Oxidative cross-coupling of allenyl ketones and organoboronic acids: Expeditious synthesis of highly substituted furans

Article abstract of DOI:10.1002/anie.201400500

Allenyl ketones are employed as coupling partners in palladium-catalyzed oxidative cross-coupling reactions with organoboronic acids. This reaction constitutes an efficient methodology for the synthesis of highly substituted furan derivatives. Palladium-carbene migratory insertion is proposed as the key step in this transformation. Migration patterns: Allenyl ketones are employed as a coupling partner in a palladium-catalyzed oxidative cross-coupling reaction with organoboronic acids. This reaction constitutes an efficient methodology for the synthesis of highly substituted furan derivatives. Palladium-carbene migratory insertion is proposed as the key step in this transformation. BQ=1,4-benzoquinone.

Full text of DOI:10.1002/anie.201400500

Angewandte
Chemie
DOI: 10.1002/anie.201400500
Synthetic Methods
Oxidative Cross-Coupling of Allenyl Ketones and Organoboronic
Acids: Expeditious Synthesis of Highly Substituted Furans**
Ying Xia, Yamu Xia, Rui Ge, Zhen Liu, Qing Xiao, Yan Zhang, and Jianbo Wang*
Abstract: Allenyl ketones are employed as coupling partners
in palladium-catalyzed oxidative cross-coupling reactions with
organoboronic acids. This reaction constitutes an efficient
methodology for the synthesis of highly substituted furan
derivatives. Palladium-carbene migratory insertion is proposed
as the key step in this transformation.
F
urans are highly important structural motifs because they
exist in a variety of natural products and synthetic pharma-
ceuticals, and some furan derivatives have been found to have
potential uses in materials science or as building blocks in
synthetic chemistry.[1] Therefore, concise and efficient syn-
thesis of highly substituted furans has attracted extensive
attention over the decades.[2,3] Among the various methods to
synthesize furans, the transition-metal-catalyzed cycloisome-
rization of 1,2-allenyl ketones is particularly attractive.[4–9]
The reaction may proceed via cyclic metal carbene inter-
mediates. Marshall et al. and Hashmi et al. demonstrated that
allenyl ketones could undergo a formal 1,2-hydride shift to
produce furan derivatives (G = H, M = RhI, AgI, PdII; Scheme
1a).[5,6] Gevorgyan and co-workers have developed a series of
transition-metal-catalyzed cyclizations of allenyl ketones
which undergo a 1,2-shift to form functionalized furans
(G = alkyl, aryl, halide, acyloxy, silyl, etc., M = AuI, AgI,
CuI, etc.; Scheme 1a).[7] Recently, Wu and co-workers
reported that the palladium-catalyzed reaction of allenyl
ketones bearing a cyclopropyl moiety (C) proceeds through
a 1,2-carbon shift of the carbene species B, thus giving the
furan-fused cyclobutene D (Scheme 1b).[8] These pioneering
studies demonstrate that the metal-mediated cycloisomeriza-
tion of allenyl ketones is a general approach to the cyclic
metal carbene species.
Scheme 1. Furan synthesis by transition-metal-catalyzed reactions of
allenyl ketones.
been demonstrated to be an important methodology in
organic synthesis.[10,11] Generally, the common carbene pre-
cursors in these transformations are diazo compounds.[12,13]
With palladium as the catalyst, diazo compounds can
participate in coupling reactions with a wide range of
electrophiles, such as aryl halides, benzyl halides, and allyl
halides. In the presence of suitable oxidants, diazo compounds
can also be employed as carbene precursors to participate in
palladium-catalyzed oxidative cross-coupling reactions with
various nucleophiles, which include aryl boronic acids,[13a–c]
terminal alkynes,[13d] and others.[13e–g] In addition, a series of
cascade reactions have also been developed.[10f,14] The palla-
dium carbene formation and the subsequent migratory
insertion are considered to be key steps in these trans-
formations (Scheme 1c).
In contrast, transition-metal-catalyzed cross-coupling
reactions involving carbene migratory insertion have recently
[*] Y. Xia, Dr. Y. Xia, R. Ge, Z. Liu, Dr. Q. Xiao, Dr. Y. Zhang,
Prof. Dr. J. Wang
Based on the importance of the furan synthesis and our
interest in palladium carbene chemistry, we envisioned that
novel methods for furan synthesis might be achieved by
integrating transition-metal-catalyzed cycloisomerization of
allenyl ketones with carbene migratory insertion reactions.
We have recently demonstrated that conjugated enynones can
be employed as carbene precursors to participate in palla-
dium-catalyzed cross-coupling reactions with organohalides
as electrophiles.[15,16] Herein we show that by utilizing allenyl
ketones as carbene precursors, palladium-catalyzed oxidative
cross-coupling of allenyl ketones with organoboronic acids
can be developed, thus providing an alternative method for
highly substituted furans (Scheme 1d).
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
E-mail: wangjb@pku.edu.cn
Dr. Y. Xia
College of Chemical Engineering, Qingdao University of Science and
Technology, Qingdao 266042 (China)
[**] This research was supported by the National Basic Research
Program of China (973 Program, No. 2012CB821600) and the
Natural Science Foundation of China (Grants 21332002, 21272010,
and 21172005).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400500.
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At the outset of this investigation, we used the 1,2-allenyl
ketone 1a and p-tolylboronic acid (2a) as the substrates to
optimize the reaction conditions (Table 1). Reaction condi-
tions similar to those we previously reported, in our study on
palladium-catalyzed oxidative cross-coupling reactions
between aryl boronic acids and diazo compounds, were first
used for the current reaction.[13a] To our delight, we obtained
the target furan product 3a in 25% yield (Table 1, entry 1).
Encouraged by this initial result, we further optimized the
reaction conditions. Changing the solvent from toluene to
other solvents failed to improve the reaction (Table 1,
entries 2–4). Then the effect of base was examined, and
iPr2NEt was found to be superior to iPr2NH, and the yield
could be improved to 52% (Table 1, entries 5–7). Changing
the ratio of the two substrates from 1:1 to 1:1.5 resulted in
a higher yield (Table 1, entries 8–10). Furthermore, we found
that the reaction was slightly affected by temperature
(Table 1, entries 11 and 12). Further lowering the temperature
led to a diminished yield (Table 1, entry 13). Subsequently, we
inspected the effect of the reaction concentration and found
that the reaction could be improved at low concentration,
thus affording 3a in 67% yield at 0.03 molLÀ1 (Table 1,
entries 14–16). Slow addition of 1a, over 5 minutes, to the
reaction system increased the yield to 76%, presumably
because the low concentration of the carbene precursor
reduced the occurrence of side reactions (Table 1, entry 17).
Finally, lowering the palladium catalyst loading from 5 mol%
to 2.5 mol% afforded a similar result (Table 1, entry 18), and
only a slightly diminished yield was observed when the
loading of catalyst was further reduced to 1 mol% (Table 1,
entry 19).
Thus, the reaction conditions given in entry 18 of Table 1
were chosen as the optimized reaction conditions for this
reaction, and a variety of allenyl ketone derivatives (1a–h)
were used to investigate the scope of the reaction (Table 2).
As illustrated in Table 1, good to excellent yields were
obtained in all the cases. We were delighted to find that the
tenfold scaled-up reaction afforded a comparable result,
wherein the product 3a was obtained in 90% yield (Table 2,
entry 1). The substituent on the aromatic ring appended to the
allene moiety has little influence on this reaction. Both
electron-donating and electron-withdrawing groups are well
tolerated in this transformation (Table 2, entries 2 and 3).
Apart from aryl-substituted allenes, a methyl-substituted
allene and a terminal allene were both suitable substrates
for this reaction (Table 2, entries 4 and 5). It is noteworthy
that tetrasubstituted furans could also be synthesized by this
method in 81–93% yields (Table 2, entries 6 and 7). In
addition, the substituents appended to the keto moiety are not
limited to aryl groups. For example, the allene 1h, bearing
a methyl ketone moiety, can be smoothly converted into the
corresponding product under the optimized reaction condi-
tions (Table 2, entry 8).
Table 2: Scope of the reaction of allenyl ketones with p-tolylboronic acid
(2a).[a]
Table 1: Optimization of reaction conditions.[a]
Entry
1
Allenyl ketone
Product
Yield [%][b]
75 (90)[c]
Entry
Solvent
Base
1a/2a
T
[8C]
c
Yield
[%][b]
[molLÀ1
]
1
2
3
4
5
6
7
8
9
toluene
1,4-dioxane
MeCN
iPr2NH
iPr2NH
iPr2NH
iPr2NH
iPr2NEt
K2CO3
1:1
1:1
1:1
1:1
1:1
80
80
80
80
80
80
80
80
80
80
90
70
50
70
70
70
70
70
70
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
25
21
trace
15
52
32
41
57
61
55
61
62
53
48
62
67
76
75[f]
69
2
3
69
69
DCE
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1:1
1:1
Et3N
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
iPr2NEt
1:1.3
1:1.5
1.5:1
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
4
5
6
7
72
63
93
81
10
11
12
13
14
15
16
17[c]
18[c,d]
19[c,e]
0.05
0.03
0.03
0.03
0.03
8
79
[a] The reaction was carried out on a 0.1 mmol scale, and 1a was added
as a solution with a concentration of about 0.5m. [b] Yield of product
isolated after column chromatography. [c] 1a was added as a solution
over 5 min. [d] The loading of [Pd(PPh3)4] is 2.5 mol%. [e] The loading of
[Pd(PPh3)4] is 1 mol%. [f] Average yield of three runs. BQ=1,4-
benzoquinone; DCE=1,2-dichloroethane.
[a] The reaction was carried on an 0.1 mmol scale with 1 equiv of 1a–h
and 1.5 equiv of 2a. [b] Yield of product isolated after column
chromatography. [c] The yield within parentheses was obtained on
a 1.0 mmol scale.
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We next explored the reaction scope with a series of aryl
boronic acids (2b–r) by using 1a as carbene precursor
(Scheme 2). To our delight, the reactions proceeded well
under the optimized reaction conditions, thus affording
versatile furan derivatives with moderate to good yields.
Scheme 2. Reaction scope of aryl boronic acids with the allenyl ketone
1a. Reactions were carried out under the reaction conditions described
in Table 2. Yields are those of the products isolated after column
chromatography.
Scheme 3. Reaction scope of allenyl ketones with alkenyl boronic
acids. Reactions were carried out under the reaction conditions
described in Table 2. Yields are those of the products isolated after
column chromatography.
The position of the substituents on the aromatic ring of the
aryl boronic acids has little effect on this transformation. Aryl
boronic acids bearing para (4b–g), meta (4h–l), and ortho
(4m–n) substituents with different electronic properties can
be employed in this reaction, thus smoothly generating the
cross-coupling products in 57–81% yields. It is noteworthy
that the compatibility of the functional groups is rather good
for this reaction. Even a terminal olefin (4d) and an aryl
boronic acid bearing halogen substituents (4e,j,k), groups
which are generally sensitive to palladium-catalyzed reaction
conditions, are well tolerated. Moreover, naphthyl boronic
acids (4o,p) and a heteroarylboronic acid (4q) are also
suitable substrates, thus affording the corresponding products
in 64–76% yields.
m) groups are well tolerated. In addition, the 2-alkyl-
substituted alkenyl boronic acids can also be used as
substrates in this reaction, thus affording the corresponding
3-alkenyl-substituted furans 6n–p.
We have proposed a plausible mechanism for the palla-
dium-catalyzed oxidative cross-coupling reaction of boronic
acids and allenyl ketones (Scheme 4). The reaction is initiated
Since alkenyl boronic acids have reactivity similar to that
of aryl boronic acids in palladium-catalyzed cross-coupling
reactions, we proceeded to extend the reaction scope to
alkenyl boronic acids. To the best of our knowledge, there
have been no reports on using alkenyl boronic acids in
palladium-catalyzed carbene migratory insertion reactions.
To our delight, alkenyl boronic acids showed good reactivity
in this palladium-catalyzed reaction. This type of reaction
provides an alternative approach to the synthesis of 3-alkenyl-
substituted furans.
As illustrated in Scheme 3, a variety of alkenyl boronic
acids (5a–h) reacted smoothly with allenyl ketones under the
same reaction conditions, thus affording 3-alkenyl-substituted
furans in 58–83% yields. E-styrylboronic acid (5a) was firstly
subjected to the reaction with a series of allenyl ketones (1a–
h), thus producing 3-alkenyl-substituted furans bearing differ-
ent substituents on the furyl rings (6a–h). The substituents on
the styryl boronic acids show little influence on the reaction as
both electron-donating (6i–k) and electron-withdrawing (6l–
Scheme 4. Proposed reaction mechanism.
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by the oxidation of the Pd0 species E by 1,4-benzoquinone
(BQ), thus generating the PdII species F which undergoes
transmetallation with aryl boronic acids to afford the aryl
palladium intermediate G.[13a] The species G coordinates to 1a
to give H, in which the allene moiety is bent, thus resulting in
a decrease of the distance between the carbonyl oxygen atom
and the terminal allene carbon atom.[6a] At this point, an
oxypalladation of the double bond would lead to the
formation of the zwitterionic intermediate I, which is
a resonance structure to the g-donor substituted vinyl carbene
complex J. Next, the aryl migration from palladium to the
carbenic carbon atom occurs in a stereoselective manner to
generate the intermediate K,[10] in which the steric hindrance
between palladium ligand and the phenyl group is avoided (in
contrast, M is not favored). The intermediate K has a cis-b-
hydrogen, which then undergoes b-hydride elimination to
form the furan product 3. Finally, the generated PdII species L
undergoes reductive elimination to regenerate E with the aid
of the base, thus completing the catalytic cycle.
During the optimization of this reaction, 2,5-diphenyl-
furan (7) was observed as a side product, and is likely
generated from the direct 1,2-H shift or b-hydride elimination
followed by reductive elimination of the palladium carbene
intermediate J.[6a,17] Previous mechanistic investigations using
DFT calculations have suggested that palladium carbene
migratory insertion is a highly facile process with an energy
barrier of less than 5 kcalmolÀ1, which is favorable to that of
the 1,2-H shift process.[15] This data is in consistent with the
experimental results of this study. Furthermore, under
reaction conditions similar to those of our previously reported
palladium-catalyzed oxidative cross-coupling reaction
between terminal alkynes and diazo compounds,[13d] phenyl-
acetylene (8) can also react with 1a to afford the 2-alkynyl
furan 9 in 35% yield along with the formation of the 1,2-H
shift byproduct 7 in 17% yield [Eq. (1)]. These preliminary
results also support the proposed mechanism involving
carbene migratory insertion process. Nevertheless, a mecha-
carbene migratory insertion is a rather general process
regardless of the source of carbene precursors. This work
should open up more possibilities in the future.
Received: January 16, 2014
Published online: March 11, 2014
Keywords: allenes · carbenes · cross-coupling · furans ·
.
palladium
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