Highly regioselective synthesis of N-acyl-2,3-dihydropyrrole or N-sulfonyl-2,3-dihydropyrrole derivatives by nano-palladium catalyzed cycloisomerization of 1,6-dienes

Article abstract of DOI:10.1016/j.catcom.2009.12.018

Porous palladium nanoparticles were prepared using a solution-phase approach. Their structures and shapes were analyzed by XRD, EDS, TEM and HRTEM. These palladium nanoparticles were used to catalyze the cycloisomerization reaction of N,N-diallylamide or N,N-diallylsulfonamide, and N-acyl-2,3-dihydropyrrole or N-sulfonyl-2,3-dihydropyrrole derivatives were therefore synthesized with very high regioselectivity and in good yields. The nano-palladium catalyst showed high efficiency and it can be reused several times. A hydropalladation mechanism was proposed for this reaction.

Full text of DOI:10.1016/j.catcom.2009.12.018

Catalysis Communications 11 (2010) 555–559
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Highly regioselective synthesis of N-acyl-2,3-dihydropyrrole
or N-sulfonyl-2,3-dihydropyrrole derivatives by nano-palladium
catalyzed cycloisomerization of 1,6-dienes
*
*
Lizhe Feng, Zhiyong Gan, Xiaopeng Nie, Peipei Sun , Jianchun Bao
Jiangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 October 2009
Received in revised form 7 December 2009
Accepted 13 December 2009
Available online 21 December 2009
Porous palladium nanoparticles were prepared using a solution-phase approach. Their structures and
shapes were analyzed by XRD, EDS, TEM and HRTEM. These palladium nanoparticles were used to cata-
lyze the cycloisomerization reaction of N,N-diallylamide or N,N-diallylsulfonamide, and N-acyl-2,3-dihy-
dropyrrole or N-sulfonyl-2,3-dihydropyrrole derivatives were therefore synthesized with very high
regioselectivity and in good yields. The nano-palladium catalyst showed high efficiency and it can be
reused several times. A hydropalladation mechanism was proposed for this reaction.
Ó 2009 Published by Elsevier B.V.
Keywords:
Palladium nanoparticles
Catalysis
Cycloisomerization
N-acyl-3,4-dimethyl-2,3-dihydropyrrole
N-sulfonyl-3,4-dimethyl-2,3-
dihydropyrrole
1. Introduction
tions, while other transition metals simply mediate the formation
of B selectively.
The synthesis of polysubstituted pyrroles remains an attractive
goal as they occur in biological, pharmaceutical and organic chem-
istry [1,2]. N-acyl-2,3-dihydropyrroles can be prepared from N-
formylpyrrolidine by anodic methoxylation and subsequent elimi-
nation of methanol [3]. These compounds were also synthesized by
the isomerization of N-acyl-2,5-dihydropyrroles, utilizing
HRhCO(PPh3)3 or Fe(CO)5 as catalysts [4]. A palladium catalyzed
isomerization of the corresponding N-acyl-2,5-dihydropyrroles to
N-acyl-2,3-dihydropyrroles was reported. In this procedure, a
phosphoric ligand dppp had been used [5].
Transition metal-catalyzed cyclization reactions of unsaturated
substrates constitute a powerful and atom economic route for the
construction of five-membered ring systems in recent years [6–10].
The alternative synthetic strategy starting from dienes (A) would
seem to hold interesting potential as several catalyst systems
based on Ni [11–14], Pd [15–18], Pt [19], Rh [20] and Ru [21–24]
were described, forming five-membered ring products B–D
through a cycloisomerization (Scheme 1). As that was described
in previous report [16], palladium can mediate the formation of
B, C and D quite selectively under the appropriate reaction condi-
In recent years, there has been growing interest in the fabrica-
tion of transition-metal nanoparticles and their applications in or-
ganic reactions especially the formation of carbon–carbon bonds
[25–28]. For example, palladium nanoparticles stabilized by tetra-
alkylammonium salts were obtained and used as heterogeneous
catalyst for the Suzuki and Stille cross-coupling reactions [29]. In
our previous works, bimetallic hollow palladium–cobalt nanopar-
ticles have been prepared and successfully used to catalyze Sono-
gashira reaction in aqueous media; cobalt hollow nanospheres
were also prepared by a wet chemical method and used to catalyze
the Heck reaction and Sonogashira reaction with good results [30–
32]. In this work, porous palladium nanospheres were prepared
and used to catalyze the cycloisomerization of N,N-diallylamide
or N,N-diallylsulfonamide and N-acyl-2,3-dihydropyrrole or N-sul-
fonyl-2,3-dihydropyrrole derivatives were therefore synthesized
with very high regioselectivities and good yields (Scheme 2).
2. Results and discussion
Palladium nanoparticles were prepared using a solution-phase
approach. Fig. 1 shows the XRD pattern of the sample prepared
by the reaction of hydrazine with [PdCl4]2À ions using acetone
and water as mixed solvents and hexadecylpyridinium chloride
as a surfactant. The diffraction peaks in the range of 30 < h < 85°
* Corresponding authors. Tel./fax: +86 25 83598280.
E-mail address: sunpeipei@njnu.edu.cn (P. Sun).
1566-7367/$ - see front matter Ó 2009 Published by Elsevier B.V.
doi:10.1016/j.catcom.2009.12.018

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L. Feng et al. / Catalysis Communications 11 (2010) 555–559
4000
Catalyst
3500
3000
2500
2000
1500
1000
500
X
+
X
+
X
X
Pd
D
B
C
A
Scheme 1. Cycloisomerization reactions of dienes.
Pd
Nano Pd, BTBAB
R
N
R
N
DMF
Pd
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
R = acyl or sulfonyl
keV
Scheme 2. Nano-palladium catalyzed cycloisomerization of N,N-diallylamide or
N,N-diallylsulfonamide.
Fig. 2. EDS image of the Pd nanoparticles.
can be indexed as cubic palladium (1 1 1), (2 0 0), (2 2 0) and
(3 1 1) and the lattice parameter is a = 3.90 Å, which all are in good
accordance with the values on the standard card (JCPDS 5-0681).
The broader peaks indicate the particles having a small size. EDS
analysis (Fig. 2) for the sample shows that they are composed of
palladium.
Fig. 3A shows the TEM image of the Pd particles. It can be seen
that the particles are of porous spherical shape and the average
diameter is about 40 nm mainly ranging from 30 to 50 nm. Further
information about the Pd nanoparticles comes from the highly
magnified TEM analysis (Fig. 3B). It can be seen from the figure that
there are uniform pores with a diameter of about 5 nm on the sur-
face of these Pd nanospheres. Fig. 3C shows the HRTEM image of
the Pd nanosphere. It clearly reveals the particles being well crys-
tallized and the lattice fringes with d spacings of about 0.22 nm,
which correspond to (1 1 1) reflection of the cubic Pd.
2,3-dihydropyrrole or N-sulfonyl-3,4-dimethyl-2,3-dihydropyrrole
(C) as products with very high regioselectivity. For optimizing the
reaction conditions, N,N-diallylbenzamide was chosen as a model
reactant. GC–MS and 1H NMR analyses showed that N-benzoyl-
3,4-dimethyl-2,3-dihydropyrrole (C) was generated with a high
yield and no other isomers (B and D) were detected. The experi-
ment showed that 1 mol% of Pd catalyst was sufficient to catalyze
the cycloisomerization reaction with a good result. Polar protic sol-
vent such as ethanol was disadvantageous to this reaction. In non-
polar solvent such as toluene, it gave a low yield. In polar aprotic
solvent N,N-dimethylformamide (DMF), it gave the satisfactory
yield. At room temperature, the reaction nearly could not proceed.
However, the yields improved up to 90% when the reaction was
carried out at 70 °C.
Stabilizers for metal nanoparticles to prevent their agglomera-
tion were very important in many preparations and applications
of nanoparticles [33,34]. Several quaternary ammonium salts
(QAS) were therefore tried to be used in this cycloisomerization
reaction to stabilize the nano-palladiums and enhance their cata-
lytic capability, and it led to a dramatic improvement of catalytic
activity. Among several quaternary ammonium salts or surfactants
such as tetra-n-butylammonium bromide (TBAB), dodecyltrimeth-
ylammonium bromide (DTMAB) and benzyltri-n-butylammonium
bromide (BTBAB), BTBAB produced the best effect. When the
amount of BTBAB reached to 20 mol%, the activity of this catalytic
system attained the highest (Table 1).
The measured Brunauer–Emmett–Teller (BET) specific surface
area for these porous Pd nanospheres, based on nitrogen absorp-
tion and desorption isotherm measurements, is 18 m2 gÀ1
.
Then we tried to use these palladium nanoparticles to catalyze
the cycloisomerization of N,N-diallylamide or N,N-diallylsulfona-
mide. It was quite different from the most transition-metal cata-
lyzed cycloisomerization of dienes reported previously which
mainly produced the product B (Scheme 1), these palladium nano-
particles catalyzed cycloisomerization giving N-acyl-3,4-dimethyl-
Under the selected reaction conditions, the reaction of a series
of reactants, both N,N-diallylamides and N,N-diallylsulfonamides
gave the corresponding compounds with very high selectivities.
No other isomers, except N-acyl-3,4-dimethyl-2,3-dihydropyrrole
or N-sulfonyl-3,4-dimethyl-2,3-dihydropyrrole were detected, as
the reaction of N,N-diallylbenzamide described above. It was inter-
esting that an even higher activity was observed with N,N-diallyl-
sulfonamide as substrate (Table 2, entries 1–4). At a lower
temperature of 30 °C, and within 4–7 h, the reaction could produce
N-sulfonyl-3,4-dimethyl-2,3-dihydropyrroles with perfect yields.
Both aromatic and aliphatic reactants gave the corresponding
cyclization products with good yields, and the substituted groups
of aromatic ring with electron donating or withdrawing effect
did not affect the reaction obviously.
Another notable advantage with this reaction was the recycla-
bility of the nano-Pd catalyst. The catalyst could be reused after
being separated from the solution and washed sufficiently with
methanol and ether. After being reused five times, the TEM image
showed that the modalities of the Pd nanospheres did not change
obviously, and the yield of the cycloisomerization did not reduce
4000
111
3500
3000
2500
2000
200
1500
311
220
1000
500
0
30 35 40 45 50 55 60 65 70 75 80 85
2θ (o)
Fig. 1. XRD pattern of Pd nanoparticles prepared by the reaction of hydrazine with
[PdCl4]4À ions using acetone and water as mixed solvents and hexadecylpyridinium
chloride as a surfactant.

L. Feng et al. / Catalysis Communications 11 (2010) 555–559
557
Fig. 3. TEM (A), highly magnified TEM (B), and HRTEM (C) images of the Pd nanospheres.
Table 1
Table 2
Optimization of the reaction conditions for the nano-palladium catalyzed cycloiso-
Nano-palladium catalyzed cycloisomerization of dienes.a
merization reaction.a
Nano Pd, BTBAB
O
O
R
N
R
N
Nano Pd, quaternary ammonium salt
Solv., Temp., 10 h
DMF
PhC
PhC
N
N
2 (a-n)
1 (a-n)
Entry
QAS
Solvent
Temperature (°C)
Yield (%)b
Entry
R
Temperature
(°C)
Time
(h)
Yieldb (%)
1
2
3
4
5
6
7
8
None
TBAB
TBAB
DMF
Toluene
DMF
Toluene
DMF
DMF
DMF
DMF
Toluene
Ethanol
DMF
70
70
70
70
70
70
70
70
70
70
50
90
35
55
72
75
86
90
78
90
66
25
76
90
1
2
PhSO2 (1a)
4-CH3C6H4SO2
(1b)
4-ClC6H4SO2 (1c) 30
CH3SO2 (1d)
PhCO (1e)
30
30
4
4
99
99
DTMAB
DTMAB
BTBAB
BTBABc
BTBABd
BTBAB
BTBAB
BTBAB
BTBAB
3
4
5
4
7
10
98
95
30
70
90 (89, 85, 86, 83,
74)c
83
9
6
7
8
4-CH3C6H4CO
(1f)
3-CH3C6H4CO
(1g)
2-CH3C6H4CO
(1h)
4-ClC6H4CO (1i)
2-ClC6H4CO (1j)
4-NO2C6H4CO
(1k)
70
70
70
10
10
10
10
11
12
85
82
DMF
a
b
c
1 mol% Pd and 20 mol% quaternary ammonium salt were used.
Isolated yields.
10 mol% quaternary ammonium salt was used.
25 mol% quaternary ammonium salt was used.
9
10
11
70
70
70
8
8
10
90
95
85
d
12
13
14
4-FC6H4CO (1l)
CH3CO (1m)
CH3CH2CH2CO
(1n)
70
70
70
12
7
7
80
90
92
more than 8%, which indicated that the catalytic activity did not
decrease significantly. The yield was reduced to 74% for the sixth
use which indicated that the catalyst can be used at least for five
times (Table 2, entry 5).
a
b
c
Reaction conditions: 1 mol% Pd, 20 mol% BTBAB, DMF.
Isolated yields.
The catalyst was reused five times.
Lloyd-Jones [7,35] and Widenhoefer et al. [36,37] have studied
transition-metal catalyzed cycloisomerization of 1,6-dienes by the
kinetic isotope experimental methods, and a hydropalladation
mechanism was reported convincingly. According to the related
works, we presume the product generated through a hydrometala-
tion of the diene, intramolecular carbometalation, isomerization
via reversible b-hydride elimination/addition, and turnover-limit-
ing displacement of the dihydropyrrole derivatives from palla-
dium. The general procedure is described as Scheme 3. It was
noteworthy that under the selected reaction conditions described
above, there was almost no reaction with N,N-diallylaniline. It sug-
gested that the acyl or sulfonyl group played a definite role, as indi-
cated in Scheme 3. Intramolecular displacement of the coordinated
olefin from intermediate with a pendant carbonyl or sulfonyl group
formed the palladium–carbonyl (sulfonyl) intermediate, which can
also explain the high selectivity of this reaction. Our experiment
also showed that under our nano-palladium catalyzed reaction
conditions, N-benzoyl-3-methyl-4-methylenepyrrolidin was con-
verted to N-benzoyl-3,4-dimethyl-2,3-dihydropyrrole, which sus-
tains this mechanism.

558
L. Feng et al. / Catalysis Communications 11 (2010) 555–559
Pd+
H
Pd+
Pd+
R N
R N
R N
R N
Pd+
H
R N
PdH+
N
R N
R N
R N
O
H
Pd+
Pd+
+
Pd+
Pd
R N
H
H
Scheme 3. The possible hydropalladation mechanism for nano-palladium catalyzed cycloisomerization of dienes.
ered. After being reused for five times, the yield of the cycloisomer-
3. Experimental
ization did not reduce more than 8%.
Spectroscopic data for several selected products are shown
below.
3.1. General
N-benzenesulfonyl-3,4-dimethyl-2,3-dihydropyrrole (2a) light
yellow oil. 1H NMR (400 MHz, CDCl3) d 7.78–7.80 (m, 2H), 7.59–
7.61 (m, 1H), 7.52–7.56 (m, 2H), 6.06 (s, 1H), 3.65–3.70 (m, 1H),
3.01–3.06 (m, 1H), 2.63–2.65 (m, 1H), 1.60 (s, 3H), 0.82 (d,
J = 6.89 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 135.7, 132.8, 129.0,
128.9, 127.7, 127.4, 127.3, 123.8, 55.1, 40.2, 18.2, 11.5. MS m/z
(%) 238 (52, M++1), 237 (100, M+), 222 (46), 96 (49). Anal. Calc.
for C12H15NO2S: C, 60.73; H, 6.37; N, 5.90. Found: C, 60.93; H,
6.48; N, 5.83%.
The reagents used in the experiments were purchased from Al-
drich or the Shanghai Chemical Reagent Company. Phase character-
izationwasperformedbymeansof X-raydiffraction(XRD)usinga D/
Max-RA diffractometer with Cu Ka radiation. The morphology and
particle sizes of the samples were characterized by JEM-200CX
transmission electron microscopy (TEM) and a JEOL 2010 high-reso-
lution transmission electron microscopy (HRTEM) working at
200 kV. Energy dispersive spectroscopy (EDS) was also performed
on the HRTEM. The measurement of surface area of the sample
was performed using ASAP 2020 Micromeritics. 1H NMR and 13C
NMR spectra were determined on a Bruker AV 400 spectrometer
(400 MHz) with TMS as the internal standard. MS spectra were
determined on a Varian 3800 GC–MS apparatus. Elemental analysis
was performed on a Perkin–Elmer 240C elemental analyzer.
N-(4-methylbenzoyl)-3,4-dimethyl-2,3-dihydropyrrole
(2f)
light yellow oil. 1H NMR (400 MHz, CDCl3) d 7.29–7.40 (m, 2H),
7.23–7.26 (m, 2H), 5.72 (s, 1H), 4.19–4.25 (m, 1H), 3.59–3.64 (m,
1H), 2.88–2.93 (m, 1H), 2.35 (s, 3H), 1.63 (s, 3H), 1.07 (d,
J = 6.92 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 166.3, 140.3, 129.2,
129.0, 128.4, 127.8, 126.9, 126.7, 124.6, 53.8, 38.8, 21.4, 18.7,
11.7. MS: m/z (%) 216 (73, M++1), 215 (20, M+), 119 (100), 91
(14). Anal. Calc. for C14H17NO: C, 78.10; H, 7.96; N, 6.51. Found:
C, 78.01; H, 7.87; N, 6.55%.
3.2. Typical experimental procedure for the preparation of palladium
nanoparticles
In a mixture of 1.0 mL of concentrated HCl and 2 mL of deion-
ized water, 0.02 g (0.11 mmol) of PdCl2 was dissolved. The pH va-
lue of the solution was then adjusted to 7.0 using 0.1 M NaOH.
Under magnetic stirring at 50 °C, 0.11 g of hexadecylpyridinium
chloride (0.32 mmol) and 15 mL of acetone were added to this
solution. After stirring for about 10 min, 6 mL of 80% hydrazine hy-
drate was added. The color of the solution then changed from yel-
low-brown to black, indicating the formation of the Pd colloid.
Finally, the mixture was separated by centrifugation. The deposit
was washed with deionized water and ethanol for several times.
After vacuum drying, the porous Pd nanospheres were obtained.
4. Conclusions
In conclusion, we have successfully prepared porous Pd nano-
spheres and used these nanoparticles in the synthesis of N-acyl-
3,4-dimethyl-2,3-dihydropyrrole or N-sulfonyl-3,4-dimethyl-2,3-
dihydropyrrole from dienes. The reaction was proposed through
a hydropalladation mechanism which led to the high regioselectiv-
ity. Though the method has a limitation that it is only effective to
N-acyl or sulfonyl dienes in the described reaction conditions, it
gave a very high selectivity as well as the convenient preparation
of catalyst. Further advantages of this method are the high effi-
ciency and reusability of the catalyst. The application of this new
catalyst in organic synthesis will be further expanded. This work
is under way in our laboratory.
3.3. Typical experimental procedure for the cycloisomerization of
dienes catalyzed by palladium nanoparticles
To 2 mL of DMF were added 1 mmol of N,N-diallylamide or N,N-
diallylsulfonamide, then 0.01 mmol of palladium nanoparticles
(1 mol%), 0.2 mmol benzyltri-n-butylammonium bromide were
added in turn. The mixture was heated at appropriate temperature
with stirring under a nitrogen atmosphere for the desired reaction
time (monitored by TLC) till reaction was completed, then centri-
fuged. The solution was separated and the precipitate was washed
with ether (5 Â 3 mL). The solutions were combined and washed
with water, dried over anhydrous Na2SO4 and purified by column
chromatography on silica gel with hexane–ethyl acetate (15:1) as
eluent to yield the pure product. The structures of all products
were characterized by 1H NMR, 13C NMR, GC–MS and elemental
analysis.
Acknowledgements
This work was supported by the National Natural Science Foun-
dations of China (Project 20772057, 20972068, 20871070) and the
Leading Academic Discipline Program, 211 Project for Nanjing Nor-
mal University (the 3rd phase).
Appendix A. Supplementary data
The precipitate was further washed sufficiently with methanol
and ether, and then dried, the palladium nanoparticles were recov-
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.catcom.2009.12.018.

L. Feng et al. / Catalysis Communications 11 (2010) 555–559
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