCO (10 bar),
a
c
b
For experimental details, see Table S1, ESI. Conversion of phenol. MPC production was more than 14 times higher, when MF
d
e
Selectivity to MPC.
Solvent: dimethyl formamide.
Solvent:
was used (0.202 mmol) compared to CO/CH OH (0.014 mmol).
3
f
tetrahydrofuran. Salen: N,N′-bis(salicylidenyl)-ethylenediamine.
At high PCO (67 bar) this effect was less pronounced; the use of
MF still resulted in a 2 times higher MPC production
(
0.137 mmol) compared to CO/CH OH (0.065 mmol). These
3
Cu(OAc) or PdCl (CH CN) –Cu(OAc) as the catalyst (entries
2
2
3
2
2
results suggest that under our reaction conditions, MF is a
better carbonylation agent compared to the conventional CO/
CH OH system. Further, it allowed us to propose a mechanistic
1
2
0 and 11) provided higher conversions of phenol (3.6% and
.3%, respectively), while the selectivities to MPC were lower
(57–59%).
3
model for the carbonylation of phenol by MF (vide infra).
As mentioned above, the pressure at the beginning of
Step 2 representing the equilibrium CO pressure was generated
The decarbonylation of MF was also performed using
1
1
KOCH , which is a stronger base than NaOCH . Lower
amounts of KOCH
by the decarbonylation of MF with NaOCH (Step 1). In other
3
3
3
3
were required to achieve similar PCO. By
words, this pressure was the initial CO pressure (PCO) for the
carbonylation reaction. The effect of PCO on the carbonylation
of phenol by MF was studied by adjusting the amount of
varying the amount of KOCH
PCO was adjusted in the range of 13–67 bar. As for the reactions
with NaOCH , an increase (from below 1% to more than 3%)
in phenol conversion and a decrease (from 75% to 60%) in
MPC selectivity were observed upon reducing PCO from 67 to
3
within the range of 2.5–6 mmol,
3
NaOCH in the range of 4.68–9.36 mmol, thereby controlling
3
the rate of the decarbonylation of MF. In consequence, PCO
was controlled within the range of 10–67 bar. As shown in
Fig. 1, by reducing PCO from 67 to 10 bar, the conversion of
phenol increased, while the selectivity to MPC decreased.
Reducing the NaOCH3 amount to lower than 4.68 mmol,
resulting in a PCO of less than 10 bar (ca. 8 bar), led to an even
higher conversion of phenol (up to 6%), while the high
1
3 bar (see Fig. S1, ESI†). The reaction did not proceed using
1
1
3 3
LiOCH , most likely, as it is a weaker base than NaOCH ,
and was not able to activate/decarbonylate MF at 100 °C.
Other than MPC and PAC, dimethyloxalate (DMO) and
dimethylcarbonate (DMC) were identified as side-products by
GC-MS. An overview of the reaction network involving the path-
ways to each product is shown in Scheme 1. DMO is the result
of oxidative self-carbonylation of MF, while DMC is the result
of the carbonylation of methanol (formed by decarbonylation
of MF). Thus, the Pd catalyst enables four parallel catalytic
reactions. The overall turnover number (TON) with respect to
Pd was as high as 70 (P 67 bar) comparable to related stu-
CO
1
b,12
dies.
Note that DMO and DMC can be converted to DPC
via transesterification with phenol. Nonetheless, DPC was
not detected in the final reaction mixture (vide infra).
1
a
Concerning the MPC production and its side products, a
mechanism is proposed as shown in Scheme 2. MF is activated
by MOCH via carbonyl–hydrogen bond dissociation, which in
3
the presence of Pd(II) leads to the formation of methoxy-
1
3
carbonyl–palladium intermediate 1. This complex is very
stable and depending on the type of ligand can be formed
1
4
even at room temperature. The corresponding phenoxycarbo-
Fig. 1 Conversion of phenol and selectivity to MPC in the carbonylation of
nyl–palladium complex proposed as an intermediate in the for-
phenol by MF or CO/CH
ditions: phenol (9.09 mmol), PdBr
1.7 mmol), BQ (0.25 mmol), CH
carbonylation with MF: MF (45 mL), NaOCH
ation with CO/CH OH: CH OH (45 mL), NaOCH
3
OH varying the initial CO pressure (PCO). Reaction con-
(0.11 mmol), Mn(acac) (0.688 mmol), TBAB
(7% of PCO), 100 °C;
15
mation of DPC is apparently not formed. Note that we have
2
3
not observed DPC in the reaction mixture. The activation of
(
2
Cl
2
(10 mL), O
2
■
(4.68–9.36 mmol); ▲
3
carbonyl- MF by MOCH can be represented by the equilibrium equation
3
3
3
3
(4.68 mmol), CO (10–70 bar).
in Scheme 1 (Step 1). A methoxycarbonyl ion is formed as an
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§
Scheme 1 Reaction pathways during the oxidative carbonylation of phenol by methyl formate; B: base (methoxide); *oxidation; isomerization–esterification.
At low PCO, the isomerization of MF to acetic acid is more
plausible, leading to formation of higher amounts of PAC.
•
DMO is produced by the coordination of a second
methoxycarbonyl group to Pd(II), followed by coupling of the
two methoxycarbonyl groups via reductive elimination (see
2
0
Scheme S1, ESI†). At PCO lower than 33 bar, the formation of
DMO is drastically reduced (see Fig. S2, ESI†). Probably, a rela-
2 3 2
tively high PCO is required for generating the [L Pd(COOCH ) ]
intermediate.
•
DMC is formed by reaction of a methoxide ion with 1.
Subsequent reductive elimination of the methoxycarbonyl and
the methoxide group results in the DMC formation.
In summary, a new protocol for the formation of MPC by
oxidative carbonylation of phenol with MF was developed,
which does not require the use of toxic CO gas. The rate of the
decarbonylation of MF is controlled by the concentration of
Scheme 2 Mechanism of the carbonylation of phenol by MF to MPC using
−
−
Pd(II) catalysts (L: neutral ligand; X: ionic ligand like Br , OAc ) and Mn(acac)
the co-catalyst.
3
as
3 3
NaOCH . At low concentrations of NaOCH (low PCO), the acti-
1
6
vation of MF to a methoxycarbonyl ion dominates and higher
phenol conversions and MPC productivities are obtained. In
comparison, MF is a much more efficient carbonylation agent
intermediate by dissociation of the carbonyl–hydrogen bond.
When low amounts of MOCH are used (e.g., 4.68 mmol
NaOCH corresponding to a P of ca. 10 bar), a relatively long
lifetime of the methoxycarbonyl species is expected. In con-
sequence, the kinetics of MF activation (Scheme 1, Step 1a)
dominates and the methoxycarbonyl ion is the main agent for
the formation of 1. When larger amounts of NaOCH3 are
employed, the formation of CO and CH OH (Step 1b) is faster
3
and 1 is also formed by a consecutive methoxylation–
carbonylation. Since MPC production by the comparative reac-
tion using CO/CH OH was less efficient, it is concluded that 1
3
3
3
CO
3
compared to the conventional CO/CH OH system. Most note-
worthy, the carbonylation of phenol by MF proceeds via
an activated MF intermediate (most likely, a methoxycarbonyl
ion) and not via molecular CO. Two further oxidative
carbonylation reactions occur in parallel, resulting in the for-
mation of DMO and DMC, two other useful starting materials
for DPC production. Thus, MF emerges as a safe and promis-
ing substitute for gaseous CO in carbonylation reactions.
is formed mostly via the methoxycarbonyl ion, which readily
explains the higher MPC productivity of MF compared to CO/
CH OH. In the next step, 1 reacts with phenol to form the
3
intermediate 2. Finally, MPC is formed by reductive elimin-
ation of the methoxycarbonyl and the phenolate group. Simul-
taneously, Pd(II) is reduced to Pd(0), which is then re-oxidized
by Mn(III) to complete the cycle. The general aspects of this
mechanism are similar to the mechanism proposed for the
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1
9
•
MF can undergo isomerization to acetic acid, which
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1
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