Catalytic activity of a zirconium(IV) Schiff base complex in facile and highly efficient synthesis of indole derivatives

Article abstract of DOI:10.1007/s11243-011-9519-6

Eight Schiff base compounds were prepared by condensation of 1-amino-2-propanol with different benzaldehydes in water. One of the Schiff bases, (z)-N-bezylidene-2-hydroxypropane-1-amine (HL¹), was used as a bidentate ligand for preparation of a

Full text of DOI:10.1007/s11243-011-9519-6

Transition Met Chem (2011) 36:685–690
DOI 10.1007/s11243-011-9519-6
Catalytic activity of a zirconium(IV) Schiff base complex in facile
and highly efficient synthesis of indole derivatives
•
Maasoumeh Jafarpour Abdolreza Rezaeifard
•
•
Somayeh Gazkar Maryam Danehchin
Received: 4 May 2011 / Accepted: 18 July 2011 / Published online: 5 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Eight Schiff base compounds were prepared
by condensation of 1-amino-2-propanol with different
benzaldehydes in water. One of the Schiff bases, (z)-N-
bezylidene-2-hydroxypropane-1-amine (HL1), was used as
a bidentate ligand for preparation of a zirconium complex
(Zr(L1)2Cl2). The complex has been used as a catalyst for
efficient synthesis of wide variety of indole derivatives in
EtOH under mild conditions. The turnover number and
reusability of the catalyst indicate that it has high efficiency
and is fairly stable under the reaction conditions.
oxygen- and nitrogen-containing ligands, and zirconium
salen complexes have proved to be efficient catalysts
both in homogeneous and in heterogeneous reactions. The
activities of these complexes vary markedly with the type
of ligands and coordination sites [8–17].
Indole derivatives have attracted much interest due to
their manifold applications in materials science, agro-
chemicals and pharmaceuticals. In particular, compounds
containing the bis (indolyl) motif, such as secondary
metabolites and marine sponge alkaloids, are of great
interest. Indoles can readily undergo electrophilic substi-
tution reactions with the oxygen atom of carbonyl com-
pounds in the presence of a suitable catalyst to form bis-
and tris (1H-indol-3-yl) methanes. Different compounds
are known to catalyze this reaction [18].
Introduction
The chemistry of transition metal complexes with Schiff
base ligands has been widely investigated in organome-
tallic chemistry and catalysis. Despite the extensive
investigations on complexes of salen-type ligands with
middle and late d-block transition metals as catalysts for
diverse organic reactions, very few reports have appeared
describing the synthesis [1, 2], crystal structures [3, 4] or
catalytic properties of group IV metal complexes with such
ligands [5, 6].
In continuation of our ongoing research on the
development of new eco-friendly organic synthesis, par-
ticularly by mediation of zirconium salts [19–27], herein,
we report the facile preparation of some new Schiff
bases and the catalytic performance of a zirconium(IV)
complex of one of these Schiff base ligands for the mild
and facile synthesis of indole derivatives in ethanol
(Schemes 1, 2).
Zirconium(IV), with a high charge-to-size ratio (Z2/r
is 22.22 e2m-10) [7], has a strong coordinating ability with
Experimental
Carbonyl compounds, indoles, ZrCl4 and amino alcohol
were purchased from Merck or Fluka. The progress of each
reaction was monitored by TLC using silica gel SIL G/UV
254 plates and also by GC-FID on a Shimadzu GC-16A
instrument using a 25-m CBP1-S25 (0.32 mm ID, 0.5 lm
coating) capillary column. NMR spectra were recorded on
Bruker Avance DPX 250 MHz and 400 MHz spectrome-
ters. Mass spectra were recorded on a Shimadzu GC–MS-
QP 5050A instrument.
M. Jafarpour (&) ꢀ A. Rezaeifard (&) ꢀ S. Gazkar ꢀ
M. Danehchin
Catalysis Research Laboratory, Department of Chemistry,
Faculty of Science, University of Birjand, Birjand 97179-414,
Iran
e-mail: mjafarpour@birjand.ac.ir
A. Rezaeifard
e-mail: rrezaeifard@gmail.com; rrezaeifard@birjand.ac.ir
123

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Transition Met Chem (2011) 36:685–690
Scheme 1 Synthesis
of Zr-complex (ZrL12Cl2)
CHO
H2O
N
H2N
+
reflux/1 hr
OH
OH
N
Cl
O
ZrCl4
dry CH2Cl2
N
Zr
O
Cl
2
N
OH
Zr-complex (ZrL12Cl2)
2
2
Scheme 2 Synthesis of indole derivatives in the presence Zr-complex (ZrL12Cl2) (2 mol%) in ethanol at room temperature
General procedure for the preparation of the Schiff
base ligands (HL1–8
6.25 Hz), 2.49 (s, 1H, OH), 3.50–3.58 (m, 1H), 3.72–3.77
(m, 1H), 4.08–4.20 (m, 1H), 7.87 (d, 2H, J = 8.25 Hz),
8.16(d, 2H, J = 8.25 Hz), 8.39 (s, 1H, CH=N); 13C NMR
(CDCl3, 63 MHz) d (ppm) = 20.74, 67.37, 68.66, 123.87,
128.84, 141.26, 149.09, 160.39 ppm; Anal. Calcd for
(C10H12N2O3): C, 57.7; H, 5.8; N, 13.4. Found: C, 57.7; H,
5.8; N, 13.4; MS (m/e) = 208 [M]?.
)
The appropriate aldehyde (1 mmol) was added to a solu-
tion of 1-amino-2-propanol (0.17 mL, 1.5 mmol) in dis-
tilled water (0.2 mL). The mixture was refluxed for 1 h.
The precipitate which formed upon cooling was filtered off
and washed with cold water. The Schiff bases were purified
by recrystallization from n-hexane (Table 1). [Note: some
of Schiff base ligands precipitated after hours or days (HL2,
HL3, HL6, HL7)].
HL1; M.P: 73–75 °C; IR (KBr,): (C=N) 1,638 cm-1; 1H
NMR (250 MHz, CDCl3) dppm = 1.33 (d, 3H,
J = 6.25 Hz), 2.69 (s, 1H, OH), 3.43–3.51(m, 1H),
3.66–3.72 (m, 1H), 4.04–4.11 (m, 1H), 7.40–7.74 (m, 5H,
Ar), 8.3 (s, 1H, CH=N); 13C NMR (CDCl3, 63 MHz) d
(ppm) = 20.6, 67.47, 68.59, 128.18, 128.62, 130.88, 135.85,
162.77; Anal. Calcd for (C10H13NO): C, 73.6; H, 8.0; N, 8.6.
Found: C, 73.6; H, 8.1; N, 8.6; MS (m/e) = 163 [M]?.
HL2; M.P: 77–78 °C; IR (KBr,): (C=N) 1,639 cm-1; 1H
NMR (250 MHz, CDCl3) dppm = 1.28 (d, 3H, J =
1
HL3; M.P: 81–82 °C; IR (KBr,): (C=N) 1,644 cm-1; H
NMR (250 MHz, CDCl3) dppm = 1.29 (d, 3H,
J = 6.25 Hz),2.55(s, 1H, OH),3.50–3.58(m, 1H), 3.71–3.77
(m, 1H), 4.07–4.17 (m, 1H), 7.51 (m, 1H), 8.03 (d, 1H,
J = 7.5 Hz), 8.23(d, 1H, J = 8 Hz), 8.38(s, 1H), 8.55(s, 1H,
CH=N); 13C NMR (CDCl3, 63 MHz) d (ppm) = 20.74,
67.37, 68.66, 123.77, 124.20, 128.84, 135.20, 141.16, 148.89,
160.40 ppm; Anal. Calcd for (C10H12N2O3): C, 57.7; H, 5.8;
N, 13.4. Found: C, 57.7; H, 5.8; N, 13.4; MS (m/e) = 208
[M]?.
1
HL4; M.P: 61–62 °C; IR (KBr,): (C=N) 1,644 cm-1; H
NMR(CDCl3, TMS, 250 MHz): d 1.25 (d, 3H, J = 6.25 Hz),
2.51 (s, 1H, OH), 3.42–3.50 (m, 1H), 3.64–3.71 (m, 1H),
4.01–4.13(m, 1H), 7.49(m, 4H), 8.25(s, 1H, CH=N)ppm;13C
123

Transition Met Chem (2011) 36:685–690
687
Table 1 Synthesis of Schiff base ligandsa
CHO
+
N
H2O
H2N
R
R
HLx
OH
m.p (°C)
OH
Entry
Aldehyde
Productb
Isolated yield (%)
IR (cm-1
)
1
2
3
4
5
6
7
8
R=H
HL1 (R=H)
75
82
90
90
85
75
74
90
73–76
77–78
81–82
61–62
56–57
51–52
101–103
50–56
1,638
1,639
1,644
1,644
1,644
1,641
1,630
1,637
R=p-NO2
R=m-NO2
R=p-Br
HL2 (R=p-NO2)
HL3 (R=m-NO2)
HL4 (R=p-Br)
HL5 (R=p-Cl)
HL6 (R=p-Me)
R=p-Cl
R=p-Me
R=p-NMe2
HL7 (R=p-NMe2)
CHO
OH
N
HL8
a
The reactions were run at reflux and the molar ratio of aldehyde/aminoalcohol was 1:1.5 in 0.2 mL H2O
All products were identified by their spectroscopic data
b
NMR (CDCl3, 63 MHz) d (ppm) = 20.64, 67.40, 68.53,
162.63 ppm; Anal. Calcd for (C12H18N2O): C, 69.9; H, 8.8;
N, 13.6. Found: C, 69.9; H, 8.8; N, 13.6; MS (m/e) = 206
[M?].
124.31,130.07, 131.12, 137.20, 161.35 ppm; Anal. Calcd for
(C10H12BrNO): C, 49.6; H, 5.0; N, 5.8. Found: C, 49.6; H, 5.1;
N, 5.8; MS (m/e) = 242 [M?].
HL8; M.P: 50–56 °C; IR (KBr,): (C=N) 1,637 cm-1; 1H
NMR(CDCl3, TMS, 250 MHz): d 1.34 (d, 3H, J =
6.25 Hz), 2.66 (s, 1H, OH), 3.53–3.65 (m, 1H), 3.79–3.85
(m, 1H), 4.17–4.18 (m, 1H), 7.49–7.61 (m, 3H, Ar),
7.80–8.00 (m, 3H, Ar), 8.85 (d, 1H, J = 8 Hz), 8.96(s, 1H,
CH=N) ppm; 13CNMR(CDCl3, TMS, 62.9 MHz): 20.71,
67.62, 69.58, 124.13, 125.21, 126.08, 127.21, 128.68,
128.89, 131.25, 133.83, 162.42 ppm; Anal. Calcd for
(C14H15NO): C, 78.8; H, 7.1; N, 6.6. Found: C, 78.9; H,
7.1; N, 6.6; MS (m/e) = 213 [M?].
HL5; M.P: 56–57 °C; IR (KBr,): (C=N) 1,644 cm-1; 1H
NMR(CDCl3, TMS, 250 MHz): d 1.25 (d, 3H, J =
6.25 Hz), 2.68 (s, 1H, OH), 3.41–3.50 (m, 1H), 3.64–3.70
(m, 1H), 4.01–4.13 (m, 1H), 7.34 (d, 2H, J = 8.5 Hz), 7.62
(d, 2H, J = 8.25 Hz), 8.23 (s, 1H, CH=N) ppm;13C NMR
(CDCl3, 63 MHz) d (ppm) = 20.64, 67.40, 68.53, 128.87,
129.35, 134.31, 136.80, 161.35 ppm; Anal. Calcd for
(C10H12ClNO): C, 60.8; H, 6.1; N, 7.1. Found: C, 60.8; H,
6.1; N, 7.1; MS (m/e) = 197 [M?].
HL6; M.P: 51–52 °C; IR (KBr,): (C=N) 1,641 cm-1
;
1HNMR(CDCl3, TMS, 250 MHz): d 1.24 (d, 3H, J =
6.25 Hz), 2.37 (s, 3H), 2.93 (s, 1H, OH), 3.40–3.48 (m, 1H),
3.63–3.68 (m, 1H), 4.04–4.08 (m, 1H), 7.18 (d, 2H,
J = 7.5 Hz), 7.59 (d, 2H, J = 7.5 Hz), 8.24 (s, 1H, CH=N)
ppm;13C NMR (CDCl3, 63 MHz) d (ppm) = 20.61, 21.18,
67.45, 68.62, 128.17, 129.04, 133.29, 141.15, 162.67 ppm;
Anal. Calcd for (C11H15NO): C, 74.5; H, 8.5; N, 7.9. Found:
C, 74.5; H, 8.5; N, 7.9; MS (m/e) = 177 [M?].
Preparation of ZrL12Cl2
ZrCl4 (0.233 g, 1 mmol) was added to a solution of (z)-N-
bezylidene-2-hydroxypropane-1-amine (HL1) (2 mmol,
0.326 g) in dry CH2Cl2 (10 mL). The mixture was stirred
under N2 for 2 h at room temperature. The precipitate was
filtered off and washed with cold, dry CH2Cl2 then with dry
n-hexane. The solid was dried under reduced pressure to
give the complex as a deep yellow powder in 80% yield.
HL7; M.P: 101–103 °C; IR (KBr,): (C=N) 1,630 cm-1
1HNMR(CDCl3, TMS, 250 MHz):
1.23 (d, 3H,
;
d
M.P.: 143 °C; IR (KBr,): (C=N) 1,665 cm-1
;
1H NMR
J = 6.25 Hz), 2.85 (s, 1H, OH), 3.00 (s, 6H), 3.39–3.44 (m,
1H), 3.65–3.67 (m, 1H), 4.01–4.06 (m, 1H), 6.66 (d, 2H,
J = 8.75 Hz), 7.58 (d, 2H, J = 8.75 Hz), 8.16 (s, 1H,
CH=N) ppm;13C NMR (CDCl3, 63 MHz) d (ppm) = 20.51,
40.18, 67.59, 68.47, 111.56, 123.98, 129.67, 152.20,
(400 MHz, d6-DMSO) dppm = 1.17 (d, 6H, J = 6.4 Hz),
3.58–3.88 (m, 4H, CH2), 4.26 (m, 2H), 7.49–7.62 (m, 10H,
Ar), 9.05(s, 2H, CH=N). 13CNMR(100.64 MHz, d6-
DMSO): 21.31, 63.46, 64.16, 128.21, 129.03, 130.06,
137.07, 171.72; Anal. Calcd for (C20H24N2O2ZrCl2): C,
123

688
Transition Met Chem (2011) 36:685–690
49.4; H, 5.0; N, 5.8. Found: C, 49.4; H, 5.0; N, 5.7. MS (m/
e) = 484 [M?].
bisindolylmethane in 95% yield within 5 min at room
temperature.
Based on the optimized reaction conditions, the present
protocol was applied to a variety of carbonyl compounds
and indoles. As shown in Table 2, aromatic aldehydes
having both electron-donating and electron-withdrawing
groups afforded bis(indolyl)methanes in excellent yields.
Rapid condensation of various carbonyl compounds with
2-methyl indole under the same reaction conditions was
observed (Table 2, entries 11–16). Moreover, when 3-for-
mylindole was employed as a carbonyl compound, the
corresponding trisindolylmethane was obtained in excellent
yield (Table 2, entry 9, 15).
General procedure for the condensation of indoles
with carbonyl compounds catalyzed by the complex
The appropriate indole (10.5 mmol) was added to a
solution of the carbonyl compound (5 mmol) and ZrL12Cl2
complex (0.1 mmol, 0.048 g) in ethanol (2.5 mL). The
mixture was stirred at room temperature for an appro-
priate time, as monitored by TLC (Table 2). After com-
pletion of the reaction, the product was extracted by
plate chromatography eluted with n-hexane/EtOAc (3/1).
Structural assignments of the products are based on their
1H NMR, 13C NMR and MS spectra and elemental
analysis.
When we applied similar reaction conditions for the
condensation of isatin and indoles, the desired 3,30-diind-
olyloxindole derivatives were secured in 85–90% yields
(Table 2, entries 10, 16).
However, the reaction of indole with acetophenone in
the presence of 2 mol% of catalyst was not successful, and
after a prolonged reaction time, most of the starting
materials were observed intact (Table 2, entry 18).
The high yields of condensation products (80–97%)
obtained by this catalytic system in short reaction times
(\30 min) indicates the high efficiency of this Zr(IV)
Schiff base complex catalyst. This was further supported
by the high turnover numbers obtained for the catalyst in
the condensation of benzaldehyde with indole (3,300/24 h)
using a 10,000:21,000:2 molar ratio of benzaldehyde/
indole/catalyst.
Results and discussion
The Schiff base ligands (Table 1) were prepared by the
condensation of 1-amino-2-propanol with a number of
aldehydes with different electronic and steric substituents
and purified by recrystallization from n-hexane.
One of the Schiff bases, namely (z)-N-bezylidene-2-
hydroxypropane-1-amine (HL1), was chosen for reaction
with ZrCl4.
Analytical and spectroscopic data for the complex are
given in the experimental section. Spectroscopic data and
elemental analyses are consistent with a monomeric com-
plex with a ligand/Zr ratio of 2:1. When the synthesis
reaction was performed using three equivalents of HL1, the
same product was formed, as observed with two equiva-
lents of L1. The infrared spectrum of ZrL21Cl2 shows that
the imine band of HL1, observed at 1,638 cm-1 for the free
ligand, shifts to higher wavenumbers after coordination
of the azomethine nitrogen to the Zr center, appearing at
The catalyst proved to be reusable for at least three
times in multiple sequential condensations by the addition
of new samples of benzaldehyde and indole to the reaction
mixture.
The reactions are also scalable; thus, the condensation of
benzaldehyde and indole as substrates in a semi-scale-up
procedure (20 times) gave the desired product in 95%
yield.
In order to show the merit of ZrL12Cl2 in comparison
with the other catalysts used for similar reactions, we have
compared the results obtained using condensation of
benzaldehyde and indole with some of those reported in the
literature in Table 3.
1
1,665 cm-1. In the HNMR, a singlet at 8.3 ppm that is
assigned to the azomethine proton of HL1is shifted
downfield to 9.05 ppm, resulting from the coordination of
the azomethine nitrogen. The absence of a broad singlet at
2.75 ppm in the spectrum of the complex compared with
the free Schiff base HL1 indicates coordination of the
hydroxyl group after deprotonation.
Conclusion
Catalytic activity of the complex
In conclusion, the catalytic activity of ZrL12Cl2 in the
synthesis of wide variety of indole derivatives under mild
conditions has been established. The employment of eth-
anol as an environmentally benign solvent in this high
yielding method, along with high turnover numbers and
reusability of the catalyst providing ready scalability,
makes it appropriate for practical applications.
Our preliminary experiments were addressed to the con-
densation of benzaldehyde with indole as a model reaction,
to determine the best conditions with respect to tempera-
ture and reaction time. The use of 1 mmol of benzaldehyde
and 2.1 mmol of indole in ethanol in the presence of a
catalytic amount of the complex (2 mol%) gave the desired
123

Transition Met Chem (2011) 36:685–690
689
Table 2 Condensation of
indoles with carbonyl
compounds catalyzed by
Zr-complex (ZrL12Cl2)a
Carbonyl
Time
(min)
Isolated
Yield%
Entry
Indole
Productb
Compounds
X-Ph
H
CHO
X
N
H
N
H
1
2
3
X=H
X=H
5
95
97
96
N
H
X=4-Me
X=4-OMe
X=4-Me
10
15
N
H
X=4-OMe
X=4-OH
N
H
4
X=4-OH
5
97
N
H
5
6
X=4-Br
X=4-Br
30
20
95
90
N
H
X=4-NO2
X=4-NO2
N
H
CHO
7
25
85
N
H
N
H
N
H
O
8
9
20
20
95
78
CHO
O
N
H
N
H
N
H
H
C
CHO
N
H
N
H
N
H
N
2
H
O
O
HN
O
10c
20
97
N
H
N
H
N
H
N
H
X-Ph
H
CHO
N
H
X
N
H
HN
11
12
X=H
X=H
<1
<1
<1
95
96
95
N
H
X=4-OMe
X=4-NO2
X=4-OMe
N
H
13
14
X=4-NO2
X=4-Br
N
H
X=4-Br
<1
4
97
96
N
H
a
The reactions were run at
H
C
CHO
25–27 °C, and the molar ratio of
carbonyl compound/indole/
catalyst was 100:210:2 in
0.5 mL ethanol
15
Me
N
H
N
H
N
H
N
H
2
HN
O
O
b
All products were identified
10
97
16c
17
18
O
N
H
by their spectroscopic data and
their comparison with known
samples [20, 26, 28]
N
N
N
H
H
H
Br
Br
Br
Ph
H
CHO
95
20
25
c
N
H
The reactions were run at
N
H
HN
25–27°C and the molar ratio of
carbonyl compound/indole/
catalyst was 100:210:2 in 1 mL
ethanol
Ph
O
120
N
H
N
HN
H
123

690
Transition Met Chem (2011) 36:685–690
Table 3 Catalytic activity of Zr-complex (ZrL12Cl2) in comparison with other catalysts used for condensation of benzaldehyde and indole
Entry
Catalyst
Catalyst (mol %)
Time (h)
Yield (%)
Ref.
a
1
2
3
4
5
6
7
a
Zr-complex (ZrL12Cl2)/ethanol
ZrCl4/CH3CN
2
5 min
31 min
35 min
12
95
91
89
95
86
98
92
–
5
[29]
[30]
[31]
[32]
[33]
[34]
ZrOCl2.8H2O/CH3CN
Dy(OTf)3/EtOH.H2O
HMTABb/H2O
10
10
0.1 g
5
2.5
FeCl3.6H2O/[omim]PF6
Al(HSO4)3
1.5
100
2.5
This work
b
Hexamethylenetetraamine-Bromine
Acknowledgments Support for this work by Research Council of
University of Birjand is highly appreciated.
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