Organostannoxane-supported Pd(0) nanoparticles as efficient catalysts for Heck-coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2011.05.006

A new functional organostannoxane cage, SnP, that contains phosphine ligands in its periphery has been structurally utilized as support palladium(0) nanoparticles SnPPd. The latter was shown to catalyze the Heck coupling reactions of wide variety of functionalities efficiently.

Full text of DOI:10.1016/j.tetlet.2011.05.006

Tetrahedron Letters 52 (2011) 3527–3531
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Organostannoxane-supported Pd(0) nanoparticles as efficient catalysts
for Heck-coupling reactions
⇑
Vadapalli Chandrasekhar , Ramakirushnan Suriya Narayanan
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new functional organostannoxane cage, SnP, that contains phosphine ligands in its periphery has been
structurally utilized as support palladium(0) nanoparticles SnPPd. The latter was shown to catalyze the
Heck coupling reactions of wide variety of functionalities efficiently.
Received 1 April 2011
Revised 1 May 2011
Accepted 3 May 2011
Available online 8 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Organostannoxane
Cage
Nanoparticles
Palladium
Catalysis
Palladium nanoparticles (PdNPs) are increasingly being used
as catalysts in various organic transformations and their advan-
tages stem from the fact they can be dispersed in an organic sol-
vent and can be reused.1 In general, prevention of agglomeration
of PdNPs is achieved by the use of stabilizers that include mi-
celles,2 microemulsions,3 surfactants,4 polymers,5 dendrimers,6
solid supports (SiO2, Al2O3, zeolites, etc.),7 polyoxo-metallates8,
and ionic liquids.9 Surprisingly, there has been less attention to
the stabilization of PdNPs by phosphine ligands, particularly
since the latter are very effective in molecular catalysis involving
Pd(0).10 Although Hyeon and co-workers have reported the sta-
bilization of nearly monodisperse PdNPs using various phos-
phine ligands these have not been tested for their effectiveness
as catalysts.11 On the other hand, recently, the research groups
of Chaudret12 and Fujihara13 have reported the asymmetric cat-
alytic applications of chiral phosphorous ligand-stabilized PdNPs.
More recently, Fan and co-workers have reported the prepara-
tion and use of PdNPs that were stabilized by a phosphine-con-
taining dendron.14 In a search for alternative stabilizers for NPs
we were intrigued by the possibility of using inorganic rings
and were encouraged by some preliminary reports in the
literature.15
encapsulation of PdNPs and recently we had successfully demon-
strated this strategy.16 Considering the importance of the Heck
reaction in organic synthesis17 we were interested in testing the
effectiveness of SnPPd, the Pd(0) nanoparticles stabilized by SnP.
These aspects are discussed herein.
As reported recently the reaction of [n-BuSn(O)OH]n with
4-PPh2-C6H4-COOH
afforded
[n-BuSn(O)OC(O)-C6H4-p-PPh2]6
(SnP) (Supplementary data).The phosphine unit in SnP undergoes
a slow oxidation in solution (chlorinated solvents) to the phos-
phine oxide as confirmed by the presence of two signals at À4.6
(-PPh2) and 29.5 (-P(O)Ph2) ppm. However, the shelf life of SnP,
in the solid-state, in air, is quite high. A fresh solution of an aged
sample does not reveal oxidation. Also, oxidation does not seem
to occur rapidly in solvents such as toluene. PdNPs prepared using
SnPPd possess an average size of 3 nm as revealed by HRTEM
experiments.16 The palladium content was estimated to be
19.9 wt %.18 The unique advantage of the SnP ligand is that it not
only is a spherical bulky ligand but also possesses additional phos-
phine appendages; these two factors are responsible to stabilize
PdNPs.
PdNPs stabilized by SnP, SnPPd, were used in the Heck reac-
tion. The reaction of iodobenzene with styrene was optimized
(100 °C, K2CO3, 4 h, 1,4-dioxane, Tables S1 and S2, Supplementary
data). A variety of substrates could be used in the Heck coupling
reaction with good yields and high TON and TOF were observed
(Table 1 and Figure S1). Substrates such as vinyl pyridines (entries
2 and 3, Table 1) could be readily used. A variety of acrylates could
Our experience in organostannoxane chemistry suggested to us
that we can utilize inorganic-cored star-type ligands for effective
⇑
Corresponding author. Tel.: +91 512 259 7259; fax: +91 512 259 0007/7436.
E-mail address: vc@iitk.ac.in (V. Chandrasekhar).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.006

3528
V. Chandrasekhar, R. S. Narayanan / Tetrahedron Letters 52 (2011) 3527–3531
Table 1
Heck-coupling reaction of different alkenes with iodobenzene under optimized conditions
R
R
Entry
Time (h)
Yeild (%)
TON 104 molÀ1
TOF 104 molÀ1 hÀ1
Ph
1
2
3
4
7
7
99
74
89
10.06
7.91
9.52
2.65
1.13
1.36
1a
1b
1c
N
N
N
N
O
O
O
O
O
O
4
5
6
5
99
99
10.6
10.6
1.76
2.12
O
O
1d
O
O
1e
n-butyl
O
n
-butyl
6
7
4
7
99
88
10.6
9.41
2.65
1.34
O
O
1f
NH2
NH2
O
1g
CN
O
8a
9b
8
6
55 (69)
30 (70)
0.74
0.54
0.74
0.54
CN
O
1h
1i
Reactions and Conditions: (a) PhI (1 equiv), alkene (1.25 equiv), K2CO3 (2 equiv), PdNPs (9.6 Â 10À4mol %) and solvent (5 mL) at 100 °C. (b) Yields reported are the isolated
yields and progress of the reaction was monitored by TLC. (c) The values in brackets of the column of yields of entries 8 and 9 are the overall yield of the reaction
a
E:Z is 79:21
E:Z is 42:58
b
also be used successfully. Although in most cases only E isomers
are formed (entries 1–8, Table 1) in the case of ethylvinylether
the E/Z ratio is nearly 1 presumably because of the unhindered nat-
ure of the substrate.
The Heck-coupling reaction involving arylbromides was more
effective in DMF (Tables S3–S4, Supplementary data). A large vari-
ety of arylbromides, including those that are sterically hindered as
well as those containing electron-releasing substituents could be
used (Table 2 entries 2–4, 9). Difficult substrates such as 1-bromo-
naphthalene could also be used. Similarly the versatility of SnPPd
can be gauged by the fact that substrates such as 2-bromopyridine
2,6-dibromopyridine, as well as bromo-substituted benzaldeydes
could be used.
Aryl chlorides were also tested as substrates with mixed results.
Firstly, the reactions required longer times and higher tempera-
tures (Table 3). Secondly, using 4-chloro-1-nitrobenzene as the
substrate resulted in only 35% of the Heck-coupled product (entry
9, Table 3). The major product was the Buchwald product involving
dimethylamination of the substrate, 4-dimethylamino-1-nitroben-
zene.19 However, in spite of this, a number arylchlorides do under-
go Heck reaction under these conditions (entries 1–11, Table 3).
For example, coupling of 2-chlorobenzaldehyde reacts with sty-
rene to give the Heck-coupled product in about 90% yield (entry
5, Table 3).
SnPPds were tested in recycling experiments involving the
coupling of 4-bromotoluene and styrene as a representative
example (entry 4, Table 2) by adopting the in situ recycling
methodology (Table S5, Supplementary data).13 It was found that
the reaction could be recycled up to 8 cycles, beyond which the
recyclability was not tested. The fate of the catalyst was exam-
ined after the catalysis. The catalyst particles showed virtually
no change in the dimension of the nanoparticles except for a
slight increase from 3.0 to 3.6 nm even after 8 consecutive cycles
(Fig. 1).
During this whole process formation of the palladium black
was not observed indicating the efficacy of the sterically
bulky dendrimer-like organostannoxane, SnP in stabilizing
PdNPs.
The nature of SnPPd was further examined to rule out any
molecular species. The 31P NMR of SnPPd was flat (Supplementary
data) consistent with the absence of molecular complexes. A neg-
ative mercury20 and -thiophene poisoning effect21 was observed
indicating that the nanoparticles are encapsulated effectively by
the organostannoxane ligand. Finally, the solutions of the nanopar-
ticles in many solvents including dimethylformamide and dioxane
were always clear and no indication of any palladium black forma-
tion was observed. Prevention of formation of the latter is one of
the challenges in this area.

V. Chandrasekhar, R. S. Narayanan / Tetrahedron Letters 52 (2011) 3527–3531
3529
Table 2
Heck-coupling reaction of different aryl bromides with styrene under optimized conditions
Ar
Entry
1
ArX
Time (h)
4
Yeild (%)
99
TON 104 molÀ1
10.6
TOF 103 molÀ1 hÀ1
Ph
Br
Br
2.65
1.12
2a
2
8
84
8.98
2b
2c
Br
3
4
8
6
89
99
9.52
10.6
1.19
1.76
Br
2d
Br
CHO
5
6
5
4
90
99
10.6
10.6
1.93
2.65
2e
CHO
Br
OHC
OHC
O
2f
OHC
Br
7
8
9
4
5
8
99
99
90
9.63
10.6
9.63
2.65
2.12
1.2
OHC
O
2g
Br
2h
O
Br
O
2i
Br
Br
10
11
12
9
99
90
87
10.6
9.63
9.3
1.18
0.80
0.66
2j
12
14
2k
Br
2l
Br
13
7
95
10.2
1.45
2m
14
15
9
85
70
9.09
7.49
1.01
0.53
2n
N
Br
N
14
Br
N
Br
N
2o
Reactions and Conditions: (a) ArX (1 equiv), Styrene (1.25 equiv), K2CO3 (2 equiv), PdNPs (9.6 Â 10À4mol %) and solvent (5 mL) at 100 °C. (b) Yields reported are the isolated
yields and progress of the reaction was monitored by TLC.

3530
V. Chandrasekhar, R. S. Narayanan / Tetrahedron Letters 52 (2011) 3527–3531
Table 3
Heck-coupling reaction of different aryl chlorides with styrene under optimized conditions
Ar
Entry
1
ArX
Time (h)
15
Yeild (%)
87
TON 104 molÀ1
TOF 103 molÀ1 hÀ1
Ph
Cl
Cl
9.3
6.203
3a
2
24
33
3.53
1.47
3b
3c
Cl
3
4
20
24
52
70
5.58
7.49
2.78
3.12
Cl
3d
Cl
5
6
12
22
90
45
9.63
4.81
8.02
2.18
CHO
CHO 3e
Cl
OHC
OHC
OHC
OHC
3f
Cl
7
15
32
10
48
76
15
35
75
8.13
1.6
5.41
0.5
3g
O
Cl
8
O
3h
3i
O2N
Cl
9
3.74
8.02
3.74
1.67
O2N
10
N
Cl
Cl
3j
N
Cl
11
36
30
3.21
8.91
3k
Reactiona and Conditions: (a) ArX (1 equiv), Styrene (1.25 equiv), K2CO3 (2 equiv), PdNPs (9.6 Â 10À4mol %) and solvent (5 mL) at 100 °C. (b) Yields reported are the isolated
yields and progress of the reaction was monitored by TLC.
of the Department of Metallurgical Engineering and Materials Sci-
ence, IIT Kanpur, for TEM measurements.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.05.006.
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