Efficient Catalyst for Both Suzuki and Heck Cross-Coupling Reactions: Synthesis and Catalytic Behaviour of Geometry- Constrained Iminopyridylpalladium Chlorides

Article abstract of DOI:10.1002/adsc.201600294

A series of geometry-constrained iminopyridyl-palladium chlorides were synthesized and characterized. These phosphine-free palladium complexes were explored for their catalytic activities in both Suzuki and Heck cross-coupling reactions, achieving turnover numbers as high as 10⁶towards various aryl bromides, even those containing various functionalities. In addition, the influence of substituents with steric and electronic factors was reflected by the differences observed in their activities. (Figure presented.).

Full text of DOI:10.1002/adsc.201600294

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DOI: 10.1002/adsc.201600294
Efficient Catalyst for Both Suzuki and Heck Cross-Coupling
Reactions: Synthesis and Catalytic Behaviour of Geometry-
Constrained Iminopyridylpalladium Chlorides
Yujie Tang,a Yanning Zeng,b Qingxia Hu,a Fang Huang,a,b Liqun Jin,a,*
Weimin Mo,a Nan Sun,a Baoxiang Hu,a Zhenlu Shen,a Xinquan Hu,a,c,*
and Wen-Hua Sunb,c,*
a
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, Peopleꢀs Republic of China
.
E-mail: liqunjin@zjut.edu.cn or xinquan@zjut.edu.cn
Key laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry,
b
Chinese Academy of Sciences, Beijing 100190, Peopleꢀs Republic of China
E-mail: whsun@iccas.ac.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation Lanzhou Institute of Chemical Physics Chinese
c
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
Received: March 16, 2016; Revised: May 20, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600294.
Abstract: A series of geometry-constrained imino- functionalities. In addition, the influence of substitu-
pyridyl-palladium chlorides were synthesized and ents with steric and electronic factors was reflected
characterized. These phosphine-free palladium com- by the differences observed in their activities.
plexes were explored for their catalytic activities in
both Suzuki and Heck cross-coupling reactions, ach-
ieving turnover numbers as high as 106 towards vari- Keywords: Iminopyridyl complexes; Palladium;
ous aryl bromides, even those containing various Heck reaction; Suzuki reaction
Introduction
gands such as phosphorus ligands[14–25] and N-hetero-
cyclic carbene (NHCs),[26–29] capable of providing
Palladium-catalyzed coupling reactions have been re- facile coordination to palladium as well as facilitating
garded as one of the most powerful transformations the oxidative addition process, are mostly employed.
in organic synthesis.[1–3] Despite the tremendous prog- Alternatively, the palladacycles pioneered by Herr-
ress, there remains a great demand for economic and mann and Beller in the 1990s[30–39] were considered as
practicable coupling processes with ultra-low catalyst highly active precursors with high TONs. Hence, vari-
loadings and higher turnover numbers due to the em- ous palladacycles bearing phosphine-free nitrogen li-
ployment of precious metal catalysts.[4–7] As described gands, such as quinoline,[40] oxazolines,[41,42] pyri-
by Farina from an industrial point of view, “any Pd- dines,[43–45] imidazoles,[46–48] pyrazoles[49] and ureas,[50,51]
based methodology with TONs of 105–106 and ade- were developed, ligands with bidentate or multiden-
quate turnover frequency (TOF) will be very practi- tate coordination were employed to elongate the life-
cal…… Beyond this level it is rather meaningless, time of the active Pd(0) species.[52] Structure/activity
from a practical standpoint, to venture”.[8] There thus correlations indicated that the pyridine-imine ligand
remains a significant interest in developing efficient could intervene in the catalytic cycle in two ways:[53]
palladium catalyst with ultra-low loadings.
(i) coordination of the pyridine makes the palladium
In order to develop a palladium catalyst with high center electron-rich, increasing the ability for oxida-
TONs, researchers have direted most efforts to en- tive addition and (ii) coordination of imino group
hancing the stability of the active palladium species could make the palladium center more electrophilic,
by using auxiliary ligands, which can be finely tuned allowing facile transmetalation or reductive elimina-
with regard to their steric and electronic properties by tion. On the basis of this rationale, a series of palladi-
installing various substituents.[9–13] Electron-rich li- um complexes bearing geometry-constrained imino-
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nolin-8-one and n-butylamine made it difficult to
obtain the pure desired ligand. Gratifyingly, the one-
pot condensation of 5,6,7-trihydroquinolin-8-one,
amine, and PdCl2 at same equivalent amounts in re-
fluxing toluene effectively formed the palladium com-
plexes Pd1–Pd6 in moderate to good yields
(Scheme 2). Therefore, this is a practical way to pre-
pare these palladium complexes in a one-pot reaction
procedure. Complexes Pd1–Pd6 were stable in air in
both solid and solution states; moreover, their melting
Scheme 1. The geometry-constrained iminopyridyl palladi-
um complexes.
pyridine derivatives (8-aryl- or 8-alkylimino-5,6,7-tri- points were measured to be higher than 2308C (dec.),
hydroquinolines) were synthesized (Scheme 1). The indicating good thermal stability. The analytical data
ring-fused framework is able to establish a strained from their NMR and IR spectra as well as elemental
environment and maintain the planar positions of the analyses were in good agreement at every aspect with
two nitrogen atoms, allowing the palladium complex the proposed structure.[32,59–61] To further elucidate the
to be more tightly ligated and stable. According to structures, all complexes were individually investigat-
their analogs in ethylene polymerization,[54–57] the ed by the single crystal X-ray diffraction.
ring-strain of the fused framework significantly affect-
Single crystals of all palladium complexes were ob-
ed the activities and the properties of the metal cata- tained by the slow diffusion of diethyl ether into di-
lyst. In this work, the palladium complexes of the imi- chloromethane solutions, respectively.[62] The solid-
nopyridine ligands were synthesized, their applica- state structures of all palladium complexes by X-ray
tions as catalysts in both Suzuki and Heck couplings diffraction established the square-planar coordination
were demonstrated, and TONs as high as 4.5ꢂ106 geometry around the palladium center with a slight
were achieved.
distortion being attributed to the non-aromatic six-
membered framework, in which each Pd atom is coor-
dinated with two nitrogen atoms of the imino-pyri-
dine ligand as well as two chlorides. Their molecular
structures are shown in Figure 1, Figure 2, Figure 3,
Figure 4, Figure 5, and Figure 6, respectively, and se-
lected bond lengths and angles are tabulated in
Table 1.
Results and Discussion
Syntheses and Characterization of Palladium
Complexes
The palladium complexes could be stepwise prepared
Regarding the structure of the ligands in the palla-
À
through
the
stoichiometric
coordination
of dium complexes, the bond lengths of N-2 C-8 charac-
PdCl2(CH3CN)2 with ligands, which were prepared by teristically revealed the double-bonding imino group
condensation of 5,6,7-trihydroquinolin-8-one and aryl- in the range of 1.275(16) ꢃ for Pd1 and 1.293(12) for
amines according to our previous report.[58] The Pd6, while the single-bonding characteristics were ob-
À
ligand compounds were prepared as the mixture of served for N-2 C-10 with lengths between
isomeric imine and enamine due to the inherent facile 1.419(13) ꢃ for Pd6 and 1.478(3) ꢃ for Pd2. The
tautomerization (Scheme 2).[58] Moreover, the reversi- bond lengths of Pd N ranged within 2.00 ꢃ and
À
ble nature of the reaction between 5,6,7-trihydroqui- 2.04 ꢃ as the typical bonds between Pd2+ and sp2-hy-
Scheme 2. Two methods for the iminopyridine palladium complexes
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Figure 4. ORTEP drawing of Pd4 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity.
Figure 1. ORTEP drawing of Pd1 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity.
Figure 5. ORTEP drawing of Pd5 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity.
Figure 2. ORTEP drawing of Pd2 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity.
Figure 3. ORTEP drawing of Pd3 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity.
Figure 6. ORTEP drawing of Pd6 with thermal ellipsoids at
30% probability. Hydrogen atoms have been omitted for
clarity
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Table 1. Selected bond lengths [ꢃ] and angles [8] for palladium complexes.
Pd1
Pd2
Pd3
Pd4
Pd5
Pd6
Bond lengths [ꢃ]
À
Pd(1) N(1)
2.004(11)
2.021(11)
2.292(4)
2.0173(19)
2.0285(19)
2.2989(7)
2.2841(7)
1.334(3)
1.357(3)
1.289(3)
1.478(3)
2.033(2)
2.030(2)
2.2905(8)
2.2791(8)
1.328(3)
1.362(3)
1.289(3)
1.446(3)
2.0256(15)
2.0192(15)
2.2952(5)
2.2709(5)
1.333(2)
1.360(2)
1.291(2)
1.444(2)
2.0252(18)
2.0213(17)
2.2938(6)
2.2672(6)
1.331(3)
1.365(3)
1.291(3)
1.440(3)
2.035(8)
2.022(8)
2.290(3)
2.268(3)
1.309(13)
1.360(13)
1.293(12)
1.419(13)
À
Pd(1) N(2)
À
Pd(1) Cl(1)
À
Pd(1) Cl(2)
2.276(4)
À
N(1) C(1)
1.333(16)
1.334(17)
1.275(16)
1.463(16)
À
N(1) C(9)
À
N(2) C(8)
À
N(2) C(10)
Pd1
Pd2
Pd3
Bond angles [8]
Pd4
Pd5
Pd6
À
À
N(1) Pd(1) Cl(1)
94.2(3)
94.22(6)
94.02(6)
174.97(6)
173.84(7)
95.67(7)
80.18(8)
90.22(3)
95.38(4)
171.78(4)
171.42(4)
94.40(4)
80.23(6)
90.727(19)
95.23(5)
172.68(5)
172.75(5)
94.28(5)
80.33(7)
90.61(2)
94.8(3)
À
À
N(1) Pd(1) Cl(2)
175.4(3)
174.0(3)
95.8(3)
79.8(4)
90.19(17)
175.68(6)
174.29(6)
95.81(6)
80.20(8)
89.73(3)
175.4(2)
175.7(2)
94.5(2)
80.9(3)
89.78(11)
À
À
N(2) Pd(1) Cl(1)
À
À
N(2) Pd(1) Cl(2)
N(2)-Pd(1)-N(1)
À
À
Cl(1) Pd(1) Cl(2)
bridized nitrogen.[44,45,60] Although there was no signif- whereby 60%, 60% and 69% conversions were ob-
icant difference or clear tendency regarding these served, respectively. The reactions with the same con-
bond lengths, slight influences of the substituents ditions as entry 2 but with additional 0.09 mL of H2O
linked to Nimino were observed in that the bond
À
lengths of Pd Npyridine were slightly longer than that of
Pd Nimino with aryl groups (Pd3–Pd6) and the reverse
Table 2. Pd-catalyzed couplings between 4-bromotoluene
À
and phenylboronic acid by using different catalyst precur-
effects were seen with aliphatic group (Pd1 and Pd2);
moreover, the ligands with the aryl-Nimino bond coun-
terpoised the relative bonds of N-2 C-8, N-1 C-9,
Pd-1 N-1 and Pd-1 N-2 due to the delocalized elec-
trons of the conjugated aromatic system. In addition,
the Pd N-2 bond lengths were slightly elongated
from Pd4 to Pd6 along with the reverse effects to N-
2 C-10 bonds due to the increasing bulkiness and
electron-donating influences from methyl to isopropyl
substituents.
AHCTUNGTRENNUNG
sors.[a]
À
À
À
À
À
À
Catalytic Application of Pd Complexes for Suzuki
Coupling
Typical reaction conditions for the Suzuki coupling at
0.01 mol% palladium loading are as follows:[37,48]
a mixture of 20 mmol of 4-bromotoluene, 22 mmol of
phenylboronic acid, and 40 mmol of K2CO3 in 25 mL
toluene was heated at reflux for 18 h (Method A). In
all cases high conversions were observed and are col-
lected in entry 1 of Table 2. A higher efficiency was
achieved by using a Young tube under a sealed envi-
ronment;[63] the above reactions were conducted in
parallel employing the Young tube for three hours
(Method B), indicating perfect conversion yields
(entry 2, Table 2). Under the same conditions as
entry 2, the reactions with PdCl2, PdCl2(CH3CN)2 and
Pd(OAc)2 as the precatalyst were also performed,
[a]
Reaction conditions: 4-bromotoluene (20 mmol), phenyl-
boronic acid (22 mmol), K2CO3 (40 mmol), toluene
(25 mL), Pd (10À4 M in toluene). Method A: reflux;
Method B: in a Young tube, 1108C in oil bath; Method
C: 1108C in oil bath, H2O (0.09 mL) was added, in
a Young tube.
[b]
Conversions and yields were determined by GC.
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(Method C, entry 3) were conducted and still showed for Suzuki cross-coupling, being interpreted as the
high activities. Subsequently, a higher molar ratio of positive influence of the constrained iminopyridine
substrates to the palladium adapted from 104 to 106 palladium complex.
along with gradiently extending the reaction time
were probed, the results are tabulated in Table 2 as
entries 4 to 6. In general, high conversions were ob- Heck Reactions by Pd6
served even with molar ratios of substrate to palladi-
um of up to a million (entries 4 to 6). However, the The Heck reaction is considered to be as equally im-
significantly lower yield by Pd1 was observed in portant as the Suzuki cross-coupling under palladium
entry 6 due to the aliphatic substituent of the imino catalysis. After screening of the catalysts, Pd6 was em-
group.[53] Further evaluation of the catalyst activity by ployed as the preferential choice. Simple optimization
increasing the molar ratio of substrates to the palladi- of the reaction conditions suggested that when
um up to 107 differentiated Pd5 or Pd6 from Pd2 to 0.001 mol% of Pd6 was utilized, the Pd-catalyzed
Pd4 as the preferential catalysts for further examina- Heck reaction between 4-bromotoluene 1a and sty-
tion.
rene occurred smoothly in DMF at 1308C with K2CO3
Beside the above results, the compound Pd6 is as the base. It was reported that a cationic palladium
easily prepared through either two-step synthesis or intermediate was involved because of dissociation of
one-pot synthesis (described in the Experimental Sec- one halide group and such a palladium species with
tion) because of the easier condensation of ketone a chelated ligand promoted the Heck reaction in
with aniline bearing electron-donating substituents. DMF.[66] Under similar reaction conditions as for 4-
Subsequently Pd6 was chosen as the precursor in ex- bromotoluene, different aryl bromides, particularly
ploring the scope of substances for the Suzuki cross- those with electron-donating substituents, were tested.
coupling with a lower loading of 0.001 mol% of Pd. As shown in Table 4, 4-bromoaniline 1d and 4-bro-
The results are collected in Table 3. In general, high moanisole 1e could react with styrene to deliver the
conversions were achieved in all cases. Electron-with- Heck reaction products with high isolated yields, 94%
drawing aryl bromide 1b reacted smoothly with 2a to and 96%, respectively (Table 4, entry 3 and entry 4).
yield the coupling product in high yield (entry 2) due However, the regioselectivity for 1d and 1e was lower
to the active substrate in the oxidative addition than with other substrates, which could be rational-
step;[8] meanwhile, 4-bromoaniline 1d coupled with 2a ized from the fact that geminal substitution is strongly
to also form its product in high isolated yield (95%) favoured by electron-donating substituents for the
(entry 4). Although electron-donating aryl bromides cationic Heck reaction.[30,67] A more sterically hin-
are sometimes deactivated in Suzuki cross-cou- dered substrate, 2-bromotoluene (1f), was also toler-
plings,[64] 4-bromoanisole 1e reacted with 2a affording ated at 0.001 mol% of catalyst loading. (E)-1-Methyl-
the corresponding product in a good yield. Notewor- 2-styrylbenzene was isolated with 87% yield in high
thy, the addition of H2O was necessary to promote selectivity (Table 4, entry 5). Moreover, fluoro-substi-
the transformation for 1e.[65] The coupling of 1a with tuted substrates such as 3,4,5-trifluorobromobenzene
electron-rich arylboronic acids, such as 2b, 2e, 2f and (1g) and 4-fluorobromobenzene 1l were also suitable
electron-deficient 2c and 2d led to the desired biaryl with satisfactory conversions and yields, although
products in good to high yields (86–100%, entries 9– higher reaction temperatures and longer reaction
13). Moreover, sterically hindered 2-bromotoluene 1f times were necessary (Table 4, entry 6 and entry 8).
underwent coupling with 2a to produce the target Similarly, a high reaction temperature as high as
product in 90% yield (entry 6). 1,3-Dibromobenzene 1508C and 0.01 mol% of catalyst loading were also
1h was also efficiently converted to the double cou- crucial for a high yield when 3-bromopyridine was
pling product terphenyl 3ha with 2a in a facile process employed (Table 4, entry 7). To our surprise, the elec-
(entry 8). In addition, this coupling could be extended tron-deficient 4-bromoacetophenone 1b was less effi-
to benzyl bromide (entry 15), in which the diphenyl- cient. We proposed that the oxidative addition was
methane was obtained in 94% yield. More interesting- not likely to be the rate-limiting step in the mechanis-
ly the couplings with heteroaryl bromides and hetero- tic cycle, and alkene insertion could be the slowest
arylboronic acids were also achieved albeit with low due to the chelation effect of the ligand.[66,68,69] This
conversions. On further increasing the catalyst loading phenomenon further indicated that the palladium
to 0.02 mol% and 0.01 mol% for 2-bromothiophene center was strongly stabilized by the constrained imi-
1j (entry 16) and 3-bromopyridene 1k (entry 17), the nopyridine ligand. Furthermore, the reaction of 4-bro-
expected products were isolated in excellent yields of motoluene 1a with ethyl acrylate could also provide
97% and 93%, respectively. 2-Bromopyridene reacted the Heck coupling product selectively in 93% yield
with 2a to form the product in a lower conversion due and only a 0.01 mol% amount of Pd6 complex was re-
to the possible competitive coordination effect to pal- quired.
ladium. The results demonstrated the high efficiency
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Table 3. Pd-catalyzed Suzuki cross-coupling of aryl bromides and arylboronic acid derivatives.[a]
[a]
Reaction conditions: ArBr 1 (10 mmol), ArB(OH)2 (11 mmol), K2CO3 (20 mmol), catalyst (10À4 mmol, 0.001 mol%), tolu-
ene (10 mL), Young tube, 1108C, purge with N2 for 3 min, 18 (36) h.
Isolated yields.
[b]
[c]
0.1 mL H2O was added.
1j (5 mmol), 2a (6 mmol), K2CO3(10 mmol), catalyst (10À4 mmol, 0.002 mol%), toluene (5 mL), H2O (0.1 mL) Young
[d]
tube.
[e]
ArBr 1 (10 mmol), ArB(OH)2 (11 mmol), K2CO3 (20 mmol), catalyst (10À3 mmol, 0.01 mol%), toluene (10 mL).
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Table 4. Heck reactions between aryl bromides and alkenes by Pd6.[a]
[a]
Reaction conditions: ArBr (10 mmol), alkene (12 mmol), K2CO3 (12 mmol), catalyst (10À4 mmol, 0.001 mol%), DMF
(10 mL), Young tube, 1308C, purge with N2 for 3 min.
[b]
Isolated yields including 1,1-disubstituted ethylene.
The selectivity was determined by 1H NMR spectrometry.
[c]
[d]
The isomers are 4-acetyl-trans-stilbene and 4-acetyl-cis-stilbene, respectively.
recorded on a Bruker ADVANCE III 500 MHz spectrome-
ter in deuteriated solvents with tetramethylsilane (TMS) as
Conclusions
internal standard. NMR multiplicities are abbreviated as fol-
In summary, a series of the geometry-constrained imi-
lows: s=singlet, d=doublet, t=triplet, q=quartet, sept=
nopyridylpalladium complexes were synthesized and
septet, m=multiplet, br=broad signal. Chemical shifts are
characterized. These phosphine-free palladium com-
plexes have provided excellent reactivities in both
Suzuki and Heck cross-couplings for most common
substrates with extremely high turnover numbers (>
106). The successful development of these iminopyri-
dine ligands for cross-coupling reactions should be
helpful for the design of more efficient and robust
palladium catalyst with even higher TONs.
given in ppm and are referenced to TMS (1H, 13C). All spec-
tra were obtained at 258C in the solvent indicated. Coupling
constants J are given in Hz. GC analyses were performed on
an Agilent 6890 instrument with FID detector using an HP-
5 capillary column [30 mꢂ0.32 mm (i.d.), 0.25 m]. High-reso-
lution mass spectra were recorded in the EI mode on
a Waters Synapt-G2 HDMS. Flash column chromatography
was performed on neutral silica gel (200–300 mesh) with
ethyl acetate/petroleum ether as eluent. Melting points were
determined on Buchi M-565 apparatus. X-ray crystallogra-
phy was performed on an Oxford Xcalibur Eos Gemini
Ultra with MoKa radiation. Details of the X-ray structure
determinations and refinements are provided in Table 1. IR
spectra were recorded on a Thermo Nicolet AVATAR 370
FT-IR. Elemental analysis was carried out using a Flash EA
1112 microanalyzer. All solvents and reagents were used di-
rectly from commercial sources without further purification.
Experimental Section
General Procedures
All reactions were carried out under an air atmosphere
1
unless otherwise noted. All H and 13C NMR spectra were
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6,7-dihydroquinolin-8(5H)-one was prepared following the
J2 =5.4 Hz, 1H), 7.34 (t, J=7.5 Hz, 2H), 7.28 (t, J=7.5 Hz,
1H), 5.28 (s, 2H), 2.98 (m, 4H), 2.06 (m, 2H); 13C NMR
(DMSO-d6, 125 MHz): d=180.5, 153.3, 148.0, 143.0, 141.1,
136.2, 128.5, 127.6, 127.3, 54.1, 29.8, 27.0, 21.0; HR-MS
(ESI+): m/z=418.0300, calcd. for C18H19ClN3Pd [MÀCl+
CH3CN]+: 418.0302; anal. calcd. for C16H16Cl2N2Pd: C 46.46,
H 3.90, N 6.77; found: C 46.23, H 3.90, N 6.73.
reported procedure.[70]
Syntheses and Characterization of the Complexes
Typical synthesis of complex Pd6 by two-step procedure:
Toluene (150 mL) was added to a mixture of 6,7-dihydroqui-
nolin-8(5H)-one (2.94 g, 20 mmol), 2,6-diisopropylaniline
(1.77 g, 10 mmol) and TsOH·H2O (285 mg, 1.5 mmol). The
reaction mixture was stirred for 12 h under reflux tempera-
ture and then allowed to cool to room temperature. After
column chromatography with petroleum ether as the eluent,
N-(5,6,7-trihydroquinolin-8-ylidene)-2,6-diisopropylaniline
was obtained as a yellow solid in quantitative yield, which
was then utilized for the next step directly.
Complex Pd3: Yield: 88%; mp 2708C (dec.); 1H NMR
(CDCl3, 500 MHz): d=9.11 (d, J=5.4 Hz, 1H), 7.89 (d, J=
8.0 Hz, 1H), 7.54 (dd, J1 =8.0 Hz, J2 =5.4 Hz, 1H), 7.40 (t,
J=7.4 Hz, 2H), 7.33 (t, J=7.4 Hz, 1H), 7.20 (d, J=7.4 Hz,
2H), 3.05 (t, J=6.0 Hz, 2H), 2.73 (t, J=6.3 Hz,2H), 2.00
(m, 2H); 13C NMR (DMSO-d6, 125 MHz): d=80.0, 153.0,
148.0, 144.9, 143.5, 141.0, 128.9, 128.3, 127.1, 123.2, 31.5,
27.3, 21.2; HR-MS (ESI+): m/z=404.0155, calcd. for
C17H17ClN3Pd [MÀCl+CH3CN]+: 404.0146; anal. calcd for
C15H14Cl2N2Pd: C 45.08, H 3.53, N 7.01; found: C 45.39, H
3.57, N 6.83.
The solution of N-(5,6,7-trihydroquinolin-8-ylidene)-2,6-
diisopropylaniline (673 mg, 2.2 mmol) in dichloromethane
(25 mL) was added slowly to the mixture of PdCl2(CH3CN)2
(519 mg, 2.0 mmol) in dichloromethane (25 mL). The result-
ing mixture was stirred for 24 h at room temperature. After
removal of the volatiles under vacuum, the complex Pd6
was obtained by column chromatography (dichloromethane:
ethanol=10:1) as a yellow solid; yield: 764 mg (79%).
One-pot synthesis: Toluene (30 mL) was added to a mix-
ture of 6,7-dihydroquinolin-8(5H)-one (323 mg, 2.2 mmol),
2,6-diisopropylaniline (425 mg, 2.4 mmol), PdCl2 (354 mg,
2.0 mmol) and TsOH·H2O (114 mg, 0.6 mmol). The reaction
mixture was stirred for 12 h at reflux temperature and then
allowed to cool to room temperature. After filtration and
washing by toluene and ethanol, the solid was dried under
reduced pressure affording the desired complex Pd6. The
complex was further purified after column chromatography
(dichloromethane: ethanol=10:1); yield: 550 mg (56%); mp
Complex Pd4: Yield: 57%; mp 2758C (dec.); 1H NMR
(CDCl3, 500 MHz): d=9.33 (d, J=5.5 Hz, 1H), 7.94 (d, J=
7.8 Hz,1H), 7.68 (dd, J1 =7.8 Hz, J2 =5.5 Hz, 1H), 7.17 (m,
1H), 7.11 (d, J=7.5 Hz, 2H), 3.05 (t, J=6.0 Hz, 2H), 2.46
(d, J=6.6 Hz, 2H), 2.29 (s, 6H), 2.01 (m, 2H); 13C NMR
(DMSO-d6, 125 MHz): d=180.2, 152.2, 148.1 143.9, 142.6,
142.1, 141.1, 129.6, 127.8, 127.0, 30.3, 27.2, 21.4, 18.0; HR-
MS (ESI+): m/z=432.0463, calcd. for C19H21ClN3Pd
[MÀCl+CH3CN]+:
432.0459;
anal.
calcd.
for
C17H18Cl2N2Pd: C 47.74, H 4.24, N 6.55; found: C 47.45, H
4.13, N 6.45.
Complex Pd5: Yield: 77%; mp 2778C (dec.); 1H NMR
(CDCl3, 500 MHz): d=9.35 (d, J=5.5 Hz, 1H), 7.94 (d, J=
8.0 Hz, 1H), 7.68 (dd, J1 =8.0 Hz, J2 =5.5 Hz, 1H), 7.29 (t,
J=7.7 Hz,1H), 7.17 (d, J=7.7 Hz, 2H), 3.04 (t, J=6.0 Hz,
2H), 2.85 (m, 2H), 2.49 (m, 4H), 1.99 (m, 2H), 1.31 (t, J=
7.6 Hz, 6H); 13C NMR (DMSO-d6, 125 MHz): d=180.5,
152.1, 148.2, 144.1, 141.4, 141.2, 134.8, 129.3, 127.5, 125.4,
30.7, 27.3, 23.6, 21.4, 13.2; HR-MS (ESI+): m/z=460.0778,
calcd. for C20H25ClN3Pd [MÀCl+CH3CN]+: 460.0772; anal.
calcd. for C19H22Cl2N2Pd: C 50.08, H 4.87, N 6.15; found: C
50.20, H 4.97, N 6.15.
1
2828C (dec.). H NMR (CDCl3, 500 MHz): d=9.37 (d, J=
5.6 Hz, 1H), 7.95 (d, J=7.9 Hz, 1H), 7.69 (dd, J1 =7.9 Hz,
J2 =5.6 Hz, 1H), 7.34 (t, J=7.8 Hz, 1H), 7.22 (d, J=7.8,
2H), 3.15 (sept, J=6.8 Hz, 2H), 3.06 (t, J=5.9 Hz, 2H),
2.50 (t, J=6.6 Hz, 2H), 1.98 (m, 2H), 1.42 (d, J=6.8 Hz,
6H), 1.15 (d, J=6.8 Hz, 6H); 13C NMR (DMSO-d6,
125 MHz): d=180.7, 152.1, 148.1, 144.1, 141.2, 139.9, 139.8,
129.3, 128.0, 123.4, 31.2, 27.9, 27.3, 23.6, 23.4, 21.4; HR-MS
(ESI+): m/z=488.1088, calcd. for C23H29ClN3Pd [MÀCl+
CH3CN]+: 488.1085; anal. calcd. for C21H26Cl2N2Pd: C 52.14,
H 5.42, N, 5.79; found: C 52.14, H 5.51, N 5.80.
Complexes Pd1, Pd2, Pd3, and Pd4 were prepared follow-
ing the one-pot synthetic procedure as described for com-
plex Pd6. Pd5 was prepared following the two-step proce-
dure as described for complex Pd6.
General Procedure for Pd-Catalyzed Suzuki Cross-
Coupling Reactions
To a Young tube, were added aryl bromide (10 mmol),
K2CO3 (20 mmol), and arylboronic acid (11 mmol), then
complex Pd6 (10À4 M in toluene, 1 mL) and toluene (9 mL)
were added. The mixture was degassed with N2 bubbling for
3 min. Then, the sealed Young tube was set into the pre-
heated 1108C oil bath; after stirring for 18 h, the Young
tube was allowed to cool to room temperature. After filtra-
tion and extraction with toluene (50 mL), the resulting solu-
tion was concentrated under vacuum and the desired biaryl
was isolated by column chromatography.
Complex Pd1: Yield: 58%; mp 2338C (dec.); 1H NMR
(CDCl3, 500 MHz): d=8.82 (d, J=5.5 Hz, 1H), 7.82 (d, J=
7.7 Hz, 1H), 7.39 (dd, J1=7.7 Hz, J2=5.5 Hz, 1H), 3.78 (t,
J=7.9 Hz, 2H), 3.11 (t, J=6.5 Hz, 2H), 3.06 (t, J=6.1 Hz,
2H), 2.11 (m, 2H), 1.70 (m, 2H), 1.36 (m, 2H), 0.93 (t, J=
7.4 Hz, 3H); 13C NMR (DMSO-d6, 125 MHz): d=177.8,
153.6, 147.8, 142.5, 141.0, 128.1, 51.9, 30.8, 28.8, 27.1, 21.1,
19.5, 13.8; HR-MS (ESI+): m/z=384.0459, calcd. for
C15H21ClN3Pd [MÀCl+CH3CN]+: 384.0459; anal. calcd. for
C13H18Cl2N2Pd: C 41.13, H 4.78, N 7.38; found: C 41.26, H
4.85, N 7.48.
General Procedure for Pd-Catalyzed Heck Reactions
To
a Young tube, aryl bromide (10 mmol), K2CO3
(12 mmol), alkene (12 mmol), complex Pd6 (10À4 M in DMF,
1 mL) and DMF (9 mL) were added. The mixture was de-
gassed for 3 min. After stirring for 24 h at 1308C, the reac-
tion temperature was allowed to cool to room temperature
and water (100 mL) was added. The resulting mixture was
Complex Pd2: Yield: 44%; mp 2758C (dec.); 1H NMR
(CDCl3, 500 MHz): d=9.02 (d, J=5.4 Hz, 1H), 7.80 (d, J=
7.9 Hz, 1H), 7.56 (d, J=7.5 Hz, 2H), 7.45 (dd, J1 =7.9 Hz,
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extract by ethyl acetate (50 mL) for 3 times. After drying
and concentration, the desired product was isolated by
column chromatography.
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Acknowledgements
This work was supported by the National Natural Science
Foundations of China (21473160 and 21376224). Jin is also
thankful for the support from Zhejiang University of Tech-
nology and Qianjiang Scholar Program funded by Zhejiang
Province, China.
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Efficient Catalyst for Both Suzuki and Heck Cross-Coupling
Reactions: Synthesis and Catalytic Behaviour of Geometry-
Constrained Iminopyridylpalladium Chlorides
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Adv. Synth. Catal. 2016, 358, 1 – 11
Yujie Tang, Yanning Zeng, Qingxia Hu, Fang Huang,
Liqun Jin,* Weimin Mo, Nan Sun, Baoxiang Hu,
Zhenlu Shen, Xinquan Hu,* Wen-Hua Sun*
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