Palladium/Carboxylic Acid-catalyzed Alkenylation of Furfural and its Derivatives Using Alkynes

Article abstract of DOI:10.1002/cctc.202001685

Furfural and its derivatives underwent alkenylation with alkynes via α-C?H activation in the presence of a palladium/carboxylic acid catalyst to give the corresponding single and double alkenylated products. The reactive aldehyde group remained intact during this reaction. This catalytic system allowed selective alkenylation of furan substrates having electron-withdrawing substituents.

Full text of DOI:10.1002/cctc.202001685

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Palladium/carboxylic acid-catalyzed Alkenylation of Furfural and
its Derivatives Using Alkynes
Authors: Yasunori Minami, Hitomi Miyamoto, and Yumiko Nakajima
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10.1002/cctc.202001685
ChemCatChem
COMMUNICATION
Palladium/carboxylic acid-catalyzed Alkenylation of Furfural and
its Derivatives Using Alkynes
Yasunori Minami,*[a] Hitomi Miyamoto,[a] and Yumiko Nakajima*[a]
[a]
Dr. Y. Minami, Ms. H. Miyamoto, Dr. Y. Nakajima
Interdiscplinary Research Center for Catalytic Chemistry (IRC3)
National Institute of Advanced Industrial Science and Technology (AIST)
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
E-mail: yasu-minami@aist.go.jp, yumiko-nakajima@aist.go.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: Furfural and its derivatives underwent alkenylation with
alkynes via α-C–H activation in the presence of
a
A) Ru-catalyzed alkenylation of furans under CO atmosphere
R'
palladium/carboxylic acid catalyst to give the corresponding single
and double alkenylated products. The reactive aldehyde group
remained intact during this reaction. This catalytic system allowed
selective alkenylation of furan substrates having electron-
withdrawing substituents.
R
R'
R
Ru3(CO)12 cat.
CO
O
O
O
H
220 °C
O
excess
B) Previous Work: Pd/carboxylic acid-catalyzed alkenylation of arene C-H bonds
Ar Ar
Pd cat.
RCO2H cat.
Effective uses of biomass and its derivatives are of growing
importance in attaining sustainable development. Furfural is one
of the important products derived from cellulosic biomass, and is
utilized extensively for the preparation of organic acids such as
maleic acid. Furfural is composed of a furan ring with an
aldehyde group at the α position. The furan ring is a useful and
versatile building block and a key component for functional
molecules such as pharmaceuticals, bioactive natural products,
Ar
Ar
H
Ar
n
Ar
n
thiophene, benzofuran, electron-deficient arenes
C) This work: Alkenylation of furfural C-H bonds
R
R'
Pd cat.
RCO2H cat.
R'
O
O
O
H
O
R
and emissive materials.[1,2,3] Thus, development of
a
straightforward catalytic functionalization method of furfural and
its derivatives may contribute to advances in syntheses of high-
value added chemicals from biomass as a means of diminishing
our dependence on fossil fuel resources.
Figure 1. A) Hong’s study. B) Our previous study. C) This work
We first examined the reaction of furfural (1a, 1.0 equiv)
with diphenylacetylene (2a, 1.0 equiv) under typical conditions
(Pd(dba)2 (5 mol%), tricyclohexylphosphine (PCy3) (10 mol%),
and 2,2-dimethylbutanoic acid (t-AmylCO2H) (10 mol%) as
catalysts) at 100 °C for 17 h.[8a] The reaction occurred to give the
desired adduct, 5-(1,2-diphenylethenyl)-furfural (3a) (Z/E =
1.2/1), in moderate yield (Table 1, Run 1).[10] To enhance the
yield of the product, we optimized the conditions (Runs 2-6). As
a result, the reaction using 4 equivalents of 1a at a longer
reaction time gave 3a in good yield with Z-selectively (Run 5). In
this scenario, the reactive aldehyde group was retained during
the alkenylation, and did not interfere with the addition reaction.
In fact, this stereoselectivity is in contrast to the previous case[8a]
in which syn-addition proceeds to form the E-adduct kinetically,
and the subsequent isomerization occurred to provide the Z-
form smoothly.[4] The catalyst loading was successfully reduced
to 1 mol% with a comparable yield (Run 7). No reaction occurred
in the absence of t-AmylCO2H (Run 8), PCy3 (Run 9), or both
Pd(dba)2 and PCy3 (Run 10), showing that the combination of
Pd(dba)2, PCy3, and t-AmylCO2H is essential for this reaction.
Use of Pd(OAc)2 instead of Pd(dba)2 and t-AmylCO2H
decreased the yield (Run 11). Notably, although an excess
amount of 1a was required for enhancing the yield of 3a,
residual 1a remained intact after the reaction. This starting
material could be recovered by an isolation procedure.
At present, catalytic C–C bond-forming reaction via inert C–
H bond cleavages is regarded as the most important and
effective method for the facile construction of various highly
functionalized molecules with atom- and step-economies. In fact,
several examples of those reactions employing furyl C–H bonds
in furfural were reported,[4,5] in which the reactive aldehyde group
was left intact.[6] Among them, alkenylation of arenes using
alkynes via a C–H bond cleavage is a straightforward method to
construct π-conjugated arylene-vinylene units without byproduct
formation.[7] However, to the best of our knowledge, only one
example of the alkenylation of furfural using alkynes was
reported by Hong and co-workers.[4] They showed that an
excess amount of furfural underwent α-alkenylation with alkynes
using
a ruthenium catalyst under CO pressure at high
temperature (Figure 1A). Recently, we demonstrated that a
simple palladium(0)/carboxylic acid catalyst system is highly
effective for alkenylation of an equimolar amount of thiophenes
and electron-deficient arenes using alkynes, which gives
variously substituted alkenylated (hetero)arenes (Figure 1B).[8,9]
We expected that this palladium(0)/acid catalyst system has the
potential to employ furfural for alkenylation by alkynes without
excess use of substrates under mild conditions and avoid any
reactions involving the reactive aldehyde group. Herein, we
report the facile alkenylation of furfural and its analogues using
alkynes to form α-alkenylated products (Figure 1C).
1
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10.1002/cctc.202001685
ChemCatChem
COMMUNICATION
Table 1. Optimization of the conditions[a]
carbonyl group, that is bulkier than the aldehyde group, at the
C3 position probably prefers the syn-addition product.
Pd(dba)2 (5 mol%)
PCy3 (10 mol%)
t-AmylCO2H (10 mol%)
Ph
Ph
Ph
Table 2. Scope of the alkenylation[a]
O
O
+
O
O
1,4-dioxane (0.5 M)
100 °C
Pd(dba)2
PCy3
t-AmylCO2H
Ph
R
R
1a
(X equiv)
2a
(1 equiv)
3a
R
R
+
O
O
1,4-dioxane
100 °C
R'
R'
Run
X (equiv. of 1a)
Temp (°C)
Time (h)
3a, Yield (Z/E)[b]
1
3
(Z:E)
(4 equiv)
2
1
2
3
4
5
1
100
100
100
100
100
17
17
16
16
35
52% (1.2:1)
31% (1.2:1)
70% (4.1:1)
79% (9.4:1)
83% (>20:1)
74% (>20:1)[c]
48% (8.7:1)
82% (12:1)
0%
R
0.5
2
OMe
O
4
O
O
O
4
6
4
4
4
4
4
4
80
100
100
100
100
100
17
17
21
21
16
17
OMe
7[d]
8[e]
9[f]
10[g]
11[h]
3b
3c
(R = CF3), 84% (>20:1) (44 h)
3d (R = F), 62% (>20:1) (44 h)
, 80% (4.4:1) (44 h)
0%
Ph
O
0%
iPr3Si
O
O
O
O
41% (2.0:1)
[a] A mixture of 1a, 2a (1 equiv), Pd(dba)2 (5 mol%), PCy3 (10 mol%), t-
AmylCO2H (10 mol%), and 1,4-dioxane (0.5 M) was stirred. [b] Determined by
1H NMR. [c] Isolated yield. [d] Pd(dba)2 (1 mol%), PCy3 (2 mol%), and t-
AmylCO2H (2 mol%). [e] Absence of t-AmylCO2H. [f] Absence of PCy3. [g]
Absence of both Pd(dba)2 and PCy3. [h] Use of Pd(OAc)2 instead of Pd(dba)2
and t-AmylCO2H.
, 29%[b] (44 h)
, 48% (1:>20) (91 h)
3e
3f
Ph
Ph
Ph
Ph
O
O
O
O
OMe
Ph
Ph
3g
3h
3i
,
, 69% (4.5:1) (42 h)
,
0%
0%
57% (3.0:1) (16 h)[c]
45% (1.7:1) (19 h)[d]
Under the optimized conditions, we examined the scope of
alkynes, and the results are summarized in Table 2. Substituted
diarylalkynes having methoxy, trifluoromethyl, or fluorine groups
reacted with 1a to give corresponding adducts 3b-3d in good
yields. Unsymmetrical 1-(1-naphthyl)-2-phenylethyne underwent
the reaction to afford Z-3e as a main product containing a 1-
naphthyl group at the α-position of the alkenyl moiety. Moreover,
O
F
OMe
MeO
O
O
O
F
+
O
R
O
F
R
when
the
electronically
and
sterically
biased
F
3j
(R = H), 74% (1:1.5) (46 h)
3k (R = CF3), 71% (1:1.1) (43 h)
aryloxy(triisopropylsilyl)ethyne was used, the reaction took place
with complete regioselectivity to give the syn-addition product Z-
3f in a manner similar to that from the previously reported
hydroarylation.[8a] We next investigated the scope of furan
substrates as shown in Figure 2. Methyl furan-2-carboxylate (1b)
reacted with 2a to form the product 3g with the Z-isomer being
favored. In the case of normal furan (1c), no reaction was
observed under the optimized conditions. On the other hand, the
reaction using neat furan as a solvent underwent the reaction at
the C2 position to afford the corresponding product 3h. The
same tendency was observed in the reaction employing 2-
methylfuran (1d), which provided 3i in a moderate yield. These
results showed that furans having an electron-withdrawing group
are highly reactive substrates in this catalytic system. This trend
is in contrast to the previous alkenylation reaction using furan
derivatives.[4,7h] 3-Furancarboxaldehyde (1e) underwent the
reaction with alkynes via selective cleavage of the 2-C–H
bond[5d,11] to give 2-alkenylated products 3j and 3k. The 3-
aldehyde group acted as a directing group to affect the selective
2-C–H bond cleavage. The poor ratios of stereoselectivities of 3j
and 3k are likely due to steric repulsion between the 3-aldehyde
group and the β-aryl group on the alkenyl moiety. Methyl furan-
3-carboxylate (1f) also reacted at the 2-position in preference to
the 5-position to form E-3l as a major product. The methoxy
3l 3'l
, 35% (6.3 : 1) (43 h)
+
[a] A mixture of 1 (4 equiv), 2 (1 equiv), Pd(dba)2 (5 mol%), PCy3 (10 mol%), t-
AmylCO2H (10 mol%), and 1,4-dioxane (0.5 M) was stirred at 100 °C. Isolated
yields based on 2. [b] In a crude mixture, four regio- and stereo-mixtures were
observed (64 : 21 : 10 : 5). However, no isomers except the major product Z-
3e were isolated in substantial amounts. [c] Furan (1c, 28 equiv) was used as
a solvent instead of 1,4-dioxane. [d] 2-Methylfuran (1d, 27 equiv) was used as
a solvent instead of 1,4-dioxane.
CO2Me
O
CO2Me
O
O
O
O
O
1b
1c
1d
1e
1f
Figure 2. Furan substrate 1
Double alkenylation was examined by using 1e and 2.5
equivalents of 2a. Although the conditions for double
alkenylations were not optimized, the desired double adduct 4
was successfully produced (Scheme 1).[12]
2
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10.1002/cctc.202001685
ChemCatChem
COMMUNICATION
Pd(dba)2 (5 mol%)
PCy3 (10 mol%)
AmylCO2H (10 mol%)
early stage of this reaction. More E-isomer, the syn-adduct of 3a,
was observed than in the cases in Table 1, Runs 4 and 5
(Scheme 4). The same trend was observed in the reaction using
1b. Supported by the syn-conformation of 3f derived from the
bulky silylethynyl ether, these results suggest that syn-addition
proceeds and the subsequent stereoisomerization occurs under
thermodynamic control.[17]
O
Ph
O
t
+
Ph
Ph
1,4-dioxane (0.5 M)
100 °C, 46 h
H
H
O
O
Ph
, 2.5 equiv
Ph
Ph
1e
2a
4
, 57%
Scheme 1. Double alkenylation of 1e
The present α-alkenylation was used in the sequential
functionalization of furfural. According to Oble and Poli’s
report,[5f] we prepared 3-phenylated furfural 5, and then
subjected it to the present alkenylation to produce the 5-
alkenylation product 6 (Scheme 2).
O
Br Ph
O
2a (1.5 equiv)
Pd(dba)2 (5 mol%)
PCy3 (10 mol%)
tAmylCO2H (10 mol%)
Ru cat.
O
Ph
Ph
1,4-dioxane (0.5 M)
100 °C, 43 h
O
O
O
O
O
Ref. 5f
1a
5
6
Z E
, 41%( : = 1.4:1)
Scheme 4. Early stage of this reaction
Scheme 2. Sequential introduction of furfural
Based on results from previous reports,[8,9] we propose the
reaction mechanism shown in Figure 3. After coordination of the
alkyne to the palladium(0) complex forms 7, the carboxylic acid
reacts to generate alkenylpalladium carboxylate 8.[18] The
regioselectivity is probably derived from a steric repulsion
between bulky substituents on the alkynyl carbon and the
palladium center and the polarization of the carbon-carbon triple
bonds.[19] Subsequent C–H bond activation of 1 proceeds via the
CMD pathway to give alkenylarylpalladium 9.[16] Reductive
elimination produces the syn-addition product and regenerates
the palladium(0) complex. The syn-adduct is isomerized to the
anti-adduct possibly mediated by the carboxylic acid.
As shown above, furan derivatives having electron-
withdrawing substituents are effective substrates in this reaction
rather than normal furan and electron-rich furans. Since it is
highly advantageous to use substrates selectively, we performed
a
competition experiment between furfural (1a) and 2-
methylfuran (1d) under the optimized conditions. Indeed, 1a
underwent the reaction preferentially and 3i was hardly formed
(Scheme 3A).[13] This result suggests that the C–H bond
cleavage proceeds not via an attack of the furan to the alkyne
mediated by
metalation-deprotonation (CMD) pathway.[16] Moreover,
a a concerted-
metal complex[14,15] but by
a
competition reaction of equimolar mixtures of 1a and 3-
furancarboxaldehyde (1e) was conducted, which gave 3j in
preference to 3a (Scheme 3B). This result suggests that an
electron-deficient directing group enhanced the reactivity of the
2-C–H bond over other substrates having less-hindered α-C–H
bonds.
A)
2a
(1 equiv)
Pd(dba)2
PCy3
-AmylCO2H
Ph
Ph
Ph
Ph
t
O
O
+
+
Figure 3. Plausible reaction mechanism
O
O
O
O
1,4-dioxane
100 °C, 38 h
1a
3a
, 57%
(Z:E = 1.5:1)
(1 equiv)
1d
3i
, <5%
(1 equiv)
In conclusion, the alkenylation of furfural and similar furan
derivatives using alkynes was made possible under a simple
palladium/carboxylic acid catalyst via α-C–H bond cleavage.
Various furan substrates were used in this reaction to form the
corresponding α-alkenylated products. This catalytic method
enabled the selective transformation of furans having electron-
withdrawing groups such as furfural. Site-selective α-C–H bond
cleavage was also achieved by using carbonyl-directing groups
at the β position.
B)
2a
Pd(dba)2
(1 equiv)
O
Ph
O
PCy3
t
-AmylCO2H
O
O
Ph
+
+
O
O
O
O
1,4-dioxane
100 °C, 15 h
Ph
Ph
1a
1e
3a
3j
, 47%
(Z:E = 14:1)
(2 equiv)
(2 equiv)
, 14%
(Z:E = 1:1.6)
Scheme 3. Competition experiments
To obtain information about the stereochemistry of this
addition, we monitored the Z/E ratios of the product 3a at the
Acknowledgements
3
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10.1002/cctc.202001685
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COMMUNICATION
alkenes, see: h) A. Vasseur, C. Laugel, D. Harakat, J. Muzart, J. Le
Bras, Eur. J. Org. Chem. 2015, 944;
This research was supported financially by Grants-in-Aid for
Scientific Research (C) (19K05481 to Y.M.) from the JSPS and
the Shorai Foundation for Science and Technology (Y.M.).
[8]
[9]
a) Y. Minami, Y. Furuya, T. Kodama, T. Hiyama, Chem. Lett. 2018, 47,
674; For related catalytic transformations, see: b) Y. Minami, M. Sakai,
T. Sakamaki, T. Hiyama, Chem. Asian J. 2017, 12, 2399; c) Y. Minami,
Y. Noguchi, T. Hiyama, J. Am. Chem. Soc. 2017, 139, 14013; d) Y.
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1151; e) Y. Minami, Y. Furuya, T. Hiyama, Chem. Eur. J. 2020, 26,
9471.
Keywords: Alkynes • C–H activation • Heterocycles • Insertion •
Palladium
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[10] The stereochemistry of 3a was determined using the 1H NMR spectrum
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4
structures of other addition products were determined likewise.
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10.1002/cctc.202001685
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Furfural and its derivatives underwent alkenylation with alkynes via α-C–H activation in the presence of a simple palladium/carboxylic
acid catalyst with atom- and step-economies. The corresponding single and double alkenylated products were obtained. In the case
of furfural, the reactive formyl group was retained during the reaction. This catalytic system allowed selective alkenylation of furan
substrates having electron-withdrawing substituents.
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