Simple, efficient and reusable Pd-NHC catalysts for hydroamination

Article abstract of DOI:10.1039/c3ra42990c

A series of chelating NHC-palladium complexes with different alkane-bridges of the type Pd[NHC-(CH₂)_n-NHC]X₂ (X = Br or Cl, n = 2-4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor ligand. The bromi

Full text of DOI:10.1039/c3ra42990c

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Simple, efficient and reusable Pd–NHC catalysts for
hydroamination3
Cite this: RSC Advances, 2013, 3,
18359
Qian Chen,a Lanlan Lv,a Meng Yu,a Yanhui Shi,*a Yuling Li,a Guangsheng Pangb
and Changsheng Cao*a
A series of chelating NHC–palladium complexes with different alkane-bridges of the type Pd[NHC–(CH2)n–
NHC]X2 (X = Br or Cl, n = 2–4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor
ligand. The bromide complexes with n = 2 and 3 were characterized by X-ray crystallography. The effects of
the length of the bridge and halide ligand on the catalytic reactivity in the hydroamination reaction were
investigated. The best catalytic performance was observed with the propylene-bridged Pd–NHC bromide
complex 2. The hydroamination of phenylacetylene or 4-methylphenylacetylene with various substituted
anilines catalyzed by 2 yielded only Markovnikov addition products in good yield. Interestingly, this
homogenous catalyst can be reused three times without significant loss in activity.
Received 14th June 2013,
Accepted 22nd July 2013
DOI: 10.1039/c3ra42990c
www.rsc.org/advances
metal complexes with inexpensive ligands. In addition, none
of these homogeneous catalysts are recyclable, which make
these methods more expensive from an economic point of
view. As recyclability is of great importance for applying a
catalyst in industrial processes, making the catalyst recyclable
is of particular value.
Owing to their wide application as potentially useful
catalysts in organic synthesis, the chemistry of palladium–
NHC (N-heterocyclic carbene) complexes has become an area
of great interest and has been extensively studied.6 Although
Pd–NHC complexes have been successfully used in many
organic transformations, very little work has been reported on
hydroamination using NHC palladium complexes.7 Herein, we
report on an easily synthesized, air- and moisture-stable Pd(II)
complex supported by an inexpensive chelating di-carbene
ligand based on 1,2,4-triazoles with varying bridge length. The
hydroamination of phenylacetylene can be efficiently catalyzed
by the palladium complexes under mild reaction conditions,
and we demonstrated in one example that the catalyst is
reusable in the process of hydroamination.
Introduction
Amines and their derivatives are widely found in biologically
active and industrially useful molecules. One of the most
attractive routes for C–N bond formation is the transition
metal catalyzed hydroamination reaction, which involves the
atom-economic addition of N–H bonds across unsaturated C–
C multiple bonds.1 However, diverse thermodynamic and
kinetic effects hinder the direct nucleophilic addition of this
transformation, and the reaction requires that a catalyst
activates the amine and/or the carbon–carbon bond for the
coupling process.2 A plethora of catalytic systems have been
developed to overcome the high activation barrier during the
last two decades, including a number of early- and late-
transition-metal complexes.3,4 Recently, much effort has been
focused on the use of late transition metal complexes because
they are easily accessible, simple to handle, and can tolerate
more functional groups. Nevertheless, it must be pointed out
that most of the hydroamination reactions catalyzed by late
transition metals suffer from problems due to sensitive
ligands and metal complexes, the high catalyst loadings, the
high cost of the expensive precious metal, such as Rh, Ir and
Au, and patented phosphine ligands.5 Consequently, it is
necessary to develop air-stable, robust and well-defined late
Results and discussion
Synthesis and characterization of the palladium complexes
aSchool of Chemistry and Chemical Engineering and Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou,
Jiangsu 221116, PR China. E-mail: yhshi@jsnu.edu.cn; Fax: (+86)516 8350 0349;
Tel: (+86)516 8350 0349
The preparation of the (CH2)n-linked (n = 2–4) bis-triazolium
salts as ligand precursors was straightforward and high-
yielding.8 Palladation of triazolium bromides or chlorides
was routinely carried out with Pd(OAc)2 in DMSO at elevated
temperature as shown in Scheme 1. The yield of the complex
changes significantly with varying bridge length. Complexes 2
and 5 with a propylene bridge were achieved in good yield
(85% and 77%), and complexes 1 and 4 with an ethylene
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin
University, Changchun, Jilin 130012, PR China
Electronic supplementary information (ESI) available. CCDC 909117 and
3
909118. For crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ra42990c
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Scheme 1 Synthesis of compounds 1–5.
bridge were in moderate yield, whereas a yield of only 36% for
complex 3 with a butylene bridge was isolated. The Pd complex
bearing chloride ligands with a butylene-bridge cannot be
isolated in a pure form at all. Furthermore, the bridge length
has an effect on the color and solubility of palladium
complexes too. The complexes with ethylene and propylene
spacers are white powders; however, complex 3 with butylene
spacer is a light yellow solid. All complexes have poor
solubility in common solvents and the solubility is in the
order of 2, 5 (n = 3) > 1, 4 (n = 2) > 3 (n = 4). The complexes 1–2
and 4–5 are slightly soluble in DMSO, MeOH and CH3CN,
whereas, complex 3 is only slightly soluble in DMSO. All the
complexes are air- and moisture-stable and can be stored in an
air atmosphere for more than 6 months without any noticeable
decomposition.
Fig. 2 ORTEP structual drawing of 2. Ellipsoids are draw at 30% probability
level, and hydrogen atoms and solvent are omitted for clarity.
crystallographic data for complexes 1 and 2 are given in
Table 1. Selected bond lengths and angles are given in Table 2.
In both compounds, the geometry at the palladium is square
planar with the triazole rings of the chelating ligand at a
dihedral angle of ca. 88u. Complex 1 contains a seven-
membered metallacycle, and complex 2 contains an eight-
membered metallacycle. The coordination plane of the two
bromides was twisted out of the C2Pd plane by 8.14u for 1,
which is much larger than that for 2 (1.55u). The C1–Pd–C2
bite angle increased slightly from 83.6(9)u for 1 with the ethyl
bridge to 87.0(4)u for 2 with the propyl bridge, however, these
bite angles are less than 90u due to a chelate effect. The
distortions of the Pd–C(carbene)–N bond angles are reflected
in the yaw distortion angle h (Table 2). The angle is decreased
with increasing bridge length (h = 8.8 or 2.2 for 1; h = 1.35 for
2), which was proposed to be a consequence of the ring strain.
It can also be seen from complex 1 that the yaw distortion
mainly affects one of the two Pd–NHC bonds, which appears to
be energetically favorable. The length of the Pd–C bonds is
slightly shorter than those in a similar iodide complex,9a thus
reflecting the lower trans influence of bromide versus iodide,
and are comparable with analogue dibromide complexes.9b All
other distances and angles lie in the expected range.
The proton signal of NCHN from the triazolium salts was
absent in the 1H NMR of the palladium complexes, confirming
carbene generation. In addition, the 13C NMR of the Pd
complexes provide direct evidence of the metalation of the
ligand, as seen by the signal in the typical range of 158–164
ppm. The 1H NMR spectra reflect the rigidity of the dicarbene
complexes. For example, two different multiplets of the linker
1
protons are present in the H NMR spectrum for 1, indicating
that these protons are chemically but not magnetically
equivalent. This splitting can be explained as an AA9XX9 spin
system caused by a fixed C1-symmetric arrangement of
ligands.
The molecular structures of 1 and 2 were determined by
means of X-ray diffraction studies. Single crystals for the solid-
state structure determination of 1 could be obtained by slow
evaporation of a DMSO saturated solution, and for 2 were
obtained by a CH3CN solution. The molecular diagrams of 1
and 2 are shown in Fig. 1 and Fig. 2, respectively. Selected
Catalysis in hydroamination
Initial studies for hydroamination were conducted with the
intermolecular hydroamination reaction of phenylacetylene
with aniline and the results are summarized in Table 3. The
influence of the bridge length on the activity can be seen for
complexes 1–3 containing identical heterocycles, wingtip and
bromide ligands, differing only in bridge length.
In all cases, the production of the imine was clean and
efficient at 2 mol% of Pd complex loading in the presence of 4
mol% silver triflate as an additive, and only the Markovnikov
product was obtained in 99% yield within 2 h. Therefore, the
amount of Pd catalyst and silver triflate was reduced by a
factor of 2 in two additional experiments. With 1 mol% of
loading of the catalyst, 99% yield was still observed for all
three complexes within 2 h. However, when the amount was
lowered to 0.5 mol%, only complex 2 with the propene bridge
was found to be still very efficient. Even though it required an
increased reaction time to achieve the same level of yield, it is
still significant that the reaction was carried out with only 0.5
Fig. 1 ORTEP structural drawing of 1. Ellipsoids are drawn at 30% probability
level, and hydrogen atoms are omitted for clarity.
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Table 1 Selected crystallographic data for complexes 1 and 2
Complexes
1
2
Formulaa
C14H24Br2N6Pd
542.61
C15H26Br2N6Pd?C2H3N
597.67
M/g mol21 a
T/K
298(2)
0.71073
296(2)
0.71073
Wavelengthb/Å
Crystal system
Space group
Monoclinic
P21/c
Orthorhombic
Pnma
a/Å
b/Å
c/Å
16.0644(18)
9.4857(11)
13.3115(16)
90
100.765(3)
90
8.1484(11)
18.284(2)
15.475(2)
90
90
90
2305.6(5)
a/u
b/u
c/u
V/Å3
1992.7(4)
Z
4
4
r/g cm23
F(000)
1.809
1064
1.722
1184
Crystal size/mm
h range/u
Limiting indices
0.40 6 0.30 6 0.20
2.51 to 24.99u
219 ¡ h ¡ 18
211 ¡ k ¡ 11
214 ¡ l ¡ 15
16 867/3314
3314/0/210
1.245
0.33 6 0.27 6 0.08
1.72 to 25.00u
29 ¡ h ¡ 9
221 ¡ k ¡ 20
218 ¡ l ¡ 18
22 406/2099
2091/2/133
1.025
Reflections collected/unique
Data/restraints/parameters
GOF
R1c, wR2 [I . 2s(I)]
R1 = 0.1349, wR2 = 0.3353
R1 = 0.1493, wR2 = 0.3444
1.988 and 23.076
R1 = 0.0421, wR2 = 0.1361
R1 = 0.0497, wR2 = 0.1536
0.704 and 21.139
d,e
R1 , wR2 (all data)
Largest diff. peak and hole/e Å3
a
b
c
d
Including solvate molecules. Mo-Ka radiation. R1 = g(|Fo| 2 |Fc|)/g(|Fo|) for observed reflections. w = 1/[s2(Fo2) + (aP)2 + bP] and P =
[max(Fo2,0) + 2Fc2]/3. wR2 = {g[w(Fo 2 Fc2)2]/g[w(Fo2)2]}1/2 for all data.
e
2
mol% of catalyst. The yield was decreased dramatically to 52%
for 1 (n = 2) and only 28% for 3 (n = 4). These results indicate
that the highest yield of the hydroamination product was
achieved using the propene-bridged complex, and no clear
trend is found with regard to the bridge length.
The influence of halide ligands on the reactivity of the
palladium complex was investigated by comparing complexes
1 and 2 bearing bromides vs. complexes 4 and 5 bearing
chlorides. Both of the complexes with chlorides were not as
good as their analogues with bromides. For example, an
Table 2 Selected bond lengths (Å) and angles (u) for 1 and 2
1
Table 3 The hydroamination of phenylacetylene with anilines with Pd com-
plexes 1–5
Pd(1)–C(1)
Pd(1)–C(2)
Pd (1)–Br(1)
Pd (1)–Br(2)
N(4)–C(2)
N(6)–C(2)
N(1)–C(1)
N(3)–C(1)
2.00(2)
1.99(2)
2.476(3)
2.477(3)
1.38(3)
1.29(3)
1.36(3)
1.31(3)
N(6)–C(2)–Pd(1)
N(4)–C(2)–Pd(1)
N(1)–C(1)–Pd(1)
N(3)–C(1)–Pd(1)
C(2)–Pd(1)–C(1)
C(2)–Pd(1)–Br(2)
C(1)–Pd(1)–Br(1)
triazole dihedral anglea
h
136.4(19)
118.8(16)
129.4(19)
125.0(17)
83.6(9)
172.7(7)
172.8(7)
88.21
Entrya Pd catalyst (mol%) AgOTf (mol%) Time (h) Yieldb (%)
8.8 or 2.2
1
2
3
4
5
6
7
8
1 (2)
2 (2)
3 (2)
1 (1)
2 (1)
3 (1)
1 (0.5)
2 (0.5)
3 (0.5)
4 (0.5)
5 (0.5)
4
4
4
2
2
2
1
1
1
1
1
2
2
2
2
2
2
12
12
12
12
12
99
99
99
99
99
99
52
99
28
42
65
2
Pd (1)–C(5)
Pd (1)–Br(1)
N(1)–C(5)
N(3)–C(5)
C(5A)–Pd(1)–C(5)
1.974(7)
C(5)–Pd(1)–Br(1A)
C(5A)–Pd(1)–Br(1)
N(3)–C(5)–Pd(1)
N(1)–C(5)–Pd(1)
triazole dihedral angleb
h
176.39(18)
176.39(19)
126.6(5)
129.3(5)
88.39
2.4797(8)
1.328(9)
1.348(9)
87.0(4)
1.35
9
10
11
a
Triazole dihedral angle = angle between LS planes: one through
the C1 triazole ring; one through the C2 triazole ring. h = 1/2[/N(1)–
C(1)–Pd(1)2/N(3)–C(1)–Pd(1)] or 1/2[/N(6)–C(2)–Pd(1)2/N(4)–C(2)–
a
b
Reaction conditions: phenylacetylene (0.6 mmol), aniline (0.5
Pd(1)]. Triazole dihedral angle = angle between LS planes: one
through the C5 triazole ring; one through the C5A triazole ring. h =
1/2[/N(1)–C(5)–Pd(1)2/N(3)–C(5)–Pd(1)].
mmol), Pd complex (0.5–2 mol%) and AgOTf (1–4 mol%) in toluene
(1 mL) at 100 uC. GC yield measured with dodecane as internal
standards.
b
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excellent yield of 99% was achieved when the reaction was
catalyzed by 2, while a markedly lower yield of 65% was
obtained for 5 under the same conditions (Table 3, entry 11).
Based on the results in Table 3, 0.5 mol% loading of
complex 2 was chosen in the addition reaction of phenylace-
tylene and 4-methylphenylacetylene with various anilines, and
the results are depicted in Table 4. Good results were obtained
for the anilines containing either electron-withdrawing or
electron-donating substituents, and corresponding amines
from the reduction of the hydroamination products were
mostly isolated in about 80% yield. Disappointingly, the
reaction with 2,6-dimethyl aniline gave lower yields (54%),
due to the enhanced steric bulk at the nitrogen atom.
Recycling of catalysts is amongst the main goals in
organometallic chemistry, and is one of the main principles
of green chemistry.10 However, generally it is very difficult to
recover and reuse a homogenous catalyst because of the
difficult separation of the catalyst from the reaction mixture.
After carefully analyzing our catalytic system, we proposed that
the real catalyst in the hydroamination reaction was
[Pd(NHC)]2+[(OTf)2]22, which was generated in situ from the
reaction of the precatalyst 2 with AgOTf. As we know, normally
ionic compounds have poor solubility in diethyl ether,
however, the more organic compounds are very soluble in
ether. We assumed that the product imine would be separated
from the reaction mixture by adding diethyl ether to the
reaction mixture. To our delight, the catalyst did precipitate
out from the reaction solution when 3 mL of diethyl ether was
added to the cooled resulting mixture after the hydroamina-
tion. The catalyst was recovered by carefully removing the
organic layer, and washing with ether (3 6 3 mL). The
recyclability of the catalyst was investigated in the reaction of
phenyl acetylene with aniline. Subsequently, the isolated yield
of N-(1-phenylethyl)benzenamine was checked in repeated
cycles 1–5 (81%, 80%, 80%, 67%, 65%) and the results are
shown in Table 5. The results indicated that there is not a
significant loss of activity of the catalyst after recycling 3 times.
Table 4 Hydroamination of aniline with phenylacetylene
Entrya
1
Aryl halide
Amine
Product
Yield(%)b
81
6a
6b
6c
6d
6e
6f
2
3
4
5
6
80
76
74
72
83
7
6g
89
8
6h
6i
81
74
81
77
83
81
9
10
11
12
13
6j
6k
6l
Experimental section
General considerations
6m
All reagents were commercially available and were used
without further purification. 1H and 13C NMR spectra were
recorded on a Bruker DPX 400 MHz spectrometer at room
temperature and referenced to the residual signals of the
solvent. Elemental analyses were performed on a EuroVektor
Euro EA-300 elemental analyzer. GC-MS was performed on an
Agilent 6890-5973N system with electron ionization (EI) mass
spectrometry.
14
15
6n
6o
80
54
X-ray crystallographic studies
Intensity data were collected with a Rigaku Mercury CCD area
detector in v scan mode by using Mo-Ka radiation (l =
0.71075 Å). The diffracted intensities were corrected for
Lorentz polarization effects and empirical absorption correc-
tions. Details of the intensity data collection and crystal data
are given by full-matrix least-squares procedures based on F2.
a
Reaction conditions: (1) phenylacetylene (2.2 mmol), aniline (2
mmol), 2 (0.5 mol%) and AfOTf (1 mol%) in toluene (2 mL) at 100
uC for 12 h; (2) LiAIH4 (3 mmol), THF (5 mL) at 50 uC for 3 h.
b
Isolated yield from an average of two runs.
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Syntheses of complexes
Table 5 Recycling study of the precatalyst 2 for hydroamination of phenylace-
tylene with aniline
[1,2-Ethanediylbis(1-butyl-1H-1,2,4-triazol-4(5H)-yl-5-ylide-
ne)]dibromopalladium (1). 1,19-Di-n-butyl-4,49-(1,2-ethanediyl)-
bistriazolium dibromide (1 mmol, 0.44 g) was dissolved in
DMSO (10 mL) and Pd(OAc)2 (0.96 mmol, 0.22 g) was added in
small portions. The reaction mixture was heated at 90 uC
overnight with stirring. The volatile component was removed
under vacuum after the mixture was filtered over Celite. The
resulting yellow solid was rinsed with DCM 3 times and gave
the product as a white solid (0.33 g, 63%). 1H NMR (400 MHz,
DMSO-d6): d = 8.77 (s, 2H, NCHN), 5.21 (m, 2H, NCH2), 4.73
(m, 2H, NCH2), 4.55 (m, 2H, 1-CH2 of Bu), 4.32 (m, 2H, 19-CH2
of Bu), 1.85 (m, 4H, 2-CH2 of Bu), 1.31 (m, 4H, 3-CH2 of Bu),
0.90 (t, J = 7.6 Hz, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): d =
162.8, 143.9, 51.8, 44.7, 30.6, 19.2, 13.5. Anal. calc. for
C14H24Br2N6Pd (542.61 g mol21): C, 30.99; H, 4.46; N, 15.49.
Found: C, 30.72; H, 4.34; N, 15.61%.
Number of cycles
Yieldb
1a
81%
2
80%
3
80%
4
67%
5
65%
a
Reaction conditions: (1) phenylacetylene (2.2 mmol), aniline (2
mmol), 2 (2 mol%) and AgOTf (4 mol%) in toluene (2 mL) at 100 uC
b
for 12 h; (2) LiAlH4 (3 mmol), THF (5 mL) at 50 uC for 3 h. Isolated
yield from an average of two runs.
[1,2-Propanediylbis(1-butyl-1H-1,2,4-triazol-4(5H)-yl-5-ylide-
ne)]dibromopalladium (2). 1,19-Di-n-butyl-4,49-(1,2-propane-
diyl)bistriazolium dibromide (1 mmol, 0.45 g) was dissolved
in DMSO (10 mL) and Pd(OAc)2 (0.96 mmol, 0.22 g) was added
in small portions. The reaction mixture was heated at 90 uC
overnight with stirring. The volatile component was removed
under vacuum after the mixture was filtered over Celite. The
resulting yellow solid was rinsed with DCM 3 times and gave
the product as a white solid (0.45 g, 85%). 1H NMR (400 MHz,
DMSO-d6): d = 8.72 (s, 2H, NCHN), 4.79 (m, 2H, NCH2), 4.61 (m,
4H, NCH2), 4.30 (m, 2H, NCH2), 2.41 (m, 1H, NCH2CH2), 1.94
(m, 3H, NCH2CH2), 1.77 (m, 2H, CH2), 1.41 (m, 4H, CH2CH3),
0.90 (t, J = 7.6 Hz, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): d =
160.1, 144.8, 52.2, 44.7, 31.0, 19.1, 13.4. Anal. calc. for
C15H26Br2N6Pd (556.64 g mol21): C, 32.37; H, 4.71; N, 15.10.
Found: C, 32.18; H, 4.63; N, 15.25%.
[1,2-Butanediylbis(1-butyl-1H-1,2,4-triazol-4(5H)-yl-5-ylide-
ne)]dibromopalladium (3). 1,19-Di-n-butyl-4,49-(1,2-butane-
diyl)bistriazolium dibromide (1 mmol, 0.46 g) was dissolved
in DMSO (10 mL) and Pd(OAc)2 (0.96 mmol, 0.22 g) was added
in small portions. The reaction mixture was heated at 90 uC
overnight with stirring. The volatile component was removed
under vacuum after the mixture was filtered over Celite. The
resulting yellow solid was rinsed with DCM 3 times and gave
the product as a light yellow solid (0.20 g, 36%). 1H NMR (400
MHz, DMSO-d6): d = 8.80 (s, 2H, NCHN), 4.97 (m, 2H, NCH2),
4.56 (m, 2H, NCH2), 4.31 (m, 4H, NCH2), 2.30 (s, 1H,
NCH2CH2), 2.07–1.81 (m, 7H, NCH2CH2), 1.39 (m, 4H,
CH2CH3), 0.93 (t, J = 7.2 Hz, 6H, CH3). 13C NMR (100 MHz,
DMSO-d6): d = 164.2, 145.0, 51.6, 48.8, 30.5, 19.2, 13.5. Anal.
calc. for C16H28Br2N6Pd (570.66 g mol21): C, 33.68; H, 4.95; N,
14.73. Found: C, 33.91; H, 4.86; N, 14.64%.
All the non-hydrogen atoms were refined anisotropically. The
hydrogen atoms in these complexes were all generated
geometrically (C–H bond lengths fixed at 0.95 Å), assigned
appropriate isotropic thermal parameters, and allowed to ride
on their parent carbon atoms. All the hydrogen atoms were
held stationary and included in the structure factor calculation
in the final stage of the full-matrix least-squares refinement.
The structure was solved by directed methods using the
SHELXS-97 program and absorption correction was performed
by SADABS program. CCDC-909117 and CCDC-909118 contain
the supplementary crystallographic data for this paper.
General procedures for hydroamination
A vial was charged with AgOTf (0.02 mmol, 5.1 mg), and
palladium catalyst (0.01 mmol, 4.8 mg) under argon.
Phenylacetylene (2.2 mmol, 146 ml), aniline (2 mmol, 182 ml)
and toluene (2 mL) were syringed into the vial. The mixture
was heated at 100 uC for 12 h and then filtered through Celite.
The filtrate was collected and dissolved in THF (5 mL). LiAlH4
(3 mmol, 113.8 mg) was added to THF solution in small
portions at 0 uC, and then the mixture was heated at 50 uC for 3
h. The remaining LiAlH4 was quenched by NaOH aqueous
solution (2 M) in an ice bath until the solid disappeared. The
organic layer was separated and the aqueous layer was
extracted by DCM (3 6 10 mL). The combined organic phase
was dried over MgSO4 and concentrated. The pure compound
was isolated by column chromatography (petroleum ether/
diethyl ether = 100 : 1).
General procedures for the recycling of the catalyst
The reaction conditions for hydroamination and reduction of
imine to amine were the same as those mentioned above. After
the hydroamination, diethyl ether (3 mL) was added to the
cooled reaction solution. The mixture was stirred at room
temperature for 10 min, and then was settled to enable the
separation of solid (catalyst) and liquid (product) phases. The
catalyst was recovered by removing the liquid, and washing
with ether (3 6 3 mL). The used catalyst was added to the new
substrates directly for the next run. The combined organic
phase, including the imine product of hydroamination, was
collected carefully and reduced by LiAlH4 in THF.
[1,2-Ethanediylbis(1-butyl-1H-1,2,4-triazol-4(5H)-yl-5-ylide-
ne)]dichloropalladium (4). 1,19-Di-n-butyl-4,49-(1,2-ethanediyl)-
bistriazolium dichloride (1 mmol, 0.35 g) was dissolved in
DMSO (10 mL) and Pd(OAc)2 (0.96 mmol, 0.22 g) was added in
small portions. The reaction mixture was heated at 90 uC
overnight with stirring. The volatile component was removed
under vacuum after the mixture was filtered over Celite. The
resulting yellow solid was rinsed with DCM 3 times and gave
the product as a white solid (0.28 g, 63%). 1H NMR (400 MHz,
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DMSO-d6): d = 8.76 (s, 2H, NCHN), 5.21 (m, 2H, NCH2), 4.73
(m, 2H, NCH2), 4.60 (m, 2H, NCH2), 4.32 (m, 2H, NCH2), 1.85
(m, 4H, NCH2CH2), 1.31 (m, 4H, CH2CH3), 0.90 (t, J = 7.2 Hz,
6H, CH3). 13C NMR (100 MHz, DMSO-d6): d = 158.7, 144.7, 51.8,
44.9, 31.3, 19.1, 13.4. Anal. Calc. for C14H24Cl2N6Pd (453.71 g
mol21): C, 37.06; H, 5.33; N, 18.52. Found: C, 36.89; H, 5.44; N,
18.68%.
(m, 1H, Ar–H), 6.98 (t, J = 8.0 Hz, 1H, Ar–H), 6.60 (d, 1H, Ar–H),
6.49 (s, 1H, Ar–H), 6.37 (d, 1H, Ar–H), 4.46 (q, 1H, CH), 4.11 (s,
1H, NH), 1.52 (d, 3H, CH3). 13C NMR (CDCl3, 100 MHz): 148.3,
144.5, 134.7, 130.0, 128.7, 127.1, 125.7, 117.1, 113.0, 111.4,
53.3, 24.8.
2-Chloro-N-(1-phenylethyl)benzenamine (6g). Light yellow
1
oil. H NMR (CDCl3, 400 MHz): d = 7.34–7.28 (m, 4H, Ar–H),
7.23–7.19 (m, 2H, Ar–H), 6.93 (t, J = 7.6 Hz, 1H, Ar–H), 6.54 (t, J
= 7.6 Hz, 1H, Ar–H), 6.39 (d, 1H, Ar–H), 4.68 (s, 1H, NH), 4.49
(q, 1H, CH), 1.56 (d, 3H, CH3). 13C NMR (CDCl3, 100 MHz):
144.5, 142.9, 128.9, 128.7, 127.6, 127.0, 125.6, 118.8, 117.1,
112.4, 53.3, 25.1.
N-(1-p-Tolylethyl)benzenamine (6h). Light yellow oil. 1H
NMR (CDCl3, 400 MHz): d = 7.25 (d, 2H, Ar–H), 7.12–7.06 (m,
4H, Ar–H), 6.63 (t, J = 7.2 Hz, 1H, Ar–H), 6.51 (d, 2H, Ar–H), 4.44
(q, 1H, CH), 3.99 (s, 1H, NH), 2.31(s, 3H, CH3), 1.49 (d, 3H,
CH3). 13C NMR (CDCl3, 100 MHz): 147.3, 142.1, 136.3, 129.3,
129.0, 125.7, 117.1, 113.2, 53.1, 25.0, 21.0.
4-Bromo-N-(1-p-tolylethyl)benzenamine (6i). Light yellow oil.
1H NMR (CDCl3, 400 MHz): d = 7.20 (m, 2H, Ar–H), 7.13 (m,
4H, Ar–H), 6.37 (d, 2H, Ar–H), 4.40 (q, 1H, CH), 4.05 (s, 1H,
NH), 2.31 (s, 3H, CH3), 1.49 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 146.2, 141.5, 136.6, 131.7, 129.3, 125.6, 114.8, 108.7,
53.1, 24.9, 21.0.
[1,2-Propanediylbis(1-butyl-1H-1,2,4-triazol-4(5H)-yl-5-ylide-
ne)]dichloropalladium (5). 1,19-Di-n-butyl-4,49-(1,2-propane-
diyl)bistriazolium dichloride (1 mmol, 0.36 g) was dissolved
in DMSO (10 mL) and Pd(OAc)2 (0.96 mmol, 0.22 g) was added
in small portions. The reaction mixture was heated at 90 uC
overnight with stirring. The volatile component was removed
under vacuum after the mixture was filtered over Celite. The
resulting yellow solid was rinsed with DCM 3 times and gave
1
the product as a white solid (0.35 g, 77%). H NMR (400 MHz,
DMSO-d6): d = 8.70 (s, 2H, NCHN), 4.79 (m, 2H, NCH2), 4.61 (m,
4H, NCH2), 4.33 (m, 2H, NCH2), 2.50 (m, 2H, NCH2CH2), 1.94
(m, 2H, NCH2CH2), 1.77 (m, 2H, CH2), 1.41 (m, 4H, CH2CH3),
0.92 (t, J = 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, DMSO-d6): d =
163.0, 145.0, 51.6, 48.8, 30.8, 19.2, 13.5. Anal. calc. for
C15H26Cl2N6Pd (467.73 g mol21): C, 38.52; H, 5.60; N, 17.97.
Found: C, 38.73; H, 5.44; N, 17.82%.
Characterization data for hydroamination products
4-Fluoro-N-(1-p-tolylethyl)benzenamine (6j). Light yellow oil.
1H NMR (CDCl3, 400 MHz): d = 7.24 (d, 2H, Ar–H), 7.13 (m, 2H,
Ar–H), 6.78 (t, J = 8.8 Hz, 2H, Ar–H), 6.42 (m, 2H, Ar–H), 4.38 (q,
1H, CH), 3.91 (s, 1H, NH), 2.31 (s, 3H, CH3), 1.49 (d, 3H, CH3).
13C NMR (CDCl3, 100 MHz): 156.7, 154.4, 143.62, 143.61, 142.0,
136.5, 129.3, 125.7, 115.5, 115.3, 114.0, 113.9, 53.7, 25.1, 21.0.
4-Methyl-N-(1-p-tolylethyl)benzenamine (6k). Light yellow
N-(1-Phenylethyl)benzenamine (6a). Light yellow oil. 1H
NMR (CDCl3, 400 MHz): d = 7.35 (d, 2H, Ar–H), 7.33–7.29 (t,
2H, Ar–H), 7.24–7.20 (m, 1H, Ar–H), 7.08 (t, J = 7.6 Hz, 2H, Ar–
H), 6.64 (t, J = 7.2 Hz, 1H, Ar–H), 6.51 (d, 2H, Ar–H), 4.47 (q, 1H,
CH), 4.02 (s, 1H, NH), 1.52 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 147.3, 145.2, 129.0, 128.6, 126.8, 125.8, 117.2, 113.3,
53.4, 25.0.
1
oil. H NMR (CDCl3, 400 MHz): d = 7.23 (m, 2H, Ar–H), 7.12
4-Bromo-N-(1-phenylethyl)benzenamine (6b). Light yellow
oil. 1H NMR (CDCl3, 400 MHz): d = 7.31 (d, 4H, Ar–H), 7.28 (m,
1H, Ar–H), 7.15 (d, 2H, Ar–H), 6.37 (d, 2H, Ar–H), 4.43 (q, 1H,
CH), 4.06 (s, 1H, NH), 1.52 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 146.1, 144.5, 131.7, 128.7, 127.0, 125.7, 114.8, 108.8,
53.4, 24.9.
(d, 2H, Ar–H), 6.90 (d, 2H, Ar–H), 6.44 (d, 2H, Ar–H),4.42 (q, 1H,
CH), 3.88 (s, 1H, NH), 2.31 (s, 3H, CH3), 2.17 (s, 3H, CH3), 1.48
(d, 3H, CH3). 13C NMR (CDCl3, 100 MHz): 145.0, 142.3, 136.2,
129.5, 129.2, 126.2, 125.7, 113.3, 53.3, 25.0, 21.02, 21.03.
4-Chloro-N-(1-p-tolylethyl)benzenamine (6l). Light yellow oil.
1H NMR (CDCl3, 400 MHz): d = 7.22 (m, 2H, Ar–H), 7.13 (d, 2H,
Ar–H), 7.02 (d, 2H, Ar–H), 6.42 (d, 2H, Ar–H), 4.41 (q, 1H, CH),
4.03 (s, 1H, NH), 2.31 (s, 3H, CH3), 1.49 (d, 3H, CH3). 13C NMR
(CDCl3, 100 MHz): 145.8, 141.6, 136.5, 129.3, 128.8, 125.6,
121.6, 114.3, 53.2, 25.0, 21.0.
3-Chloro-N-(1-p-tolylethyl)benzenamine (6m). Light yellow
oil. 1H NMR (CDCl3, 400 MHz): d = 7.23 (d, 2H, Ar–H), 7.13 (d,
2H, Ar–H), 6.97 (t, J = 8 Hz, 1H, Ar–H), 6.59 (d, 1H, Ar–H), 6.49
(d, 1H, Ar–H), 6.37 (d, 1H, Ar–H), 4.44 (q, 1H, CH), 4.09 (s, 1H,
NH), 2.32 (s, 3H, CH3), 1.49 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 148.4, 141.6, 136.6, 134.7, 130.0, 129.4, 125.6, 117.0,
112.9, 111.4, 53.0, 24.8, 21.0.
2-Chloro-N-(1-p-tolylethyl)benzenamine (6n). Light yellow
oil. 1H NMR (CDCl3, 400 MHz): d = 7.24 (d, 3H, Ar–H), 7.13
(d, 2H, Ar–H), 6.95 (t, J = 7.6 Hz, 1H, Ar–H), 6.55 (t, J = 7.6 Hz,
1H, Ar–H), 6.41 (d, 1H, Ar–H), 4.48 (q, 1H, CH), 4.67 (s, 1H,
NH), 2.31 (s, 3H, CH3), 1.56 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 143.0, 141.5, 136.5, 129.3, 128.9, 127.6, 125.6, 118.8,
117.0, 112.4, 53.0, 25.1, 21.1.
4-Fluoro-N-(1-phenylethyl)benzenamine (6c). Light yellow
1
oil. H NMR (CDCl3, 400 MHz): d = 7.35–7.29 (m, 4H, Ar–H),
7.24–7.20 (m, 1H, Ar–H), 6.78 (m, 2H, Ar–H), 6.41 (m, 2H, Ar–
H), 4.41 (q, 1H, CH), 3.91 (s, 1H, NH), 1.52 (d, 3H, CH3). 13C
NMR (CDCl3, 100 MHz): 156.8, 154.4, 145.0, 143.6, 128.6,
126.9, 125.8, 115.6, 115.3, 114.1, 114.0, 54.0, 25.1.
4-Methyl-N-(1-phenylethyl)benzenamine (6d). Light yellow
1
oil. H NMR (CDCl3, 400 MHz): d = 7.37–7.29 (m, 4H, Ar–H),
7.24–7.21 (m, 1H, Ar–H), 6.89 (m, 2H, Ar–H), 6.43 (m, 2H, Ar–
H), 4.44 (q, 1H, CH), 3.92 (s, 1H, NH), 2.17 (m, 3H, CH3), 1.52
(d, 3H, CH3). 13C NMR (CDCl3, 100 MHz): 145.4, 145.0, 129.6,
128.6, 126.8, 126.3, 125.8, 113.4, 53.7, 25.0, 20.3.
4-Chloro-N-(1-phenylethyl)benzenamine (6e). Light yellow
oil. 1H NMR (CDCl3, 400 MHz): d = 7.33 (m, 4H, Ar–H), 7.24 (m,
1H, Ar–H), 7.03 (d, 2H, Ar–H), 6.42 (d, 2H, Ar–H), 4.44 (q, 1H,
CH), 4.04 (s, 1H, NH), 1.51 (d, 3H, CH3). 13C NMR (CDCl3, 100
MHz): 145.7, 144.6, 128.9, 128.7, 127.0, 125.7, 121.7, 114.3,
53.5, 25.0.
3-Chloro-N-(1-phenylethyl)benzenamine (6f). Light yellow
oil. H NMR (CDCl3, 400 MHz): d = 7.34 (m, 4H, Ar–H), 7.25
2,6-Dimethyl-N-(1-phenylethyl)benzenamine (6o). Light yel-
low oil. 1H NMR (CDCl3, 400 MHz): d = 7.35 (d, 2H, Ar–H),
1
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3 For selected examples for early transition-metals, see: (a)
F. Lauterwasser, P. G. Hayes, W. E. Piers, L. L. Schafer and
7.33–7.29 (t, 2H, Ar–H), 7.24–7.20 (m, 1H, Ar–H), 7.08 (t, J = 7.6
Hz, 2H, Ar–H), 6.64 (t, J = 7.2 Hz, 1H, Ar–H), 4.48 (q, 1H, CH),
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(CDCl3, 100 MHz): 147.2, 145.2, 129.0, 128.6, 126.8, 125.8,
117.2, 113.3, 53.4, 25.0, 16.6.
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A series of triazole-based di-NHC–palladium complexes with
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structures showed a chelating coordination of the di-NHC
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Acknowledgements
We gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (21071121 and 21172188), the
Scientific Research Foundation (SRF) for the Returned
Overseas Chinese Scholars (ROCS), the State Education
Ministry (SEM), the Priority Academic Program Development
of Jiangsu Higher Education Institutions (PAPD), the 333
project for the cultivation of high-level talents (33GC10002),
the provincial project for undergraduate research
(12ssjcxzd02), and the State Key Laboratory of Inorganic
Synthesis and Preparative Chemistry at Jilin University (2013-
03) for financial support.
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