An Efficient and General Method for Formylation of Aryl Bromides with CO2 and Poly(methylhydrosiloxane)

Article abstract of DOI:10.1002/chem.201504320

The formylation of aryl halides with CO₂ to generate aryl aldehydes is challenging. Herein, we report a novel synthesis of aryl aldehydes by formylation of aryl bromides with CO₂ and a waste silane, poly(methylhydrosiloxane) (PMHS). It has been discovered that a simple combination of 1,3-bis(diphenyphosphino)propane (DPPP)-chelated Pd catalyst, Pd(DPPP)Cl₂, with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is able to effectively catalyze the reaction, leading to aryl aldehydes in moderate to excellent yields, and without any by-products in most cases. Moreover, this route could be extended to the formylation of aryl iodides with high efficiency. This approach is simple, less costly, and environmentally friendly, and also widens the applications of CO₂ to form value-added chemicals by the construction of new C-C bonds.

Full text of DOI:10.1002/chem.201504320

DOI: 10.1002/chem.201504320
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Aryl Aldehyde Synthesis |Hot Paper|
An Efficient and General Method for Formylation of Aryl Bromides
with CO2 and Poly(methylhydrosiloxane)
Bo Yu, Zhenzhen Yang, Yanfei Zhao, Leiduan Hao, Hongye Zhang, Xiang Gao, Buxing Han,
and Zhimin Liu*[a]
Abstract: The formylation of aryl halides with CO2 to gener-
ate aryl aldehydes is challenging. Herein, we report a novel
synthesis of aryl aldehydes by formylation of aryl bromides
with CO2 and a waste silane, poly(methylhydrosiloxane)
(PMHS). It has been discovered that a simple combination of
aryl aldehydes in moderate to excellent yields, and without
any by-products in most cases. Moreover, this route could
be extended to the formylation of aryl iodides with high effi-
ciency. This approach is simple, less costly, and environ-
mentally friendly, and also widens the applications of CO2 to
form value-added chemicals by the construction of new CÀC
bonds.
1,3-bis(diphenyphosphino)propane
(DPPP)-chelated
Pd
catalyst, Pd(DPPP)Cl2, with 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) is able to effectively catalyze the reaction, leading to
Introduction
formylsaccharin as a highly reactive CO surrogate and silane as
a hydrogen donor, and thereby realized reductive carbonyla-
tion of aryl bromides to aryl aldehydes catalyzed by palladium
acetate with the aid of a ligand and a base (Scheme 1a). Sub-
sequently, other CO surrogates, including isocyanide[16] and
Aromatic aldehydes are important chemical intermediates for
the synthesis of materials, pharmaceuticals, pesticides, and ag-
ricultural chemicals, and their synthesis has attracted much in-
terest from both organic and pharmaceutical chemists.[1] The
traditional synthetic methods, including reduction of carboxylic
acids or esters, Gattermann–Koch, Reimer–Tiemann, Duff, and
Vilsmeier reactions, generally have inherent drawbacks, such as
requiring harsh reaction conditions and multiple steps and
giving low yields and/or poor selectivity.[2,3] Thus, there has
been an ongoing quest for simple and clean routes. The palla-
dium-catalyzed formylation of aromatic halides using CO as
a C1 source, originally reported by Heck in 1974, provides an
alternative route for the synthesis of aryl aldehydes,[4–6] and is
one of the most efficient and widely used methodologies for
constructing carbonyl-containing compounds.[7,8] It shows high
compatibility with a wide variety of substrates bearing differ-
ent functional groups. However, this method is limited by the
high toxicity of CO gas and requires special CO detectors. For
safety and operational reasons, the employment of CO surro-
gates is a very promising alternative for small-scale reactions in
laboratories. Over the last three decades, palladium-catalyzed
CO-free carbonylation reactions have attracted great atten-
tion.[9–14] For example, Manabe and co-workers[15] adopted N-
Scheme 1. Palladium-catalyzed formylation of aryl halides to afford
aldehydes.
[a] Dr. B. Yu, Dr. Z. Yang, Dr. Y. Zhao, L. Hao, Dr. H. Zhang, X. Gao,
Prof. Dr. B. Han, Prof. Dr. Z. Liu
paraformaldehyde,[17] were successfully applied in the carbony-
lation of aryl/hetaryl iodides and bromides in the presence of
different silanes, providing access to a wide range of aryl/he-
taryl aldehydes under relatively mild conditions (Scheme 1b,
c). Although great progress has been made in the use of Pd-
catalyzed CO-free carbonylation of aromatic halides to synthe-
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: liuzm@iccas.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504320.
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size aryl(hetaryl) aldehydes, there are still inherent drawbacks,
such as the need for expensive CO surrogates and ligands, and
inevitable by-product formation. Therefore, there is continuous
motivation for the exploration of simpler, cleaner, and less
expensive routes for the synthesis of aromatic aldehydes.
Carbon dioxide is one of the main greenhouse gases respon-
sible for environmental problems such as climate change.
Meanwhile, it is also an attractive and ideal C1 source due to
its abundance, low cost, nontoxicity, and renewable and envi-
ronmentally friendly features.[18–24] In recent years, the use of
CO2 as an alternative to toxic CO has received much atten-
tion,[25–38] and in the context of formylation of aryl halides to
afford aryl aldehydes, such a switch is of great significance
from a green and sustainable point of view. In our recent
efforts, we found that Pd/C combined with 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) could catalyze the formylation
of aryl iodides with CO2 and an inexpensive silane,
poly(methylhydrosiloxane) (PMHS), producing aryl aldehydes
along with by-products from coupling and deiodination reac-
tions (e.g., biphenyls and arenes).[39] Compared to iodides, bro-
mides and chlorides are less expensive and more easily avail-
able, and hence their transformations are of greater interest.
However, the reactivities of aryl halides in their transformations
follow the order: iodides>bromides@chlorides. The formyla-
tion of aromatic bromides or chlorides with CO2 to afford aryl
aldehydes is still challenging, and has not hitherto been re-
ported. From a synthetic point of view, a methodology that
can utilize widely available substrates as starting materials is
both attractive and necessary.
Table 1. Optimization of the formylation of bromobenzene with CO2 and
PMHS to afford benzaldehydes.[a]
entry
Pd cat.
Silane
Base
Conv.
(%)
Yield
(%)
1
2
3
4
5
6
7
8[d]
9[d]
10
11
12
13
14
15
16
17
18
19
20
Pd(DPPP)Cl2
–
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
Ph2MeSiH
Et2SiH2
Et3SiH
PhSiH3
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
PMHS
DBU
DBU
–
100
<1
<1
100
100
100
<5
100
100
100
81
83
85
69
72
70
83
90
100
100
94(81)[b]
n.d.[c]
n.d.
24
23
17
n.d.
39
32
31
13
n.d.
28
36
29
30
34
62
85[e]
81[f]
Pd(DPPP)Cl2
Pd(DPPE)Cl2
Pd(DPPF)Cl2
Pd(PPh3)2Cl2
Pd/C
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Na2CO3
Cs2CO3
NaOH
NEt3
DBN
DBU
DBU
Pd(OAc)2
PdCl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
Pd(DPPP)Cl2
[a] Reaction conditions: 1a (2 mmol), Pd catalyst (2 mol%), ligand
(4 mol%), PMHS (Si-H 2.5 equiv), DBU (1.5 equiv), initial pressure of CO2
(1 MPa), DMF (5 mL), 20 h. The yields were determined by 1H NMR spec-
troscopic analysis of crude products using mesitylene as internal stan-
dard. [b] Yield of isolated product in parentheses. [c] Not detected.
[d] With DPPP ligand. [e] CH3CN instead of DMF. [f] THF instead of DMF.
Herein, we report the first synthesis of aryl aldehydes by for-
mylation of aryl bromides with CO2. It has been discovered
that
a simple combination of 1,3-bis(diphenyphosphino)-
DBN=1,5-diazabicyclo[4.3.0]non-5-ene,
DPPE=1,2-bis(diphenylphos-
propane (DPPP)-chelated Pd catalyst, Pd(DPPP)Cl2, with DBU is
capable of effectively catalyzing the formylation of bromides
with CO2 and PMHS, producing aryl aldehydes in good to
excellent yields (Scheme 1d). Moreover, this combination of
Pd(DPPP)Cl2 and DBU could be extended to catalyzing the
formylation of various aryl iodides with high efficiency. More
importantly, dehalogenation and coupling reactions could be
suppressed in most cases, leading to the exclusive formation
of the target aldehydes without any by-products.
phino)ethane, DPPF=1,1’-bis(diphenyphosphino)ferrocene.
to be essential for this reaction, with no formylation occurring
if either was omitted (Table 1, entries 2 and 3). Compared to
the previously reported CO-free approaches to synthesize aryl
aldehydes from aromatic halides (Scheme 1a, b, c), no extra
ligand was required, making this approach more simple. Simi-
lar Pd complexes with different phosphorus ligands, including
Pd(DPPE)2Cl2, Pd(DPPF)2Cl2, and Pd(PPh3)2Cl2, were examined
for this reaction. Though these catalysts were very effective for
the conversion of 1a, coupling and debromination reactions
took place, resulting in low yields of the target product
(Table 1, entries 4–6). Commonly used Pd catalysts, such as Pd/
C, Pd(OAc)2, and PdCl2, were also applied in the formylation of
1a. It was demonstrated that they were ineffective for this re-
action, whereas in the presence of DPPP they became effec-
tive, albeit affording lower product yields along with by-prod-
ucts (Table 1, entries 7–9). Further investigation demonstrated
that Pd(OAc)2 and PdCl2 could react with DPPP to form
Pd(DPPP)(OAc)2 and Pd(DPPP)Cl2, respectively, which were
responsible for catalyzing the formylation of 1a.
Results and Discussion
To optimize the conditions for the formylation of aryl bromides
with CO2, the formylation of bromobenzene (1a) as a model
reaction was investigated under various conditions as listed in
Table 1. After extensive investigation, we found that
Pd(DPPP)Cl2 (2 mol%) combined with DBU (1.5 equiv) dis-
played high efficiency in catalyzing the formylation of bromo-
benzene (2 mmol) with CO2 (1 MPa) and PMHS (2.5 equiv) in
DMF (5 mL) at 1008C, producing benzaldehyde in 94% yield
(Table 1, entry 1) without any detectable by-products. This rep-
resented great progress in the Pd-catalyzed transformation of
aryl halides because the dehalogenation and coupling reac-
tions that are generally difficult to avoid were apparently com-
pletely suppressed. Notably, both Pd(DPPP)Cl2 and DBU proved
Other hydrosilanes, including Ph2MeSiH, Et2SiH2, Et3SiH, and
PhSiH3, were also examined for the formylation of 1a (Table 1,
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entries 10–13). Among the tested hydrosilanes, PMHS showed
the best performance for this reaction. PMHS is an abundant
and nontoxic by-product of the silicone industry, making it
cost-effective, and it is also moisture stable.[40] Therefore, the
formylation of halides with CO2 and PMHS described in this
work represents a less expensive and cleaner route compared
to the previously reported CO-free approaches.
The effects of bases and solvents were investigated using
Pd(DPPP)Cl2 as the catalyst. DBU was found to be the most
effective base, which neutralized the in situ produced HBr
during the reaction process to form the ionic liquid [DBUH]Br
and simultaneously catalyzed the reaction of CO2 with
PMHS.[39] It is interesting that in the presence of inorganic
bases such as Na2CO3, Cs2CO3, and NaOH or of organic bases
such as NEt3 and DBN, the desired product was also observed,
albeit in low yields (Table 1, entries 14–18). This is in contrast
to the ineffectiveness of inorganic bases (e.g., K2CO3 and
Cs2CO3) in the Pd/C-catalyzed formylation of iodobenzene with
CO2 and PMHS,[39] implying that Pd(DPPP)Cl2 has distinct fea-
tures from those of Pd/C, which will be explored in the follow-
ing section. DMF was found to be the optimal solvent for this
formylation. When CH3CN and THF were used, some unknown
side products were observed (Table 1, entries 19 and 20).
Based on the above results, we selected the optimum reaction
conditions for the formylation of aryl bromides as follows:
Pd(DPPP)Cl2 as catalyst, DBU as base, PMHS as hydrogen
donor, and DMF as solvent.
Figure 1. Substrate scope of the various aryl and hetaryl bromides. Reaction
conditions: substrate (2 mmol), PMHS (Si-H, 5 mmol), DBU (3 mmol),
Pd(DPPP)Cl2 (0.04 mmol), CO2 (1 MPa), DMF (5 mL), 1008C. Yield of isolated
product in %.
Having established the optimum conditions, we proceeded
to examine the general scope and limitations of the formyla-
tion of a wide range of aryl bromides with different electronic
and steric properties. As shown in Figure 1, most of the sub-
strates were transformed into the corresponding aldehydes in
moderate to good yields under the standard conditions. Nota-
bly, the substrates bearing electron-donating groups such as
methyl, methoxy, and tert-butyl were well tolerated, and no
debrominated or coupling products were detectable. Further-
more, ortho, meta, and para substituents were also well tolerat-
ed (Figure 1, 3B–3I), although those in the ortho position led
to decreased yields of the target products, presumably due to
steric hindrance (Figure 1, 3C and 3G). This inference was
supported by the fact that no aldehyde was obtained with 1-
bromo-2,4,6-trimethylbenzene as the substrate (Figure 1, 3H).
The formylation of substrates bearing electron-withdrawing
groups, such as fluoro, bromo, chloro, trifluoromethyl, and al-
dehyde, also proceeded well (Figure 1, 3J–3M), affording ex-
clusively the corresponding aryl aldehydes. The formylation of
1,4-dibromobenzene afforded a mixture of 4-bromobenzalde-
hyde and terephthaldehyde without any by-products under
the experimental conditions (Scheme 2). Increasing the
amount of DBU to 2.1 equiv resulted in complete conversion
of 1,4-dibromobenzene to terephthaldehyde, suggesting that
4-bromobenzaldehyde could be further formylated with CO2
and PMHS to produce terephthaldehyde under the experimen-
tal conditions. For substrates bearing electron-withdrawing
groups (e.g., ester, formyl) at the 2-position, the formylation
was inhibited and reductive dehalogenation prevailed
(Figure 1, 3N and 3O). Interestingly, formylation proceeded
Scheme 2. The formylation of 1,4-dibromobenzene catalyzed by Pd(DPPP)Cl2
and DBU.
successfully with 4-bromobiphenyl and 1- and 2-naphthyl bro-
mides, producing the corresponding aldehydes in good yields
(Figure 1, 3P–3R). It is noteworthy that the combination of
Pd(DPPP)Cl2 with DBU was also effective for the formylation of
heterocycles (such as 3-bromothiophene and 2-bromo-
thiophene), affording the target aldehydes in good yields
(Figure 1, 3S and 3T).
Encouraged by the above results, we applied Pd(DPPP)Cl2
with DBU to a variety of aryl iodides, as shown in Figure 2. Ex-
citingly, for most of the substrates, irrespective of the presence
of electron-donating or electron-withdrawing groups, the for-
mylation reactions proceeded well, affording exclusively the al-
dehydes, and the coupling and dehalogenation reactions did
not occur. Moreover, the yields of aldehydes obtained by this
protocol were considerably improved compared to those ob-
tained over Pd/C with DBU under similar conditions.[39] As an
exception, the formylation of 1-bromo-4-nitrobenzene was ac-
companied by debromination, leading to a decreased yield of
the target aldehyde, presumably due to low electron density
(Figure 2, 4K). In addition, the steric hindrance of 2,4,6-trisub-
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(Scheme 3, [Eq. (2)]). After reaction at 1008C for the desired
time, the mixture was analyzed by 31P and 13C NMR spectros-
copies. As illustrated in Figure 3, a new singlet signal appeared
at d=30.14 ppm in the 31P NMR spectrum of the mixture of
Pd(DPPP)Cl2 and DBU, implying the formation of a new Pd0
complex,[41] denoted as Pd0L in Scheme 4. In the 31P NMR spec-
Figure 2. Substrate scope of the various aryl and hetaryl iodides. Reaction
conditions: substrate (2 mmol), PMHS (Si-H, 5 mmol), DBU (3 mmol),
Pd(DPPP)Cl2 (0.04 mmol), CO2 (1 MPa), DMF (5 mL), 808C. Yield of isolated
product in %.
Figure 3. 31P NMR spectra of: (1) Pd(DPPP)Cl2, (2) a mixture of Pd(DPPP)Cl2
and DBU, and a mixture of PhBr and silyl formate containing Pd(DPPP)Cl2
and DBU at different times using H3PO4 as an external standard in DMF:
(3) 5 min, (4) 1 h, (5) 12 h.
stitution prevented the reaction from taking place (Figure 2,
4E), similar to what was observed with bromo-substituted
2,4,6-trimethylbenzene.
In our previous work,[39] we found that DBU was able to cat-
alyze the reaction of CO2 with PMHS and further promoted the
Pd/C-catalyzed formylation of aryl iodides with CO2 to afford
aryl aldehydes, whereas inorganic bases (e.g., Cs2CO3) were in-
effective. However, besides DBU, the tested inorganic bases
listed in Table 1 proved to be effective for the formylation of
bromides in this work. In order to gain insight into the mecha-
nism of this formylation catalyzed by Pd(DPPP)Cl2 with bases,
control experiments were performed as illustrated in
Scheme 3. Interestingly, it was found that Pd(DPPP)Cl2 cata-
Scheme 4. The proposed reaction catalytic cycle.
trum of the formylation solution after reaction for 1 h, the
signal of Pd0L at d=30.14 ppm had almost disappeared, and
new signals had emerged at d=21.75 and 17.44 ppm, which
may be attributed to the intermediate complexes D or E, re-
spectively, as shown in Scheme 4. The 13C NMR spectrum of
the formylation solution after reaction for 12 h revealed the
presence of benzaldehyde (Figure 4), and the 31P signals of D/E
disappeared with re-emergence of the signal at d=30.14 ppm
(Figure 3). These findings suggest that Pd0L was involved the
reaction of PhBr with silyl formate, resulting in the formation
of benzaldehyde, and that it was regenerated after the reac-
tion. The above results imply that Pd(DPPP)Cl2 plays dual roles
Scheme 3. Control experiments.
lyzed the reaction of CO2 with PMHS in the absence of any
base to form silyl formate, as confirmed by 13C NMR analysis
(see the Supporting Information, Figure S1). Subsequently,
a formylation reaction was performed by introducing PhBr and
silyl formate into a solution of Pd(DPPP)Cl2 and DBU in DMF
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Experimental Section
Methods and characterization: CO2 (99.99%) was purchased from
Beijing Analytical Instrument Company. All aryl halides and solvents
were purchased from J&K and were used as received without fur-
ther purification. PMHS (poly(methylhydrosiloxane), mol. wt. ca.
1900) and other hydrosilanes were purchased from Alfa Aesar and
used without purification. TLC analysis was performed on silica gel
60 F254 and the spots were visualized under UV light at 254 nm or
by exposure to iodine vapor. Column chromatography was carried
1
out on silica gel (200–400 mesh). H, 13C, and 31P NMR spectra were
acquired at 400, 100, and 367 MHz, respectively, from solutions in
CDCl3 on a Bruker Avance NMR spectrometer (400 MHz) at ambient
temperature; chemical shifts are given in ppm relative to tetra
methylsilane (TMS). Abbreviations used are as follows: s, singlet; d,
doublet; t, triplet; q, quartet; m, multiplet.
Typical procedure for the formylation of aryl/hetaryl halides cat-
alyzed by Pd(DPPP)Cl2 with DBU: All reactions were carried out in
a Teflon-lined stainless steel reactor of capacity 22 mL equipped
with a magnetic stirrer. Typically, the aryl halide (2.0 mmol),
Pd(DPPP)Cl2 (2 mol%), PMHS (Si-H 2.5 equiv), DBU (1.5 equiv), and
DMF (5 mL) were loaded into the reactor. The autoclave was
charged with CO2 to 1 MPa and then transferred to an oil bath at
1008C, which was controlled by a Haake-D3 temperature control-
ler. After the reaction, the reactor was cooled in iced water and the
gas inside was carefully vented. The reaction mixture was analyzed
by 1H NMR using mesitylene as an internal standard. The pure
products were obtained following column chromatographic sepa-
ration (eluent: n-heptane/ethyl acetate, 15:1), and their isolated
yields were calculated based on the obtained masses. The isolated
Figure 4. 13C NMR spectrum of a reaction solution containing PhBr and silyl
formate in the presence of Pd(DPPP)Cl2 and DBU. Reaction conditions: PhBr
(2 mmol), DBU (1.5 equiv), Pd(DPPP)Cl2 (2 mol%), and silyl formate (2 mmol)
in DMF (5 mL), 1008C, 12 h. After the reaction, a portion of the mixture was
taken up in CDCl3 (0.5 mL) and transferred to an NMR tube. The 13C NMR
spectrum showed the formation of benzaldehyde ( ).
J
in the formylation process, catalyzing the reaction of CO2 and
PMHS to form silyl formate, and further catalyzing the reaction
of PhBr with silyl formate in the presence of DBU to afford the
target aryl aldehyde. DBU also plays multiple roles, promoting
the formation of the active Pd0L species by reaction with
Pd(DPPP)Cl2, catalyzing the reaction leading to silyl formate,[39]
and neutralizing the in situ generated HX (X=Br, I) to
regenerate the active Pd0L species.
On the basis of the above results and previous reports,[39,41]
a possible catalytic cycle for the reaction is proposed, as illus-
trated in Scheme 4. The active catalyst is formed by reduction
of PdII to Pd0L species B with DBU. Oxidative addition of the
aryl bromide to Pd0L then takes place to form the correspond-
ing organopalladium complex C, which then reacts with the
silyl formate generated from CO2 and PMHS to afford the
desired product, along with intermediates D and E. Finally, the
active Pd0L species is regenerated from the palladium
hydrobromide complex E by reaction with DBU.
1
products were identified by H and 13C NMR.
Acknowledgements
The authors thank the National Natural Science Foundation of
China for support of this work (grant nos. 21503239, 21125314,
21021003, 21373242).
Keywords: aryl aldehydes · aryl bromides · carbon dioxide ·
formylation · green chemistry · P ligands
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In summary, we have reported the first formylation of aryl (he-
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a C1 source and PMHS as a hydrogen donor, catalyzed by
Pd(DPPP)Cl2 and a base. The combination of Pd(DPPP)Cl2 and
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both bromides and iodides, affording the corresponding aryl
aldehydes in moderate to good yields, without by-products
from coupling or dehalogenation reactions in most cases.
Further studies on the extension of this protocol to the
formylation of aryl chlorides with CO2 are currently underway
in our laboratory.
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Received: October 28, 2015
Published online on December 11, 2015
Chem. Eur. J. 2016, 22, 1097 – 1102
www.chemeurj.org
1102
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