Pd/C-catalyzed direct formylation of aromatic iodides to aryl aldehydes using carbon dioxide as a C1 resource

Article abstract of DOI:10.1039/c3cc49365b

Pd/C-catalyzed direct formylation of aromatic iodides to aryl aldehydes using CO₂ as a C1 resource was realized for the first time in the presence of hydrosilanes and base DBU under mild conditions, giving a series of aldehydes in good yields. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc49365b

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Pd/C-catalyzed direct formylation of aromatic
iodides to aryl aldehydes using carbon dioxide
as a C1 resource†
Cite this: Chem. Commun., 2014,
50, 2330
Received 10th December 2013,
Accepted 8th January 2014
Bo Yu, Yanfei Zhao, Hongye Zhang, Jilei Xu, Leiduan Hao, Xiang Gao and
Zhimin Liu*
DOI: 10.1039/c3cc49365b
www.rsc.org/chemcomm
Pd/C-catalyzed direct formylation of aromatic iodides to aryl aldehydes
using CO2 as a C1 resource was realized for the first time in the
presence of hydrosilanes and base DBU under mild conditions,
giving a series of aldehydes in good yields.
Catalytic conversion of CO2 into value-added chemicals has
received significant attention in recent years because of its
economical and nontoxic characteristics, and high potential as a
renewable and environmentally friendly C1 resource alternative to
toxic CO.1–9 However, high-energy substrates, specific catalysts and
Scheme 1 Direct formylation of aromatic iodides with CO2 and PMHS
(right) compared to previous reductive formylation methods (left).
In the course of our continual investigation on chemical conver-
harsh conditions are generally required for CO2 transformation due sion of CO2 into value-added chemicals via construction of C–N,17
to the thermodynamic stability of CO2. Recently, hydrosilyla- C–S18 and C–C bonds, we herein present the Pd/C-catalyzed direct
tion of CO2 with hydrosilanes has made the CO2 conversion formylation of aromatic iodides to aryl aldehydes using CO2 and
proceed under mild conditions, showing great promise in the silanes (e.g., poly(methylhydrosiloxane), PMHS) as the resources for
synthesis of carbonyl compounds using CO2 as a C1 resource.10 constructing C–C bonds in the presence of 1,8-diazabicyclo[5.4.0]
For example, direct formylation of amines with CO2 and hydro- undec-7-ene (DBU) (Scheme 1, right). In this protocol, the commer-
silanes was achieved at 1 atm of CO2.11
cial Pd/C was used to activate aryl ioides, and DBU served as a
Aromatic aldehydes are important building blocks used as catalyst for activating CO2 to react with silanes and as a base to
reactive intermediates in C–C, C–N and C–S coupling reactions, which neutralize the produced HI. The reported reactions were carried
have found wide applications in the synthesis of materials, pharma- out under mild conditions and were successfully applied to a
ceuticals, pesticides and agricultural chemicals.12 Traditional methods wide range of aryl iodides, affording aromatic aldehydes in
for the synthesis of aromatic aldehydes include reduction of carboxylic moderate to excellent yields. To the best of our knowledge, this
acids or esters, Gattermann–Koch, Reimer–Tiemann, Duff and Vils- is the first work on the direct formylation of aromatic iodides to
meier reactions, which generally suffer from inherent drawbacks such aldehydes using CO2 as a C1 resource.
as very low yields, poor selectivity, and requirement of toxic gas and
The model substrate for the initial screening was iodobenzene
harsh conditions.13 The direct formylation synthesis of aromatic (1), leading to benzaldehyde with the formation of biphenyl as a
aldehydes from aromatic halides as a single-step alternative transfor- byproduct. The commercial Pd/C with a Pd particle size of around
mation has been developed using CO or CO alternatives as C1 sources 3 nm was used as the catalyst in this work. The preliminary reaction
(Scheme 1, left).14–16 Such progress encourages us to explore the of iodobenzene with CO2 and PMHS catalysed by Pd/C was
possibility for the direct formylation of aromatic halides into aromatic performed in CH3CN in the presence of DBU at 80 1C and CO2
aldehydes using CO2 instead of CO as a C1 building block.
pressure of 1 MPa for 12 h, and a benzaldehyde yield of 85% was
obtained with biphenyl as the sole byproduct at 91% conversion
of iodobenzene (Table 1, entry 1). Both Pd/C and DBU were
essential for the formation of benzaldehyde confirmed by the fact
that no benzaldehyde was obtained in the absence of either the
Pd/C catalyst or base (Table 1, entries 2 and 3). The influence of
the temperature and pressure on the reaction was investigated.
As the reaction temperature was increased to 100 1C, both the
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: liuzm@iccas.ac.cn
† Electronic supplementary information (ESI) available: Procedures for formyla-
tion of iodobenzenes to aryl aldehydes, the results of formylation of iodobenzene
using CO2 and various hydrosilanes, control experiments, 1H and 13C NMR data
and spectra of intermediates and products. See DOI: 10.1039/c3cc49365b
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Table 1 Pd/C-catalyzed direct formylation of 1 with carbon dioxide and
found that PMHS and phenylsilanes were effective for the
formylation reaction, and PMHS showed the best performance
(Table S1, entries 2–6, ESI†). However, in the cases of Et3SiH and
Et2SiH2, only biphenyl was formed without any detectable
benzaldehyde. In addition, in the case of PhSiH3, Ph2SiH2 and
Ph3SiH as hydrides, the iodobenzene conversion and benzalde-
hyde yield followed the order: PhSiH3 o Ph2SiH2 o Ph3SiH
(Table S1, entries 2–4, ESI†).
PMHS under different conditionsa
Entry
ArX
Base
T/1C
Conv./%
Yieldb/%
1
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5I
C6H5Br
C6H5Cl
DBU
DBU
—
DBU
DBU
DBU
DBU
Cs2CO3
K2CO3
DBU/Cs2CO3
DBU
80
80
80
100
50
80
80
80
80
80
91
0
0
85(70)c
nde
nd
2d
3
In the cases of Ph(CH3)2SiH and Ph2CH3SiH, similar phenomena
were observed (Table S1, entries 5 and 6, ESI†).
4
5
94
60
100
28
95
90
80
0
87(73)c
42
Encouraged by the results described above, various iodobenzenes
substituted with electron-donating and electron-withdrawing groups
were examined to react with CO2 and PMHS catalyzed by Pd/C and
DBU, and the results are listed in Table 2. To our delight, all the
tested substrates produced the corresponding aryl aldehydes in
moderate to excellent yields under the standard conditions, and
coupling or deiodinated products were obtained as byproducts
depending on the nature of the used substrates. The reactions
of iodobenzene derivatives with electron-donating groups, for
example, Me and MeO groups proceeded efficiently in high
aldehyde yields comparable to that of iodobenzene formylation,
and the byproducts were the corresponding coupling products and
no deiodinated products were detectable (Table 2, entries 2–6).
6f
7g
8h
9h
10
11
12
93(77)c
11
nd
nd
58
100
100
nd
nd
DBU
0
a
Reaction conditions: 1 (5 mmol), Pd/C catalyst (2.5 mol%), PMHS
(Si–H 15 mmol), base (1.1 equiv.), pressure of CO2 (1 MPa), CH3CN
b
(10 ml), 12 h. Yields determined by NMR analysis using mesitylene as
c
d
internal standard. Yields of isolated products. In the absence of
e
f
g
the Pd/C catalyst. Not detected. Reaction time, 20 h. CO2 pressure
(0.1 MPa), 24 h. The product was biphenyl.
h
iodobenzene conversion and the yield of benzaldehyde slightly Substituents at the ortho position affected the formylation reactions,
increased accordingly (Table 1, entry 4), while lowering the tempera- leading to slightly decreased yields of the corresponding aldehydes,
ture to 50 1C still resulted in a yield of 42% at the conversion of 60% presumably due to steric hindrance (Table 2, entries 2, 5 and 7).
(Table 1, entry 5). Prolonging the reaction time to 20 h afforded 93% Reactions of substrates with weakly electron-withdrawing groups,
yield of the product with complete conversion of the substrate such as F-, Cl- and Br-substituents, also proceeded well. Because
(Table 1, entry 6). Lowering the CO2 pressure to 0.1 MPa still afforded chlorobenzene and bromobenzene were proved not to react with
a yield of 11% after 24 h (Table 1, entry 7). These results indicated that CO2 and PMHS under the experimental conditions (Table 1, entries
the direct formylation of aryl iodides with CO2 into aldehydes could be 11 and 12), the selective formylation at the iodide position of the F-,
achieved under mild conditions.
Cl-, Br-substituted iodobenzenes occurred as expected, thus giving
The influence of different bases on the direct formylation of the corresponding benzaldehydes in good isolated yields (Table 2,
iodobenzene was studied. The tested inorganic bases like Cs2CO3 and entries 8–10). The formylation of 1,4-diiodobenzene occurred well,
K2CO3 were ineffective in the formation of benzaldehyde, affording giving the mixture of phenyl iodide, benzaldehyde and terephth-
biphenyl in high yields instead (Table 1, entries 8 and 9). This suggests aldehyde as illustrated in Scheme 2. The formylation reactions of
that Ullmann reactions of iodobenzene to produce diphenyl occurred iodobiphenyl and 1-iodonaphthalene took place; however, more
under the experimental conditions, in which the bases promoted the deiodinated products were produced (Table 2, entry 11 and 12).
iodobenzene conversion and neutralized the produced HI. Interest- The formylation reactions also proceeded with substrates bearing
ingly, when Cs2CO3 (1.0 equiv.) together with DBU (0.1 equiv.) was strongly electron-withdrawing groups, such as NO2, CN and CHO
used as the base, the formylation reaction proceeded well, giving 58% (Table 2, entries 13–15). However, in these cases, the corresponding
yield of benzaldehyde (Table 1, entry 10). From these findings, it can products were obtained in decreased yields, and the dehalogenation
be deduced that DBU played a crucial role in the formation of products became the major products determined by NMR
benzaldehyde though all the tested bases including DBU, Cs2CO3 analysis. The direct formylation reaction was also applied to
and K2CO3 could promote the conversion of iodobenzene. In the case substituted S- and N-containing heteroaromatic compounds
of DBU as the base, [DBU][HI] was detected as an ionic liquid, (Table 2, entries 16–19). The protocol was found to work well
suggesting that one role of DBU in the reaction process was to with iodothiophenes to produce the corresponding aldehydes in
neutralize the produced HI similar to those of the inorganic bases. good yields. It is noteworthy that in the cases of the substrates
Another role of DBU was deduced to activate CO2, consistent with that bearing electron-withdrawing substitutes, the competitive side
reported,9b thus resulting in the formylation of iodobenzene finally.
In addition, bromobenzene or chlorobenzene was examined aldehydes in declined yields.
reaction of deiodination proceeded, thus giving corresponding
as the substrate. Unfortunately, the direct formylation reaction
In order to gain an insight into the mechanism of the
of these two compounds did not occur under similar reaction Pd-catalyzed direct formylation of aromatic iodides to aldehydes,
conditions (Table 1, entries 11 and 12).
several control experiments were performed. First, the reaction of
Various hydrosilanes were also examined in the formylation iodobenzene (1 mmol) with CO2 (1 MPa) catalyzed by Pd/C (2.5 mol%)
reaction of iodobenzene with CO2 (Table S1, see ESI†). It was in the presence of DBU (1.1 equiv.) and without PMHS was
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Table 2 Synthesis of various aromatic aldehydesa
Scheme 2 Formylation of 1,4-diiodobenzene with CO2 and PMHS.
Products yieldb/%
ArCHO Ar–Ar Ar–H
Entry Substrate
Product
carried out (Scheme S1A, see ESI†). It was indicated that only a
trace amount of biphenyl was obtained and no benzoic acid was
detected. The reaction of benzoic acid with PMHS was performed
in the presence of Pd/C (2.5 mol%) and DBU (1.1 equiv.) under the
same other conditions; however, no benzaldehyde was obtained
(Scheme S1B, ESI†). These results suggested that the formylation of
aromatic iodides to aldehydes with CO2 and PMHS may not
proceed through benzoic acid as an intermediate.
1
2
77
71
5
ndc
nd
nd
8
3
4
5
67
81
66
11
It was reported that CO2 could react with organosilanes to
produce silyl formate.19 In this work, silyl formate was also
formed from CO2 and PMHS in the presence of DBU, confirmed
by 1H NMR analysis. In the 1H NMR spectrum (Fig. S1b, ESI†) the
signal assigned to the methyl proton of PMHS shifted from d = 0.22
to 0.36 ppm and a new signal appeared at d = 8.07 ppm, ascribed to
silyl formate. This silyl formate might be the key intermediate in the
formation of aldehydes. In order to prove this hypothesis, we added
iodobenzene and Pd/C to the silyl formate solution from CO2
reacting with PMHS catalyzed by DBU, and stirred it at 80 1C for
Trace nd
13
nd
6
7
80
62
Trace nd
nd
nd
12
24
1
12 h. In the H NMR spectrum of the reaction solution (Fig. S1c,
8
64
ESI†) a new signal appeared at 10.02 ppm, attributed to benzalde-
hyde. These results indicated that silyl formate formed from PMHS
with CO2 was the key intermediate in the direct formylation reaction
of iodobenzene with CO2, which further reacted with iodobenzene
catalyzed by Pd/C to form benzaldehyde.
From the above experiments, it is clear that DBU had dual
roles: catalyzing the formation of silyl formate from CO2 with
hydrosilanes and neutralizing HI produced in the reaction
process, while Pd/C catalyzed the formation of benzaldehyde
via the reaction of silyl formate with iodobenzene. On the basis of
the experimental results and the previous reports,19 a possible
reaction mechanism was proposed, as shown in Scheme 3. The
first step of the catalytic cycle is an already accepted process: the
rapid oxidative addition of aryl iodide to Pd(0) A to genetate
arylpalladium halide B, which acted as a nucleophilic agent to
9
60
57
30
nd
nd
nd
25
27
58
10
11
12
60
nd
25
13
14
12
20
nd
nd
80
75
15
29
57
nd
nd
63
16d
nd
17
64
nd
27
18
19
a
29
43
33
20
35
nd
Reaction conditions: substrate (5 mmol), PMHS (Si–H 15 mmol), DBU
(1.1 equiv.), CH3CN (10 ml), 80 1C, CO2c (1 MPa), 20 h. Conversion
b
d
was 100%. Yields of isolated products. Not detected. Conversion
was 73%.
Scheme 3 Proposed mechanism.
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iodides into benzaldehyde derivatives using CO2 as a C1 resource
under mild conditions. Notably, various functional groups are
tolerated under the experimental conditions, and a series of
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