Synthesis of (NHC)Pd(salicylaldimine)Cl complexes through template-directed ortho-aromatic metaloxylation of NHC-palladacycles derived from arylimines

Article abstract of DOI:10.1039/c7dt00007c

Reactions of imidazoliums with palladacycles afforded NHC-palladacycle complexes. The reactivity of the NHC-palladacycles was explored and the results demonstrated that they could be template-directed ortho-aromatic metaloxylated in the presence of tert-b

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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ARTICLE
Synthesis of (NHC)Pd(salicylaldimine)Cl complexes through
template-directed ortho-aromatic metaloxylation of NHC–
palladacycles derived from arylimines
Received 00th January 20xx,
Accepted 00th January 20xx
Jin Yang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reactions of imidazoliums with palladacycles afforded NHC–palladacycle complexes. The reactivity of
the NHC–palladacycles was explored and the results demonstrated that they could be template-
directed ortho-aromatic metaloxylated in the presence of tert-butyl hydroperoxide (TBHP), affording
(NHC)Pd(salicylaldimine)Cl complexes. Furthermore, the NHC–palladacycles showed efficient
catalytic activities toward ortho-aromatic hydroxylation of arylimines.
been described. It was deemed worthwhile to investigate the
reactivity of NHC–palladacycle, so that this methodology could
Introduction
Since the first stable N-heterocyclic carbene (NHC) was
isolated in 1991 by Arduengo, NHCs have been extensively
employed as supporting ligands in transition-metal-catalysed
reactions.1 In particular, NHC–Pd complexes have been
developed as highly active catalysts for coupling reactions.2
Many groups have focused on the development of more
efficient NHC–Pd catalysts.3 Based on this, there has been a
growing interest in the use of ancillary ligands to modify NHC–
Pd complexes due to the inherent hybrid character of the
ligands would be electronically more tunable to the needs of
the substrates. Various heteroatom donors (N, O, P, and S)4,
allyl,5 cinnamyl,6 as well as bidentate chelating ligands (N^N,7
N^O,8 O^O,9 C^O,10 C^N,11 and C^P12) have been involved in
the coordination with NHC–Pd complexes. Among them,
cyclopalladation of C,N-chelating ligands has gained much
attention.13 Pioneering works by Nolan’s group shown NHC–
palladacycles derived from N,N`-dimethyl-2-biphenylamine.11a
In a similar fashion, NHC–palladacycles derived from 2-
be extended to broader application. Herein, we reported the
synthesis of NHC–palladacycles derived from arylimines and
the template-directed ortho-aromatic metaloxylation of the
NHC–palladacycles in the presence of tert-butyl hydroperoxide
(TBHP), affording (NHC)Pd(salicylaldimine)Cl complexes
(Scheme 1). Further exploration of the catalytic potential of
the NHC–palladacycles for ortho-aromatic hydroxylation of
arylimines has been investigated, and the complexes exhibited
good catalytic activities to afford salicylaldimines.
R2
R1
R1
R1
R1
R4
R1
N
N
N
N
R2
R2
R1
R1
R1 R1
R2
R2
N
R1
Cl
R1
R3
Cl
O
Cl
R3
Pd
Cl
Pd
N
ꢀꢀ
ꢀꢀ
K2CO3
TBHP
toluene
N
R1
R2
N
R3
N
R3
+
toluene
Pd
R3
R3
2a-2d
2
R4
1a 1d
R4
-
1
and : R1 = R2 = R3 = R4 = Me;
and : R = R2 = Me; R3 = i-Pr; R4 = H
1a
1c
2a
1b
2b
1
R1 = i-Pr; R2 = H; R3 = R4 = Me;
and : R
=
R3 = i-Pr; R2 = R4 =H
2c
:
1d
2d
and
Scheme 1 Overview the synthesis and functionalization of NHC–palladacycles
phenylpyridine,11c
N,N`-dimethylbenzylamine,11d
and
ferrocenylimine11g have been developed. The construction of
NHC–palladacycles has aroused more interest due to their
stable coordination geometries and unique properties. In
order to obtain more information on this process, we focused
our attention on the development of NHC–palladacycles
derived from arylimines, which enriches the already versatile
NHC chemistry. In addition, to the best of our knowledge,
further functionalization of the NHC–palladacycles has not
Results and discussion
Synthesis and Characterization of the Complexes
The NHC–palladacycles (1a–1d) could be prepared by reactions
of imidazoliums with palladacycles [Pd{C6H4CH=N–R}(μ-Cl)]2 in
the presence of excess K2CO3 in toluene. The formation of the
complexes is evident from stoichiometric proton signal
1
resonances of NHCs and palladacycles in H NMR spectra. The
imino proton signals (–HC=N) appear in a range of 7.70–7.78
ppm, similar to the related signals in the palladacycles derived
from arylimines.14 The chemical shifts of Pd–Ccarbene peaks
which appear in the range 158.7–160.2 ppm in 13C NMR
spectra are compared with other NHC–Pd analogues.
a.Department of Chemistry, Huaibei Normal University, Huaibei, 235000, P R China;
E-mail: yangjinlz@163.com
†
Electronic supplementary information (ESI) available: Crystallographic data, 1H
and 13C NMR spectra of the compounds. CCDC reference numbers
1007359−1007366. See DOI: 10.1039/b000000x/.
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The structures of 1a–1d were further confirmed by single- to tune the electronic property of the coordination sphere and
crystal X-ray diffraction. As shown in Fig. 1, all of the modulate the chelation ability of the complex. Benzylidene-
complexes crystallized as cis configuration, in which each arylamines have a similar property with 2-phenylpyridine, the
palladium centres was surrounded by an NHC, a C^N-chelating presence of an NHC bonding with palladium centre can make
ligand, and a chloride in a slightly distorted square planar the C^N chelating ligands in the palladacycle potentially
fashion. The Pd–Ccarbene bonds [1.9773(19)–1.993(3) Å] and Pd– hemilabile. Herein, NHC–palladacycles
1a–
1d
were
Cphenyl bonds [1.988(3)–2.015(2) Å] were within the expected investigated for ortho-aromatic metaloxylation. To our
range, relative to other NHC–palladacycles.11 Notably, the Pd– pleasure, when complex 1a was refluxed in toluene in the
N bonds in 1a
slightly longer than that observed in NHC-palladacycle derived yellow to yellow was observed. Indeed, the ortho-aromatic
from 2-phenylpyridine,11c but were slightly shorter than N,N`- metaloxylation
occurred with 1a affording
–1d ranging from 2.089(2) to 2.0980(17) Å were presence of TBHP over 8 h, a change of colour from pale-
,
dimethyl-2-biphenylamine and N,N`-dimethylbenzylamine (NHC)Pd(salicylaldimine)Cl complex 2a. The effect of oxidant
analogues,11a,11d which indicated the moderate chelation sources was further investigated. Several oxidant sources were
between the palladium centres and the arylimine ligands. The screened in the reaction. It was found that C6H5I(OAc)2, K2S2O8
five membered chelate rings were oriented approximately and H2O2 were much inferior and generated the product in
perpendicularly to the mesityl and diisopropylphenyl planes lower yields (less than 20%). Other oxidants, such as (t-
with the smallest dihedral angle of 76.05o, and were quasi- C4H9O)2, dicumyl peroxide and O2 were no longer effective in
coplanar with the coordination planes with the biggest this reaction and no product was isolated. The scope of the
dihedral angle of 3.44o. The dihedral angles between the reaction for NHC–palladacycles 1b
carbene ring planes and the coordination planes varied from and the corresponding products 2b
67.45 to 77.88°, which were typical for NHC complexes to structures of 2a 2d were confirmed by NMR spectra, high-
–
1d was further investigated
2d were achieved. The
–
–
relieve steric congestion.
resolution mass spectra, and single-crystal X-ray diffraction.
1
The H NMR spectra showed singlets at 7.44–7.52 ppm for
imine protons, which shifted slightly upfield compared to 1a
–
1d. The 13C NMR spectra showed sharp singlets (160.3–162.8
ppm) for the carbene carbons, which shifted slightly downfield
relative to 1a–1d. The high-resolution mass spectra of 2a–2d
showed predominant signals for their [M+H+]+ fragments and
subordinate signals for their [M−Cl–]+ fragments.
Fig. 1 ORTEP diagrams of 1a–1d with thermal displacement parameters drawn at
30% probability. Hydrogen atoms have been omitted for clarity.
Reactivity studies of the NHC–palladacycles 1a–1d
Palladium-mediated σ-chelation directed C–H activation is the
attractive process in organic chemistry, in which palladacycles
have been thought to be the important intermediates.15 Jiao’s
group has reported the palladium catalysed ortho-
hydroxylation of 2-phenylpyridine via C–H activation, in which,
the palladacycle derived from 2-phenylpyridine was proposed
to be the reaction intermediate.16 However, when the
palladacycle was directly employed in the reaction, no desired
product was formed. The low solubility of the palladacycle or
the strong chelation between the palladium and substrate may
prevent further hydroxylation of 2-phenylpyridine. However,
Templeton has reported the ortho-aromatic metaloxylation of
Fig. 2 ORTEP diagrams of 2a
–2d with thermal displacement parameters drawn at
30% probability. Hydrogen atoms have been omitted for clarity.
The identities of 2a–2d were confirmed by single-crystal X-ray
diffraction. As shown in Fig. 2, all of the complexes crystallize
as trans configuration and each palladium centres adopt a
characteristic, slightly distorted square planar geometry, which
are coordinated by an NHC, a N^O-chelating ligand, and a
chloride. In order to satisfy the square-planar geometry of the
palladium centres, the salicylaldimine units are slightly twisted
with the dihedral angles between the six membered chelate
rings and the coordination planes are in the range 1.31–8.96o.
[Rh(Cp*)(2-phenylpyridine)
and
it
catalysed
ortho-
hydroxylation of 2-phenylpyridine.17 The introduction of an
ancillary ligand into metal centre could be an effective strategy
2 | J. Name., 2017, 00, 1-3
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Table 1. NHC–palladacycles catalysed ortho-hydroxylation of aryliminesa
The Pd–C bonds [1.977(2)–2.007(6) Å] and Pd–N bonds
[2.0521(17)–2.102(5) Å] in 2a 2d are within the expected
range, relative to NHC–palladacycles 1a 1d. The carbene ring
–
–
planes are twisted out of the adjacent coordination planes
with the dihedral angles ranging from 72.35 to 86.63°.
The oxygen atom inserted into the Pd−C bonds of the
Entry
1
Catalyst
1a
1b
1a
1b
1c
Additive
--
Arylimine
R3=R4=R5=H
Yieldb
20
coordinated arylamine (1a
–
1d) results in the formation of
2d) This insertion occurs
2
--
R3=R4=R5=H
24
coordinated salicylaldimines (2a
–
3
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
R3=R4=R5=H
86
probably involving two possible operative pathways. The first
is a concerted insertion of oxygen into the Pd−C bond, with
simultaneous release of t-BuOH. A second possible pathway
would be a stepwise insertion and involve redox chemistry at
palladium centre. Initiatelly, a hydroxyl radical was generated
from a homolytic cleavage of TBHP, the obtained hydroxyl
radical reacted with NHC-palladacycle to afford either Pd(IV)18
or Pd(III)19 intermediate. In the final step, C–O bond formation
via reductive elimination affords (NHC)Pd(salicylaldimine)Cl.
Apparently, the second mechanism is more favorable in C–H
activation, although our results cannot confirm it. It is clearly
difficult to access the Pd(IV) or Pd(III) oxidation state.
4
R3=R4=R5=H
88
5
R3=R4=R5=H
85
6
1d
1a
1b
1c
R3=R4=R5=H
90
7
R3=R4=Me, R5=H
R3=R4=Me, R5=H
R3=R4=Me, R5=H
R3=R4= Me, R5=H
R3=i-Pr, R4=R5=H
R3=i-Pr, R4=R5=H
R3=i-Pr, R4=R5=H
R3=i-Pr, R4=R5=H
R3=R4=H, R5=Cl
R3=R4=H, R5=Cl
R3=R4=H, R5=Cl
R3=R4=H, R5=Cl
R3=R4=H, R5=OMe
R3=R4=H, R5=OMe
R3=R4=H, R5=OMe
R3=R4=H, R5=OMe
R3=R4=H, R5=NO2
R3=R4=H, R5= NO2
R3=R4=H, R5= NO2
R3=R4=H, R5= NO2
R3=R4=H, R5= COOH
R3=R4=H, R5= COOH
R3=R4=H, R5= COOH
R3=R4=H, R5= COOH
87
8
89
9
90
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
1d
1a
1b
1c
91
90
92
91
1d
1a
1b
1c
94
88
90
NHC-palladacycles catalysed ortho-hydroxylation of arylimines
91
1d
1a
1b
1c
94
Further examination of the catalytic performance of NHC–
palladacycles in ortho-hydroxylation of arylimines was done.
Firstly, a model reaction of benzylidene-arylamine with NHC–
palladacycle 1a (5%) in the presence of TBHP in toluene at
110 °C was done, no desired salicylaldimine was obtained. It
was possibly difficult to cleave the chelating ligand from the
palladium centre. To further modify the hemilability of the C^N
80
80
83
1d
1a
1b
1c
85
90
92
94
−
1d
1a
1b
1c
94
ligands in NHC–palladacycle, a weakly coordinated anion SbF6
trace
trace
trace
trace
was introduced in the reaction, the expected salicylaldimine
were obtained in good yield. With the optimized conditions in
hand, the scope of the NHC–palladacycle-catalysed
1d
hydroxylation reaction with
a series of arylamines was
a
investigated. As can be seen from Table 1, when the NHC–
palladacycles were used as catalysts, substituted arylimines
with either electron-donating or withdrawing groups attached
to the benzene rings were able to undergo ortho-hydroxylation
and generate the corresponding salicylaldimines in high yields.
Importantly, similar yields were obtained with the various
NHC–palladacycle catalysts. However, the carboxyl-substituted
Reaction conditions: arylimine (0.50 mmol), NHC-palladacycle (0.025 mmol),
AgSbF6 (0.05 mmol), K2CO3 (0.75 mmol), TBHP (1.0 mmol) in toluene (2.0 mL) at
110oC for 6 h. b Isolated yield.
Experimental section
General Remarks
arylimine afforded only trace amounts of product under the The cyclopalladated arylimines were prepared according to
present reaction conditions. The possible reason was that the literature method.14 NMR were performed in CDCl3 and
substrate bearing ligating atoms may deactivate the catalyst.
recorded on a Bruker Avance 400 NMR spectrometer with
tetramethylsilane (TMS) as an internal standard. High-
resolution mass spectra were recorded on Agilent 6550
iFunnel Q-TOF MS System. IR spectra were recorded on a
Bruker IFS 120HR spectrometer as KBr disks. Elemental
analyses were performed on a Vario El III elementar.
Conclusions
In summary, we have described the synthesis and structures of
four NHC–palladacycles derived from arylimines. The reactivity
of the NHC–palladacycles has been investigated and the
template-directed ortho-aromatic metaloxylation the NHC–
palladacycles in the presence of TBHP, affording the
(NHC)Pd(salicylaldimine)Cl complexes has been achieved.
Furthermore, the NHC–palladacycles exhibited good catalytic
activities in ortho-hydroxylation of arylimines.
General procedures for synthesis of NHC–palladacycles (1a–1d)
A mixture of imidazolium chloride (0.22 mmol), K2CO3 (0.40
mmol), cyclopalladated arylimine (0.10 mmol), and toluene (3
mL) were charged into a round bottom flask. The mixture was
o
heated at 110 C for 10 h and then the solvent was removed.
The crude product was purified by flash chromatography on a
short silica gel using CH2Cl2 as eluent. The combined eluent
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2009, 109, 3612; (f) E. A. B. Kantchev, C. J. O’Brien and M. G.
Organ, Angew. Chem. Int. Ed., 2007, 46, 2768.
was evaporated, and the residue was recrystallized from n-
hexane/CH2Cl2 to produce a pale-yellow solid.
3
(a) M. G. Organ, S. Avola, I. Dubovyk, N. Hadei, E. A. B.
Kantchev, C. J. O’Brien and C. Valente, Chem. Eur. J., 2006,
12, 4749; (b) N. Marion, O. Navarro, J. Mei, E. D. Stevens, N.
M. Scott and S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101;
(c) N. Marion, O. Navarro, E. D. Stevens, E. C. Ecarnot, A. Bell,
General procedures for synthesis of (NHC)Pd(salicylaldimine)Cl
complexes 2a–2d
A sealable reaction tube was charged with NHC-palladacycle
(0.15 mmol), TBHP (70 wt. % in H2O) (0.30 mmol) and toluene
(2.0 mL). The reaction vessel was heated at 110 oC for 8 h.
The crude product was purified by flash chromatography on a
short silica gel using CH2Cl2 as eluent. The combined eluent
was evaporated and the residue was recrystallized from n-
hexane/CH2Cl2 to produce a yellow solid.
D. Amoroso and S. P. Nolan, Chem. Asian J., 2010, 5, 841; (d)
O. Diebolt, V. Jurcik, R. Correa da Costa, P. Braunstein, L.
Cavallo, S. P. Nolan, A. M. Z. Slawin and C. S. J. Cazin,
Organometallics, 2010, 29, 1443;
For selected examples see: (a) F. E. Hahn and M. C. Jahnke,
Angew. Chem. Int. Ed., 2008, 47, 3122; (b) O. Kühl, Chem.
Soc. Rev., 2007, 36, 592; (c) E. A. B. Kantchev, C. J. O’Brien
and M. G. Organ, Aldrichimica Acta., 2006, 39, 97; (d) A. John
and P. Ghosh, Dalton Trans., 2010, 39, 7183; (e) M.
Bierenstiel and E. D. Cross, Coord. Chem. Rev. 2011, 255, 574;
(f) D. Yuan and H. V. Huynh, Molecules, 2012, 17, 2491; (g) D.
4
Catalyst screening reaction
A sealable reaction tube was charged with arylimine (0.5
mmol), NHC-palladacycle (0.025 mmol), AgSbF6 (0.05 mmol),
TBHP (70 wt. % in H2O) (1.0 mmol) and toluene (2.0 mL). The
Yuan, Q. Teng and H. V. Huynh, Organometallics, 2014, 33
1794.
,
5
M. S. Viciu, R. F. Germaneau, O. Navarro-Fernandez; E. D.
Stevens and S. P. Nolan, Organometallics 2002, 21, 5470.
G. Bastug and S. P. Nolan, Organometallics, 2014, 33, 1253.
(a) S. Gu and W. Chen, Organometallics, 2009, 28, 909; (b)
X.-Q. Xiao, A.-Q. Jia, Y.-J. Lin and G.-X. Jin, Organometallics,
2010, 29, 4842; (c) C. Chen, W. Chen and H. Qiu, Dalton
Trans., 2012, 41, 13405.
(a) Z. Jin, L.-L. Qiu, Y.-Q. Li, H.-B. Song and J.-X. Fang,
Organometallics, 2010, 29, 6578; (b) Y.-J. Li, J.-L. Zhang, X.-J.
Li, Y. Geng, X.-H. Xu and Z. Jin, J. Organomet. Chem., 2013,
737, 12.
(a) O. H. Winkelmann, A. Riekstins, S. P. Nolan and O.
Navarro, Organometallics, 2009, 28, 5809; (b) S. Meiries, A.
Chartoire, A. M. Z. Slawin and S. P. Nolan, Organometallics,
2012, 31, 3402; (c) S. Meiries, K. Speck, D. B. Cordes, A. M. Z.
Slawin and S. P. Nolan, Organometallics, 2013, 32, 330; (d) G.
o
reaction vessel was heated at 110 C for 8 h, then the solvent
was removed and the residue was purified by silica gel column
chromatography with petroleum ether/EtOAc as eluent to give
the products.
6
7
X-Ray crystallography
8
9
Data collection was performed on a Bruker SMART APEX II
CMOS area detector diffractometer at 296 K using ω rotation
scans with a scan width of 0.5° and Mo-Kα radiation (λ =
0.71073 Å). Multi-scan corrections were applied using
SADABS.20 Structure solutions and refinements were
performed with the SHELX-97 program.21 All non-hydrogen
atoms were refined anisotropically by full-matrix least-squares
on F2. The hydrogen atoms to carbon were included in
idealized geometric positions with thermal parameters
equivalent to 1.2 times those of carbon atoms. The selected
L. Duc, S. Meiries and S. P. Nolan, Organometallics, 2013, 32
7547.
,
10 (a) D. Katayev, Y.-X. Jia, A. K. Sharma, D. Banerjee, C. Besnard,
R. B. Sunoj and E. P. Kündig, Chem. Eur. J., 2013, 19, 11916;
(b) A. K. de K. Lewis, S. Caddick, O. Esposito, F. G. N. Cloke
and P. B. Hitchcock, Dalton Trans. 2009, 7094; (c) B. Xiao, T.-
J. Gong, Z.-J. Liu, J.-H. Liu, D.-F. Luo, J. Xu and L. Liu, J. Am.
Chem. Soc., 2011, 133, 9250.
bond lengths and bond angles are listed in Table S2 and
summary of the crystallographic data, data collection, and
a
refinement parameters for complexes 1a–1d and 2a–2d is
11 (a) M. S. Viciu, R. A. Kelly III, E. D. Stevens, F. Naud, M. Studer
and Nolan, S. P. Org. Lett., 2003, 5, 1479; (b) J. Broggi, H.
provided in Table S3.
Clavier and S. P. Nolan, Organometallics, 2008, 27, 5525; (c) J.
Nasielski, N. Hadei, G. Achonduh, E. A. B. Kantchev, C. J.
Acknowledgments
O’Brien, A. Lough and M. G. Organ, Chem. Eur. J., 2010, 16
,
Financial support from the National Natural Science
Foundation of China (No. 21301061) is gratefully
acknowledged.
10844; (d) E. A. B. Kantchev, G.-R. Peh, C. Zhang and J. Y.
Ying, Org. Lett., 2008, 10, 3949; (e) E. A. B. Kantchev and J. Y.
Ying, Organometallics, 2009, 28, 289; (f) G.-R. Peh, E. A. B.
Kantchev, C. Zhang and J. Y. Ying, Org. Biomol. Chem., 2009,
7
, 2110; (g) J. Li, M. Cui, A. Yu, Y. Wu, J. Organomet. Chem.,
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DOI: 10.1039/C7DT00007C
Abstract
Synthesis of (NHC)Pd(salicylaldimine)Cl complexes through template-
directed ortho-aromatic metaloxylation of NHC-palladacycles derived
from arylimines
Jin Yang*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Reactions of imidazoliums with palladacycles afforded NHC–palladacycle complexes. The reactivity
of the NHC–palladacycles was explored and the results demonstrated that they could be templateꢀ
10 directed orthoꢀaromatic metaloxylated in the presence of tertꢀbutyl hydroperoxide (TBHP), affording
(NHC)Pd(salicylaldimine)Cl complexes. Furthermore, the NHC–palladacycles showed efficient
catalytic activities toward orthoꢀaromatic hydroxylation of arylimines.
15
20
a Department of Chemistry, Huaibei Normal University, Huaibei, Anhui
25 235000, P R China; E-mail: yangjinlz@163.com
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