Palladacycles of unsymmetrical (N,C-,E) (E = S/Se) pincers based on indole: Their synthesis, structure and application in the catalysis of Heck coupling and allylation of aldehydes

Article abstract of DOI:10.1039/c6dt00060f

Unsymmetrical (N,C,E)-type pincer ligand precursors [L1 and L2: E = S/Se] with an indole core were synthesized for the first time by the condensation of 1-(2-phenylsulfanyl/selenylethyl)-1H-indole-3-carbaldehyde with benzyl amine. The synthetic protocols

Full text of DOI:10.1039/c6dt00060f

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Dalton Transactions
Palladacycles of unsymmetrical (N,C ‾,E) (E = S / Se) pincers based on indole:
Synthesis, structure and catalysis of Heck coupling and allylation of aldehydes
Mahabir P. Singh, Fariha Saleem, Gyandshwar K. Rao, S. Kumar, Hemant Joshi and Ajai K Singh*.
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Unsymmetrical (N,C,E)-type pincer ligand precursors [L1 and L2 : E = S / Se] with indole core were
synthesized for the first time by condensation of 1-(2-phenylsulfanyl/selenylethyl)-1H-indole-3-
carbaldehyde with benzyl amine. The synthetic protocols are easy and give good yields (>85%). The L1
and L2 on reaction with sodium tetrachloropalladate(II) in the presence of CH COONa result in
3
−
1
0
complexes [Pd(L1/L2−H)Cl] (1 / 2), where they bind in a tridentate (N,C , E) mode. The L1 and L2,
1
13
1
their aldehyde precursors and Pd(II)-complexes, 1 and 2 have been characterized with H, C{ H}and
7
7
1
Se{ H} NMR and HR-MS. Palladium(II) complexes 1 and 2 and precursor aldehydes of L1 and L2
were authenticated with single crystal X-ray diffraction. The catalytic activities of complexes 1 and 2
were investigated for Heck coupling and allylation of aldehydes. The two reactions require 0.1−0.3 and 1
mol% loading of complexes as catalysts, respectively.
1
5
is expected to be stronger than that of its benzene analogue. Thus
Introduction
ligands with indole as a pincer core are worth exploring, as they
have been scantly explored so far. Only one example of (S,C,S)
The interest in research on metal complexes of pincer ligands
5
6
6
7
7
8
5
0
5
0
5
0
1
2
continues due to their versatile applications, arising from their
stability and unique reactivity resulting probably due to tridentate
coordination mode of such ligands and the presence of a metal–
9
pincer having this core and non-identical arms is known.
Herein, first examples of indole core based unsymmetrical
2
2
3
0
5
0
(N,C,E) type (E = S / Se) pincer ligand precursors (L1 and L2,
3
carbon σ-bond in many cases. The fine tuning of steric and
Scheme 1), which have different arms as well as donor atoms and
their Pd(II) complexes are reported. Interestingly these ligands
make both five and six membered chelate rings. Such kind of
electronic properties of metal–pincer ligand complexes and
consequently their catalytic properties is possible by changing
donor atoms, substituent on them and central ring system. The
4
10
pincers known, are fewer in number than those which make two
donor groups attached to arms of known pincer ligands include
PR , AsR , NR , OR, SR and SeR. Benzene, pyridine or triazole
1,5-6
five or six membered rings.
The applications of these
2
2
2
complexes in catalysis of Heck coupling and allylation of
aldehydes have been found promising and are described here.
Heck coupling continues to get attention of both
5
is a core structural unit in many of them. The known
unsymmetrical pincer structures are fewer in number than
symmetrical ones (both arms identical), which are easy to
synthesize. In unsymmetrical pincer ligands two arms differ in
one or more of the following three ways: (i) donor atoms in them
11
industry and academia due to its synthetic importance. The
designing of various pincer and bulky electron-rich phosphine
12
ligands has fuelled investigations on it. High stability of
(ii) substituents on donor atoms and (iii) length of linkers
palladium-pincer ligand complexes has made them important for
6
between donor atom and core. When donor atoms are different
i.e. E and E), metal complex of an unsymmetrical pincer has M–
1f
Heck reaction under strident conditions. In 2003 Szabό and co-
(
workers reported pincer ligand based palladium complexes as
highly active and selective catalysts for allylation of aldehydes or
activated imines with allylstannanes and showed better results
3
4
4
5
5
0
5
0
E and M–E bonds of different strength. In this situation out of
two donor atoms, the one forming more labile bond may
dissociate from the metal leaving the relatively inert one bonded,
13a,b
compared to bis(allyl)palladium intermediates.
The
7
resulting in a condition favorable for catalytic activity. However,
homoallylic alcohols obtained by allylation of aldehydes are used
extensively in the synthesis of biologically active compounds and
major problem with unsymmetrical pincer ligands is their
syntheses, as most of such ligands reported so far, are designed
by less convenient multi step reactions, which consequently result
in low overall yield.
unsymmetrical pincer-palladacycles for some chemical processes
1
3c-e
natural products.
For allylation of aldehydes and Heck
coupling no complex of palladium(II) with indole based pincer
ligand is in our knowledge. The 1 and 2 are probably the first
examples.
5
h
However, catalytic efficiencies of
7
are better than those of symmetric ones. Thus, unsymmetrical
pincer ligands may be envisaged as important candidates for
designing of new and efficient catalysts for organic synthesis.
Recently, Pd(II) complexes of chalcogen donor containing
symmetrical pincer ligands have emerged as a family of efficient
Results and discussion
The syntheses of L1 and L2 and their Pd(II)-complexes (1 - 2)
are summarized in Scheme 1. 1-(2-Chloroethyl)-1H-indole-3-
catalysts for various organic transformations including C-C 85 carbaldehyde (A) was synthesized by reacting indole-3-
8
coupling.
The C-H of indole’s five membered ring is more acidic than
carboxaldehyde with 1,2-dichloroethane (DCE) in the presence of
1
4
K CO and n-Bu NBr using a reported procedure with some
2
3
4
−
−
those of benzene. Consequently M-C bond of indole based pincer
modifications. The nucleophiles PhS and PhSe , generated in
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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DOI: 10.1039/C6DT00060F
Dalton Transactions
Page 2 of 9
situ by reaction of PhSH with NaOH and reduction of diphenyl
refinement parameters are given in Tables S1 and S2 of ESI.
diselenide with NaBH4 in EtOH under inert atmosphere
respectively, on reaction with compound A formed 1-(2-
phenylsulfanylethyl)-1H-indole-3-carbaldehyde
phenylselanylethyl)-1H-indole-3-carbaldehyde
and
respectively.
1-(2-
5
These aldehydes on condensation with benzyl amine resulted in
N,C,E)-type pincer ligand precursors L1 and L2, which on
palladation with Na PdCl in the presence of CH COONa in
(
2
4
3
methanol formed complexes 1 and 2 respectively. The L1 and
L2 have good solubility in common organic solvents: viz., CHCl3,
CH Cl and EtOAc but their palladium complexes are soluble in
1
0
2
2
them moderately.
45
Fig. 1 Molecular structure of A. Selected bond distances (Å).
Cl(1)−C(1) 1.784(3); N(1)−C(3) 1.358(3); N(1)−C(10) 1.392(3);
N(1)−C(2) 1.459(3); C(8)−C(7) 1.393(4); C(11)−O(1) 1.212(3).
Selected bond angles (°): C(3)−N(1)−C(10) 108.59(19);
50
C(10)−N(1)−C(2)
125.5(2);
O(1)−C(11)−C(4)
126.2(3);
N(1)−C(3)−C(4) 110.4(2); C(6)−C(7)−C(8) 121.3(2).
Scheme 1. Synthesis of L1 and L2 and their complexes, 1 and 2
1
5
NMR spectra
The pincer ligand precursors L1 and L2 and their diamagnetic
Fig. 2 Molecular structure of B1. Selected bond distances (Å).
S(1)−C(1) 1.770(3); S(1)−C(7) 1.792(3); N(1)−C(9) 1.352(3);
N(1)−C(12) 1.395(3); N(1)−C(8) 1.451(3); C(1)−C(6) 1.381(4);
C(17)−O(1) 1.215(4). Selected bond angles (°): C(1)−S(1)−C(7)
103.82(15); C(9)−N(1)−C(12) 108.1(2); C(9)−N(1)−C(8)
126.4(2); C(12)−N(1)−C(8) 125.4(2); C(6)−C(1)−C(2) 117.9(3).
1
palladium(II) complexes (1 and 2) were characterized with H,
1
3
1
77
1
2
2
3
3
4
0
5
0
5
0
C{ H}, and
Se{ H} NMR spectroscopy and mass
55
spectrometry (See ‘ESI’ for scans of various spectra; Figs S4–
S29). The NMR and mass spectral data are consistent with the
structures depicted for them in Scheme 1. The singlet of CH
1
(
imine) in H NMR spectra of L1 and L2 appearing at 8.56 and
8.41 ppm respectively, on complexation has been found shifted to
.86 and 7.88 ppm, shielded by ∼0.70 and 0.52 ppm
respectively. In C{ H} NMR spectra, the CH carbon signals
7
1
3
1
appearing at 155.8 ppm for L1 and L2 both, are deshielded by
∼
10 ppm on complexation, appearing at 166.1 and 165.8 ppm in
7
7
1
the spectra of 1 and 2 respectively. The signal in Se{ H} NMR
spectrum of 2 appears at 316.0 ppm. It is deshielded by ~30.8
ppm with respect to that of free L2 (285.2 ppm), implying
1
5
coordination of Se with palladium.
Crystal structures
Aldehydes A and B1 and complexes 1 and 2 were authenticated
by X-ray diffraction on their single crystals. The crystals of A and
B1 were obtained by slow evaporation of their solutions made in
EtOH. Ethyl acetate – chloroform (1:1) mixture was used to grow
single crystals of 1 and 2 in a similar fashion. Figs. 1−4 show the
molecular structure of A, B1, 1 and 2 respectively with selected
bond lengths and angles. Their crystal data and structure
6
0
Fig. 3 Molecular structure of 1. CHCl molecule is omitted for
clarity. Selected bond distances (Å): Pd(1)−C(10) 1.957(8);
Pd(1)−N(2) 2.082(7); Pd(1)−S(1) 2.283(2); Pd(1)−Cl(1) 2.376(2).
3
Selected
C(10)−Pd(1)−S(1)
bond
angle
93.4(2);
(°):C(10)−Pd(1)−N(2)
N(2)−Pd(1)−S(1)
79.1(3);
171.4(2);
65
C(10)−Pd(1)−Cl(1) 174.4(2).

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Page 3 of 9
Dalton Transactions
non-covalent interactions of precursor aldehyde of L1 and L2 and
complexes 1 and 2 are given in ESI (Tables S7 and S8). The
C−H∙∙∙∙Cl interactions are intra as well as intermolecular and
chlorine atoms of both CHCl3 and complex are involved.
Chloroform intercalates chains of complexes formed due to
C−H∙∙∙∙π interactions. The precursor aldehydes A and B1 have
O∙∙∙∙H secondary interactions which result in three dimensional
structural network (ESI: Figs. S2-S3)
3
4
4
5
5
0
5
0
Applications of complexes 1 and 2 in Heck coupling and
allylation of aldehydes
The catalytic properties of complexes 1 and 2 have been studied
for Heck coupling of various aryl bromides with n-butylacrylate
(
Scheme 2). K CO has been found best base for this purpose.
2 3
Optimum reaction temperature and time were found 140 °C and
4 h respectively.
Fig. 4 Molecular structure of 2. CHCl molecule is omitted for
3
2
clarity. Selected bond distances (Å): Pd(1)−C(10) 1.942(7);
Pd(1)−N(2) 2.070(5); Pd(1)−Se(1) 2.3654(10); Pd(1)−Cl(1)
2.358(2). Selected bond angle (°): C(10)−Pd(1)−N(2) 79.5(3);
C(10)−Pd(1)−Se(1) 94.2(2); N(2)−Pd(1)−Se(1) 172.18(15);
C(10)−Pd(1)−Cl(1) 175.4(2).
5
Scheme 2. Heck coupling Catalyzed with 1 and 2
The coupling of 4-bromoanisole with n-butylacrylate was carried
out under N2 atmosphere using complex 1 as a catalyst for
optimization of solvent for reaction. The results are summarized
in Table 1. The stability of complexes under ambient conditions
was found good but for good conversions, the coupling reactions
were carried out under nitrogen atmosphere.
55
a
60
Table 1. Optimization of reaction conditions for Heck
coupling
1
0
Fig. 5 Non-covalent C−H∙∙∙∙Cl interaction in 1
b
Entry
Solvent
Base
% Yield
In Tables S3−S6 of ESI more selected bond lengths and angles
for A, B1, 1 and 2 are given. Both complexes crystallize with one
molecule chloroform (per molecule of 1 or 2)
No.
1
DMF
K CO3
2
26
49
2
3
4
5
DMA
K
2
CO
CO
CO
3
3
3
Palladium has a distorted square planar geometry in both
complexes 1 and 2 with Cl and C(indolyl) disposed trans to each
other. The Pd−N bond lengths in 1 and 2 (2.082(7) and 2.070(5)
Å respectively) are consistent with the reported value, 2.056(4)
NMP
K
2
29
1
2
2
3
5
0
5
0
DMF+ Water
Toluene
K
2
< 10
< 10
2 3
K CO
‒
16a
for a palladacycle of (N, Se, C ) ligand.
The Pd−C bond
a
Reaction conditions: 4-bromoanisole, 1.0 mmol, n-
CO , 2.0 mmol, solvent, 3 mL,
distances 1.957(8) and 1.942(7) Å in 1 and 2 respectively are
1
6a
16b
butylacrylate, 1.2 mmol, K
2
3
somewhat shorter than the values 1.973(5) and 1.970(7) Å
b
complex catalyst, 0.1 mol%, temperature 140 °C. isolated
yield.
reported for Pd−C bonds of palladacycles of reduced
chalcogenated Schiff bases. The Pd−Cl bond lengths, 2.376(2)
and 2.358(2) Å for 1 and 2 respectively are consistent with each
1
6a
other and somewhat longer than the reported values 2.325(16)
and 2.319(2) Å
The yield of coupled product was found maximum in the
1
6b
for palladacycles of reduced chalcogenated 65 presence of 0.1 mol% of 1, when N,N-dimethyl acetamide
Schiff bases. The Pd−S (2.285(2) Å) and Pd−Se (2.3654(10) Å)
bond distances of complexes 1 and 2 are consistent with the
values reported for palladium(II)-complexes of 1-phenylthio-2-
(DMA) was used as a solvent (Table 1; Entry 2). In DMF, N-
methyl-2-pyrrolidone (NMP) and toluene, the yield of coupled
product diminished significantly (Table 1). The coupling in DMA
of 4-bromoanisole with n-butylacrylate resulted in 49% yield
(Table 2; Entry 1) in 24 h at 0.1 mol% of 1. On increasing the
catalyst loading to 0.3 mol% the yield of coupled product
increased to 94% in the same reaction time. The yields of coupled
1
6c
arylthioethane [2.285(14) Å] and tridentate selenated Schiff
1
6d
base [2.3669(11) Å] respectively. The C−H∙∙∙∙Cl and C−H∙∙∙∙π
secondary interactions in the two complexes result in three
dimensional structures (Fig. 5 and Fig. S1 in ESI). Distances of
70

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Dalton Transactions
Page 4 of 9
products were good for 0.1 mol% catalyst loading when an
electron withdrawing group was present in ArBr at a position
para to bromo (Table 2; Entry 6 and 7). For example for reaction 40 and 2
Table 3. Allylation of aldehydes catalyzed with complexes 1
a
of 1-bromo-4-nitrobenzene / 4-bromobenzaldehyde with n-
butylacrylate in the presence of 0.1 mol % catalyst loading
coupled product resulted in yield up to 95%. No product was
obtained when attempts were made to couple chlorobenzene with
n-butylacrylate in the presence of even 1 mol % of 1/2.
Entry
No.
Complex 1 Complex 2
Aryl aldehyde
b
b
5
Yield
Yield
1
2
3
4
5
6
7
8
4-Bromobenzaldehde
2-Bromobenzaldehde
4-Chlorobenzaldehyde
Benzaldehyde
77
80
93
90
95
92
1
0
Table 2. Heck C−C coupling reaction catalyzed with
94
96
a
palladacycle
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
Furfural
72
95
Entry
No.
Aryl halide
Complex 1
Yield
Complex 2
Yield
b
b
51
50
77
80
c
1
4-Bromoanisole
49/94
55
Pyridine-2- carbaldehyde
95
95
2
3
4
5
4-Bromoacetophenone
4-Bromotoluene
89
95
60
90
96
95
a
Reaction conditions: Aryl / heteroaryl aldehyde, 1.0 mmol,
c
33/96
allyltributyl stannane, 1.2 mmol, DMF, 2 mL, reaction time
1
chromatography, complex catalyst equiv. to 1 mol% of Pd
b
6 h, bath temperature 50 °C. Isolated yield after column
Bromobenzene
91
95
93
.
1-Bromo-4-nitrobenzene
4-Bromobenzaldehyde
d
6
Some complexes of category II are reported little active for Heck
coupling at 0.1 mol % catalyst loading, found to be good for 1
and 2. The optimum loading for good conversion in case of some
complexes of type I is up to 5 mol %, much higher than those of
present (N,C‾,E)-pincer complexes for similar conversions. In
a
Reaction conditions: Aryl bromide, 1.0 mmol, n-butylacrylate, 1.2 mmol,
CO , 2.0 mmol, reaction time, 24 h, DMA, 3 mL, complex catalyst 0.1
mol%, bath temperature 140 °C. Isolated yield after column
4
5
5
6
6
7
7
5
0
5
0
5
0
5
K
2
3
b
c
d
chromatography, complex catalyst equiv. to 0.3 mol% of Pd, bath
temperature, 110 °C.
1
7c
comparison to (NHC)-pincer complex
of Pd reported for
catalysis of Heck-coupling at 0.5 mol % loading (reaction time 20
h, temp. 165 °C), 1 and 2 appear to be better as required catalyst
loading is only 0.1 mol% at 140 °C (bath temperature). The 0.5
mol % Pd-(N,C,P)-pincer complex
Heck coupling of bromoanisole is somewhat higher than 0.3 mol
1
7d
required for catalysis of
1
5
Scheme 3. Allylation of aldehydes catalyzed with 1 and 2
%
of present complexes 1 and 2 needed for the coupling of same
substrate. Palladium complexes of (C,N,C) pincer ligands
1
7e
Allylation (Scheme 3) of aryl as well as heteroaryl aldehydes was
explored in the presence of 1 and 2 as catalyst. The results are
summarized in Table 3. DMF was found best solvent and 1 mol%
of each complex as optimum loading for the catalytic process.
The yield of allylated product was dependent on electron
withdrawing / donating properties of the substituents present at a
position ortho / para to CHO, as the catalytic activity enhanced in
the presence of electron withdrawing group. For deactivated 4-
methoxybenzaldehyde, the reaction in the presence 1 mol% of
catalyst 1, resulted in its low conversion (51%; Table 3: Entry 6)
to allylated product. Allylation of heteroaryl halides was achieved
with excellent yields (Table 3: Entry 7 and 8). Among Pd based
catalysts for Heck coupling, Pd complexes of pincer ligands have
been reported very promising. Therefore, complexes 1 and 2 of
(N,C ,E)-pincer ligands, are compared with Pd-complexes of
other pincer ligands. They have been found competitive or better
in efficiency for Heck coupling than many pincer ligand based
palladium catalysts reported to be efficient.
complexes of some (O,N,N) (I)
ligands are reported as good catalysts for coupling of aryl
iodides and but not as effective ones for nonactivated
bromobenzene substrates.
17f
based on NHC moieties and (N,C,N) pincers having bis(azole)
framework are more catalytically efficient for activated aryl
bromides than unactivated ones when their loading is 1 mol %,
more than the optimum loading of the present pincer complexes.
Palladium complex of (N,C,N)-pincer having hexahydro-1H-
pyrrolo[1,2-c]imidazolone group in its two side arms, as
catalyst
coupling at a loading of 0.1 mol %, but fails in case of ArBr,
which undergoes Heck coupling easily in the presence of 1 and 2.
With respect to palladium complex of symmetric (Se,C,Se)-
pincer reported for allylation of p-anisaldehyde, the catalytic
efficiency of 1 and 2 appears to be comparable as required
catalyst loading is 1 mol % in both the cases. The reaction time,
8 h and temperature, 60 °C for Pd complex of (Se, C, Se) pincer
are also not much different from those of 1 and 2. Palladacycle of
P,C,P)-pincer catalyzes allylation at 0 ºC to room temperature
but for good conversion, 5 mol% catalyst loading and 18-120 h
reaction time are required. However, with 1 and 2 allylation in
good yield is achieved with 1 mol % loading of catalyst at 50-55
ºC in 16 h. Palladium(II) complex of unsymmetrical (C,N,N)-
2
2
3
3
0
5
0
5
a
17g
shows good conversion of aryl iodides in Heck
18a
1
‾
4c
(
1
7a-c
Palladium-
1
7a
and (N,C‾,N) (II)-pincer
1
7b
18b
pincer has been reported as effective catalyst for allylation of

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Dalton Transactions
p-anisaldehyde at 5 mol % loading (reaction time 24 h and
temperature 50 ºC). The present (N,C‾,E)-pincer palladacycles do
bases, aldehydes and aryl bromides were procured from local
sources. Commercial nitrogen gas was used after passing it
allylation of p-anisaldehyde at 1 mol % loading under almost 60 successively through traps containing solutions of alkaline
similar reaction conditions. For allylation of p-anisaldehyde 2.5
anthraquinone, sodium dithionite, alkaline pyrogallol,
concentrated H SO and KOH pellets. The products of catalytic
1
8c
5
0
5
0
5
0
5
0
5
0
5
mol % loading of (P,P,P)-pincer based Pd catalyst was found
insufficient but good conversion resulted with 5 mol % of
2
4
Heck coupling and allylation were authenticated by matching
1
8d
1
(
P,C,P)-pincer Pd-catalyst. Thus present complexes 1 and 2
their H NMR data with those reported in the literature. Yields of
1
may be placed among efficient catalysts known for Heck 65 isolated coupled products [purity ≥ 95% established by H NMR]
coupling and allylation.
1
13
1
77
1
are reported. The H, C{ H}, and Se{ H} NMR spectra were
recorded on a Bruker Spectrospin DPX-300 NMR spectrometer
at 300.13, 75.47, and 57.24 MHz, respectively with chemical
shifts reported in ppm relative to internal standards. Carbon-13
1
1
2
2
3
3
4
4
5
5
Generally for Heck coupling Pd(0)/Pd(II) mechanism is
1
2
believed, particularly for pincer palladacycles. Under harsh and
basic reaction conditions pincer palladacycle undergoes reduction
and dispenses catalytically active Pd(0) complex with the 70 DEPT NMR experiments were used routinely to determine the
1
f
cleavage of the Pd-C bond. The catalytic efficiencies of these
Pd(0) species is finely tuned by ligands and consequently pincer
complexes differ in activity. Palladium(0) complex enters in the
catalytic cycle which includes oxidative addition, β-hydride
number of hydrogen atoms linked to carbon atoms. Melting
points were determined in an electrically heated apparatus by
taking the sample in a glass capillary sealed at one end. High-
resolution mass spectral (HR-MS) measurements were performed
elimination and base-promoted reductive elimination. In our case 75 with an instrument Bruker Micro TOF-Q II, based on electron
also Pd(0)/Pd(II) mechanism appears to be operative, as with
onsetting of catalytic process, colour of reaction mixture changes
most probably due to the formation of Pd(0) based species.
Szabό and co-workers have suggested mechanism for
spray ionization (10 eV, 180 °C source temperature, and sodium
formate as calibrant), taking the sample in CH CN. Suitable
3
single crystals of 1 and 2 were obtained by slow evaporation of
their solutions made in ethylacetate – chloroform (1:1) mixture.
allylation of aldehydes and imines catalyzed with pincer 80 The slow evaporation of solutions of A and B1 made in ethanol
1
8d
1
complexes of palladium. η -Allyl-coordinated pincer species is
generated in first step through transmetalation of allylstannane
with catalyst. Nucleophilic σ-allylpalladium species undergoes
addition to aldehyde (via a cyclic transition state with the metal)
also gave their single crystals. X-ray diffraction data for all these
crystals were collected at 298(2) K on a Bruker AXS SMART
Apex CCD diffractometer using Mo-Kα (0.71073 Å) radiation.
Frames were collected at T = 298 K by ω, φ, and 2θ- rotations
and in final step alkoxide-coordinated palladium undergoes 85 with full quadrant data collection strategy (four domains each
exchange reaction with SnBu X to give homoallyl alcohol.
Presumably a similar mechanistic process occurs in our case also.
with 600 frames) at 10s per frame with SMART. The measured
3
2
intensities were reduced to F and corrected for absorption with
2
0
SADABS. Structure solution and refinement were carried out
2
1
4
. CONCLUSION
with the SHELXTL package by direct methods. Non-hydrogen
The precursors of unsymmetrical (N,C‾,E)-pincer ligands based 90 atoms were refined anisotropically. All hydrogen atoms were
on indole core have been synthesized for the first time. The
protocols are easy and give good yields. These precursors on
reaction with sodium tetrachloropalladate(II) in the presence of
included in idealized positions and a riding model was used for
the refinement. Images were created with the program Diamond.
CH COONa have resulted in [Pd(L1/L2−H)Cl] (1 / 2) where
ligands bind in a tridentate (N,C ,E) mode, making five and six 95 Indole-3-carbaldehyde (2.18 g, 15 mmol), 1,2-dichloroethane (75
Synthesis of 1-(2-chloro-ethyl)-1H-indole-3-carbaldehyde.
3
−
membered chelate rings (one each). Such pincer complexes of
palladium are known scantly. The precursor aldehydes, ligand
precursors L1 and L2 and complexes of L1/L2−H have been
characterized with multinuclear NMR spectroscopy and mass
mL), n-Bu NBr (4.83 g, 15 mmol), K CO (18 g) and 40 mL of
4 2 3
distilled water were taken in a 250 mL round bottom flask. The
o
mixture was heated on an oil bath maintained at 80 C with
vigorous stirring for 2 h. The reaction mixture was cooled and
spectrometry. Both Pd(II) complex 1 and 2 and two precursor 100 diluted with 100 mL of water and extracted with dichloromethane
aldehydes have been characterized with single crystal X-ray
diffraction. The catalytic properties of the two complexes have
been investigated for Heck coupling and allylation of aldehydes.
Optimum loading of 1 or 2 as catalyst has been found 0.1−0.3
(2 × 50 mL). The organic layer was washed with water (5 × 50
mL) and dried over anhydrous sodium sulfate. The solvent of
extract was evaporated off under reduced pressure on a rotary
evaporator to get 1-(2-chloroethyl)-1H-indole-3-carbaldehyde as
and 1 mol% for catalysis of Heck coupling and allylation 105 yellow solid which was recrystallized with hot ethanol to get
respectively. A range of functional groups present on substrates is
tolerable in the two catalytic reactions.
white solid.
A: white solid; Yield 2.95 g, (95%). Anal. Found: C, 63.60; H,
4
.83; N, 6.73%. Calcd for C H ClNO: C, 63.62; H, 4.85; N,
11 10
1
Experimental
6.75%. H NMR (300 MHz, CDCl
, 25 ºC, TMS): δ (ppm): 3.81
3
1
10 (t, 2H, J = 6.0 Hz, H ), 4.46 (t, 2H, J = 6.0 Hz, H ), 7.25–7.29
1
2
Materials and instrumentation. Tributylstannane, diphenyl
diselenide, sodium borohydride and sodium tetrachloropalladate
were procured from Sigma-Aldrich (USA). Indole-3-
carboxaldehyde, thiophenol, dichloroethane, n-butylacrylate,
(m, 3H), 7.73 (s, 1H), 8.25–8.28 (m, 1H), 9.96 (s, 1H, H ).
11
1
3
1
C{ H} NMR (75 MHz, CDCl , 25 ºC, TMS): δ (ppm): 42.1,
3
48.5, 109.5, 118.3, 122.2, 123.0, 124.1, 125.2, 136.6, 139.0,
184.6.

View Article Online
DOI: 10.1039/C6DT00060F
Dalton Transactions
Page 6 of 9
Synthesis of 1-(2-phenylsulfanyl/selanylethyl)-1H-indole-3-
carbaldehyde. 1-(2-Chloroethyl)-1H-indole-3-carbaldehyde
2.07g, 10 mmol) dissolved in 20 mL of ethanol was added drop 60 NMR (CDCl , 25 °C, Me Se): δ (ppm): 285.2. HR-MS [M + H]
109.1, 114.6, 121.2, 122.1, 123.0, 126.3, 126.5, 127.6, 127.7,
7
7
1
128.3, 128.4, 129.3, 131.4, 133.2, 136.5, 140.4, 155.8. Se{ H}
(
3
2
wise to a solution of PhSNa or PhSeNa generated in situ by the
reaction of NaOH (0.40 g, 10.0 mmol) with thiophenol (1.10 g,
(m/z) = 419.1012; calcd. value for C H N S = 419.1022 (ppm
24 23 2
error δ: 2.4).
Synthesis of palladium complexes 1 and 2. A solution of ligand
L1 or L2 (0.2 mmol) made in 20 mL of methanol was heated at
5
0
5
0
5
0
5
0
5
0
5
1
0.0 mmol) or NaBH4 (0.37 g, 10 mmol) reduction of
diphenyldiselenide (1.52 g, 5 mmol) at 80 °C under nitrogen
atmosphere. The reaction mixture was heated on an oil bath kept 65 70 °C for 15 minutes. Anhydrous sodium acetate (0.02
at 80 °C for 6 h. It was allowed to cool to room temperature,
poured in 100 mL of distilled water and extracted with
chloroform (100 mL). The extract was washed with water (3 × 50
mL) and dried over anhydrous sodium sulfate. The solvent was
g, 0.24 mmol) was added to it with stirring and the reaction
mixture was further stirred for 15 min. Sodium
tetrachloropalladate(II) (0.059 g, 0.2 mmol) was added to the
reaction flask and the resulting mixture was further heated at
1
1
2
2
3
3
4
4
5
5
evaporated off under reduced pressure on a rotary evaporator to 70 70°C for 6 h. The solvent of the mixture was removed completely
obtain the product B1 or B2.
on a rotary evaporator. The green residue was dissolved in 30 mL
of chloroform and filtered through celite. The volume of solution
was reduced to ~3 mL and it was mixed with 50 mL of hexane to
obtain complex 1 or 2 as yellowish green colored powder.
B1: Light yellow solid; Yield: 2.6 (93%). Anal. Found: C, 72.50;
H, 5.35; N, 4.95%. Calcd for C H NOS: C, 72.57; H, 5.37; N,
1
7
15
1
4
.98%. H NMR (300 MHz, CDCl , 25 ºC, TMS): δ (ppm): 3.22
3
(
8
t, 2H, J = 6.9 Hz, H ), 4.24 (t, 2H, J = 6.9 Hz, H ), 7.12–7.26 (m, 75 Complex 1: Yellowish green solid; Yield: 0.082 g (80%). m.p.
7
8
13
1
H), 7.55 (s, 1H), 8.18–8.21 (m, 1H), 9.85 (s, 1H, H ). C{ H}
153 °C(d). Anal. Found: C, 56.32; H, 4.17; N, 5.42%. Calcd for
1
7
1
NMR (75 MHz, CDCl , 25 ºC, TMS): δ (ppm) 33.6, 46.4, 109.6,
C H ClN PdS: C, 56.37; H, 4.14; N, 5.48%. H NMR (300
3
24 21
2
1
1
18.2, 122.2, 122.9, 124.0, 125.3, 127.1, 129.2, 130.3, 133.8,
36.7, 138.6, 184.5.
MHz, CDCl , 25 ºC, TMS): δ (ppm): 3.29 (br, 2H, H ), 4.37 (br,
3 18
2H, H ), 5.06 (br, 2H, H ), 7.05–7.16 (m, 3H), 7.30–7.52 (m,
1
7
7
1
3
1
B2: Light yellow solid; Yield: 2.95g (90%). Anal. Found: C, 80 9H), 7.86 (s, 1H, H ), 8.01–8.03 (m, 2H). C{ H} NMR (75
8
6
2.28; H, 4.64; N, 4.21%. Calcd for C H NOSe: C, 62.20; H,
MHz, CDCl , 25 ºC, TMS): δ (ppm): 37.8, 42.5, 60.3, 109.0,
1
7
15
3
1
4.61; N, 4.27%. H NMR (300 MHz, CDCl , 25 ºC, TMS): δ
117.6, 120.5, 120.6, 122.1, 125.8, 127.4, 127.6, 128.7, 129.1,
129.8, 130.9, 134.0, 136.5, 138.4, 165.6, 166.1. HR-MS [M − Cl]
(m/z) = 475.0474; calcd. value for C H N PdS = 475.0463
3
(ppm): 3.17 (t, 2H, J = 6.9 Hz), 4.31 (t, 2H, J = 6.9 Hz), 7.08–
7
1
.18 (m, 1H), 7.15–7.23 (m, 5H), 7.37–7.40 (m, 2H), 7.55 (s,
H), 8.17–8.23 (m, 1H), 9.85 (s, 1H). C{ H} NMR (75 MHz, 85 (ppm error δ: 2.2).
2
4
21
2
1
3
1
CDCl , 25 ºC, TMS): δ (ppm) 26.5, 47.3, 109.6, 118.2, 122.2,
Complex 2: Yellowish Green Solid; Yield: 0.093 g (83%). m.p.
3
122.9, 124.0, 125.4, 127.8, 128.0, 129.4, 133.3, 136.7, 138.2,
165 °C(d). Anal. Found: C, 51.60; H, 3.82; N, 5.00%. Calcd for
7
7
1
1
1
84.4. Se{ H} NMR (CDCl , 25 °C, Me Se): δ (ppm): 287.2.
C H ClN PdSe: C, 51.63; H, 3.79; N, 5.02%. H NMR (300
3
2
24 21
2
Synthesis of benzyl-[1-(2-phenylsulfanyl/selanylethyl)-1H-
indol-3-ylmethylene]amine (L1/L2) 1-(2-Phenylsulfanyl/ 90 3.26 (m, 1H), 3.82–3.91 (m, 1H), 4.67–4.72 (m, 1H), 4.81–4.85
selanylethyl)-1H-indole-3-carbaldehyde (2.0 mmol) was stirred in
dry ethanol (5 mL) at room temperature for 0.5 h. Benzylamine
MHz, CDCl , 25 ºC, TMS): δ (ppm): 3.04–3.13 (m, 1H), 3.25–
3
(m, 1H), 4.97–5.12 (m, 1H), 7.03–7.16 (m, 3H), 7.26–7.52 (m,
1
3
1
9H), 7.88 (s, 1H, H ), 8.00–8.03 (m, 2H). C{ H} NMR (75
8
(0.214 g, 2.0 mmol) dissolved in dry ethanol (2 mL) was added to
MHz, CDCl , 25 ºC, TMS): δ (ppm): 29.3, 43.3, 60.1, 109.1,
3
it drop wise with stirring. The reaction mixture was stirred further
at room temperature for 12 h. It was then cooled in refrigerator 95 130.0, 130.5, 134.8, 136.7, 138.5, 165.0, 165.8. Se{1H} NMR
overnight. The precipitate was filtered off and washed with cold
ethanol.
117.5, 120.6, 121.3, 122.0, 124.2, 125.8, 127.4, 128.7, 129.1,
7
7
(CDCl , 25 °C, Me Se): δ (ppm): 316.0. HR-MS [M − Cl] (m/z)
3 2
= 522.9893; calcd. value for C H N PdS = 522.9931 (ppm error
2
4
21
2
L1: White solid; Yield: 0.65 g (88%). Anal. Found: C, 77.83; H,
δ: 3.8).
General procedure for Heck coupling. Aryl bromide (1.0
.56%. H NMR (300 MHz, CDCl , 25 ºC, TMS): δ (ppm): 3.29 100 mmol), n-butylacrylate (1.2 mmol) and K CO (2.0 mmol) were
5
7
.90; N, 7.52%. Calcd for C H N S: C, 77.80; H, 5.98; N,
24 22 2
1
3
2
3
(
t, 2H, J = 6.9 Hz, H ), 4.32 (t, 2H, J = 7.5 Hz, H ), 4.83 (s, 2H,
added to a 50 mL three neck round bottom flask and purged with
nitrogen. An appropriate amount of complex (1 or 2) from a stock
solution made in DMA was added drop wise and the reaction
mixture was heated for 24 h on an oil bath maintained at 140 °C.
1
8
17
H ), 7.20–7.42 (m, 14H), 8.34–8.37 (m, 1H), 8.56 (s, 1H, H ).
7
8
1
3
1
C{ H} NMR (75 MHz, CDCl , 25 ºC, TMS): δ (ppm) 33.8,
3
4
1
6.0, 65.5, 109.1, 114.8, 121.3, 122.2, 123.0, 126.4, 126.6, 127.0,
27.8, 128.3, 129.2, 130.3, 131.6, 134.4, 136.6, 140.5, 155.8. 105 The mixture was cooled, diluted with 20 mL of water and
HR-MS [M + H] (m/z) = 371.1578; calcd. value for C H N S =
extracted with ethylacetate (2 × 25 mL). The organic layer was
washed with water (3 × 50 mL), dried over Na SO and its
2
4
23
2
371.1576 (ppm error δ: 0.3).
L2: White solid; Yield: 0.71 g (85%). Anal. Found: C, 69.01; H,
2
4
solvent evaporated off on a rotary evaporator. The crude product
obtained as a residue was purified by column chromatography on
5
6
.36; N, 6.77%. Calcd for C H N Se: C, 69.06; H, 5.31; N,
24 22 2
1
.71%. H NMR (300 MHz, CDCl , 25 ºC, TMS): δ (ppm): 3.07 110 silica gel using ethylacetate and hexane as eluent.
3
(
t, 2H, J = 7.5 Hz, H ), 4.19 (t, 2H, J = 7.5 Hz, H ), 4.70 (s, 2H,
General procedure for allylation. Solution of aldehyde (1
mmol) and Pd complex 1 or 2 (1.0 mol%) in DMF (2 mL) was
flushed with nitrogen for 10 min. Allyltributyltin (1.2 mmol) was
added drop wise and the reaction mixture was stirred at 50 °C (oil
1
8
17
H ), 7.00–7.03 (m, 1H), 7.08–7.18 (m, 6H), 7.21–7.31 (m, 5H),
7
13
1
7
.37–7.43 (m, 2H), 8.22–8.25 (m, 1H), 8.41 (s, 1H, H8). C{ H}
NMR (75 MHz, CDCl , 25 ºC, TMS): δ (ppm) 26.7, 46.7, 65.4,
3

View Article Online
DOI: 10.1039/C6DT00060F
Page 7 of 9
Dalton Transactions
bath temperature) for 24 h. After cooling, the reaction was
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6
6
7
0
5
0
quenched by the addition of 10% aqueous KF solution (30 mL)
and the reaction mixture stirred further at room temperature for 3
h. Thereafter the mixture was extracted with ethyl acetate (2 × 25
mL). The organic layer was washed with water (2 × 50 mL),
dried over Na SO and its solvent evaporated off on a rotary
5
2
4
evaporator. The residue (crude product) was purified by column
chromatography on silica gel using ethylacetate and hexane as
eluent.
(6) (a) D. V. Aleksanyanm, V. A. Kozlov, Y. V. Nelyubina, K. A.
1
0
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Authors thank Department of Atomic Energy (BRNS), India and
Council of Scientific and Industrial Research (CSIR), India for
financial support. MPS and SK thanks UGC (India) for the award
of JRF/SRF.
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Palladacycles of unsymmetrical (N,C ‾,E) (E = S / Se) pincers based on indole:
Synthesis, structure and catalysis of Heck coupling and allylation of aldehydes
Mahabir P. Singh, Fariha Saleem, Gyandshwar K. Rao, S. Kumar, Hemant Joshi and Ajai K.
Singh*
Palladacycles of Schiff bases of 1-(2-phenylsulfanyl/selenylethyl)-1H-indole-3-carbaldehyde with benzyl
amine catalyze Heck coupling and allylation at 0.1-0.3 and 1.0 mol% respectively.