Mono- and binuclear palladacycles via regioselective C-H bond activation: Syntheses, mechanistic insights and catalytic activity in direct arylation of azoles

Article abstract of DOI:10.1039/c5ra18289a

Regioselective C-H bond palladation of the hybrid pincer-type ligands, 3-R₂PO-C₆H₄-1-CH₂NⁱPr₂ [R₂POCNiPr₂-H; R = Ph (1a), R = Et₂N (1b)] has been described

Full text of DOI:10.1039/c5ra18289a

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Mono- and binuclear palladacycles via regioselective C‒H bond
activation: syntheses, mechanistic insights and catalytic activity in
direct arylation of azoles†
Received 00th January 20xx,
Accepted 00th January 20xx
a
Dilip K. Pandey,a Shrikant M. Khake,a Rajesh G. Gonnadeb and Benudhar Punji∗
DOI: 10.1039/x0xx00000x
Regioselective C‒H bond palladation of the hybrid pincer-type ligands, 3-R2PO-C6H4-1-CH2NiPr2 [R2POCNiPr2‒H; R = Ph (1a), R
= Et2N (1b)] has been described to accomplish mono- and binuclear palladacycles. The reactions of the ligands
www.rsc.org/
R2POCNiPr2‒H (1a, 1b) with Pd(COD)Cl2 in the presence of K3PO4 afforded the mononuclear pincer complexes {κP
(R2PO)-C6H3-6-(CH2NiPr2)}PdCl (R2POCNiPr2)PdCl [R = Ph (2a), R = Et2N (2b)], whereas the similar reactions in the presence of
Et3N produced the chloro-bridged binuclear palladacycles [{κP κC-2-(R2PO)-C6H3-4-(CH2NiPr2)}(
-Cl)Pd]2 {R = Ph (4a), R =
, ,
κC κN-2-
,
ꢀ
Et2N (4b)} via the regioselective ligands C(2)–H and C(4)–H bond activation, respectively. Similarly, the reaction of a
previously reported ligand iPr2POCNiPr2‒H (1c) with Pd(COD)Cl2 in the presence of Et3N affords chloro-bridged binuclear
complex [{κP
using Pd(OAc)2 instead of Pd(COD)Cl2 in the palladation reaction of 1a, in presence or absence of base, leads to the
exclusive formation of acetate-bridged binuclear palladacycle [{κP κC-2-(Ph2PO)-C6H3-4-(CH2NiPr2)}(
-OAc)Pd]2 (5a).
, ꢀ-Cl)Pd]2 (4c) via the regioselective C(4)–H bond palladation. On contrary,
κC-2-(iPr2PO)-C6H3-4-(CH2NiPr2)}(
,
ꢀ
Mechanistic detail for this regioselective C–H bond activation to achieve mono- and binuclear palladacycles has been
demonstrated, which highlights that the coordinating ability of external base, steric between coordinated base and N-arm
of POCN ligand as well as the electrophilicity of the palladium center are crucial to the regioselective palladation. The
palladium complex 4c was shown to be the active catalyst for the direct arylation of azoles with aryl iodides under mild
reaction conditions.
Recently, the unsymmetrical ECE’-pincer palladium system
having two different donor arms, typically one soft P-donor
and one hard N- or O-donor, such as PCN,7 POCN,8 and PCO9
Introduction
Palladacycles containing C-anionic four-electron donor (CE) or
six-electron donor (ECE) ligand (E = donor group) have many
attractive structural features.1 The stable chelating structure
has given particular attention, as such system renders
complementary properties of both the hard/soft donor as well
as that of the distinct electron-donor/acceptor ability. In
addition, the hemilabile palladacycles could provide suitable
sterics, electronics, and coordination demands needed on the
different steps of a catalytic reaction, and might produce the
interesting structural motifs on stoichiometric reactions.
and strong metal-carbon σ-bond provides them with high air
and thermal stability, whereas the appropriate tuning of the
substituents on donor-atom furnishes distinct sterics and
electronics around the palladium; which make the
palladacycles extraordinary catalysts for various important
organic transformations.2,3 Though, each of these palladacycles
has its own unique chemistry, the ECE-pincer (NCN,4 PCP,5
SCS,6 etc.) palladium system has attracted considerable
interest, as it benefits from the advantage of terdentate
coordination and hence, the well-protected palladium center.
NiPr2
L
L
Cl
Cl
[PdII]
Et3
[PdII], L
R = H
Pd
Pd
Cl
R'2
N
NR'2
2
N
Pd
O
P
R'2
H
R = H
(R' = Me, Et)
(R' = Ph, NEt2, iPr)
X
E
R'2
R
ER'2
a.Organometallic Synthesis and Catalysis Group, Chemical Engineering Division,
CSIR–National Chemical Laboratory (CSIR–NCL), Dr. Homi Bhabha Road, Pune –
411 008, Maharashtra, INDIA. Phone: + 91-20-2590 2733, Fax: + 91-20-2590
2621, E-mail: b.punji@ncl.res.in.
[PdII]
[PdII
R = SiMe3
]
O
R'2P
NiPr2
K3PO4
Pd
Cl
Me2
N
NMe2
Pd
Cl
[PdII] = Pd(COD)Cl2
or Li2[PdCl4]
(R' = Ph, NEt2
)
b.Centre for Material Characterization, CSIR–National Chemical Laboratory (CSIR–
NCL), Dr. Homi Bhabha Road, Pune – 411 008, Maharashtra, INDIA.
(A)
Previous reports
(B)
ref.12,13
Described in this paper
(
)
(
)
†
Electronic Supplementary Information (ESI) available: Crystallographic
information files (CIF). NMR and mass spectra of ligands and complexes, crystal
structure data of complexes 2a 2b 3a 3b and 5a. CCDC-1063772 (2a), CCDC-
Chart 1 Representation of site-selective palladation to obtain aryl-based CE- or ECE-
palladacycles: (A) by introduction of an activation group (-SiMe3), (B) by regioselective
C–H bond activation.
,
,
,
1063769 (2b), CCDC-1063773 (3a), CCDC-1063770 (3b) and CCDC-1063774 (5a).
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
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Considering the broad applicability of the palladacycles, the
efficient and regioselective synthesis of the hemilabile ECE’-
based palladacycles (both CE- and ECE’-type) by C H bond
(1) NaH, THF
70 oC, 3 h
O
HO
(2) R2PCl, THF
70 oC, 12 h
R2P
NiPr2
NiPr2
‒
activation is indispensable. In general, the syntheses of the
arene-based ECE pincer-palladium complexes, containing 1,3-
donor group substituents, are executed by the direct
cyclopalladation at C-2 position of the arene ligands; which
usually promoted by the coordination of the donor groups on
R = Ph, 1a
1b
R = NEt2,
Scheme 1 Synthesis of POCN‒H ligands.
the –CH2 protons on –CH2NiPr2 group appeared around 3.60
ppm as singlets. Further, the assigned molecular structures of
ligand prior to the intramolecular C(2)‒
H bond activation.10,11
1
the compounds 1a and 1b are consistent with the observed H
However, in some of the pincer-type ligands, the
cyclopalladation of C-2 does not occur and instead the C-4/C-6
palladation takes place to generate CE-type palladacycles
(Chart 1A, top).12 In order to obtain the selective ECE-pincer
palladacycles, in such cases, the site selectivity of the
palladation is inverted by the introduction of an activating
group (–SiMe3) at C-2 position (Chart 1A, bottom).12b,13
Surprisingly, the synthesis of both the CE- and ECE/ECE’-
palladacycles from a single ECE/ECE’ pincer-type ligand via the
and 13C NMR data. These compounds were used for the
metallation reactions without further purification.
Synthesis of palladacycle complexes
Following our previous protocol for the synthesis of POCN
pincer-palladium complexes,14 the reactions of 1a and 1b with
Pd(COD)Cl2 in the presence of inorganic base K3PO4 in 1,4-
dioxane produced the pincer-ligated complexes {κP κC κN-2-
, ,
(R2PO)-C6H3-6-(CH2NiPr2)}PdCl [(R2POCNiPr2)PdCl; R = Ph (2a
)
selective C
the literature. We assumed that in an unsymmetrical pincer-
type ligand POCN H, as it contain two different donor groups
‒H bond palladation has not been precedented in
and R = Et2N (2b)] (Scheme 2). The 31P{1H} NMR spectra of 2a
and 2b displayed singlets at 150.0 and 142.3 ppm, respectively.
‒
1
In the H NMR spectrum of 2a, the –CH3 protons (12H) of
with the distinct donating ability, the reactivity of the ligand
towards palladium could be tuned to allow only P- or both N-
and P-coordination, which would be followed by the
isopropyl groups appeared as two set of doublets in contrast
to a single set of doublet observed for the same (12H) in 1a
,
which could be due to the magnetic non-equivalency of –CH3
protons, generated upon the coordination of N-arm of the
ligand to the palladium center. The two –CH protons on the
isopropyl groups appeared as an octet, which might be due to
the partial overlapping of the two septets for each –CH of the
–N{CH(CH3)2}2 group. Similarly, the –CH3 protons on isopropyl
groups of the 2b resonate differently and appeared as two
distinct doublets. The appearance of two set of doublets for
the –CH3 protons on –N{CH(CH3)2}2 groups in the complexes 2a
and 2b, in contrast to single set of doublet in the ligands 1a
and 1b, could be considered as an indication for the nitrogen-
arm coordination to the palladium center. Further, the peak of
–CH2 protons on the –CH2NiPr2 group of 2a and 2b deshielded
regioselective C‒H bond palladation to generate both type of
palladacycles (CE- and ECE’-type), without being the
introduction of an activating group (-SiMe3). With this in mind
and as a part of our research interest on the development of
unsymmetrical pincer-systems,14 herein, we report the
syntheses of two unsymmetrical POCN pincer-type ligands,
and their regioselective C
both the “PC”-chelate binuclear [κP
“POCN”-coordinated mononuclear (κP
type) palladacycles (Chart 1B). Furthermore, the influence of
base on the regioselective bond palladation is
‒
H bond palladation to accomplish
κC-PC-PdCl]2 (CE-type) and
κC κN-POCN)PdCl (ECE’-
,
,
,
C‒H
demonstrated, and the mechanistic aspect of the same has
been elucidated. All the palladacycles were screened as a
(∼ 0.4 ppm) compared to the respective ligands, and appeared
catalyst precursor for the direct C‒H bond arylation of azoles
around 4.0 ppm as singlets. The POCN-pincer palladium
complexes 2a and 2b were further characterized by 13C NMR,
MALDI-TOF measurements and elemental analyses. The
proton and carbon peaks chemical shift in both the complexes
were fully assigned by the 2D NMR studies, after establishing
the atom connectivity and spatial relationship. The molecular
structures of 2a and 2b were also confirmed by the single
crystal X-ray diffraction studies.
with aryl iodides and some of them were efficiently employed
in the synthesis of a variety of 2-arylated azoles.
Results and discussion
Synthesis of ligands
The R2POCNiPr2 H [3-R2PO-C6H4-1-CH2NiPr2 (R = Ph, 1a; R =
‒
Et2N, 1b)] pincer ligands were synthesized following the
procedure similar to the synthesis of analogous compounds,14
which has been recently developed in our group. Hence, the
treatment of 3-((diisopropylamino)methyl)phenol with NaH,
followed by the reaction with relevant chlorophosphine
produced the hybrid ligands 1a and 1b in good yields (Scheme
1). The 31P{1H} NMR spectra of 1a and 1b showed single
resonances at 109.7 and 131.8 ppm, respectively; which were
NiPr2
Pd(COD)Cl2
Pd(COD)Cl2
Et3
R2
P
K3PO4
N
Cl
Cl
O
O
O
1,4ꢀdioxane
70 oC, 2 h
Pd
Pd
1,4ꢀdioxane
70 oC, 2 h
NiPr2
R2
P
NiPr2
O
R2
P
Pd
Cl
P
R2
R = Ph, 1a
R = NEt2, 1b
R = Ph, 2a
R = NEt2, 2b
NiPr2
R = Ph, 4a
R = NEt2, 4b
R = iPr,
1cref 14
R = iPr,
2cref 14
R = iPr,
4c
comparable with the 31P NMR data of the similar compounds
Scheme 2 Synthesis of palladacycle complexes.
15
i.e. Ph4POCOP H3c
‒ (δP 114.0 ppm) and 4-Me-C6H4-OP(NEt2)2
(δ
P 133.0 ppm). In the 1H NMR spectra of both the compounds,
2 | RSC Adv., 2015, 00, 1-3
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NiPr2
To our surprise, the reactions of 1a
,
1b and 1c14 with
Pd(COD)Cl2 in the presence of Et3N as base in 1,4-dioxane
Me
Ph2
P
produced the binuclear ortho-palladated complexes [{κP
,
κC-2-
-Cl)Pd]2 {(R = Ph (4a), Et2N (4b), Pr
Pd(OAc)2
with/without base
O
O
O
O
i
(R2PO)-C6H3-4-(CH2NiPr2)}(
ꢀ
4c)}, with the chloride as a bridging ligand between the two
O
O
Pd
Pd
Ph2P
NiPr2
O
1,4ꢀdioxane
70 oC, 2 h
P
Ph2
(
Me
(5a)
(1a)
palladium centers (Scheme 2). In the binuclear complexes,
each palladium center is coordinated by the P-atom, and the
C-atom that is ortho to –OPR2 and para to –CH2NiPr2 group.
NiPr2
Scheme 4 Palladation of 1a with Pd(OAc)2.
Unlike the pincer-ligated complexes 2a
,
2b, and 2c 14
4b and 4c) does
,
the N-
To synthesize the derivative of a binuclear palladacycle, the
arm of the ligand in binuclear complexes (4a
,
complex 4a was treated with AgOAc which generated the
acetate-bridged binuclear complex [{κP κC-2-(Ph2PO)-C6H3-4-
,
not involved in the coordination to palladium center. It should
be noted that the triethylamine (Et3N) is commonly used as a
base for the synthesis of pincer-ligated palladium complexes;
however, in the current ligand systems the formation of non-
pincer ortho-palladated binuclear complexes were observed in
the presence of Et3N, even though the ligands are of pincer-
type. To our knowledge, this represents the first example of a
base-assisted site selective C–H bond palladation to
accomplish both the bidentate-coordinated palladacycles and
terdentate-coordinated pincer palladium complexes from the
pincer-type ligands. Similar site selective palladation has
previously been known only by the introduction of an
activating group (i.e. –SiMe3).12,13,16 The 31P NMR spectrum of
4a displayed two singlets at 152.9 and 152.2 ppm in 1.5 : 1
ratio. The two different peaks could be due to the existence of
cis and trans isomers for the complex 4a in solution.17 Though,
(CH2NiPr2)}( -OAc)Pd]2 (5a) (Scheme 3). The 31P NMR spectrum
ꢀ
of 5a displayed a singlet at 151.0 ppm, which is slightly
shielded than that observed for the chloro-bridged complex
4a. Unlike the complex 4a, the aceate-bridged binuclear
complex 5a exists as a single isomer in solution as indicated
from the 31P NMR data. The 1H NMR data of 5a resembled with
that of the 4a, except that the –CH3 protons of bridged-acetate
show a singlet at 2.17 ppm. Further, the 13C NMR data and
elemental analysis of 5a is in good agreement with the
assigned structure. The molecular structure of complex 5a was
further confirmed by X-ray diffraction study.
On a different note, the C–H bond palladation of the
R2POCNiPr2–H ligand was explored with other palladium
precursor like Pd(OAc)2. Surprisingly, the reaction of
Ph2POCNiPr2–H (1a) with Pd(OAc)2 in the presence of K3PO4
1
the 31P NMR data of 4a is suggestive of two isomers, the H
produced the ortho-palladated binuclear palladacycle 5a
,
NMR spectrum displayed a single set of peaks. The –CH3
protons on the –NiPr2 group showed only one broad singlet at
1.00 ppm, suggesting the N-arm of the ligand is not involved in
the coordination to Pd-center. Further, the peak at 3.55 ppm
for the –CH2 protons on –CH2NiPr2 group is indicative of a non-
coordinating N-arm. The MALDI-TOF spectrum of 4a displayed
the molecular ion peak at 1063.3624, which is consistence with
the existence of a binuclear complex. Similar to the complex
4a, the 31P NMR spectrum of binuclear complex 4b displayed
two singlets at 137.7 (76%) and 137.9 (24%) ppm for the two
rather than the presumed mononuclear pincer complex
(Scheme 4). Further, the same reaction in the presence of Et3N
or even in the absence of a base afforded the binuclear
complex 5a. These results indicated that the base has no
influence on the regioselective palladation of the ligand 1a
while Pd(OAc)2 is used as a palladium source. This distinct
reactivity of 1a with Pd(OAc)2 than with the Pd(COD)Cl2
towards the regioselective palladation can be attributable to
the electrophilic character of Pd(OAc)2. The high
electrophilicity of Pd(OAc)2 could accelerate the electrophilic
attack of the Pd-center on arene ring of the POCN–H ligand
after the P-arm coordination, leading to palladacycle 5a, rather
than allowing both the P- and N-arm coordination and Cipso–H
bond palladation.
1
different isomers. The H NMR spectrum showed a single set
of peaks and the splitting pattern of the proton peaks
resemble to that observed for a non-coordinating –CH2NiPr2
group. The 31P NMR spectrum of the complex 4c showed two
singlets at 200.0 and 199.0 ppm for the presumed two isomers
1
in 1.9:1 ratio. The H NMR spectrum exhibited a single set of
peaks with the distinctive of non-coordinated –CH2NiPr2 and
coordinated –OPiPr2 groups. The MALDI-TOF measurement
displayed the peaks at 1043.4058 (M+H)+ and 927.3241 (M+H)+
for the binuclear complexes 4b and 4c, respectively.
Mechanistic aspects of regioselective palladation
The reactions of the ligands 1a
-1c with Pd(COD)Cl2 in the
presence of K3PO4 produce the pincer complexes 2a-2c by the
arene C(2)–H bond activation of the 1,3-donor group
substituted ligands, whereas the similar reactions in the
presence of triethylamine exclusively furnished the non-pincer
NiPr2
NiPr2
ortho-palladated binuclear palladacycles 4a-4c through the
Me
Ph2
P
Ph2
P
C(4)–H bond activation, leaving the N-arm non-coordinated. In
view of the synthetic aspect of the palladacycle complexes,
this regioselective C–H bond palladation is unique as a base
could decides the outcome of the products formation, i.e.
mononuclear pincer complex or binuclear non-pincer complex.
O
O
O
Cl
Cl
AgOAc, THF
r.t., 3 h
O
O
Pd
Pd
Pd
Pd
O
O
O
P
P
Ph2
Ph2
Me
(5a)
NiPr2
NiPr2
(4a)
This distinct reactivity of the hybrid ligands 1a-1c motivated us
to investigate the mechanistic aspects of the regioselective
Scheme 3 Synthesis of acetate-bridged binuclear palladium complex 5a.
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palladation reaction. We have choosen 1c as a model ligand to
study it’s reactivity with Pd(COD)Cl2 under controlled reaction
conditions, and the progress of the reaction was monitored by
the 31P NMR analysis. Initially, the ligand 1c was treated with
Pd(COD)Cl2 in 1,4-dioxane in the absence of base at room
temperature (Scheme 5). After 1 h, the 31P NMR spectrum of
the crude reaction mixture displayed two singlets at 198.2 and
141.6 ppm. The peak at 198.2 ppm corresponds to the pincer-
K3PO4
70 oC, 1 h
Pd(COD)Cl2
r.t., 1 h
O
iPr2P
O
NiPr2
NiPr2
ꢀ base.HCl
iPr2P
Pd
Cl
Pd
Cl
Cl
)
(2c)[14]
6c
(
ꢀ Py.HCl
O
iPr2P
NiPr2
(1c)
O
ligated complex 2c 14
.
The phosphorous peak at δP 141.6 ppm
iPr2P
NiPr2
NiPr2
Pd
For 8c
70 oC, 5 h
Cl
Cl
was tentatively assigned for the intermediate complex 6c, as
the HRMS (ESI) measurement of the crude reaction mixture
showed a mass peak at m/z 522.0696 (m/z calcd for [6c+Na]+ =
522.0682). Several attempts to isolate the pure product of 6c
were unsuccessful, as the reaction was always resulted with
the mixture of both 2c and 6c. However, upon addition of
K3PO4 to the reaction mixture, a clean conversion of the
mixture to the compound 2c was occurred at elevated
temperature. This suggests that the chelate complex 6c is most
likely an active intermediate for the formation of pincer-
complex 2c in the presence of K3PO4. Similar chelate complex
H2
6c
)
Pd(COD)Cl2
Base (L)
(
H4
L
O
NiPr2
70 oC, 1 h
ꢀ Et3N.HCl
r.t., 1 h
iPr2P
iPr2
P
Pd
For 7c
Cl
Cl
Cl
Cl
O
Pd
Pd
O
L = Et3N; (7c)
8c
P
L = Py;
(
)
iPr2
NiPr2
(4c)
Scheme
palladacycles.
5 Proposed pathway for the formation of “PC”- and “POCN”-
intermediate [κS
,
κS-(MeS-CH2)2-C6H4-PdCl2] for the synthesis of
0.07
4c
(
from 7c
from 6c with 1 equiv of K PO )
4
)
symmetrical pincer complex [κS
,
κC κS-(MeS-CH2)2-C6H3-PdCl]
,
2c
2c
(
(
3
has previously been described.6a Further, to understand the
effect of Et3N on the palladation reaction, the ligand 1c was
treated with Pd(COD)Cl2 in the presence of Et3N (1.2 equiv) in
1,4-dioxane at room temperature and the progress of the
reaction was monitored by 31P NMR analysis. After stirring the
reaction mixture for 1 h, the ligand peak at δP 148.8 ppm
completely disappeared and the formation of a new peak at δP
143.6 ppm was observed, with the minor formation of
0.06
0.05
0.04
0.03
0.02
0.01
0.00
from 8c
)
complex 4c (δP 200.6 ppm). The peak at δP 143.6 ppm was
tentatively assigned to the intermediate palladium complex 7c
0
10
20
30
40
50
Time [min]
(Scheme 5), as the phosphorous ligands environment in 6c and
7c are almost similar and the 31P NMR chemical shifts for both Fig. 1 Reaction profile for the C–H bond palladation of 6c
of them were around in the same region (δP 141.6 and 143.6
,
7c and 8c at 70 oC.
assumed that the assigned structures for these intermediates
are appropriate. To our surprise, when the complex 8c was
heated in 1,4-dioxane at 70 oC for 5 h, the formation of pincer
complex 2c was observed, instead of the expected ortho-
palladacycle complex 4c. The formation of the pincer complex
2c from the intermediate 8c can be attributable to the easy
displacement of the less bulky pyridine ligand by the –CH2NiPr2
ligand arm leading to the formation of an intermediate species
6c, which is followed by the C(2)–H bond palladation.
However, in the case of intermediate 7c, the steric hindrance
between –CH2NiPr2 group and coordinated Et3N ligand might
keep the –CH2NiPr2 unit away from the palladium center and
ppm). An attempt to the isolation and characterization of the
presumed intermediate complex 7c was unsuccessful, as the
probable hemilabile species 7c was completely transformed
into the thermodynamically stable product 4c during the work-
up process. Assuming that the hemilabile nature of Et3N ligand
might play a role in the instability of 7c, the ligand 1c was
treated with Pd(COD)Cl2 in the presence of pyridine (1.2 equiv)
in 1,4-dioxane. After the reaction mixture was stirred at room
temperature for 1 h, the 31P NMR measurement of the
reaction mixture displayed a peak at 144.9 ppm with the
complete disappearance of the peak correspond to 1c. The 31P
NMR peak at 144.9 ppm was assigned to the complex 8c. The
compound 8c was isolated and fully characterized by the major
spectroscopic techniques. In 1H NMR spectrum of 8c, the
allow the C(4)–H bond activation to produce the complex 4c
.
In order to gain more insight into the influence of base
during the course of C–H bond palladation reaction, the
kinetics of the electrophilic palladation step in the formation of
2c and 4c from the 6c (or 8c) and 7c, respectively, were
determined. All the kinetic experiments were performed after
downfield shift (ca. 0.6 ppm) of the C(2)‒H and C(6)‒H protons
on pyridine moiety suggests the pyridine ligand coordination
to the palladium center. Further, the HRMS (ESI) analysis of
the complex 8c confirmed the assigned molecular structure.
Since the 31P NMR chemical shifts of the three intermediate
being the in situ generation of the intermediate species 6c, 7c
and 8c (For details, see SI). Initially the rates for the formation
of products 2c and 4c were determined by employing one
equiv of each base K3PO4, Et3N and pyridine using the initial
complexes (6c, 7c and 8c) were around in the same region, we
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rate approximation. The reaction profile for the C–H bond
palladation on 6c 7c and 8c over a period of 45 min is shown
AgX, THF
r.t., 3 h
,
O
O
R2P
NiPr2
R2P
NiPr2
Pd
Pd
Cl
in Fig 1. The rate for the formation of 4c from the 7c (1.14 x
10-3 Mmin-1) is determined to be approximately four times
faster than the formation of 2c from 8c (0.296 x 10-3 Mmin-1).
This indicates that the dissociation of coordinated Et3N and C–
H bond palladation in 7c is much faster than the dissociation of
OAc
R = Ph, 3a
3b
R = Ph, 2a
2b
R = NEt2,
R = NEt2,
Scheme 6 Synthesis of POCN-pincer palladium derivatives.
pyridine and palladation in the complex 8c
.
On the basis of all the experimental findings, the pathways
for the formation “POCN”-pincer palladium and “PC”-chelate
palladacycle can be drawn as shown in Scheme 5. Thus, the
formation of pincer complex 2c occurred via the chelate
intermediate species 6c in the presence of an inorganic base
K3PO4, whereas the binuclear complex 4c was formed via the
Lewis base-coordinated intermediate 7c. Notably, the Lewis
base-coordinated intermediate might leads to the formation of
pincer complex (as seen in the case of 8c), provided the
dangling non-coordinated arm (–CH2NiPr2) displaced the
Furthermore, the order of palladation reaction in each
o
base was determined individually at 70 C in 1,4-dioxane with
varying concentration of base using the initial rate
approximation. The plot was drawn between log(rate) vs.
log(conc. base), wherein the slope of the plot indicates the
order of palladation reaction with the particular base. Figure
S1 shows the rate of the palladation reaction is almost same
for the various concentration of the K3PO4, suggesting the
reaction is zeroth-order in the concentration of K3PO4.
However, the rate order of palladation reaction in the
concentration of Et3N and pyridine is -0.25 and -0.39,
respectively (Fig S2 and Fig 2). These negative fractional order
reaction rates in the concentration of Et3N and pyridine
suggest that the rate of C–H bond palladation in 7c and 8c is
actually retarded with the increased concentration of Et3N and
pyridine. Mostlikely, the complexes 7c and 8c are stabilized
with the high concentration of respective bases by restricting
the ligated-base dissociation.
coordinated-base (L) and generates the chelate species 6c
.
Hence, we proposed that the primary requirement for the
synthesis of a chloro-bridged binuclear complex could be the
use of a coordinating Lewis base (L); and secondly, the
substantial steric hindrance between the coordinated-base
and the dangling arm (–CH2NiPr2) to avoid the formation of
intermediate 6c. We assume that the steric between the N-
arm (–CH2NiPr2) and coordinated ligand (L) is crucial to the
specific palladacycle formation, rather than the electronic
factor (Et3N vs Py). This can be further outlined on the fact that
a slightly less bulky ligand Ph2PO-C6H4-CH2NEt2 upon the
reaction with PdCl2 in presence of Et3N, generated the pincer-
ligated palladium complex;8b though the ligand 1c reacted with
0.016
(A)
0.074 [M]
0.222 [M]
0.444 [M]
0.742 [M]
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
Pd(COD)Cl2 and Et3N to produces the non-pincer complex 4c
.
Syntheses of POCN-pincer palladium derivatives
The treatments of (POCN)PdCl complexes 2a and 2b with
AgOAc in THF at room temperature resulted in the formation
of complexes {κP κC κN-2-(R2PO)-C6H3-6-(CH2NiPr2)}Pd(OAc) [R
, ,
= Ph (3a) and R = Et2N (3b)] in good yields (Scheme 6). The 31P
NMR spectra of the complexes 3a and 3b displayed single
0
10
20
30
40
50
Time [min]
1
resonances at 143.8 and 142.3 ppm, respectively. In the H
ꢀ2.0
ꢀ2.5
ꢀ3.0
ꢀ3.5
ꢀ4.0
ꢀ4.5
ꢀ5.0
NMR spectra, the –OCOCH3 protons appeared as singlets at
1
Order in Pyridine
(B)
1.86 and 1.92 ppm for 3a and 3b, respectively. Other H and
13C NMR peaks for the complexes 3a and 3b as well as their
splitting patterns are largely resembled with that observed in
the respective spectra of complexes 2a and 2b. Moreover, the
MALDI-TOF and elemental analyses of both the complexes are
in accord with the assigned molecular structures. The
complexes 3a and 3b were further characterized by 13C NMR,
elemental analyses and single crystal X-ray diffraction studies.
y = ꢀ0.39336x ꢀ3.92892
2
R
= 0.692
ꢀ1.4
ꢀ1.2
ꢀ1.0
ꢀ0.8
ꢀ0.6
ꢀ0.4
ꢀ0.2
0.0
Molecular structures of palladium complexes
log(conc. Pyridine)
The crystals of the complexes 2a, 2b, 3a, 3b and 5a, suitable
Fig. 2 (A) Time-dependent yields of the palladacycle 2c at different initial concentration
of pyridine. (B) Plot of log(rate) vs log(conc. pyridine). The rates are average of two
independent measurements.
for the X-ray diffraction study were obtained from the slow
evaporation of n-hexane solution of the complexes at room
temperature. The ORTEP diagrams are shown in Fig. 3-7 and
the selected bond lengths and bond angles are given in the
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respective figure captions. In the structures of 2a and 2b, the
For the complexes 3a and 3b, the coordination geometry
geometry around the palladium is distorted square planar. The around the palladium is slightly distorted from the expected
Pd C bond lengths, 1.9514(16) and 1.960(2) Å in 2a and 2b square planar geometry. The acetate ligand in both the
respectively; are comparable with the Pd
C bond length complexes binds as η1-coordinated fashion, and the Pd
(1.957(2) Å) in the similar complex (3-MeO-Ph2POCNMe2)PdCl.8d bond lengths (2.084(2) Å in 3a, 2.1247(12) Å in 3b) are similar
The Pd Cl bond length (2.4082(6) Å) in 2b is slightly longer to those found in other examples of palladium-acetate
than the Pd Cl bond length (2.3848(4) Å) observed in 2a complexes, wherein the acetate ligand is trans to aryl
which might be due to the C bond lengths (1.952(3) and 1.9510(17) Å)
-donor strength exerted by the groups.5h,18 The Pd
(Et2N)2POCNiPr2) moiety upon the palladium in 2b being in 3a and 3b are comparable with the Pd
C bond lengths in
stronger than that of the (Ph2POCNiPr2) moiety in complex 2a
their halide counterparts 2a and 2b, which is in line with the
The Pd P bond length (2.1885(4) Å) in 2a is comparable with similar trans influence of both chloride and acetate ligands.
that of the complex (3-MeO-Ph2POCNMe2)PdCl (Pd
However, the Pd P bond lengths (2.2136(8) and 2.2140(5) Å)
2.1858(6) Å).8d However, the Pd
P bond length (2.1951(6) Å) in in the 3a and 3b are slightly longer than the corresponding
2b is significantly shorter than the Pd P bond lengths Pd P bond lengths in halide derivatives 2a and 2b. The
(2.284(2), 2.277(2) Å) observed in the symmetrical pincer P(1) Pd(1)
O(2) bond angles in 3a and 3b are 102.90(6)o and
complex ((Et2N)4POCOP)PdI.5n The Pd(1) N(1) bond length 102.48(4)o, whereas the N(1)
Pd(1) O(2) bond angles are
(2.2032(13) Å) in 2a is slightly shorter than the Pd(1)
N(1) 93.34(9)o and 95.14(5)o. This indicates that the O(2) atom of
bond length (2.2204(17) Å) in (iPr2POCNiPr2)PdCl,14 whereas the OC(O)CH3 ligand is aligned more towards N-arm than the P-
Pd(1) N(1) bond length (2.2361(16) Å) in 2b is slightly longer arm, whereas the carbonyl oxygen O(3) is closer to the P-
than that observed in complex (iPr2POCNiPr2)PdCl. For the center than the N-center in both the complexes 3a and 3b
complexes 2a and 2b, the P Pd N bond angles are 161.10(4)
and 161.30(5)o, respectively; which are smaller than the
corresponding bond angle (P Pd
N, 162.10(5)o) in the
iPr2POCNiPr2)PdCl complex.14
‒
,
‒
‒O
‒
‒
,
σ
‒
(
‒
.
‒
‒
P
=
‒
‒
‒
‒
‒
‒
‒
‒
‒
‒
‒
.
‒
‒
‒
‒
(
Fig. 5 Thermal ellipsoid plot of (Ph2POCNiPr2)PdOAc (3a). All the hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Pd(1)‒C(1), 1.952(3); Pd(1)‒P(1),
2.2136(8); Pd(1)‒N(1), 2.207(2); Pd(1)‒O(2), 2.084(2); C(26)‒O(2), 1.276(4);
C(26)‒O(3), 1.243(4). Selected bond angles (o): C(1)‒Pd(1)‒P(1), 81.21(8);
C(1)‒Pd(1)‒N(1), 82.44(10); P(1)‒Pd(1)‒N(1), 162.89(6); C(1)‒Pd(1)‒O(2), 175.64(10);
P(1)‒Pd(1)‒O(2), 102.90(6); N(1)‒Pd(1)‒O(2), 93.34(9).
Fig. 3 Thermal ellipsoid plot of (Ph2POCNiPr2)PdCl (2a). All the hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Pd(1)‒C(1), 1.9514(16); Pd(1)‒P(1),
2.1885(4); Pd(1)‒N(1), 2.2032(13); Pd(1)‒Cl(1), 2.3848(4). Selected bond angles (o):
C(1)‒Pd(1)‒P(1), 80.52(5); C(1)‒Pd(1)‒N(1), 81.72(6); P(1)‒Pd(1)‒N(1), 161.10(4);
C(1)‒Pd(1)‒Cl(1), 176.08(5); P(1)‒Pd(1)‒Cl(1), 98.672(16); N(1)‒Pd(1)‒Cl(1), 99.48(4).
Fig. 6 Thermal ellipsoid plot of ((Et2N)2POCNiPr2)PdOAc (3b). All the hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Pd(1)‒C(1), 1.9510(17); Pd(1)‒P(1),
2.2140(5); Pd(1)‒N(1), 2.2295(14); Pd(1)‒O(2), 2.1247(12). Selected bond angles (o):
C(1)‒Pd(1)‒P(1), 80.77(6); C(1)‒Pd(1)‒N(1), 81.97(7); P(1)‒Pd(1)‒N(1), 162.35(4);
C(1)‒Pd(1)‒O(2), 170.84(6); P(1)‒Pd(1)‒O(2), 102.48(4); N(1)‒Pd(1)‒O(2), 95.14(5).
Fig. 4 Thermal ellipsoid plot of ((Et2N)2POCNiPr2)PdCl (2b). All the hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Pd(1)‒C(1), 1.960(2); Pd(1)‒P(1),
2.1951(6); Pd(1)‒N(1), 2.2361(16); Pd(1)‒Cl(1), 2.4082(6). Selected bond angles (o):
C(1)‒Pd(1)‒P(1), 80.26(6); C(1)‒Pd(1)‒N(1), 81.09(8); P(1)‒Pd(1)‒N(1), 161.30(5);
C(1)‒Pd(1)‒Cl(1), 176.84(6); P(1)‒Pd(1)‒Cl(1), 97.42(2); N(1)‒Pd(1)‒Cl(1), 101.27(5).
6 | RSC Adv., 2015, 00, 1-3
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Table 1 Screening of catalysts and reaction parameters for arylation of azolesa
Entry
1
Pd-catalyst
Base
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
KOAc
K2CO3
Cs2CO3
K3PO4
K3PO4
K3PO4
K3PO4
Solvent
DMF
Yield (%)b
2a
2b
2c
4a
4b
4c
3a
3b
5a
4c
4c
4c
4c
4c
4c
4c
59
11
2
DMF
3
DMF
90 (88)
92
4
DMF
5
DMF
15
6
DMF
DMF
95 (93)
58
7
8
DMF
4
9
DMF
79
10
11
12
13
14
15
16
DMF
46
Fig. 7 Thermal ellipsoid plot of complex 5a. All the hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): Pd(1)‒C(1), 2.002(4); Pd(1)‒P(1), 2.1730(12);
Pd(1)‒O(2), 2.093(3); Pd(1)‒O(3), 2.139(3); C(26)‒O(3), 1.249(5); C(26)‒O(3A),
1.253(5). Selected bond angles (o): C(1)‒Pd(1)‒P(1), 79.51(12); C(1)‒Pd(1)‒O(2),
92.23(15); P(1)‒Pd(1)‒O(2), 169.83(9); C(1)‒Pd(1)‒O(3), 171.83(14); P(1)‒Pd(1)‒O(3),
97.33(9); O(2)‒Pd(1)‒O(3), 90.06(11).
DMF
50
DMF
46
DMSO
DMA
NMP
Toluene
85 (84)
63
5
29
a Reaction conditions: Benzothiazole (0.041 g, 0.3 mmol), 4-iodotoluene (0.098 g,
In the binuclear palladium complex 5a, each palladium
center is ligated with one P-center, one C-center and the third
and fourth coordination sites are occupied by the oxygen-
atoms of two bridged acetate ligands to form a C2-symmetry
molecule (Fig. 7). Both the palladium centers are slightly
distorted from the expected square planar geometry. The five-
membered palladacycle rings containing Pd-, P- and O-atoms
are in transoid conformation.17 The eight-membered ring
containing two palladium and two acetate ligands form a boat
conformation, with both the palladium occupying apical
positions. The distance between the two palladium centers is
3.408 Å, which is longer than the sum of their van der Waals’
radii (3.26 Å), and hence not considered as a bonding
b
0.45 mmol), Pd-catalyst (0.5 mol% per Pd), base (0.45 mmol), solvent (1.0 mL).
GC yields using para-xylene as internal standard, yields in parentheses were that
of isolated compounds.
better than 2c, the newly developed complexes were
optimized and employed for the arylation of azoles with aryl
iodides using a less expensive K3PO4 base and 1.0 mol% of CuI
co-catalyst. Initially, all the complexes (2a-5a; 0.5 mol% for
mononuclear complex, 0.25 mol% for binuclear complexes)
were screened for the C–H bond arylation of benzothiazole
(
9a, 0.3 mmol) with 4-iodotoluene (10a, 0.45 mmol),
employing CuI co-catalyst in the presence of K3PO4 in DMF
(Table 1, Entries 1-9). Interestingly, the coupled product 2-(p-
tolyl)benzothiazole (11aa) could be obtained in 93% isolated
yield, when the binuclear palladacycle 4c (0.25 mol%) was
used as a catalyst in the presence of CuI and K3PO4 in DMF
interaction. The Pd
slightly longer than the Pd
complex 3a. The Pd P(1) bond length (2.1730(12) Å) is
significantly shorter than the Pd P(1) bond length (2.216(1) Å)
found in a similar palladacycle binuclear complex.3a The
Pd O(3) bond length (2.139(3) Å) trans to the Pd C is slightly
longer than the Pd O(2) bond length (2.093(3) Å) located in
‒
C(1) bond length is 2.002(4) Å, which is
‒
C bond length (1.952(3) Å) in pincer
‒
‒
(Entry 6). Notably, the palladacycles containing iPr2POCNiPr2
ligand backbone (2c and 4c) were shown superior activities
than those having Ph2POCNiPr2-backbone (2a
3a and 4a),
whereas the palladacycles with (Et2N)2POCNiPr2-backbone (2b
-
‒
‒
,
‒
,
the cis position, which could be due to the strong trans effect
3b and 4b) exhibited very poor catalytic performances (Entries
1-9). The use of Cs2CO3 base, which shown to be ideal base for
the 2c-catalyzed arylation, was less efficient with the catalyst
4c. The arylation reaction in the polar solvents like DMSO and
DMA also gave moderate to good yields of coupled product
(Entries 13-14).
of the carbon compared to the phosphorus-atom ligand. The
P‒
Pd
‒ ‒Pd‒O(3)
C bond angle is 79.51(12)o, whereas the O(2)
bond angle is a perfect right angle (90.06(11)o).
Catalytic activity of palladacycles for the arylation of azoles
Having the optimized reaction condition in hand, a number
of substituted 5-aryl oxazoles were subjected to the direct
arylation with 4-iodotoluene (SI, Table S1, Entries 2-6). Hence,
by employing 0.25 mol% of the catalyst 4c and 1.0 mol% of
CuI, the azoles containing electronically different aryl-
substituents reacted efficiently with 4-iodotoluene to give the
coupled products in good yields. Functional groups like –Cl, –
CF3 and –OMe as well as heteroarene substituent pyridine
Recently, we have shown that the pincer-based palladium
catalyst (iPr2POCNiPr2)PdCl (2c, 0.5 mol%) can be used for the
arylation of azoles with aryl iodides, employing CuI (5 mol%)
co-catalyst and a relatively expensive Cs2CO3 base.14 Further, it
was observed that the sterically less demanding catalyst 2c is
more efficient than
tBu2POCNiPr2)PdCl. Assuming that the sterically and
electronically distinct palladacycles (2a 5a) might perform
a
sterically bulky complex
(
-
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were tolerated on the azole substrates. Furthermore, the otherwise specified; chemical shifts (
RSC Advances
δ
) are given in ppm. The
versatility of the catalyst 4c in the arylation of azoles with the 1H and 13C NMR spectra are referenced to residual solvent
functionalized aryl iodide electrophiles was tested. Thus, the signals (CDCl3:
benzothiazole was very efficiently coupled with the aryl NMR chemical shifts are referenced to an external standard,
δ H = 7.26 ppm, δ
C = 77.2 ppm) and 31P{1H}
iodides bearing a variety of important functional groups, such H3PO4 in D2O solvent (
as –OMe, –F, –Cl, –CF3 and –CN (Table S1, Entries 7-11). The
δ 0.0 ppm), in a sealed capillary tube.
Synthesis of R2POCNiPr2‒H (R = Ph, 1a; R = Et2N, 1b)
tolerability of –F, –Cl and –CN functional groups in the coupled
products (11ac 11ad 11af) are significant, as they could be To the suspension of NaH (0.30 g, 12.50 mmol) in THF (10 mL)
,
,
employed for further functionalization. The aryl iodides having was added a solution of 3-(diisopropylamino)methyl phenol
ortho-substituents were also employed for the arylation, giving (2.0 g, 9.65 mmol) in THF (20 mL) and the resulting mixture
o
the desired products in moderate to good yields (Entries 14- was refluxed at 70 C for 3 h. After the reaction mixture was
16). Further, the heteroarene electrophiles also reacted with cooled to room temperature, a solution of chlorophosphine,
benzothiazole to produce the coupled products (11ap
,
11aq) in R2PCl (2.13 g, 9.65 mmol for R = Ph; 2.03 g, 9.65 mmol for R =
moderate yields.
Et2N) in THF (20 mL) was added and resulting reaction mixture
o
Though, the palladacycle catalyst 4c showed similar activity was further refluxed at 70 C for 12 h. The reaction mixture
as the previously described pincer palladium catalyst 2c for the was cooled to ambient temperature and volatiles were
arylation of azoles, the excellent performance of the catalyst evaporated under reduced pressure. The compounds were
4c in the presence of less expensive base (K3PO4) makes this extracted with n-hexane (60 mL x 3) and the combined n-
catalyst superior. The C–H bond arylation of azoles were well hexane solutions were evaporated under vacuum to obtain
precedented by in situ generated palladium catalysts oily products of R2POCNiPr2
employing high loading of the precious metal.19 However, the Ph2POCNiPr2−H (1a): Yield = 3.20 g, 85%. H-NMR (500 MHz,
well-defined palladacycle 4c catalyzes the arylation of azoles CDCl3): = 7.61-7.56 (m, 4H, Ar−H), 7.37-7.32 (m, 6H, Ar−H),
‒H.
1
δ
only with 0.25 mol% of the catalyst loading. The mechanistic 7.21 (s, 1H, Ar−H), 7.14 (dd, J = 7.9, 7.6 Hz, 1H, Ar−H), 7.01 (d, J
details for the arylation of azoles catalyzed by the palladacycle = 7.3 Hz, 1H, Ar−H), 6.95 (d, J = 6.7 Hz, 1H, Ar−H), 3.58 (s, 2H,
4c could be interesting to investigate, which is currently CH2), 2.95 (sept, J = 6.7 Hz, 2H, CH(CH3)2, 0.96 (d, J = 6.4 Hz,
underway.
12H, CH3). 13C-NMR (100 MHz, CDCl3):
Hz, Cq), 145.4 (Cq), 141.1 (d, JP−C = 18.2 Hz, 2C, Cq), 130.7 (2C,
CH), 130.5 (2C, CH), 129.6 (2C, CH), 129.0 (CH), 128.5 (d, JP−C
δ = 157.4 (d, JP−C = 9.0
=
Experimental
6.4 Hz, 4C, CH), 121.9 (CH), 118.1 (d, J = 10.9, CH), 116.6 (d, J =
10.0 Hz, CH), 48.8 (CH2), 47.9 (2C, CH(CH3)2), 20.8 (4C,
General information
CH(CH3)2). 31
P
{
1H
}
NMR (202 MHz, CDCl3):
(Et2N)2POCNiPr2−H (1b): Yield = 3.10 g, 84%. H-NMR (500
MHz, CDCl3): = 7.14 (vt, J = 7.7 Hz, 1H, Ar−H), 7.07 (s, 1H,
δ = 109.7 (s).
All manipulations were conducted under an argon atmosphere
either in a glove box or using standard Schlenk techniques in
pre-dried glass wares. The catalytic reactions were performed
in flame-dried reaction vessels with rubber septa. Solvents
were dried over Na/benzophenone or CaH2 and distilled prior
to use. DMF was dried over CaH2, distilled under vacuum and
stored over 4 Å molecular sieves. Liquid reagents were flushed
1
δ
Ar−H), 6.97 (d, J = 7.3 Hz, 1H, Ar−H), 6.83 (d, J = 7.8 Hz, 1H,
Ar−H), 3.60 (s, 2H, CH2), 3.22-3.10 (m, 4H, CH2CH3), 3.09-2.95
(m, 4H, CH2CH3; 2H, CH(CH3)2), 1.05 (t, J = 6.8 Hz, 12H,
N(CH2CH3)2), 1.01 (d, J = 6.6 Hz, 12H, CH(CH3)2). 13C-NMR (100
MHz, CDCl3):
δ = 155.8 (d, JP−C = 7.7 Hz, Cq), 144.9 (Cq), 128.7
with argon prior to use. The POCN-H ligand 1c 14
3-
,
(CH), 121.3 (CH), 118.9 (d, JP−C = 10.8 Hz, CH), 117.4 (d, JP−C
=
((diisopropylamino)methyl)phenol,14 and 5-aryl azoles20 were
synthesized according to previously described procedures. All
other chemicals were obtained from commercial sources and
were used without further purification. Yields refer to isolated
compounds, estimated to be > 95% pure as determined by 1H-
NMR. TLC: TLC Silica gel 60 F254. Detection under UV light at
254 nm. Chromatography: Separations were carried out on
Spectrochem silica gel (0.120-0.250 mm, 60-120 mesh, for
organic compounds) and neutral alumina (for inorganic
complexes). High resolution mass spectroscopy (HRMS) mass
spectra were recorded on a Thermo Scientific Q-Exactive,
Accela 1250 pump and MALDI-TOF mass spectra on AB SCIEX
TOF/TOFTM 5800/4800 plus. M. p.: Büchi 540 capillary melting
point apparatus, values are uncorrected. NMR (1H and 13C)
spectra were recorded at 400 or 500 (1H), 100 or 125 {13C,
DEPT (distortionless enhancement by polarization transfer)},
377 (19F) and 162 or 202 MHz (31P{1H}), respectively on Bruker
AV 400 and AV 500 spectrometers in CDCl3 solutions, if not
9.3 Hz, CH), 48.9 (CH2), 47.8 (2C, CH(CH3)2), 39.1 (d, JP−C = 19.3
Hz, 4C, CH2CH3), 20.9 (4C, CH(CH3)2), 14.8 (d, JP−C = 3.1 Hz, 4C,
CH2CH3). 31 1H
P{ }NMR (202 MHz, CDCl3): δ = 131.8 (s).
Synthesis of (R2POCNiPr2)PdCl (R = Ph, 2a; R = Et2N, 2b)
A mixture of Pd(COD)Cl2 (0.058 g, 0.203 mmol for compound
2a; 0.15 g, 0.525 mmol for compound 2b), appropriate amount
of (R2POCNiPr2
)‒H (0.079 g, 0.203 mmol of 1a; 0.20 g, 0.525
mmol of 1b) and K3PO4 (0.047 g, 0.223 mmol for compound
2a; 0.122 g, 0.577 mmol for compound 2b) was taken in a
schlenk flask and 1,4-dioxane (20 mL) was added into it. The
o
reaction mixture was heated at 70 C for 2 h under argon
atmosphere. The yellow suspension formed was cooled to
ambient temperature, filtered through cannula and the
volatile were evaporated under reduced pressure. The crude
product was purified by column chromatography using neutral
alumina (n-hexane/EtOAc : 10/1). The X-ray quality single
8 | RSC Adv., 2015, 00, 1-3
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crystals were obtained by the slow evaporation of n-hexane 496.1594. Anal. calcd for C27H32NO3PPd: C, 58.33; H, 5.80; N,
solution of the compounds at room temperature. 2.52. Found: C, 58.42; H, 6.20; N, 1.84.
Ph2POCNiPr2)PdCl (2a): Yield = 0.019 g, 18%. M. p. = 209 oC. (Et2N)2POCNiPr2)Pd(OAc)
Synthesis of
1H-NMR (500 MHz, CDCl3):
= 8.03-7.99 (m, 4H, Ar−H), 7.52- representative procedure was followed, using 2b (0.058 g,
7.44 (m, 6H, Ar−H), 6.98 (dd, J = 7.6, 7.3 Hz, 1H, Ar−H), 6.72 (d, 0.111 mmol) and AgOAc (0.022 g, 0.133 mmol). Yield = 0.040 g,
(
(
(3b):
The
δ
o
1
J = 7.6 Hz, 1H, Ar−H), 6.68 (d, J = 7.3 Hz, 1H, Ar−H), 4.14 (s, 2H, 66%. M. p. = 86 C. H-NMR (500 MHz, CDCl3):
CH2), 3.61 (apparent octet, J = 6.1 Hz, 2H, CH(CH3)2), 1.69 (d, J 7.7, 1H, Ar H), 6.59 (d, J = 7.5, 1H, Ar H), 6.53 (d, J = 7.7 Hz,
= 6.1 Hz, 6H, CH(CH3)2), 1.23 (d, J = 6.1 Hz, 6H, CH(CH3)2). 13C- 1H, Ar
H), 4.02 (s, 2H, CH2), 3.38 (apparent octet, J = 6.2 Hz,
NMR (100 MHz, CDCl3): = 162.1 (d, JP-C = 10.0 Hz, Cq), 153.6 2H, CH(CH3)2), 3.32-3.21 (m, 8H, CH2CH3), 1.92 (s, 3H, COCH3),
δ = 6.93 (vt, J =
‒
‒
‒
δ
(Cq), 144.5 (Cq), 133.8 (d, JP-C = 54.0 Hz, 2C, Cq), 132.0 (4C, CH), 1.53 (d, J = 6.4 Hz, 6H, CH(CH3)2), 1.23 (d, J = 6.4 Hz, 6H,
131.9 (2C, CH), 129.0 (d, JP−C = 13.2 Hz, 4C, CH), 126.8 (CH), CH(CH3)2), 1.13 (t, J = 7.0 Hz, 12H, CH2CH3). 13C-NMR (125 MHz,
115.4 (CH), 109.2 (d, JP−C = 17.7 Hz, CH), 61.6 (CH2), 57.6 (2C, CDCl3):
δ = 175.9 (CO), 157.7 (d, JP−C = 14.3 Hz, Cq), 153.6 (d, JP−C
CH(CH3)2), 22.8 (2C, CH(CH3)2), 19.7 (2C, CH(CH3)2). = 1.9 Hz, Cq), 140.6 (Cq), 126.3 (CH), 114.6 (CH), 108.0 (d, JP−C
31P
=
{
calcd for C25H29ClNOPPd
1H
}
NMR (202 MHz, CDCl3):
δ
= 150.0 (s). MALDI-TOF: m/z 19.1 Hz, CH), 60.4 (d, JP−C = 2.9 Hz, CH2), 56.8 (d, JP−C = 2.9 Hz,
Cl]+ 496.1022; found 2C, CH(CH3)2), 40.4 (d, JP−C = 10.5 Hz, 4C, CH2CH3), 24.7
‒
Cl+ [M
‒
496.1617. Anal. calcd for C25H29ClNOPPd: C, 56.40; H, 5.49; N, (COCH3), 21.9 (2C, CH(CH3)2), 19.3 (2C, CH(CH3)2), 14.4 (d, JP−C
2.63. Found: C, 55.84; H, 5.41; N, 2.23. NMR (202 MHz, CDCl3): = 142.3
2.9 Hz, 4C, CH2CH3). 31P 1H
(Et2N)2POCNiPr2)PdCl (2b): Yield = 0.079 g, 29%. M. p. = 105 (s). MALDI-TOF: m/z calcd for C23H42N3O3PPd OAc+ [M OAc]+
= 6.94 (dd, J = 7.9, 7.6 Hz, 1H, 486.1866; found 486.2420. Anal. calcd for C23H42N3O3PPd: C,
=
{
}
δ
(
‒
‒
1
oC. H-NMR (500 MHz, CDCl3):
δ
Ar−H), 6.63 (d, J = 7.3 Hz, 1H, Ar−H), 6.55 (d, J = 7.9 Hz, 1H, 50.60; H, 7.75; N, 7.70. Found: C, 52.37; H, 8.35; N, 6.80.21
Ar−H), 4.01 (s, 2H, CH2), 3.52 (apparent octet, J = 6.4 Hz, 2H,
Procedure for synthesis of [κP κC-4-iPr2NCH2-C6H3-Pd(ꢀ-Cl)-(2-
,
CH(CH3)2), 3.36-3.22 (m, 8H, CH2CH3), 1.62 (d, J = 6.4 Hz, 6H,
CH(CH3)2), 1.17-1.13 (m, 6H, CH(CH3)2; 12H, (CH2CH3)4). 13C-
OPR2)]2 (R = Ph, 4a; R = Et2N, 4b; R = iPr, 4c)
NMR (100 MHz, CDCl3):
(d, JP−C = 2.3 Hz, Cq), 143.1 (d, JP−C = 2.3 Hz, Cq), 126.6 (CH), 1.93 mmol for 4b; 0.088 g, 0.308 mmol for 4c), (R2POCNiPr2
δ = 157.5 (d, JP−C = 15.4 Hz, Cq), 153.6 A mixture of Pd(COD)Cl2 (0.306 g, 1.07 mmol for 4a; 0.550 g,
)‒H
115.0 (CH), 108.2 (d, JP−C = 19.3, CH), 60.7 (d, JP−C = 2.3 Hz, CH2), (0.420 g, 1.07 mmol of 1a; 0.735 g, 1.93 mmol of 1b; 0.100 g,
57.0 (d, JP−C = 3.1 Hz, 2C, CH(CH3)2), 40.3 (d, JP−C = 9.3 Hz, 4C, 0.309 mmol of 1c) and Et3N (0.18 mL, 1.28 mmol for 4a; 0.32
CH2CH3), 22.6 (2C, CH(CH3)2), 19.5 (2C, CH(CH3)2), 14.4 (d, JP−C
2.3 Hz, 4C, CH2CH3). 31P 1H
NMR (202 MHz, CDCl3): = 142.3 in a schlenk flask and 1,4-dioxane (30 mL) was added into it.
(s). MALDI-TOF: m/z calcd for C21H39ClN3OPPd
Cl+ [M
486.1866; found 486.2410. Anal. calcd for C21H39ClN3OPPd: C, atmosphere. The yellow suspension formed was cooled to
=
mL, 2.30 mmol for 4b; 0.05 mL, 0.370 mmol for 4c) was taken
{
}
δ
o
‒
‒
Cl]+ The reaction mixture was heated at 70 C for 2 h under argon
48.28; H, 7.53; N, 8.04. Found: C, 48.22; H, 7.60; N, 7.62.
ambient temperature and the volatile were evaporated under
reduced pressure. The compounds was purified by column
chromatography on neutral alumina (n-hexane:EtOAc / 10:1)
Representative procedure for synthesis of (R2POCNiPr2)Pd(OAc)
Synthesis of (Ph2POCNiPr2)Pd(OAc) (3a): To the mixture of to yielded the desired complexes (4a
2,6-(Ph2PO)(C6H3)(CH2-NiPr2)PdCl, 2a (0.020 g, 0.038 mmol) solid.
and AgOAc (0.008 g, 0.046 mmol) was added THF (10 mL) and
κP
the reaction mixture was stirred at room temperature for 3 h. 0.138 g, 24%. M.p. = 114-116 oC. 1H-NMR (500 MHz, CDCl3):
, 4b, 4c) as light yellow
[
, ꢀ-Cl)-(2-OPPh2)]2 (4a): Yield =
κC-4-iPr2NCH2-C6H3-Pd(
δ
=
The solvent was evaporated under reduced pressure and the 8.02-7.76 (m, 8H, Ar−H), 7.57-7.35 (m, 14H, Ar−H), 6.99 (br s,
compound was extracted with Et2O (10 mL x 3). Upon 2H, Ar−H), 6.83 (br s, 2H, Ar−H), 3.55 (s, 4H, CH2), 3.00 (br s,
evaporation of diethyl ether, the compound 3a was obtained 4H, CH(CH3)2), 1.00 (br s, 24H, CH(CH3)2). 13C-NMR (100 MHz,
as a light yellow solid. The compound 3a was recrystallized CDCl3):
δ = 164.3 (2C, Cq), 143.3 (2C, Cq), 135.8 (2C, CH), 132.8
from n-hexane solution by slow evaporation to obtain X-ray (4C, Cq), 132.5 (4C, CH), 132.3 (d, JP−C = 13.6 Hz, 8C, CH), 132.1
quality single crystals. Yield = 0.013 g, 62%. M. p. = 182 oC. 1H- (2C, Cq), 129.0 (d, JP−C = 10.9 Hz, 8C, CH), 122.0 (2C, CH), 111.4
NMR (400 MHz, CDCl3):
(m, 6H, Ar−H), 6.92 (vt, J = 7.8 Hz, 1H, Ar−H), 6.61 (d, J = 7.3 Hz, 31P
1H, Ar−H), 6.57 (d, J = 8.1 Hz, 1H, Ar−H), 4.16 (s, 2H, CH2), 3.53 (s,
δ = 7.90-7.84 (m, 4H, Ar−H), 7.49-7.40 (2C, CH), 48.7 (2 CH2), 47.9 (4C, CH(CH3)2), 20.9 (8C, CH(CH3)2).
{
1H
minor,
}
NMR (202 MHz, CDCl3):
δ
43%). MALDI-TOF:
= 152.9 (s, major, 57%), 152.2
m/z calcd for
(apparent octet, J = 6.1 Hz, 2H, CH(CH3)2), 1.86 (s, 3H, COCH3), C50H58Cl2N2O2P2Pd2+H+ [M+H]+ 1063.1498; found 1063.3624.
1.60 (d, J = 6.4 Hz, 6H, CH(CH3)2), 1.28 (d, J = 6.6 Hz, 6H, Anal. calcd for C50H58Cl2N2O2P2Pd2: C, 56.40; H, 5.49; N, 2.63.
CH(CH3)2). 13C-NMR (100 MHz, CDCl3):
JP−C = 11.0 Hz, Cq), 153.2 (Cq) 143.0 (Cq), 135.4 (d, JP−C = 58.6 Hz,
2C, Cq), 132.7 (d, JP−C = 13.4 Hz, 4C, CH), 131.2 (d, JP−C = 2.3 Hz, = 0.320 g, 32%. M. p. = 188-190 oC. H-NMR (500 MHz, CDCl3):
2C, CH), 128.3 (d, JP−C = 12.1 Hz, 4C, CH), 126.4 (CH), 114.5 = 7.47 (dd, J = 6.8, 6.5 Hz, 2H, Ar−H), 6.89-6.77 (m, 4H, Ar−H),
δ
= 175.0 (CO), 162.3 (d, Found: C, 55.22; H, 5.15; N, 2.25.21
[
κP κC-4-iPr2NCH2-C6H3-Pd(
,
ꢀ
-Cl)-(2-OP(NEt2)2)]2 (4b): Yield
1
δ
(CH), 108.6 (d, JP−C = 16.6 Hz), 61.4 (CH2), 57.3 (2C, CH(CH3)2), 3.55 (s, 4H, CH2), 3.48-3.34 (m, 8H, CH2CH3), 3.29-3.17 (m, 8H,
31.1 (COCH3), 22.2 (2C, CH(CH3)2), 19.5 (2C, CH(CH3)2). CH2CH3), 3.04-2.96 (m, 4H, CH(CH3)2), 1.17 (t, J = 7.0 Hz, 24H,
31P
{
calcd for C27H32NO3PPd
1H
}
NMR (202 MHz, CDCl3):
δ
= 143.8 (s). MALDI-TOF: m/z CH2CH3), 1.00 (d, J = 6.0, 24H, CH(CH3)2). 13C-NMR (100 MHz,
‒
OAc+ [M
‒
OAc]+ 496.1022; found CDCl3):
δ
= 158.3 (d, JP−C = 18.1 Hz, 2C, Cq), 142.4 (2C, Cq), 136.1
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(2C, CH), 132.5 (2C, Cq), 121.5 (2C, CH), 110.7 (d, JP−C = 20.9, 2C, 7.15 (t, J = 7.9 Hz, 1H, Ar−H), 7.07 (d, J = 7.3 Hz, 1H, Ar
CH), 48.7 (2 CH2), 47.8 (4C, CH(CH3)2), 40.5 (d, JP−C = 9.1 Hz, 8C, (br s, 1H, Py−H), 6.40 (br s, 2H, Py−H), 3.55 (s, 2H, CH2), 2.96
CH2CH3), 20.9 (8C, CH(CH3)2), 14.3 (8C, CH2CH3). 31P 1H
NMR (sept, J = 6.7 Hz, 2H, NCH(CH3)2), 2.86 (app octet, J = 7.6 Hz,
(202 MHz, CDCl3): = 137.7 (s, major, 76%), 137.9 (s, minor, 2H, PCH(CH3)2), 1.48 (dd, J = 18.0, 7.3 Hz, 6H, PCH(CH3)2), 1.40
24%). MALDI-TOF: m/z calcd for C42H78Cl2N6O2P2Pd2+H+ [M+H]+ (dd, J = 16.2, 7.0 Hz, 6H, PCH(CH3)2), 0.95 (d, J = 6.7, 12H,
1043.3186; found 1043.4058. Anal. calcd = 155.5 (d, JP−C = 8.6
for NCH(CH3)2). 13C-NMR (125 MHz, C6D6):
C42H78Cl2N6O2P2Pd2: C, 48.28; H, 7.53; N, 8.04. Found: C, 47.62; Hz, Cq), 151.6 (CH), 145.7 (Cq), 137.9 (CH), 129.4 (CH), 128.7
‒H), 6.67
{
}
δ
δ
H, 7.59; N, 6.86.21
κP κC-4-iPr2NCH2-C6H3-Pd(
(2C, CH), 124.3 (CH), 123.7 (CH), 120.6 (d, JP−C = 5.7 Hz, CH),
[
,
ꢀ
-Cl)-(2-OPiPr2)]2 (4c): Yield: 118.9 (d, JP−C = 5.7 Hz, CH), 49.5 (CH2), 48.5 (2C, NCH(CH3)2),
0.102 g, 71%. M.p. = 153-154 oC. 1H-NMR (400 MHz, CDCl3):
7.60-7.36 (m, 2H, Ar H), 6.90-9.74 (m, 4H, Ar
4H, CH2), 3.01 (br s, 4H, N{CH(CH3)2}2), 2.41 (br s, 4H, MHz, C6D6):
δ
=
31.5 (d, JP−C = 31.5 Hz, 2C, PCH(CH3)2), 21.3 (4C, NCH(CH3)2),
H), 3.54 (br s, 19.2 (2C, PCH(CH3)2), 18.0 (2C, PCH(CH3)2). 31P {1H}-NMR (202
= 144.4 (s). HRMS (ESI): m/z calcd for
‒
‒
δ
P{CH(CH3)2}2), 1.44 (d, J = 12.7 Hz, 12H, PCH(CH3)2), 1.31 (dd, J C24H39Cl2N2OPPd+H [M+H]+ 579.1285; Found 579.1282.
= 15.9, 6.9 Hz, 12H, PCH(CH3)2), 1.01 (br s, 24H, NCH(CH3)2).
13C-NMR (100 MHz, CDCl3):
δ = 165.5 (d, JP−C = 6.2 Hz, 2C, Cq),
142.8 (2C, Cq), 135.8 (2C, CH), 131.9 (2C, Cq), 121.6 (2C, CH),
Representative procedure for the arylation of azoles: synthesis of
2-(p tolyl)benzo[d]thiazole (11aa)
110.5 (d, JP−C = 16.2 Hz, 2C, CH), 48.7 (2C, CH2), 47.8 (4C,
N{CH(CH3)2}2), 29.5 (d, JP−C = 29.3 Hz, 4C, P{CH(CH3)2}2), 20.9 To a flame-dried screw-capped Schlenk tube equipped with
(4C, PCH(CH3)2), 17.8 (4C, PCH(CH3)2), 17.0 (8C, NCH(CH3)2). 31
magnetic stir bar was introduced CuI in CH3CN [0.0006 g, 0.003
{1H}-NMR (202 MHz, CDCl3):
= 200.0 (s, major, 65%), 199.0 (s, mmol, 1.0 mol%, 0.1 mL from the stock solution (0.036 g in
P
δ
minor, 35%). MALDI-TOF: m/z calcd for C38H66Cl2N2O2P2Pd2+H CH3CN (6.0 mL))] and the solvent was evaporated to dryness
[M+H]+ 927.2124; found 927.3241. Anal. calcd for under vacuum. Then, 4-iodotoluene 10a (0.098 g, 0.45 mmol),
C38H66Cl2N2O2P2Pd2: C, 49.15; H, 7.16; N, 3.02. Found: C, 49.12; K3PO4 (0.096 g, 0.45 mmol) and benzothiazole 9a (0.041 g, 0.30
H, 7.34; N, 2.69.
mmol) were added under argon. The screw-capped Schlenk
tube with the mixture was then evacuated and refilled with
argon. To the above mixture was added Pd-catalyst 4c
Synthesis of [κP κC-4-iPr2NCH2-C6H3-Pd(ꢀ-OAc)-(2-OPPh2)]2 (5a)
,
To the mixture of 4a (0.040 g, 0.038 mmol) and AgOAc (0.014 (0.00075 mmol, 0.25 mol% (0.5 mol% per Pd), 1.0 mL of
g, 0.084 mmol) was added THF (10 mL) and the reaction 0.00075 M stock solution in DMF) in DMF under argon. The
o
mixture was stirred at room temperature for 3 h. The solvent resultant reaction mixture was then stirred at 120 C in a pre-
was evaporated under reduced pressure and the compound heated oil bath for 16 h. At ambient temperature, H2O (10 mL)
was extracted with Et2O (10 mL x 3). Upon evaporation of was added and the reaction mixture was extracted with EtOAc
diethyl ether, the compound 5a was obtained as a light yellow (15 mL x 3). The combined organic layers were dried over
solid. The compound 5a was further recrystallized from n- MgSO4 and the solvent was evaporated in vacuo. The
hexane solution to obtain X-ray quality crystals at room remaining residue was purified by column chromatography on
o
1
temperature. Yield = 0.024 g, 57%. M. p. = 192 C. H-NMR silica gel (n-hexane/EtOAc 30/1→ 20/1) to yield 11aa (0.063 g,
(400 MHz, CDCl3): = 7.89-7.70 (m, 6H, Ar−H), 7.51-7.04 (m, 93%) as an off-white solid.
16H, Ar−H), 6.84 (br s, 2H, Ar−H), 6.66 (d, J = 7.6 Hz, 2H, Ar−H), All the isolated coupled products
δ
(
11aa
-
11aq
)
were
3.55 (s, 4H, CH2), 3.05 (sept, J = 6.4 Hz, 4H, CH(CH3)2), 2.17 (s, characterized by 1H and 13C-NMR techniques and well
6H, COCH3), 1.04 (d, J = 6.4 Hz, 24H, CH(CH3)2). 13C-NMR (100 compared with the literature reports.
MHz, CDCl3):
141.9 (2C, Cq), 134.7 (2C, CH), 132.6-131.2 (m, 20C), 128.6-
128.3 (m, 6C), 121.8 (2C, CH), 111.0 (2C, CH), 48.7 (2C, CH2), X-ray intensity data measurements of compounds 2a
47.6 (4C, CH(CH3)2), 25.4 (2C, COCH3), 21.0 (8C, CH(CH3)2). 3b and 5a were carried out on a Bruker SMART APEX II CCD
δ = 181.4 (2C, Cq), 164.5 (d, JP−C = 14.6 Hz, 2C, Cq),
X-ray structure determination
,
2b, 3a,
31P 1H
{ }NMR (202 MHz, CDCl3): δ = 151.0 (s). HRMS (ESI): m/z diffractometer with graphite-monochromatized (MoKα=
calcd for C54H66N2O6P2Pd2+H [M+H]+ 1113.2539; found 0.71073 Å) radiation between 150(2) - 296 (2) K. The X-ray
1113.2501. Anal. calcd for C54H66N2O6P2Pd2: C, 58.23; H, 5.97; generator was operated at 50 kV and 30 mA. A preliminary set
N, 2.51. Found: C, 57.69; H, 6.02; N, 2.13.
of cell constants and an orientation matrix were calculated
from three sets of 12 frames (total 36 frames). Data were
Synthesis of {κP-(3-iPr2NCH2)-C6H4-(2-OPiPr2)}(Py)PdCl2 (8c)
collected with
ω scan width of 0.5° at eight different settings
and 2 with a frame time of 10 sec keeping the sample-
A mixture of Pd(COD)Cl2 (0.044 g, 0.154 mmol), 1c (0.050 g, of
ϕ
θ
0.155 mmol) and pyridine (0.015 mL, 0.185 mmol) was taken to-detector distance fixed at 5.00 cm for all the compounds.
in a schlenk flask and 1,4-dioxane (10 mL) was added into it. The X-ray data collection was monitored by APEX2 program
The reaction mixture was stirred at room temperature for 1 h (Bruker, 2006).22 All the data were corrected for Lorentzian,
under argon atmosphere and the volatiles were evaporated polarization and absorption effects using SAINT and SADABS
under reduced pressure to obtain light-yellow compound of programs (Bruker, 2006). SHELX-97 was used for structure
δ
8c. Yield: 0.065 g, 73%. 1H-NMR (500 MHz, C6D6): = 9.08 (br s, solution and full matrix least-squares refinement on F2.23
2H, Py−H), 7.99 (s, 1H, Ar−H), 7.62 (d, J = 7.3 Hz, 1H, Ar−H), Hydrogen atoms were placed in geometrically idealized
10 | RSC Adv., 2015, 00, 1-3
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ARTICLE
2048-2076; (j) K. J. Szabó Top Organomet. Chem., 2013, 40
203-242.
,
position and constrained to ride on their parent atoms. CCDC-
1063772 (2a), CCDC-1063769 (2b), CCDC-1063773 (3a), CCDC-
3. For some specific examples, see: (a) W. A. Herrmann, C.
Brossmer, K. Öfele, C.-P. Reisinger, T. Priermeier, M. Beller
and H. Fischer, Angew. Chem. Int. Ed. Engl., 1995, 34, 1844-
1848; (b) M. Beller, H. Fischer, W. A. Herrmann, K. Öfele and
C. Brossmer, Angew. Chem. Int. Ed. Engl., 1995, 34, 1848-
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1063770
(
3b
)
and CCDC-1063774
(
5a
)
contain the
supplementary crystallographic data.
Conclusions
In summary, two unsymmetrical POCN ligands, and a series of
mononuclear pincer palladium complexes and chloro-bridged
binuclear palladacycles have been synthesized via the
M. Sugiya, T. Imamoto and W. Zhang, Org. Lett., 2013, 15
,
regioselective C
Thus, the palladation reactions of the POCN
isopropyl substituents on N-atom produced “PC”-palladacycles
4a 4c in the presence of Et3N, whereas the same ligands in the
presence of K3PO4 or pyridine afford “POCN”-pincer palladium
complexes; via the selective activation of the arene C(4) H and
C(2) H bond, respectively. Mechanistic studies on the C
‒
H bond activation of the POCN
‒H ligands.
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‒
‒H
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Acknowledgements
This work was financially supported by DST, New Delhi
(SR/S1/IC-42/2012) and CSIR-New Delhi (Network project
ORIGIN, CSC0108). D.K.P. thanks UGC, New Delhi for a
research fellowship. We thank Dr. P. R. Rajmohanan for NMR
facility and Dr. (Mrs) Shanthakumari for HRMS analyses.
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12 | RSC Adv., 2015, 00, 1-3
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DOI: 10.1039/C5RA18289A
Monoꢀ and binuclear palladacycles via regioselective C‒H
bond activation: syntheses, mechanistic insights and catalytic
activity in direct arylation of azoles
Dilip K. Pandey,a Shrikant M. Khake,a Rajesh G. Gonnadeb and Benudhar Punji∗a
aOrganometallic Synthesis and Catalysis Group, Chemical Engineering Division, CSIR–National Chemical
Laboratory (CSIR–NCL), Dr. Homi Bhabha Road, Pune – 411 008, Maharashtra, INDIA. Phone: + 91-20-
2590 2733, Fax: + 91-20-2590 2621, E-mail: b.punji@ncl.res.in
bCentre for Material Characterization, CSIR–National Chemical Laboratory (CSIR–NCL), Dr. Homi
Bhabha Road, Pune – 411 008, Maharashtra, INDIA
Table of contents entry
Hybrid “POCNˮꢀligated monoꢀ and binuclear palladacycles have been synthesized via the
baseꢀassisted regioselective C‒H bond activation, and their mechanistic aspects and
catalytic application for the arylation of azoles have been described.