Palladium(II) phosphine-ylide complexes as highly efficient pre-catalysts in additive- and amine-free Sonogashira coupling reactions performed under aerobic and low Pd loading conditions

Article abstract of DOI:10.1016/j.tetlet.2013.06.064

Moisture and air-stable, robust palladacycle phosphine-ylide complexes as catalyst precursors were used in additive- and amine-free Sonogashira cross-coupling reactions. Various aryl halides were coupled with phenylacetylene in DMF, under air, in the pres

Full text of DOI:10.1016/j.tetlet.2013.06.064

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium(II) phosphine–ylide complexes as highly efficient
pre-catalysts in additive- and amine-free Sonogashira coupling
reactions performed under aerobic and low Pd loading conditions
a,
⇑
a
b
b
a
Seyyed Javad Sabounchei , Mohsen Ahmadi , Zahra Nasri , Esmaeil Shams , Mohammad Panahimehr
a
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran
Chemistry Department, University of Isfahan, Isfahan 81746-73441, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Moisture and air-stable, robust palladacycle phosphine–ylide complexes as catalyst precursors were used
in additive- and amine-free Sonogashira cross-coupling reactions. Various aryl halides were coupled with
phenylacetylene in DMF, under air, in the presence of 0.001 mol % of the catalyst to afford the corre-
Received 16 March 2013
Revised 25 May 2013
Accepted 14 June 2013
Available online xxxx
sponding cross-coupled products in good to excellent yields. Application of the five-membered pallada-
1
cycle [(P^C)PdCl
2
] (C ) in Sonogashira coupling reaction produced comparable catalytic activities of the
2
seven-membered [(C^C)PdCl
2
] (C ) analogs.
Keywords:
Ó 2013 Published by Elsevier Ltd.
Palladacycle phosphine–ylide complex
Additive- and amine-free conditions
Sonogashira coupling reaction
Catalyst precursor
Phenylacetylene
Low catalyst loading
The Sonogashira coupling reaction of phenylacetylene with aryl
halides catalyzed by Pd complexes in the presence of a copper re-
agent is a powerful tool in organic synthesis and has been widely
applied to various areas such as natural product synthesis, biolog-
O
O
Ph
P
2
Ph
P
2
CH
PPh
2
CH
CH
PPh
2
1
Pd
Cl
Pd
R
ically active molecules, and materials science. In the Sonogashira
Cl
Cl
R
O
coupling reaction, copper salts usually play a significant role in
R
Cl
C2
2
transmetalation. The original Sonogashira reaction generally pro-
C1
R = NO2
ceeds in the presence of a large amount of homogeneous palladium
catalyst containing copper as the co-catalyst under inert condi-
tions. It has been well-documented that the Sonogashira coupling
often suffers from the Glaser-type oxidative dimerization of the
Figure 1. Structures of the palladacycle phosphine–ylide complexes.
3
8
alkyne substrate as a side-reaction in the presence of a Cu(I) co-
synthesis of palladacycle complexes and the applications of these
systems, we report here additive- and amine-free, homogeneous
Sonogashira reactions catalyzed under air, in DMF by two pallada-
cycle phosphine complexes that we recently developed (Fig. 1).
Complexes C and C were evaluated in the additive- and
amine-free Sonogashira coupling reactions of aryl halides with
phenylacetylene. We first investigated the effect of different
solvents (Table 1, entries 1–7) on the model reaction of 4-bromo-
benzaldehyde with phenylacetylene catalyzed by 0.001 mol % of
catalyst C , without copper iodide under air. K
base in this reaction. The solvent had a dramatic influence on the
product formation. Generally moderate product yields were
observed when the reactions were performed in solvents of low
polarity, such as 1,4-dioxane and toluene. High yields were
observed when highly polar solvents, such as acetonitrile,
9
catalyst. The other limitation of Sonogashira reactions arises from
4
the use of hazardous amines such as piperidine or triethylamine,
5
6
10
as solvents, and bases in the coupling reactions. In addition, there
are only a few catalytic systems that enable the Sonogashira cou-
pling reaction under air with a low catalyst loading.7 Thus, the
development of more efficient, cheap, and easily prepared cata-
lysts, which allow the Sonogashira cross-coupling to be performed
with low catalyst loadings under additive- and amine-free reaction
conditions, and without the need of exclusion of air and moisture,
is of high general interest. In continuation of the interest in the
1
2
2
2
CO
3
was used as a
⇑
Corresponding author. Tel.: +98 8118272072; fax: +98 8118273231.
E-mail addresses: jsabounchei@yahoo.co.uk (S.J. Sabounchei), Ahmadi.chem@
yahoo.com (M. Ahmadi).
0
040-4039/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2013.06.064
Please cite this article in press as: Sabounchei, S. J.; et al. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.06.064

2
S. J. Sabounchei et al. / Tetrahedron Letters xxx (2013) xxx–xxx
dimethylformamide (DMF), N-methylpyrrolidone (NMP), metha-
nol, or H O were used as the reaction medium. The yield of
cross-coupled product was reduced as the polarity of the organic
solvent decreased. As shown in Table 1, DMF gave the highest yield
Table 2
The effect of the amount of catalyst on the Sonogashira coupling of 4-bromobenz-
aldehyde with phenylacetylene
2
a
Entry
Catalyst (mol %)
Yieldb (%)
(
entry 7, 90%) after 3.5 h at 130 °C under copper-free conditions.
We next investigated the influence of various bases (Table 1, en-
1
2
3
4
5
6
7
8
9
None
—
90
90
84
85
95
91
90
90
1
C
C
C
C
C
C
C
C
(0.1)
1
1
1
2
2
2
2
2
(0.01)
(0.001)
(0.005)
(0.1)
(0.01)
(0.001)
(0.005)
tries 7–13) on the model system using 0.001 mol % of catalyst C
under air. Under identical reaction conditions, bases such as
K
2
CO
variation in the isolated yields. However, only K
gave acceptable results with DMF as the solvent. Based on the re-
sults shown in Table 1, K CO was the best choice.
3
, Et
3
N, Cs
2
CO
3
, Na
2
CO
3
, NaOAc, and NaF led to considerable
2
CO and Cs CO
3
2
3
2
3
a
Various catalyst concentrations were also tested and the results
are shown in Table 2. A control experiment indicated that the cou-
pling reaction did not occur in the absence of a catalyst (entry 1). A
catalyst loading of 0.001 mol % was found to be optimum (Table 2,
entries 4 and 8).
Reaction conditions: 4-bromobenzaldehyde (1 mmol), phenylacetylene
(1.3 mmol), K CO (2.5 mmol), DMF (2 ml), 3.5 h, 130 °C, under air.
Isolated yields.
2
3
b
Table 3
Using the optimized conditions, we examined the scope of the
Copper-free Sonogashira reactions of aryl halides with phenylacetylene catalyzed by
precursor C
reaction of phenylacetylene with various aryl halides using
1a
1
[
(P^C)PdCl
2
] (C ) as the catalyst and the results are shown in
Table 3. High yields of diaryl acetylenes were obtained in reactions
with electron-poor aryl halides, while low product yields were
obtained in the reactions with electron-rich aryl halides.11
As expected, the coupling of phenylacetylene with iodides and
bromides led to the desired products in good to high yields (Table 3,
entries 1–12). In particular, the reactions of aryl iodides substi-
tuted with electron-withdrawing groups occurred in high yield at
Productb
Yieldc (%)
Entry
Ar-X
Time (h)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1f
6
4
81
85
84
80
84
78
84
83
72
80
75
1
30 °C (Table 3, entries 2, 3, and 5). Electron neutral iodobenzene
was converted into the corresponding product in 81% yield after
h. 4-Iodotoluene as a deactivated aryl iodide clearly gave the cor-
4
6
responding product in 80% yield, after 7 h at 130 °C and did not
produce any side products. In the case of activated bromides, long-
er reaction times were required relative to activated aryl iodides in
order to obtain good yields (Table 3, entries 7, 8, and 10).
When less reactive aryl bromides with electron-donating
groups were used, the coupling product was obtained in an accept-
able yield after a longer reaction time of 10 h (Table 3, entry 9,
7
4.5
8
5
7
2%). As expected, satisfactory results were obtained with 2-bro-
mothiophene and 1-bromonaphthalene (Table 3, entries 11 and
2). The catalyst effect was also evident for chlorides, where only
5
1
10
5.5
5
those substituted with electron-withdrawing groups gave accept-
able yields of the coupling products (Table 3, entries 14 and 16).
A non-activated aryl chloride gave a moderate yield in the coupling
1
0
1
1
Table 1
Optimization of the base and solvent for the copper-free Sonogashira reaction of
a
4
-bromobenzaldehyde with phenylacetylene
12
1g
8
76
1
3
4
1a
1b
1d
1e
12
10
16
10
67
74
Entry
Base
Solvent
Time (h)/Temp (°C)
Yieldb (%)
1
1
2
3
4
5
6
7
8
9
0
1
2
3
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
1,4-Dioxane
NMP
MeOH
4/110
3/130
8/60
8/80
60
71
40
53
65
Trace
90
24
75
52
47
32
12
15
58 (66)d
70
CH
3
CN
16
Toluene
4/110
H
2
O
24/100
3.5/130
12/100
3.5/130
3.5/130
12/130
12/130
24/130
a
Reaction conditions: aryl halides (1 mmol), phenylacetylene (1.3 mmol), cata-
DMF
1
lyst precursor C (0.001 mol %), K
2
CO
3
(2.5 mmol), DMF (2 ml), 130 °C, under air.
Et
Cs
Na
NaOAc
NaF
3
N
H
2
O
b
For characterization of the corresponding products (1a–1g) refer to Supple-
2
CO
CO
3
DMF
DMF
DMF
DMF
DMF
mentary data.
1
1
1
1
2
3
c
Isolated yield.
Using 0.01 mol % of the catalyst C .
d
1
3
Et N
a
reaction using a 0.01 mol % catalyst loading and longer reaction
time (Table 3, entry 15, 58%).
Reaction conditions: 4-bromobenzaldehyde (1 mmol), phenylacetylene
1.3 mmol), catalyst C2 (0.001 mol %), base (2.5 mmol), solvent (2 ml), 130 °C, under
(
air.
We next examined the copper-free reactions of aryl halides cat-
b
2
Isolated yield.
2
alyzed by 0.001 mol % of [(C^C)PdCl ] (C ) (Table 4).
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S. J. Sabounchei et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
Electron-neutral, electron-rich, and electron-poor aryl iodides
reacted with phenylacetylene to generate the corresponding
cross-coupling products in excellent yields (Table 4, entries 1–5).
The mononuclear phosphine–ylide palladacycle complexes cata-
lyzed the copper-free reactions such that all of the aryl halides were
converted into the desired products within 2.5–10 h, which is excel-
lent catalytic efficiency compared to the results of the previous
4-methylchlorobenzene (Table 4, entry 6, 73%) without addition
of Cu(I) as a co-catalyst. As can be seen in Tables 3 and 4, when
the reactions with 4-chloroacetophenone were carried out at
1
130 °C, catalyst C gave the coupling product in 70% yield (Table 3,
2
entry 16), while C gave a 76% product yield (Table 4, entry 16),
2
1
suggesting that C again shows higher catalytic activity than C .
Increasing the reactivity of the catalytic system by using bulkier
phosphine ligands has been shown often as an appropriate way
to perform copper-free Sonogashira reactions. Palladium systems
modified with a bulky and electron rich phosphine ligand, can be
used to provide unusually high activity in Sonogashira coupling
1
2
homogenous system. Interestingly, the Sonogashira coupling of
phenylacetylene with aryl bromides bearing electron-donating
and electron-withdrawing groups gave the corresponding biaryl-
acetylenes in excellent yields (Table 4, entries 7–10). The electron-
ically neutral bromobenzene (Table 4, entry 6) produced a good
yield of the desired product. 2-Bromothiophene and 1-bromonaph-
thalene were also efficient substrates (Table 4, entries 11 and 12).
We next investigated the coupling of various aryl chlorides
with phenylacetylene. The coupling products of 4-chlorobenzalde-
hyde and 4-chloroacetophenone with phenylacetylene were ob-
1
c,6b,13
reactions.
It was found that bulky and symmetrical phos-
2
phorus ylide containing complex C was the most active catalyst
in these C–C cross-coupling reactions. Electron-rich compounds fa-
vor the oxidative addition step, whereas the steric bulkiness favors
the formation of low-coordinate and highly active palladium
1
4
complexes.
2
tained in good yields by using 0.001 mol % of C . However, for
Although acetylene homo-coupling reactions are generally use-
chlorobenzene, catalyst C2 showed low activity (Table 4, entry
ful in synthetic applications, the formation of homo-coupling
15
1
3). For 4-methylchlorobenzene with the as electron-donating
products while targeting cross-coupling is undesirable. This is be-
cause the terminal acetylene is either very expensive or is synthe-
sized via several steps. Therefore, the formation of a dimer is
considered wasteful. In this protocol, no copper salt was used
and the undesired formation of oxidative homo-coupling diyne
products was avoided.
group, a low yield was observed (Table 4, entry 15, 60%). Increas-
ing the catalyst loading to 0.01 mol % gave a moderate yield for
Table 4
Copper-free Sonogashira reactions of aryl halides with phenylacetylene catalyzed by
precursor C2
a
Electrochemistry is a convenient technique to detect Pd(0) com-
16
plexes that are generated in situ from Pd(II) complexes. Electro-
1
2
chemical studies of
C
and
C
were carried out by cyclic
voltammetry (CV) in acetonitrile, containing 0.1 M tetrabutylam-
monium tetrafluoroborate (TBATFB) as the supporting electrolyte
with Au as the working electrode and Pt wire as the counter elec-
trode. All the potentials were referenced to Ag/AgCl reference elec-
trode. Cyclic voltammograms were used to elucidate the oxidation
states of Pd in the catalytic process (Fig. 2).
Entry b
Ar–X
Time (h)
Yieldc (%)
87
1
2
3
4
5
6
7
8
9
4.5
3
91
1
For complex C the cathodic potential peak corresponding to
the redox pair appeared at À0.83 V versus Ag/AgCl. The reduction
was due to a one-electron transfer process. When the potential
was scanned further in the cathodic direction, a second cathodic
potential peak at À1.03 V was observed, which corresponds to
the Pd(I)/Pd(0). Another cathodic peak was observed at À1.49 V
3
88
5
86
3.5
6.5
3.5
3
87
1
7
2
corresponding to reduction of the nitro group. For C , the cyclic
voltammogram displayed two reduction peaks at À0.81 V and
À1.08 V, respectively (Fig. 2). The first peak was assigned to the
Pd(II)/Pd(I). Second was assigned to the reduction of Pd(I)/
84
90
9
b,10a,18
2
Pd(0).
In addition, the CV of the C exhibited an irreversible
87
8
80
1
0
1
5
84
1
2.5
90
1
2
7
85
13
14
15
16
9
8
71
76
10
7
60 (73)d
76
a
b
c
All reactions were carried out under air.
The reaction progress was monitored by thin layer chromatography (TLC).
Isolated yield.
Figure 2. Cyclic voltammograms of 1 mM of C1 and C2 in acetonitrile containing
0.1 M TBATFB as the supporting electrolyte. Scan rate = 100 mV/s.
d
2
Using 0.01 mol % of the catalyst C .
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4
S. J. Sabounchei et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Table 5
Catalytic activity of palladacycle complexes that promote Sonogashira couplings between phenylacetylene and aryl halides
Entry
Ar–X
-I
4-COMe-C
4-COMe-C
4-COMe-C
4-COMe-C
[Pd] catalyst
[(P^C) Pd Cl
[Pd(dmba)Cl(PTA)]
mol %
Conditions
Et
N, ionic liquid,a under air, 80 °C, 1 h
Cs CO , CH CN, under air, 80 °C, 24 h
Piperidine, under air, 70 °C, 10 min
Yield (%)
1
2
3
4
5
6
7
8
6
C H
5
2
2
2
]20a
1
2.5
4
0.5
0.004
0.8
3
3
41–56
100
99
93
82
50
91
90
b,20b
21b
6
6
6
6
H
H
H
H
4
4
4
4
-Br
-Br
-Cl
-Cl
2
3
3
2
1a
[(PPh
[(PPh
3
)
)
2
PdCl
PdCl
2
2
2
]
t
c
3
2
], P Bu
/Fc(P)
3
Cs
2
CO
3
, DMF, DBU, under nitrogen, 150 °C, 10 min
, DMF, under argon, 130 °C, 20 h
t
2
i
d,22
[Pd(C
[(N^C)
[(PPh
3
H
5
)Cl]
Pd
PdCl
Bu(P Pr)
K
2
CO
3
2
3
C
C
6
H
H
5
-Br
-Br
2
2
Br
2
]
]
Piperidine, NMP, under air, 100 °C, 10 h
e,24a
t
6
5
)
3 2
2
KO Bu, THF, under argon, 65 °C, 2 h
f,24b
4-Me-C
-Br
6
H
4
-Br
[Pd]
1
2
Cs CO
3
, DMF, CuI, PPh
3
, under air, 80 °C, 24 h
g
2
84 (8 h/78%)h
76 (10 h/70%)
9
6
C H
5
[(C^C)PdCl
[(C^C)PdCl
[(C^C)PdCl
2
2
2
] (C )
0.001
0.001
0.001
2
K CO
2
K CO
2
K CO
3
, DMF, under air, 130 °C, 6.5 h
3
, DMF, under air, 130 °C, 7 h
3
, DMF, under air, 130 °C, 5 h
2
h
1
1
0
1
4-COMe-C
4-COMe-C
6
H
H
4
-Cl
-Br
] (C )
2
84 (4.5 h/84%)h
6
4
] (C )
a
b
c
[
bmim][PF
PTA = 1,3,5-triaza-7-phosphaadamantane.
,8-Diazabicyclo[5.4.0]undec-7-ene.
Palladium tridentate ferrocenyl phosphine complex.
6 4
] and [bmim][PF ].
1
d
e
f
Used with 18-crown-6 and dihydroimidazolium hexafluorophosphate salt.
Carbamoyl-substituted N-heterocyclic carbene Pd(II) complexes.
This work (entries 9–11).
g
h
1
Using [(P^C)PdCl
2
] (C ) as the catalyst.
reduction peak at À1.42 V corresponding to reduction of the nitro
group. Therefore, we conclude that the reaction mechanism occurs
via reduction of Pd(II) to Pd(0) in the Sonogashira cycle.
General experimental procedure for the Sonogashira cross-
coupling
To evaluate the homogeneous nature of the catalysts and pro-
posed reduction of the palladium(II) phosphine–ylide complexes
to Pd(0) species, the mercury drop test was utilized, since mercury
leads to amalgamation of the surface of a heterogeneous catalyst.
In contrast, Hg(0) is not expected to have a poisoning effect on
homogeneous palladium complexes.19 When two drops of Hg(0)
were added to the reaction mixture of 4-bromobenzaldehyde and
phenylacetylene under the optimized conditions and heated using
an oil bath, the conversion of the reaction was not affected, which
suggests that the catalysis is homogeneous in nature (please see
Supplementary data: Table S1). The fact that metallic Hg does
not stop the catalysis is a good indication that no metallic Pd is
being produced. The data obtained confirmed that the Pd(0):Pd(II)
cycle was accruing.
A
mixture of an aryl halide (1 mmol), phenylacetylene
(1.3 mmol), catalyst (0.001 mol %), K CO (2.5 mmol), and DMF
2
3
(2 ml) was heated to 130 °C. The mixture was then cooled to room
temperature and the solvent was removed under reduced pressure.
The combined organic extracts were washed with brine and dried
over CaCl2 or MgSO . The solvent was evaporated and liquid resi-
dues were purified by silica gel column chromatography (n-hex-
ane:EtOAc, 80:20) and solid residues were purified by
recrystallization from EtOH and H O. Products were identified by
comparison of their H and C NMR spectral data those reported
4
2
1
13
2
6–35
in the literature.
In conclusion, we have used five-membered complex C1 (con-
2
taining P,C-donors ligand) and seven-membered complex C (con-
taining C,C-donors ligand) as efficient and well-defined catalysts
for the Sonogashira coupling of various aryl halides. Also, we have
developed optimized conditions for additive- and amine-free
Sonogashira reactions catalyzed by these palladacycle complexes
under aerobic conditions. The catalysts show high activity, and offer
many practical advantages such as low catalyst loading and air
stability.
Although several catalytic systems have been reported to sup-
port additive- and amine-free Sonogashira C–C coupling reactions,
1
homogeneous catalyst precursors of the type [(P^C)PdCl
2
] (C ) and
2
[
(C^C)PdCl
2
] (C ) are novel with respect to the P and CH phospho-
rus ylide environment. A comparison of palladacycles C and C
1
2
with other similar palladacycle systems in Sonogashira reactions
is presented in Table 5. Utilizing the conventional copper-free
Sonogashira cross-coupling reactions of phenylacetylene with var-
ious aryl halides, comparisons are made of the efficiencies of the
catalyst systems based on palladacycle bisphosphine monoylide
Acknowledgment
We gratefully acknowledge the funding support received for
this project from the Bu-Ali Sina University, I. R. Iran.
1
2
(
C ), bisphosphine bisylide (C ) with monophosphine (Table 5,
2
0
21
entries 1 and 2), bisphosphine (Table 5, entries 3 and 4), and
tetraphosphine (Table 5, entry 5)22 Pd(II) complexes. Of note is that
the best results for the coupling of phenylacetylene with aryl
halides were obtained with phospha-palladacycles (Table 5, entry
Supplementary data
Supplementary data ( H and 13C NMR spectra of Sonogashira
coupling reaction products and the results of Sonogashira cross-
coupling reactions catalyzed under mercury drop conditions) asso-
ciated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.06.064.
1
2
3
6
7
), or with palladacycles modified with carbenes (Table 5, entries
and 8), or phosphorus containing ligands (Table 5, entry 4).21b
2
4
This might be rationalized by the extra stability provided by these
ligands for the stabilization of the low-ligated catalytically active
Pd(0) species involved in the main catalytic cycle.25
The ease of preparation of the complexes as homogeneous
catalysts, their high solubility in organic solvents, low catalyst
loading, and stability toward air make them ideal complexes for
Sonogashira coupling. This method proved successful in most cases
to give the product in >99% purity. In all of the successful reactions,
References and notes
1.
(a) Sonogashira, K. Metal-Catalyzed Cross-Coupling Reactions; Wiley–VCH: New
York, NY, 1998; (b) Sonogashira, K. Comprehensive Organic Synthesis; Pergamon:
New York, NY, 1991; (c) Hundertmark, T.; Littke, A. F.; Fu, S. L.; Buchwald, G. C.
Org. Lett. 2000, 2, 1729; (d) Paterson, I.; Davies, R. D. M.; Marquez, R. Angew.
Chem., Int. Ed. 2001, 40, 603; (e) Cosford, N. D. P.; Tehrani, L.; Roppe, J.;
1
13
the H and C NMR data of the products indicated no homo-
coupling diyne products.
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S. J. Sabounchei et al. / Tetrahedron Letters xxx (2013) xxx–xxx
5
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Please cite this article in press as: Sabounchei, S. J.; et al. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.06.064