A phosphine-free, atom-efficient cross-coupling reaction of triorganoindiums with acyl chlorides catalyzed by immobilization of palladium(0) in MCM-41

Article abstract of DOI:10.1039/c7ra08355f

The first phosphine-free heterogeneous palladium(0)-catalyzed cross-coupling of triorganoindiums with acyl chlorides has been developed that proceeds smoothly in THF at 68 °C and provides a general and powerful tool for the synthesis of various valuable aryl ketones and α,β-acetylenic ketones with high atom-economy and high yield. This phosphine-free heterogeneous palladium(0) catalyst can be easily prepared from commercially available reagents and recovered by a simple filtration of the reaction solution and used for at least 10 consecutive trials without any decreases in activity. Our system not only avoids the use of phosphine ligands, but also solves the basic problem of palladium catalyst recovery and reuse.

Full text of DOI:10.1039/c7ra08355f

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A phosphine-free, atom-efficient cross-coupling
reaction of triorganoindiums with acyl chlorides
catalyzed by immobilization of palladium(0) in
MCM-41†
Cite this: RSC Adv., 2017, 7, 42570
*
Jiankang Miao, Bin Huang, Haiyi Liu and Mingzhong Cai
The first phosphine-free heterogeneous palladium(0)-catalyzed cross-coupling of triorganoindiums with
acyl chlorides has been developed that proceeds smoothly in THF at 68 ꢀC and provides a general and
powerful tool for the synthesis of various valuable aryl ketones and a,b-acetylenic ketones with high
atom-economy and high yield. This phosphine-free heterogeneous palladium(0) catalyst can be easily
prepared from commercially available reagents and recovered by a simple filtration of the reaction
solution and used for at least 10 consecutive trials without any decreases in activity. Our system not only
avoids the use of phosphine ligands, but also solves the basic problem of palladium catalyst recovery and
reuse.
Received 28th July 2017
Accepted 28th August 2017
DOI: 10.1039/c7ra08355f
rsc.li/rsc-advances
the minimization of byproduct formation6 and the choice of
chemical processes with little or no risk associated to humans
1. Introduction
Both aryl ketones1 and a,b-acetylenic ketones2 are important
building blocks for a large number of natural products and
pharmaceutical compounds. The development of various
approaches to these unsymmetrical ketones is of great interest
and many methods for their preparation have been reported.
One general and straightforward route to aryl ketones is the
Friedel–Cras acylation of aryl cycles with acid halides or acid
anhydrides.3 However, this methodology suffers from some
drawbacks such as the use of more than a stoichiometric
amount of aluminum trichloride, which is incompatible with
many functional groups, production of a large amount of highly
toxic and corrosive waste, and narrow scope of substrates.
Traditionally, a,b-acetylenic ketones are usually prepared via
the acylation of alkynylmetal reagents with acid chlorides4 or
palladium-catalyzed coupling of acyl chlorides with terminal
alkynes.5 Generally, the preparation of unsymmetrical ketones
by transition-metal-catalyzed cross-coupling reaction is severely
limited, since the organometallic partners oen react with the
product ketone.
or the environment are major contemporary concerns in the
chemistry community.7 Some organometallic reagents used in
cross-coupling reactions have shown limitations concerning the
use of alkyl (sp3) organometallics as coupling partners, the
toxicity associated with the metal, and undesirable side reac-
tions. During the past two decades, the applications of indium
in organic synthesis have increased steadily due to the apparent
low toxicity8 associated with indium as well as other interesting
chemical properties, such as low nucleophilicity and hetero-
philicity or similarities with magnesium, zinc, and tin.9 Tri-
organoindium compounds (R3In, R ¼ alkyl, alkenyl, alkynyl,
aryl) are increasingly gaining attention as efficient coupling
partners for transition-metal-catalyzed cross-couplings with
a variety of organic electrophiles such as aryl (or alkenyl) halides
(or pseudohalides) and acid chlorides10 and the carbonylative
cross-couplings11 because of their high efficiency, versatility,
atom-economy, and chemoselectivity. However, in most cases,
homogeneous palladium complexes such as Pd(PPh3)4,
PdCl2(PPh3)2, and Pd(dppf)Cl2 are usually used as catalysts for
these cross-coupling reactions. The problem with homogeneous
catalysis is the difficulty to separate the expensive palladium
catalyst from the reaction mixture and the impossibility to reuse
it in consecutive reactions. In addition, homogeneous catalysis
might result in unacceptable heavy metal contamination of the
desired isolated product due to the leaching of the metal. These
problems may have a very serious negative impact on their
possible industrial applications, especially the pharmaceutical
industry. In contrast, heterogeneous catalysts can be easily
separated from the reaction mixture by a simple ltration of the
The development of novel transition-metal-catalyzed cross-
coupling reactions with high degrees of conversion of the
organometallic species, high chemoselectivity, and the ability to
form carbon–carbon bonds between all of the different carbon
types (sp3, sp2, and sp) is still of considerable interest. Besides,
Key Laboratory of Functional Small Organic Molecules, Ministry of Education, College
of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang 330022,
P. R. China. E-mail: mzcai@jxnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08355f
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development of phosphine-free supported palladium complex
catalysts having a high activity and good stability is a topic of
enormous importance. The MCM-41-immobilized palladiu-
m(II)–Schiff base complex [MCM-41-N,N-Pd(OAc)2] could easily
be prepared starting from mesoporous material MCM-41 and
commercially available 3-aminopropyltriethoxysilane, pyridine-
Scheme 1 Synthesis of aryl ketones and a,b-acetylenic ketones via
heterogeneous Pd(0)-catalyzed cross-coupling of triorganoindiums
with acid chlorides.
2-carboxaldehyde, and Pd(OAc)2 as shown in Scheme
2
according to our previous procedure.21 Firstly, the mesoporous
MCM-41 was condensed with 3-aminopropyltriethoxysilane in
ꢀ
toluene at 100 C for 24 h, followed by the silylation with Me3-
reaction solution and reused in successive reactions provided
that the active sites have not become deactivated. Heteroge-
neous catalysis also helps to reduce wastes derived from reac-
tion workup, contributing to the development of green and
sustainable chemical processes.12 From the viewpoints of
economical and environmental concern, the development of
immobilized transition-metal catalysts is challenging and
important. So far, supported palladium catalysts have success-
fully been used in many carbon–carbon and carbon–hetero-
atom bond formation reactions,13 however, to the best of our
knowledge, no example of a heterogeneous palladium-catalyzed
cross-coupling reaction of triorganoindiums with organic elec-
trophiles has been described until now.
The discovery of mesoporous MCM-41 materials has
provided a new possible candidate for an ideal solid support to
immobilize homogeneous catalysts and given an enormous
stimulus to research in heterogeneous metal catalysis.14 The
hexagonally-ordered MCM-41 material possesses large and
uniform pore size, ultrahigh surface area, big pore volume and
rich silanol groups in the inner walls.15 To date, some func-
tionalized MCM-41-supported palladium,16 rhodium,17 molyb-
denum,18 gold19 and copper20 complexes have been successfully
used as potentially green and sustainable catalysts in organic
reactions. Recently, we have reported the rst preparation of an
MCM-41-immobilized palladium(II)–Schiff base complex [MCM-
41-N,N-Pd(OAc)2] and its successful application to the atom-
efficient cross-coupling of triarylbismuths with aryl iodides.21
In continuing our efforts to develop greener synthetic pathways
for organic transformations,16d–f,19d,20 herein we wish to report
the rst phosphine-free heterogeneous palladium(0)-catalyzed
cross-coupling reaction of triorganoindiums with acid chlo-
rides by using an MCM-41-immobilized palladium(0)–Schiff
base complex [MCM-41-N,N-Pd(0)] as a recyclable catalyst. The
reactions proceeded in THF smoothly under mild conditions,
yielding a variety of aryl ketones and a,b-acetylenic ketones in
good to excellent yields with high atom-economy and recycla-
bility of the catalyst (Scheme 1).
SiCl in toluene at room temperature for 24 h to give 3-
aminopropyl-functionalized MCM-41 material (MCM-41-NH2).
The latter was subsequently treated with pyridine-2-
carboxaldehyde in dry EtOH at 80 ꢀC for 12 h to afford the
Schiff base-functionalized MCM-41 (MCM-41-N,N), which was
then reacted with Pd(OAc)2 in acetone under reux for 72 h to
generate the MCM-41-immobilized palladium(II)–Schiff base
complex [MCM-41-N,N-Pd(OAc)2] as a light yellow powder.
Finally, the treatment of MCM-41-N,N-Pd(OAc)2 with hydrazine
hydrate in EtOH at 30 ꢀC for 3 h afforded the MCM-41-
immobilized palladium(0)–Schiff base complex [MCM-41-N,N-
Pd(0)] as a gray powder.
Elemental analyses and X-ray photoelectron spectroscopy
(XPS) were used to characterize the MCM-41-immobilized
palladium(0)–Schiff base complex. The N : Pd mole ratio of
the MCM-41-N,N-Pd(OAc)2 was determined to be 5.2. The XPS
data for MCM-41-N,N-Pd(0) (fresh), MCM-41-N,N-Pd(OAc)2,
MCM-41-N,N, Pd(OAc)2, metal Pd, and MCM-41-N,N-Pd(0)
(used) are listed in Table 1. It can be seen that the binding
energies of Si2p and O1s of MCM-41-N,N-Pd(OAc)2 were similar
to those of MCM-41-N,N. However the difference of N1s binding
energies between MCM-41-N,N-Pd(OAc)2 and MCM-41-N,N was
1.1 eV. The binding energy of Pd3d5/2 in MCM-41-N,N-Pd(OAc)2
was 0.9 eV less than that in Pd(OAc)2, but 1.9 eV larger than that
in metal Pd. These results showed that a coordination bond
between N and Pd was formed in MCM-41-N,N-Pd(OAc)2 and the
oxidation state of palladium in MCM-41-N,N-Pd(OAc)2 was
Pd(II).22 The binding energy (336.2 eV) of Pd3d5/2 of the freshly
prepared MCM-41-N,N-Pd(0) was lower than the binding energy
(337.4 eV) of Pd3d5/2 of MCM-41-N,N-Pd(OAc)2. The Pd3d5/2
binding energy depends strongly on the nature of the ligands.
Consequently, it is impossible to identify the reduced complex
as a zerovalent one on the basis of its Pd3d5/2 binding energy
only. However, the shi (lower) of Pd3d5/2 binding energy
together with the change in color from light yellow to gray
suggests that the reduction of the starting palladium(II) complex
to the zerovalent state has taken place. The binding energy of
Pd3d5/2 of the used MCM-41-N,N-Pd(0) catalyst was determined
to be 336.3 eV, which indicating that the oxidation state of
palladium in used MCM-41-N,N-Pd(0) catalyst aer reaction was
2. Results and discussion
Although phosphine ligands can stabilize palladium and still Pd(0).
inuence its catalytic activity, undoubtedly, the simplest and
The MCM-41-immobilized palladium(0)–Schiff base complex
cheapest palladium catalysts are still the phosphine-free [MCM-41-N,N-Pd(0)] was then used as the catalyst for the cross-
systems. Furthermore, the procedure for the synthesis of coupling reaction of triorganoindiums with acid chlorides. In
immobilized phosphine palladium complexes is rather our initial screening experiments, the cross-coupling of benzoyl
complicated since the preparation of the heterogeneous phos- chloride with Ph3In (0.37 equiv.) in THF was chosen as the
phine ligands requires multi-step sequences. Therefore, the model reaction to optimize the reaction conditions and the
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Scheme 2 Preparation of MCM-41-N,N-Pd(0) complex.
Table
1 XPS data for MCM-41-N,N-Pd(0) (fresh), MCM-41-N,N-
MCM-41-immobilized palladium(II)–Schiff base complex [MCM-
41-N,N-Pd(OAc)2] was used as catalyst, benzophenone 3a was
isolated in 71% yield (entry 1). To our delight, the yield of 3a
could be improved to 88% by replacing the MCM-41-N,N-
Pd(OAc)2 complex with the MCM-41-immobilized palladium(0)–
Schiff base complex [MCM-41-N,N-Pd(0)] (entry 2). We also
tested the catalytic activity of the MCM-41-supported thioether
palladium(0) complex [MCM-41-S-Pd(0)],23 which is an efficient
and phosphine-free heterogeneous palladium catalyst for
Suzuki coupling reaction reported by our group, but a lower
yield of 3a was obtained (entry 3), so the MCM-41-N,N-Pd(0)
complex was nally selected as the catalyst for the reaction. The
result means that all three of phenyl groups attached to the
indium were transferred to benzoyl chloride in the heteroge-
neous coupling reaction. Control experiments showed that
palladium catalyst is essential for this ketone formation
Pd(OAc)2, MCM-41-N,N, Pd(OAc)2, metal Pd, and MCM-41-N,N-Pd(0)
(used)a
Sample
Pd3d5/2
N1s
Si2p
O1s
MCM-41-N,N-Pd(0) (fresh)
MCM-41-N,N-Pd(OAc)2
MCM-41-N,N
Pd(OAc)2
Metal Pd
336.2
337.4
400.9
400.8
399.7
103.3
103.4
103.3
533.2
533.2
533.1
338.3
335.5
336.3
MCM-41-N,N-Pd(0) (used)
400.7
103.4
533.3
a
The binding energies are referenced to C1s (284.6 eV) and the energy
differences were determined with an accuracy of ꢁ0.2 eV.
results are summarized in Table 2. First, the effect of various
supported palladium complexes on the model reaction was
examined in THF at 68 ꢀC (Table 2, entries 1–3). When the
Table 2 Reaction condition screening for the coupling reaction of Ph3In (0.37 equiv.) with benzoyl chloridea
Entry
Palladium catalyst
Temp. (ꢀC)
Time (h)
Isolated yield (%)
1
2
3
4
5
6
MCM-41-N,N-Pd(II)
MCM-41-N,N-Pd(0)
MCM-41-S-Pd(0)
—
68
68
68
68
68
25
50
68
68
2
2
2
12
2
12
8
71
88
65
0
89
0
67
76
89
Pd(PPh3)4
MCM-41-N,N-Pd(0)
MCM-41-N,N-Pd(0)
MCM-41-N,N-Pd(0)
MCM-41-N,N-Pd(0)
7
8b
9c
5
1
a
All reactions were performed using 1.0 mmol of benzoyl chloride, 0.37 mmol of Ph3In, 1 mol% of palladium catalyst in 5.0 mL of THF under Ar.
b
0.5 mol% of MCM-41-N,N-Pd(0) was used. c 2 mol% of MCM-41-N,N-Pd(0) was used.
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reaction (entry 4). When Pd(PPh3)4 was used as the catalyst,10a (1a) with a variety of aromatic acyl chlorides bearing either
the desired product 3a was also isolated in 89% yield (entry 5), electron-donating or electron-withdrawing groups proceeded
which indicating that catalytic activity of the phosphine-free smoothly under the optimized conditions, generating the cor-
heterogeneous palladium(0) complex [MCM-41-N,N-Pd(0)] was responding substituted benzophenones 3b–3f in excellent
comparable to that of Pd(PPh3)4. The heterogeneous cross- yields. The results indicated that the electronic properties of the
coupling reaction did not occur at 25 ꢀC (entry 6). Lowering substituents on benzene ring have limited inuence on this
the reaction temperature to 50 ꢀC resulted in a decreased yield heterogeneous palladium-catalyzed cross-coupling reaction.
(entry 7). Finally, the amount of the immobilized palladium Reactions of sterically hindered 2-methylbenzoyl chloride and
catalyst was screened, and 1 mol% loading of palladium was 2-methoxybenzoyl chloride also proceeded effectively to
found to be optimal, a lower yield was observed and a longer produce the desired 2-methylbenzophenone 3g and 2-methox-
reaction time was required when the amount of the catalyst was ybenzophenone 3h in 83% and 86% yield, respectively. Bulky 1-
decreased to 0.5 mol% (entry 8). Increasing the amount of the naphthoyl chloride and 2-naphthoyl chloride could afford the
palladium catalyst could shorten the reaction time, but did not expected products 3i and 3j in high yields. It is noteworthy that
improve the yield of 3a obviously (entry 9). Therefore, the the reactions of heteroaroyl chlorides such as thiophene-2-
optimal catalytic system involved the use of MCM-41-N,N-Pd(0) carbonyl chloride and pyridine-3-carbonyl chloride with Ph3In
ꢀ
(1 mol%) in THF at 68 C under Ar for 2 h (Table 2, entry 2).
also gave the desired phenyl heteroaryl ketones 3k and 3l in
With the optimal reaction conditions established, we tried to 90% and 84% yield, respectively. In addition to aroyl chlorides,
investigate the scope and limitations of this heterogeneous phenylacetyl chloride proved to be also suitable coupling
palladium(0)-catalyzed cross-coupling reaction of triorganoin- partner and could afford the expected 1,2-diphenylethanone 3m
diums with acid chlorides and the results are summarized in in 89% yield. However, other aliphatic acyl chlorides such as
Table 3. As shown in Table 3, the coupling reactions of Ph3In butyryl chloride and hexanoyl chloride were not reactive under
Table 3 Synthesis of aryl ketones via heterogeneous palladium(0)-catalyzed cross-coupling of acid chlorides with Ph3In and trialkylindiumsa,b
a
All reactions were performed using 1.0 mmol of acid chloride, 0.37 mmol of R3In, 1 mol% of MCM-41-N,N-Pd(0) in 5.0 mL of THF at 68 ꢀC under Ar
b
for 2 h. Isolated yield based on acid chloride used.
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acetophenones 3n–3r in 86–95% yields. The reactions of bulky 1-
naphthoyl chloride and thiophene-2-carbonyl chloride with
Me3In (1b) also proceeded smoothly to furnish the desired
coupling products 3s and 3t in high yields. In addition, a variety
of butyl aryl ketones 3u–3z could be obtained in high yields by the
cross-coupling of n-Bu3In (1c) with various aroyl chlorides under
the optimized conditions. In all these reactions, the tertiary
alcohol resulting from double addition was not detected due to
the high chemoselectivity of triorganoindiums. The present
method provides a quite general and practical route for the
synthesis of a variety of biaryl ketones, substituted acetophe-
nones and butyl aryl ketones. A range of electron-donating and
electron-withdrawing groups such as methyl, methoxy, chloro,
uoro, nitro, and cyano on aroyl chlorides were well tolerated.
Encouraged by the above results, the heterogeneous cross-
coupling reactions of various trialkynylindiums 1d–1i with
a range of acid chlorides were then examined under optimized
reaction conditions and the results are listed in Table 4. The
Scheme 3 Heterogeneous Pd(0)-catalyzed cross-coupling of Me3In
and n-Bu3In with acid chlorides.
the conditions optimized for aromatic acyl chlorides and no
desired cross-coupling products were detected. The similar
observation was made by our group in heterogeneous palladium-
catalyzed cross-coupling of acyl chlorides with triarylbismuths.24
Under the reaction conditions optimized for Ph3In (1a), Me3In
(1b) and n-Bu3In (1c) also exhibited high reactivity in cross-
coupling reactions with acid chlorides (Scheme 3). For
example, the reactions of Me3In (1b) with aroyl chlorides bearing
various substituents, regardless of their electronic properties and
substitution positions, produced the corresponding substituted
Table 4 Synthesis of a,b-acetylenic ketones via heterogeneous palladium(0)-catalyzed cross-coupling of acid chlorides with trialkynylindiumsa,b
a
All reactions were performed using 1.0 mmol of acid chloride, 0.37 mmol of (RC^C)3In, 1 mol% of MCM-41-N,N-Pd(0) in 5.0 mL of THF at 68 ꢀC
b
under Ar for 2 h. Isolated yield based on acid chloride used.
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coupling reactions of tri(phenylethynyl)indium (1d) with
a range of electron-neutral, electron-rich and electron-decient
aroyl chlorides proceeded smoothly in THF at 68 ꢀC to afford the
corresponding a,b-acetylenic ketones 4a–4d in 89–92% yields.
Heteroatoms turned out to be compatible with the employed
reaction conditions. The reactions of heteroaroyl chlorides such
as furan-2-carbonyl chloride and thiophene-2-carbonyl chloride
with 1d produced the expected coupling products 4e and 4f in
good yields. Besides aroyl and heteroaroyl chlorides, an
aliphatic phenylacetyl chloride was also good coupling partner
and gave the desired 1,4-diphenylbut-3-yn-2-one 4g in 90%
yield. The coupling reaction worked equally well with tri(p-tol-
ylethynyl)indium (1e), providing the desired coupling products
4h–4j in 86–91% yields. In addition to aromatic trialkynylin-
diums 1d and 1e, three aliphatic trialkynylindiums 1f, 1g and 1h
also showed high reactivity in the cross-couplings with acid
chlorides. The reactions of trialkynylindiums 1f–1h with a wide
range of electron-neutral, electron-rich and electron-decient
aroyl chlorides or heteroaroyl chlorides proceeded effectively
under the optimized reaction conditions to afford the corre-
sponding a,b-acetylenic ketones 4k–4u in good to excellent
Scheme 4 Proposed catalytic cycle.
yields. Interestingly,
a
trimethylsilyl-functionalized tri-
alkynylindium reagent 1i also underwent the cross-coupling
For the practical application of a heterogeneous catalyst
with various aroyl chlorides smoothly to afford the desired system its stability and reusability are important factors. The
trimethylsilyl-functionalized alkynyl ketones 4v–4x in excellent MCM-41-N,N-Pd(0) complex could be easily separated and
yields. The developed methodology provides a quite general and recovered by a simple ltration of the reaction solution. We next
practical route for the preparation of a,b-acetylenic ketones investigated the recyclability of the catalyst by using the cross-
having various functionalities.
coupling reaction of benzoyl chloride with (Me3SiC^C)3In
To verify whether the observed catalysis was due to the (1i). Aer carrying out the reaction, the catalyst was recovered by
heterogeneous MCM-41-N,N-Pd(0) complex or to a leached simple ltration and washed with DMF and diethyl ether. Aer
palladium species in solution, we performed the hot ltration being air-dried, it can be reused directly without further puri-
test.25 We focused on the coupling reaction of Ph3In with cation. The recovered palladium catalyst was used in the next
benzoyl chloride. We ltered off the MCM-41-N,N-Pd(0) run, and almost the same yield of 4v was obtained for ten
complex aer 0.5 h of reaction time and allowed the ltrate to consecutive cycles (Fig. 1). In addition, palladium leaching in
react further. The catalyst ltration was performed at the reac- the heterogeneous catalyst was also determined by ICP-AES
tion temperature (68 ꢀC) in order to avoid possible recoordi- analysis on the recovered catalyst aer ten consecutive runs,
nation or precipitation of soluble palladium upon cooling. We which revealing almost the same palladium content as the fresh
found that, aer this hot ltration, no further reaction was one. The high catalytic performance and excellent recyclability
observed. We also determined the palladium content in the of the MCM-41-N,N-Pd(0) catalyst may relate to the efficient site
ltrate by ICP analysis, and only 0.3 ppm of palladium was isolation, to the optimal dispersion of the active sites on the
found in the clear solution. These results rule out any contri- inner channel walls and to the relatively strong interaction
bution to the observed catalysis from a homogeneous palla-
dium species indicating that the catalyst was stable during the
reaction and the observed catalysis was intrinsically
heterogeneous.
A plausible mechanism for this heterogeneous palladium(0)-
catalyzed cross-coupling reaction of triorganoindiums with acid
chlorides is illustrated in Scheme 4. First, oxidative addition of
R1COCl (2) to the MCM-41-N,N-Pd(0) complex provides an
MCM-41-bound acylpalladium(II) complex (A), which should be
the rate-limiting step.10a Subsequent transmetalation between
intermediate A and triorganoindium reagent (1) produces
intermediate B and R2InCl, which can re-enter in a new catalytic
cycle and effectively transfer its organic groups. Finally, reduc-
tive elimination of intermediate B affords the desired ketone (3
or 4) and regenerates the MCM-41-N,N-Pd(0) complex to
complete the catalytic cycle.
Fig. 1 Recycle of the MCM-41-N,N-Pd(0) catalyst.
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between bidentate Schiff base ligand and the palladium centre
anchored on the MCM-41. The result is important from the
standpoint of green and sustainable chemistry.
General procedure for preparation of triorganoindium
reagents
A 25 mL round-bottomed ask containing a stirrer bar was
charged with InCl3 (0.37 mmol) and dried under vacuum with
a heat gun. The ask was cooled, a positive argon pressure was
established and anhydrous THF (2 mL) was added. The result-
ing solution was cooled to ꢂ78 ꢀC and a solution of RLi or
RMgBr (1.1 mmol, 1.0–2.5 M in hexane, THF, or Et2O) was
slowly added (15–30 min). The reaction mixture was stirred for
30 min, the cooling bath was removed, and the mixture was
warmed to room temperature over 30 min.
3. Conclusions
In summary, we have successfully developed a novel, atom-
efficient, practical and environmentally friendly method for
the synthesis of aryl ketones and a,b-acetylenic ketones through
the cross-coupling of triorganoindiums with acid chlorides by
using an MCM-41-immobilized palladium(0)–Schiff base
complex [MCM-41-N,N-Pd(0)] as catalyst. This heterogeneous
phosphine-free palladium(0) catalyst could be easily prepared
by a simple procedure from commercially readily available
reagents and exhibited the same catalytic activity as Pd(PPh3)4.
The reactions generated a variety of valuable aryl ketones and
a,b-acetylenic ketones in good to excellent yields and were
applicable to various triorganoindiums and a wide range of acid
chlorides. In addition, this methodology offers the competi-
tiveness of recyclability of the palladium catalyst without
signicant loss of activity, and the palladium catalyst can be
easily recovered by a simple ltration of the reaction solution
and reused at least 10 cycles, thus making this procedure
economically and environmentally more acceptable.
General procedure for the heterogeneous palladium(0)-
catalyzed cross-coupling of acid chlorides with
triorganoindiums
A solution of R3In (0.37 mmol, ca. 0.18 M in dry THF) was added
to a mixture of MCM-41-N,N-Pd(0) (24 mg, 1 mol%) and acid
chloride (1 mmol) in dry THF (3 mL) under Ar. The resulting
mixture was reuxed under Ar until the starting material had
been consumed (TLC). Aer being cooled to room temperature,
the mixture was diluted with Et2O (30 mL) and ltered. The
palladium catalyst was washed with DMF (2 ꢃ 5 mL), Et2O (2 ꢃ
5 mL) and reused in the next run. The ltrate was washed with
sat. aq NaHCO3 (5 mL), water (3 ꢃ 10 mL) and dried over
MgSO4, ltered, and concentrated under vacuum. The residue
was puried by ash chromatography on silica gel
(EtOAc : hexane ¼ 1 : 25) to give the desired cross-coupling
product.
4. Experimental
All chemicals were reagent grade and used as purchased. All
solvents were dried and distilled before use. The MCM-41-N,N-
Pd(OAc)2 catalyst was prepared according to our previous
procedure,21 the palladium content was determined to be
0.42 mmol gꢂ1. The products were puried by ash chroma-
tography on silica gel. Mixture of EtOAc and hexane was
generally used as eluent. All coupling products were character-
ized by comparison of their spectra and physical data with
authentic samples. 1H NMR spectra were recorded on a Bruker
Avance 400 MHz spectrometer with TMS as an internal standard
in CDCl3 as solvent. 13C NMR spectra (100 MHz) were recorded
on a Bruker Avance 400 MHz spectrometer in CDCl3 as solvent.
HRMS spectra were recorded on a Q-Tof spectrometer with
micromass MS soware using electrospray ionization (ESI).
Melting points are uncorrected. X-ray photoelectron spectra was
recorded on XSAM 800 (Kratos). Palladium content was deter-
mined with inductively coupled plasma atom emission Atom-
scan16 (ICP-AES, TJA Corporation).
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21462021) and Key Laboratory of Functional Small Organic
Molecule, Ministry of Education (No. KLFS-KF-201409) for
nancial support.
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