Switching the Cleavage Sites in Palladium on Carbon-Catalyzed Carbon-Carbon Bond Disconnection

Article abstract of DOI:10.1021/acs.joc.5b02632

We have demonstrated a palladium on carbon-catalyzed approach to regioselectively alter the cleavage sites of the C-C bonds of cinnamaldehyde derivatives by a slight change in the reaction conditions in isopropanol under an O₂ atmosphere. Styrene derivatives could be selectively formed by the addition of Na₂CO₃ in association with the dissociation of carbon monoxide, while benzaldehyde derivatives were generated by the addition of CuCl and morpholine instead of Na₂CO₃. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.joc.5b02632

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Article
Switching the cleavage sites in palladium on carbon-
catalyzed carbon–carbon bond disconnection
Tomohiro Hattori, Ryoya Takakura, Tomohiro Ichikawa,
Yoshinari Sawama, Yasunari Monguchi, and Hironao Sajiki
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02632 • Publication Date (Web): 04 Mar 2016
Downloaded from http://pubs.acs.org on March 5, 2016
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Switching the cleavage sites in palladium on carbonꢀ
catalyzed carbon–carbon bond disconnection
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Tomohiro Hattori, Ryoya Takakura, Tomohiro Ichikawa, Yoshinari Sawama, Yasunari Monguchi,* and
Hironao Sajiki*
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Laboratory of Organic Chemistry, Gifu Pharmaceutical University, 1ꢀ25ꢀ4 Daigakuꢀnishi, Gifu 501ꢀ
1196, Japan
Phone/Fax: (+81)ꢀ58ꢀ230ꢀ8109; email: monguchi@gifuꢀpu.ac.jp; sajiki@gifuꢀpu.ac.jp
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
CORRESPONDING AUTHORS
Yasunari Monguchi and Hironao Sajiki
*Eꢀmail: monguchi@gifuꢀpu.ac.jp; sajiki@gifuꢀpu.ac.jp
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ABSTRACT
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Na2CO3
Pd/C, iPrOH, O2
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O
O
CuCl, Molpholine
We have demonstrated a palladium on carbon catalyzed approach to regioselectively alter the cleavage
sites of the C–C bonds of cinnamaldehyde derivatives by the slight change of the reaction conditions in
isopropanol under O2 atmosphere. Styrene derivatives could be selectively formed by the addition of
Na2CO3 in association with the dissociation of carbon monoxide, while benzaldehyde derivatives were
generated by the addition of CuCl and morpholine instead of Na2CO3.
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INTRODUCTION
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Carbon–carbon (C–C) bonds are the most fundamental and important structural unit of organic
compounds and the selective cleavage of the C–C bonds of molecules could be used to construct
different types of structural units. Therefore, the development of selective C–C bond cleavage methods
is exceedingly important for synthetic organic chemistry, including the total synthesis of natural
products,1 while the thermodynamic stability of the C–C bonds makes their cleavage difficult, especially
in a regioselective manner. A couple of C–C cleavage reactions based on the decarbonylation of
aromatic and aliphatic aldehydes using a homogeneous rhodium2 or iridium3 catalyst have been
reported. Furthermore, Maiti and coworkers recently accomplished the decarbonylation of aldehydes
using a palladium catalyst [Pd(OAc)2],4 and the heterogeneous decarbonylation of biomassꢀderived
molecules, such as furfural, was reported as the only example using palladium nanoparticles deposited
on SBAꢀ15 mesoporous silica as a catalyst.5 Among the available supported catalysts, palladium on
carbon (Pd/C) is one of the most frequently used catalysts for industrial processes due to its low price,
high stability, easy removal from the reaction mixture, and reusable properties.
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We now report an additiveꢀdependent regioselective control method between two types of Pd/Cꢀ
catalyzed C–C cleavage reactions of cinnamaldehyde derivatives (Scheme 1; a and b). The addition of
Na2CO3 promotes the decarbonylation for the formation of styrene derivatives (Scheme 1, a), although
the C=C bonds are cleaved in the presence of Cu species and morpholine as additives to obtain the
corresponding benzaldehyde derivatives (Scheme 1, b).
Scheme 1. This work: Regioselective C–C bond cleavage catalyzed by 10% Pd/C.
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RESULTS AND DISCUSSION
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We initially realized that a C–C bond of 4ꢀmethoxycinnamaldehyde was cleaved leading to the
formation of a styrene derivative in the presence of 10% Pd/C and Na2CO3 in N,Nꢀdimethylacetamide
(DMA) under O2 atmosphere (Table 1, entries 1–4), while the use of Pd(OAc)2, substituted for 10%
Pd/C, was not effective (entry 5). The reaction efficiency was improved in isopropanol (iPrOH) as a
solvent and the conversion yield increased to 89% (entries 3 and 6–8). Furthermore, the temperature of
the external heating equipment (aluminum block) could be lowered to 120 °C from 140 °C in iPrOH
without any loss of reaction efficiency (entries 8 and 9), although further reduction of the temperature
caused a significant decrease in the reaction efficiency (entries 10 and 11).
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Table 1. The effect of solvent and additive on the 10% Pd/Cꢀcatalyzed decarbonylation of (E)ꢀ4'ꢀ
methoxycinnamaldehyde.
entry
1
2
additive
–
NaOtBu
Na2CO3
TFA
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
solvent
DMA
DMA
DMA
DMA
DMA
DMSO
MeOH
iPrOH
iPrOH
iPrOH
iPrOH
temperature (°C)
yield (%)a
140
140
140
140
140
140
140
140
120
100
80
3
0
75
0
39
3
59
89
86c
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5b
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10
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aDetermined by 1H NMR using terephthalonitrile as an internal standard. Pd(OAc)2 was used instead of 10%
b
Pd/C. cIsolated yield.
Encouraged by these results, we applied the present optimal conditions to the decarbonylation of
various cinnamaldehydes (Table 2). The cinnamaldehydes bearing either the electronꢀdonating methoxy
and/or hydroxyl groups or electronꢀwithdrawing nitro group on the benzene ring could be efficiently
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converted into the corresponding styrene derivative (entries 2–6).6 The phenyl substituent at the βꢀ
position of the alkene moiety never suppressed the decarbonylation, while the βꢀmethylated
cinnamaldehyde had a critical influence on the reaction efficiency (entries 7 and 8).7 Furthermore, 4ꢀ
phenylbenzaldehyde, which has an aldehyde group directly on the aromatic ring, smoothly underwent
the corresponding decarbonylation (entry 9).
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Table 2. 10% Pd/Cꢀcatalyzed decarbonylation of cinnamaldehydes to styrene derivatives.a
entry
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aldehyde
product
yield (%)b,c
76
2
86
3
4
5
80
72
41
OMe
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94
7d
8d
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34
Me
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99
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aReaction conditions: aldehyde (0.25 mmol), Na2CO3 (0.5 mmol), 10% Pd/C (0.025 mmol) in iPrOH (1.5 mL) at
d
120 ˚C under O2 for 24 h. bIsolated yield. 1 mmol of Na2CO3 was added. cAll reactions cleanly proceeded, and
any byproducts were not detected. Substrates were completely consumed in entries 1–4, 6, 7, and 9 only except
1
entries 5 and 8 (51% and 41% of the substrates remained unchanged, respectively, by the H NMR analysis of
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the crude reaction mixture).
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The decarbonylation of the dihydrocinnamaldehyde derivatives also smoothly proceeded in
association with the dehydrogenation of the ethylene moiety to afford the corresponding styrene
derivatives in good yields (Table 3, entries 1 and 2). For the substrates possessing a methyl or phenyl
substituent at the benzylic position of dihydrocinnamaldehyde, 1,1ꢀdisubstituted ethenes were obtained
along with the formation of ca. 10% yield of saturated 1,1ꢀdisubstituted ethane derivatives as minor
products (entries 3 and 4).8 The decarbonylation could also be completed under argon atmosphere
without oxygen gas, although the styrene yield significantly decreased due to the generation of the
corresponding ethylbenzene (entry 5).
Table 3. 10% Pd/Cꢀcatalyzed decarbonylation of dihydrocinnamaldehydesa
entry
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aldehyde
product
yield (%)b,c
68
2
78
3d
71
9
Me
4d
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aReaction conditions: aldehyde (0.25 mmol), Na2CO3 (0.5 mmol), 10% Pd/C (0.025 mmol) in iPrOH (1.5 mL) at
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b
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120 ˚C under O2 for 24 h. Isolated yield. All reactions cleanly proceeded, and any other products were not
detected. Substrates were completely consumed in entries 1–3 and 5 only except entry 4 (41% of the substrate
1
d
remained unchanged by the H NMR analysis of the crude reaction mixture). 1 mmol of Na2CO3 was added.
eUnder Ar atmosphere instead of O2.
The synthesis of benzaldehyde from cinnamaldehyde via the C=C cleavage could be achieved using
hydrogen peroxide (H2O2).9ꢀ11 Several methods without H2O2 in the presence of water were also
reported using cyclodextrins,12 hydrotalcite as a catalyst,13 microwave heating,14 or secondary amine.15
We then investigated the benzaldehyde synthesis by the Pd/Cꢀcatalyzed C=C cleavage of
cinnamaldehyde in anticipation of switching the cleavage sites of the same compound by the use of
different additives.
The effect of amines as additives was specifically investigated in the presence of CuBr. The presence
of amines was found to be essential for the C=C bond cleavage reaction (Table 4, entry 1). The addition
of cyclic aliphatic amines was especially effective (entries 2–5). The screening of copper species
revealed that 1) the reaction hardly took place without copper species (entry 6); and 2) the reaction was
completed within 24 h by using CuBr2 (entry 7) or monovalent copper species, such as CuBr and CuCl
(entries 5 and 9). CuCl was finally selected as the appropriate copper salt after comparison of the
conversion ratios of 4ꢀmethoxybenzaldehyde for 5 h in the presence of CuBr2, CuBr, or CuCl (entries
10–12) and the reaction efficiency decreased by the reduction of the morpholine usage to 0.5 equiv
(entry 13). No reaction took place without 10% Pd/C in iPrOH (entry 14), while the reaction partially
proceeded in H2O regardless of the presence or absence of 10% Pd/C (entries 15 and 16). These results
indicated that a Pd/Cꢀcatalyzed mechanism, which is different from the waterꢀmediated reactions,12ꢀ15
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would have strong control over the present reaction effectively proceeding only in the presence of Pd/C
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and amine in iPrOH.
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Table 4. The effect of amine and copper as a C=C cleavage reaction.
entry
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amine
–
NEt3
copper
CuBr
CuBr
CuBr
CuBr
CuBr
–
CuBr2
CuCl2
CuCl
CuBr2
CuBr
CuBr
CuCl
CuCl
CuCl
CuCl
Ratio (substrate : product)
100 : 0
yield (%)a
99b
–
99 : trace
99 : trace
0 : 100
0 : 100
87 : 13
0 : 100
7 : 93
0 : 100
80 : 20
55 : 45
16 : 84
38 : 62
100 : 0
51 : 49
66 : 34
Propylamine
Pyrrolidine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
–
38
84
–
84
–
85
–
–
–
–
–
–
9
10c
11c
12c
13d
14e
15f
16e, f
–
b
c
d
e
aIsolated yield. Yield of recovered starting material. 5 h. 0.5 equiv of morpholine was used. Without 10%
Pd/C. fH2O was used as a solvent instead of iPrOH.
Electronꢀneutral and electronꢀsufficient cinnamaldehydes were smoothly converted to the
corresponding benzaldehydes in high yields under the present conditions (Table 5, entries 1–6).16 A
bromine as a substituent on the aromatic ring of the substrate was tolerant under the reaction conditions
(entry 7). While the replacement of the benzene ring of cinnamaldehyde with a furan ring hardly affected
the reaction progress, the desired furfural could be obtained in 54% yield (entry 9). Interestingly,
benzaldehyde was never obtained using dihydrocinnamaldehyde as the substrate (entry 10).
Table 5. Scope of cinnamaldehyde derivatives for the benzaldehyde synthesis.a
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entry
aldehyde
product
yield (%)b,c
87
O
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H
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0
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aReaction conditions: aldehyde (0.25 mmol), CuCl (0.25 mmol), morpholine (0.25 mmol), 10% Pd/C (0.025
mmol) in iPrOH (1.5 mL) at 100 ˚C under O2 for 24 h. bIsolated yield. cAll reactions cleanly proceeded, and any
other products were not detected. Substrates were completely consumed in entries 1–3 and 6–8 only except
1
entries 4, 5, and 9 (13%, 38%, and 31% of the substrates remained unchanged, respectively, by the H NMR
analysis of the crude reaction mixture). dNo reaction took place.
We recently reported that the Pd/Cꢀ, Rh/Cꢀ or Pt/Cꢀcatalyzed dehydrogenation of iPrOH effectively
generate H2 gas and acetone,17 and Pd/C catalyzed the hydrogenation of oxygen gas to directly generate
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H2O2.18 The generation of H2O2 in iPrOH in the presence of 10% Pd/C and O2, CuCl, and morpholine at
100 °C was confirmed by iodometry of the filtrate obtained after the removal of 10% Pd/C by hot
filtration, indicating that the Pd/Cꢀcatalyzed hydrogenation of O2 gas by the H2, which was generated via
the Pd/Cꢀcatalyzed dehydrogenation of iPrOH, took place during the present C=C cleavage reaction of
cinnamaldehydes to benzaldehydes. Furthermore, 4ꢀmethoxybenzaldehyde could be synthesized from 4ꢀ
methoxycinnamaldehyde in 80% yield in the presence of 30% aqueous H2O2 (2 equiv) even without
10% Pd/C [eq. 1]. Since the reaction was strongly suppressed by using Nꢀmethylmorpholine instead of
morpholine that significantly impeded the reaction, the secondary amine moiety of morpholine should
be important [eq. 2]. In addition, the in situ generated H2O2 would not oxidize morpholine during the
reaction, since the addition of morpholine Nꢀoxide was not efficient [eq. 3]. While 51% CO was
detected in the reaction vessel during the styrene synthesis by gas chromatography [eq. 4], CO was
hardly detected under the benzaldehyde synthesis conditions [eq. 5].
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The decarbonylation of cinnamaldehyde would proceed similar to the Rhꢀcatalyzed reaction reported
in the literature, although the role of Na2CO3 is unclear.17,19 Pd(0)/C would be oxidatively inserted into
the C(O)ꢀH bond of the aldehyde to form an acyl Pd(II) hydride complex (A). Subsequently, Pd would
migrate to form a vinyl Pd(II) hydride complex (B), and reductive elimination of styrene would take
place to generate CO and Pd(0)/C (Scheme 2). Similarly, phenethyl Pd(II) hydride (B') (Scheme 3)
would form from dihydrocinnamaldehyde, and styrene would be afforded together with H2 via the βꢀ
hydrogen elimination. The alkene moiety could be partially hydrogenated by the H2 gas generated from
B' and/or iPrOH. For the benzaldehyde formation, we propose a H2O2ꢀmediated mechanism, thus
epoxyꢀiminium intermediate (C) could be initially generated from cinnamaldehyde in the presence of
morpholine and CuCl via the in situꢀgenerated H2O2ꢀmediated oxidation of the alkene moiety as with
the organocatalytic epoxidation of α,βꢀunsaturated aldehydes using H2O2.20 A further nucleophilic attack
of H2O2 to the epoxide and iminium moieties of C would lead to the formation of a fiveꢀmembered ring
intermediate, and the subsequent ring opening reaction gave the desired benzaldehyde, glyoxal, and
morpholine.
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Scheme 2. Proposed catalytic cycle for the C–C cleavage reactions.
Scheme 3. Proposed mechanism for styrene synthesis from dihydrocinnamaldehyde.
10% Pd/C could be recovered and reused for both decarbonylation (Table 6) and C=C cleavage
reaction (Table 7) of (E)ꢀ4'ꢀmethoxycinnamaldehyde at least until the third runs without any change of
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the catalyst activity, although the decreased reaction efficiency was observed in the fourth run of only
decarbonylation reaction.
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Table 6. Reuse test of 10% Pd/C for the decarbonylation of (E)ꢀ4'ꢀmethoxycinnamaldehyde.
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run
1st
0 : 100
91
2nd
0 : 100
89
3rd
0 : 100
88
4th
42 : 58
50
ratio 1 : 2a
yield (%)b
aDetermined by 1H NMR. Any byꢀproducts were never observed. bIsolated yield.
Table 7. Reuse test of 10% Pd/C for the C=C cleavage reaction of (E)ꢀ4'ꢀmethoxycinnamaldehyde.
run
1st
0 : 100
92
2nd
0 : 100
90
3rd
0 : 100
91
4th
0 : 100
89
ratio 1 : 3a
yield (%)b
aDetermined by 1H NMR. Any byꢀproducts were never observed. bIsolated yield.
CONCLUSION
In conclusion, we have developed two types of Pd/Cꢀcatalyzed efficient regioselective carbon–carbon
bond cleavage reactions of cinnamaldehyde derivatives. Styrene and benzaldehyde derivatives could be
selectively synthesized in high yields using different additives, i.e., Na2CO3 for the synthesis of the
styrene derivatives and CuCl and morpholine for the synthesis of the benzaldehyde derivatives.
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EXPERIMENTAL SECTION
General
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All solvents and reagents were obtained from commercial sources and used without further purification.
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Chemical shifts (δ) of H NMR (400 MHz, CDCl3) are expressed in ppm and are internally referenced
1
(0.00 ppm for tetramethylsilane). Presented products in this manuscript were all known and their H
NMR and MS spectra were identical to those in the literature.
General procedure for synthesis of styrene derivatives (Tables 1–3).
A mixture of 10% Pd/C (26.6 mg, 25.0 ꢁmol), a cinnamaldehyde derivative (250 ꢁmol), Na2CO3 (53.0
mg, 50.0 ꢁmol) in iPrOH (1.5 mL) in a 17ꢀmL test tube was stirred using a personal organic synthesizer
ChemistationTM (EYELA, Tokyo) or ChemistPlazaTM (Shibata Scientific Technology, Ltd., Tokyo) at
120 °C under O2 atmosphere for 24 h. The mixture was passed through a membrane filter (Millipore
Corp., Billerica, MA; MillexꢀLH, 0.45 ꢁm) to remove the insoluble catalyst, and the filtered residue was
washed with Et2O (30 mL). The combined filtrate was washed with H2O (3 × 20 mL), dried (MgSO4),
filtered, and concentrated in vacuo. The residue was purified by silica gel column chromatography
(hexane) or preparative TLC (hexane) to afford the corresponding styrene derivative.
Styrene (Table 2, entry 1, Table 3, entries 1 and 5)21
1H NMR δ 7.40 (d, J = 7.5 Hz, 2H), 7.31 (t, J = 7.5 Hz, 2H), 7.24 (t, J = 7.5 Hz, 1H), 6.71 (dd, J = 17.2,
11.2 Hz, 1H), 5.74 (d, J = 17.2 Hz, 1H), 5.23 (d, J = 11.2 Hz, 1H); MS (EI) m/z 104 (M+, 100), 78 (76).
4ꢀMethoxystyrene (Table 2, entry 2, Table 3, entry 2)22
1H NMR δ 7.35 (d, J = 8.4 Hz, 2H), 6.87 (d, J = 8.4 Hz, 2H), 6.67 (dd, J = 18.0, 10.4 Hz, 1H), 5.52(d, J
= 18.0 Hz, 1H), 5.13 (d, J = 10.4 Hz, 1H), 3.81 (s, 3H); MS (EI) m/z 134 (M+, 100), 119 (51), 103 (2),
91 (71).
3ꢀMethoxystyrene (Table 2, entry 3)22
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1H NMR δ 7.24 (t, J = 7.8 Hz, 1H), 7.00 (d, J = 7.8 Hz, 1H), 6.95 (s, 1H), 6.81 (d, J = 7.8 Hz, 1H), 6.69
(dd, J = 17.6, 11.2 Hz, 1H), 5.74 (d, J = 17.6 Hz, 1H), 5.24 (d, J = 11.2 Hz, 1H), 3.81 (s, 3H); MS (EI)
m/z 134 (M+, 100), 119 (3), 104 (25), 91 (62).
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2ꢀMethoxystyrene (Table 2, entry 4)22
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1H NMR δ 7.50 (d, J = 8.0 Hz, 1H), 7.26 (t, J = 8.0 Hz, 1H), 7.08 (dd, J = 18.0, 10.8 Hz, 1H), 6.96 (t, J
= 8.0 Hz, 1H), 6.89 (d, J = 8.0 Hz, 1H), 5.76 (d, J = 18.0 Hz, 1H), 5.29 (d, J = 10.8 Hz, 1H), 3.86 (s,
3H); MS (EI) m/z 134 (M+, 27), 119 (28), 103 (3), 91 (100), 78 (9).
4ꢀHydroxyꢀ3ꢀmethoxyphenylethene (Table 2, entry 5)23
1H NMR δ 6.95–6.88 (m, 3H), 6.63 (dd, J = 18.0, 10.8 Hz, 1H), 5.63 (d, J = 8.8 Hz, 1H), 5.57 (s, 1H),
5.13 (d, J = 8.8 Hz, 1H), 3.92 (s, 3H); MS (EI) m/z 150 (M+, 100), 135 (80), 77 (66).
4ꢀNitrostyrene (Table 2, entry 6)24
1H NMR δ 8.18 (d, J = 8.8 Hz, 2H), 7.53 (d, J = 8.8 Hz, 2H), 6.78 (dd, J = 17.6, 10.8 Hz, 1H), 5.93 (d, J
= 17.6 Hz, 1H), 5.50 (d, J = 10.8 Hz, 1H); MS (EI) m/z 149 (M+, 46), 133 (1), 119 (11), 103 (11), 91
(21), 77 (100).
1,1ꢀDiphenylethylene (Table 2, entry 7)25
1H NMR δ 7.32–7.28 (m, 10H), 5.43 (s, 2H); MS (EI) m/z 180 (M+, 100), 165 (64).
1,1ꢀDiphenylethane (Table 3, entry 3)26
1H NMR δ 7.29–7.15 (m, 10H), 4.14 (q, J = 7.6 Hz, 2H), 1.63 (d, J = 7.6 Hz, 3H); MS (EI) m/z 182 (M+,
70), 167 (100), 152 (34), 77 (58).
αꢀMethylstyrene (Table 2, entry 8, Table 3, entry 4)27
1H NMR δ 7.46 (d, J = 8.0 Hz, 2H), 7.31 (t, J = 8.0 Hz, 2H), 7.24 (t, J = 8.0 Hz, 1H), 5.36 (s, 1H), 5.07
(s, 1H), 2.14 (s, 3H); MS (EI) m/z 118 (M+, 100), 103 (38), 90 (13), 77 (43).
2ꢀPhenylpropane (Table 3, entry 4)26
1H NMR δ 7.18–7.32 (m, 5H), 2.91 (sept, J = 6.9 Hz, 1H), 1.25 (d, J = 6.9 Hz, 6H) MS (EI) m/z 120
(M+, 45), 105 (100), 77 (35).
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Biphenyl (Table 2, entry 9)28
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1H NMR δ 7.62–7.58 (m, 4H), 7.48–7.25 (m, 6H); MS (EI) m/z 154 (M+, 100).
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General procedure for synthesis of benzaldehyde derivatives (Table 4 and 5).
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A mixture of 10% Pd/C (26.6 mg, 25.0 ꢁmol), a cinnamaldehyde derivative (250 ꢁmol), CuCl (25.0 mg,
250 ꢁmol), morpholine (21.6 ꢁL, 250 ꢁmol) in iPrOH (1.5 mL) in a 17ꢀmL test tube was stirred using a
personal organic synthesizer ChemistationTM (EYELA, Tokyo) or ChemistPlazaTM (Shibata Scientific
Technology, Ltd., Tokyo) at 100 °C under an O2 atmosphere for 24 h. The mixture was passed through a
membrane filter (Millipore Corp., Billerica, MA; MillexꢀLH, 0.45 ꢁm) to remove the insoluble catalyst,
and the filtered residue was washed with Et2O (30 mL). The combined filtrate was washed with H2O (3
× 20 mL), dried (MgSO4), filtered, and concentrated in vacuo. The residue was purified by silica gel
column chromatography (hexane) or preparative TLC (hexane) to afford the corresponding
benzaldehyde derivative.
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Benzaldehyde (Table 5, entry 1)29
1H NMR δ 10.02 (s, 1H), 7.88 (d, J = 7.7 Hz, 2H), 7.63 (t, J = 7.7 Hz, 1H), 7.53 (t, J = 7.7 Hz, 2H); MS
(EI) m/z 105 (M+, 57), 77 (100).
4ꢀMethoxybenzaldehyde (Table 4, Table 5, entry 2)30
1H NMR δ 9.89 (s, 1H), 7.84 (d, J = 8.8 Hz, 2H), 6.98 (d, J = 8.8 Hz, 2H), 3.89 (s, 3H); MS (EI) m/z
135 (M+, 100), 107 (9), 92 (21), 77 (64).
3ꢀMethoxybenzaldehyde (Table 5, entry 3)30
1H NMR δ 9.98 (s, 1H), 7.46–7.42 (m, 2H), 7.39 (d, J = 1.6 Hz, 1H), 7.18 (m, 1H), 3.87 (s, 3H); MS
(EI) m/z 136 (M+, 100), 119 (2), 107 (19), 92 (6), 77 (49).
2ꢀMethoxybenzaldehyde (Table 5, entry 4)30
1H NMR δ 10.47 (s, 1H), 7.83 (d, J = 7.6 Hz, 1H), 7.55 (t, J = 8.0 Hz, 1H), 7.04–6.98 (m, 2H), 3.93 (s,
1H); MS (EI) m/z 136 (M+, 56), 118 (18), 104 (5), 92 (15), 77 (100).
4ꢀHydroxyꢀ3ꢀmethoxybenzaldehyde (Table 5, entry 5)31
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1H NMR δ 9.89 (s, 1H), 7.40–7.45 (m, 2H), 7.08 (d, J = 7.8 Hz, 1H), 6.25 (s, 1H), 3.99 (s, 3H); MS (EI)
m/z 151 (M+, 100).
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4ꢀ(N, Nꢀdimethylamino)benzaldehyde (Table 5, entry 6)32
1H NMR δ 9.74 (s, 1H), 7.74 (d, J = 8.8 Hz, 2H), 6.70 (d, J = 8.8 Hz, 2H), 3.09 (s, 6H); MS (EI) m/z
149 (M+, 100).
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4ꢀBromobenzaldehyde (Table 5, entry 7)29
1H NMR δ 9.98 (s, 1H), 7.76 (d, J = 8.4 Hz, 2H), 7.69 (d, J = 8.4 Hz, 2H); MS (EI) m/z 184 (M+, 100),
156 (13), 75 (23).
4ꢀPhenylbenzaldehyde (Table 5, entry 8)33
1H NMR δ 10.06 (s, 1H), 7.97 (d, J = 8.4 Hz, 2H), 7.77 (d, J = 8.4 Hz, 2H), 7.65–7.42 (m, 5H); MS (EI)
m/z 181 (M+, 100), 152 (32).
Furanꢀ2ꢀcarbaldehyde (Table 5, entry 9)32
1H NMR δ 9.67 (s, 1H), 7.72 (d, J = 1.2 Hz, 1H), 7.29 (d, J = 3.6, 1H), 6.63 (dd, J = 3.6, 1.2 Hz, 1H);
MS (EI) m/z 96 (M+, 100), 39 (52).
Procedure for the confirmation of H2O2 generation by iodometry
A mixture of 10% Pd/C (26.6 mg, 25.0 ꢁmol), CuCl (25.0 mg, 250 ꢁmol), morpholine (21.6 ꢁL, 250
ꢁmol) in iPrOH (1.5 mL) in a 17ꢀmL test tube was stirred using a personal organic synthesizer
ChemistPlazaTM (Shibata Scientific Technology, Ltd., Tokyo) at 100 °C under O2 atmosphere for 6 h.
The mixture was passed through a membrane filter (Millipore Corp., Billerica, MA; MillexꢀLH, 0.45
ꢁm) without cooling to remove the insoluble catalyst. The remaining H2O2 in the resulting filtrate was
mesured by iodometry using 0.01 M Na2S2O3 (f = 0.996) for the titration. 6.50 mL of 0.01 M Na2S2O3
corresponding to 32.4 was consumed by the titration
.
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General procedure for reuse test of 10% Pd/C for the decarbonylation of (E)ꢀ4'ꢀ
methoxycinnamaldehyde (Table 6).
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Three test tubes were prepared, and (E)ꢀ4'ꢀmethoxycinnamaldehyde (40.6 mg, 0.250 mmol), 10% Pd/C
(26.6 mg, 25.0 µmol, 10 mol%), Na2CO3 (53.0 mg, 0.500 mmol) and iPrOH (1.5 mL) were placed in
each test tube. The mixture in each test tube was stirred under O2 (balloon) at 120 °C for 24 h, then the
combined mixture was filtered using a Kiriyama funnel (1 µm filter paper). The catalyst on the filter
paper was washed with EtOAc (20 mL), H2O (20 mL), and MeOH (20 mL), and the filtrate was
concentrated in vacuo to give 4ꢀmethoxystyrene (95.5 mg, 0.682 mmol, 91%). The recovered catalyst
was dried at room temperature under reduced pressure for 24 h, then weighed [79.7 mg, 100%, 79.7 ÷
(26.6 × 3) × 100]. The reaction for the second run was carried out in the same manner as the first run. 4ꢀ
Methoxystyrene was obtained in 89% yield (89.6 mg, 0.668 mmol), and the catalyst was recovered [70.5
mg, 88%, 70.5 ÷ (26.6 × 3) × 100]. The reactions for the third run and fourth run were carried out in the
same manner as the first run, but using two test tubes due to the slight loss of the recovered 10% Pd/C
during the filtration [total substrate amount, 81.2 mg (40.6 mg × 2), 0.500 mmol (0.250 mmol × 2); total
catalyst amount, 53.2 mg (26.2 mg × 2), 50.0 µmol (25.0 µmol × 2); total Na2CO3 amount, 106 mg
(53.0 mg × 2), 1.00 mmol (0.500 mmol × 2)]. The yields of 4ꢀmethoxystyrene in the third run and fourth
run were 88% (59.0 mg, 0.440 mmol), 50% (33.2 mg, 0.247 mmol), respectively. The catalyst was
quantitatively recovered in both third run [53.2 mg, 53.2 ÷ (26.6 × 2) × 100] and fourth run [53.0 mg,
53.0 ÷ (26.6 × 2) × 100].
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General procedure for reuse test of 10% Pd/C for the C=C cleavage reaction of (E)ꢀ4'ꢀ
methoxycinnamaldehyde (Table 7).
Three test tubes were prepared, and (E)ꢀ4'ꢀmethoxycinnamaldehyde (40.6 mg, 0.250 mmol), 10% Pd/C
(26.6 mg, 25.0 µmol, 10 mol%), CuCl (24.0 mg, 0.250 mmol), morpholine (22.0 µL, 0.250 mmol), and
iPrOH (1.5 mL) were placed in each test tube. The mixture in each test tube was stirred under O2
(balloon) at 100 °C for 24 h, then the combined mixture was filtered using a Kiriyama funnel (1 µm
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filter paper). The catalyst on the filter paper was washed with EtOAc (20 mL), H2O (20 mL), and MeOH
(20 mL), and the filtrate was concentrated in vacuo to give 4ꢀmethoxybenzaldehyde (93.8 mg, 0.690
mmol, 92%). The recovered catalyst was dried at room temperature under reduced pressure for 24 h,
then weighed [79.8 mg, 100%, 100 ÷ (26.6 × 3) × 100]. The reaction for the second to fourth run was
carried out in the same manner as the first run. The yield of 4ꢀmethoxybenzaldehyde in the second to
fourth runs were 90% (91.8 mg, 0.670 mmol), 91% (92.9 mg, 0.683 mmol), and 89% (91.3 mg, 0.671
mmol), respectively. The catalyst were quantitatively recovered in the second to fourth runs {100% for
the second run [80.0 mg, 80.0 ÷ (26.6 × 3) × 100], 100% for the third run [79.8 mg, 79.8 ÷ (26.6 × 3) ×
100], and 98% for the fourth run [78.2 mg, 78.2 ÷ (26.6 × 3) × 100]}.
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ASSOCIATED CONTENT
Supporting Information
1
Copies of H spectra of products. This material is available free of charge via the Internet at
http://pubs.acs.org.
ACKNOWLEDGMENTS
This work was partly supported by the Research Fellowship for Young Scientists of the Japan Society
for the Promotion of Science (Grant number 15J11072). 10% Pd/C was a kind gift from the N.E.
Chemcat Corporation.
REFERENCES
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propenal, and citral were not suitable as substrates for the decarbonylation. No desired products were
obtained. The amino group and pyridine ring might act as catalyst poisons of Pd(0)/C by the relatively
strong coordination"
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