Air-stable and highly efficient indenyl-derived phosphine ligand: Application to Buchwald-Hartwig amination reactions

Article abstract of DOI:10.1016/j.jorganchem.2012.02.007

2-Mesitylindenyl phosphine ligand (1) and [(2-mesitylindenyl)dicyclohexyl- phosphine]PdCl₂ (2) have been synthesized and fully characterized by NMR and elemental analysis, as well as by X-ray crystallography for 2. A Highly active catalyst system derived from a palladium precatalyst and bulky 2-mesitylindenyl phosphine ligand (1) for the Buchwald-Hartwig amination reaction of aryl halides with primary and secondary amines has been developed. This method allows for the preparation of a wide variety of amines in moderate to excellent yields and displays a high level of activity for the coupling of aryl chlorides as well as hindered aryl bromides.

Full text of DOI:10.1016/j.jorganchem.2012.02.007

Journal of Organometallic Chemistry 706-707 (2012) 99e105
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Air-stable and highly efficient indenyl-derived phosphine ligand: Application
to BuchwaldeHartwig amination reactions
,
a
a
c
b
Xiaowei Hao a, Jia Yuan a, Guang-Ao Yu a , Ming-Qiang Qiu , Neng-Fang She , Yue Sun , Cui Zhao ,
*
Shu-Lan Mao a, Jun Yin a, Sheng-Hua Liu a
,
*
a Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, People’s Republic of China
b Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
c Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei University, Wuhan 430062, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
2-Mesitylindenyl phosphine ligand (1) and [(2-mesitylindenyl)dicyclohexyl-phosphine]PdCl2 (2) have
been synthesized and fully characterized by NMR and elemental analysis, as well as by X-ray crystal-
lography for 2. A Highly active catalyst system derived from a palladium precatalyst and bulky 2-
mesitylindenyl phosphine ligand (1) for the BuchwaldeHartwig amination reaction of aryl halides
with primary and secondary amines has been developed. This method allows for the preparation of
a wide variety of amines in moderate to excellent yields and displays a high level of activity for the
coupling of aryl chlorides as well as hindered aryl bromides.
Received 28 November 2011
Received in revised form
27 January 2012
Accepted 1 February 2012
Keywords:
Amination
Cross-coupling
Halide
Ó 2012 Elsevier B.V. All rights reserved.
Palladium
Phosphine
1. Introduction
methods is still important for the development of versatile catalysts
for numerous applications of coupling reactions. We were inter-
Aromatic amines and their derivatives are of significant impor-
tance in organic chemistry and play an important role as interme-
diates for agrochemicals, pharmaceuticals, conducting polymers,
and dyes in the chemical industry. Palladium-catalyzed CeN bond-
ested in exploring indenyl-derived phosphine [24]. In this paper,
we describe the development of 2-mesitylindenyl dicyclohex-
ylphosphine as a novel ligand to activate the palladium catalyst for
the BuchwaldeHartwig amination reaction.
forming reactions have evolved into
a highly versatile and
synthetically attractive technique for targeting pharmaceutically
useful intermediates [1,2]. Since the discovery of the first catalytic
amination method [3e5], efforts have been made to increase the
reaction efficacy [6e8]. Notable ligands, such as tBu3P [9,10],
indenyl-phosphines and related phosphines based on CH-acidic
cyclopentadiene derivatives [11e13], Beller and co-workers’ PAP
[14], Buchwald and co-workers’ biaryl phosphines [15e18], Hartwig
and co-workers’ Q-Phos and CyPF-tBu [19,20], Verkade and co-
workers’ amino phosphine [21,22], and Kwong and co-workers’ 2-
indolylaryl phosphine [23] (Scheme 1) provide excellent catalytic
activity for the cross-coupling of aryl halides (especially aryl chlo-
rides) or aryl mesylate/arenesulfonate.
2. Results and discussion
2.1. Synthesis of (2-mesitylindenyl)dicyclohexylphosphine (1)
(2-Mesitylindenyl) phosphine ligand can be easily prepared
from 2-bromoindene and mesityl bromide (Scheme 2). 2-Mesity-
lindene was prepared according to the reported procedure [25],
then the straightforward deprotonation of 2-mesitylindene by
nBuLi and trapping of the lithiated intermediate by Cy2PCl afforded
the corresponding phosphine 1 in high yields. It is interesting that
this ligand is air-stable maybe because its structure is similar to that
of Buchwald’s ligand [26].
Although a variety of ligands have been introduced, the rapid
assembly of structurally diverse ligand systems via simple synthetic
2.2. Synthesis and X-ray crystal structure of [(2-mesitylindenyl)
dicyclohexyl-phosphine]PdCl2 (2)
* Corresponding authors. Tel./fax: þ86 27 67867725.
E-mail addresses: yuguang@mail.ccnu.edu.cn (G.-A. Yu), chshliu@mail.ccnu.edu.cn
(S.-H. Liu).
[(2-Mesitylindenyl)dicyclohexylphosphine]PdCl2 (2) was pre-
pared by stirring 2 equiv of 1 with PdCl2(CH3CN)2 in CH2Cl2.
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.02.007

100
X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
Table 1
Selected bond lengths (Å) and angles (ꢀ) for compound 2.
C(11)eP(1)
C(10)eC(11)
1.8280(19)
1.358(2)
C(10)eC(14)
Pd(1)eP(1)
C(20)eC(19)eP(1)
C(11)eP(1)ePd(1)
C(19)eP(1)ePd(1)
C(25)eP(1)ePd(1)
1.513(3)
2.3514(5)
113.85(13)
114.78(6)
108.44(6)
115.00(6)
Cl(1)ePd(1)eCl(1)#1
Cl(1)ePd(1)eP(1)
C(10)eC(11)eP(1)
C(12)eC(11)eP(1)
C(24)eC(19)eP(1)
180.00(2)
90.024(17)
132.25(15)
119.67(13)
110.62(13)
but a possible precursor to the active catalyst when PdCl2(CH3CN)2
is used as the precatalyst.
2.3. Pd complex of (2-mesitylindenyl)dicyclohexylphosphine in the
BuchwaldeHartwig amination reaction
With this 2-mesitylindenyl phosphine ligand, the feasibility of
promoting CeN couplings of aryl halide was investigated. Chloro-
benzene and aniline were used as the prototypical substrates in our
reaction (Table 2). In this work the base was not screened and
tBuONa was used. Of the solvents surveyed (toluene and DME),
DME provided the best product yield (Table 2, entries 1e6).
Screening of commonly used Pd sources indicated that Pd(dba)2
and PdCl2(CH3CN)2 were suitable sources and Pd(dba)2 showed the
highest catalytic ability for the chlorobenzene coupling reaction
(Table 2, entries 4 and 7). Complex 2 catalyzed amination with
a moderate efficiency, similar to the conventional Pd complexes
(Table 2, entry 6).
Scheme 1. Recent developments on effective phosphine ligands.
The scope of this reactionwas then investigated under optimized
conditions [Pd(dba)2 (1 mol %), ligand (2 mol %), tBuONa (1.4 equiv),
120 ꢀC], and the results are summarized in Table 3. In most cases, the
different aryl chloride reacted with aniline leading to the corre-
sponding products in good to excellent yields. For example, the
reaction of 4-chloroacetophenone with aniline gave rise to the
corresponding secondary amine in 66% yield (Table 3, entry 2),
while the reaction of 4-chloroanisole or 2-chlorobenzonitrile with
aniline afforded secondary amines in 51% or 16% yield, respectively
(Table 3, entries 3 and 4). The reaction of 1, 2-dichlorobenzene with
aniline afforded 48% of the diamine product (Table 3, entry 5), the
analog process with 2-chloropyrimidine occurred in moderate yield
(Table 3, entry 6). In addition, sterically hindered diphenylamine
was converted to the tertiary amine smoothly (Table 3, entry 8).
Besides aryl amine, the scope of the Pd/(2-mesitylindenyl)
dicyclohexylphosphine catalytic system can be extended to
Scheme 2. Synthesis of 1 and 2.
This complex is also indefinitely stable in air and in solution. The
molecular structure of 2 has been confirmed by single-crystal X-ray
diffraction (Fig. 1). Selected bond lengths and angles are listed in
Table 1. Complex 2 possesses a trans square planar geometry,
including a center of inversion about the palladium atom, in which
the sterically demanding 2-mesitylindene portion of the ligands are
pointed away from the crowded palladium center, the bond
distance Pd(1)eP(1) is 2.3514 Å, the C(10)eC(14) bond is in the
single bond range (distance between C(10)eC(14) is 1.513 Å) [26],
while the bond distance C(10)eC(11) is 1.358 Å, which is in the
range of a carbonecarbon double bond [26]. The distance of P(1) to
the indene plane is 0.0425 Å, so the P(1) atom is almost in the same
plane as the indene, while the distance of P(1) to the aryl ring is
2.8287 Å, the dihedral angle between the aryl ring and the indenyl
plane is 88.2ꢀ, which is similar to that of [2-(dicyclohex-
ylphosphino)-20,60-dimethoxyl-1,10-biphenyl]PdCl2 [27]. It is
important to note that 2 is not an intermediate in the catalytic cycle
Table 2
Screen of Pd sources and solvents for the coupling of phenyl chloride and aniline.a
Entry
[Pd]
Solvent
T (ꢀC)
Time [h]
Yield (%)b
1
2
3
4
5
6
7
8
9
10
Pd(dba)2
Toluene
Toluene
Toluene
DME
DME
DME
DME
DME
DME
DME
120
120
120
120
120
120
120
120
120
120
24
24
24
24
24
24
24
4
86
59
12
91
87
84
90
76
63
27
Pd(OAc)2
PdCl2(CH3CN)2
Pd(dba)2
Pd(OAc)2
2
PdCl2(CH3CN)2
Pd(dba)2
Pd(OAc)2
PdCl2(CH3CN)2
4
4
a
ArCl (1 mmol), aniline (1.2 mmol), [Pd] (1 mol%), [ligand] (2 mol%), tBuONa
Fig. 1. ORTEP drawing of 2. Thermal ellipsoids are drawn at the 30% probability level,
hydrogen atoms and solvent molecules are omitted for clarity.
(1.4 mmol), solvent (4 mL).
b
GC yields.

X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
101
Table 3
Palladium-catalyzed amination of aryl chloride and aniline.a
1% Pd(dba)2
2% Ligand
BuONa
R'
R'
HN
R''
Cl
+
N
t
R''
DME
Entry
1
ArCl
Amine
Product
Time [h]
24
Yield [%]b
89
66
2
3
24
24
51
4
5
24
24
16
48
6
7
24
24
44
10
8
24
37
a
t
ArCl (1 mmol), amine (1.2 mmol), BuONa (1.4 mmol), DME (4.0 mL), at 120 ꢀC under N2 for the indicated time.
Yield of isolated product.
b
heterocyclic amine. For example, the amination of chlorobenzene
To further demonstrate the general applicability of our catalyst
system, we studied the palladium-catalyzed BuchwaldeHartwig
aminations with primary alkylamine (Table 5). Again, we
adopted the optimized reaction conditions to the amination
of octylamine with various aryl chlorides. Chlorobenzene, 2-
chlorotoluene, 4-chlorobenzotrifluoride, and 2-chlorobenzonitrile
reacted smoothly in the amination with octylamine (Table 5,
with morpholine in DME gave the coupled product in 56% yield
(Table 4, entry 1), while the reactions of morpholine with 2-
chlorotoluene, 4-chlorobenzotrifluoride or 2-chlorobenzonitrile
afforded the corresponding products in 27e95% yields (Table 4,
entries 2e4). The reaction of morpholine with 2-chloropyridine in
24 h gave the desired product in 86% yield (Table 4, entry 5).

102
X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
Table 4
Palladium-catalyzed amination of aryl chloride and morpholine.a
Entry
1
ArX
Amine
Product
Time [h]
24
Yield (%)b
56
2
3
24
24
46
95
4
5
24
24
24
27
86
12
6
a
t
ArX (1 mmol), amine (1.2 mmol), BuONa (1.4 mmol), DME (4.0 mL), at 120 ꢀC under N2 for the indicated time.
Yield of isolated product.
b
entries 1e4). In addition, heterocycles such as 2-chloropyrimidine,
and 2-chloropyridine were aminated in moderate yield (Table 5,
entries 5 and 6).
system for BuchwaldeHartwig amination reactions. Couplings of
an electronically diverse array of aryl halides with amines are
realized in good to excellent yields. We have also described the
facile synthesis, structural characteristics, and catalytic behavior of
phosphine palladium complex 2. Explorations of the potential
applications of the new class of substituted indenyl phosphine
ligands/catalysts exemplified by 1 and its corresponding complexes
of type 2 in synthetically useful cross-coupling reactions are
underway.
The optimized reaction conditions were also effective for the
amination of sterically bulky aryl bromides with aniline, both 2-
bromomesitylene and 1-bromo-2,4,6-triisopropylbenzene gave
the expected products in good to excellent yield (Table 6, entries 1
and 2). We were pleased to find that the amination of octylamine
with 2-bromomesitylene provided the corresponding products in
moderate yield (Table 6, entry 3). Unfortunately, 1-bromo-2,4,6-
triisopropylbenzene do not work under the current reaction
conditions, the starting material remained, and the yield is too low
to be isolated (Table 6, entry 4). In addition, for reactions of these
two aryl bromides with morpholine, no tertiary amines were
observed (Table 6, entries 5 and 6).
4. Experimental section
4.1. General considerations
Unless otherwise noted, all reagents were purchased from
commercial suppliers and used without purification. All
BuchwaldeHartwig amination reactions were performed in
resealable screw cap Schlenk flask (approx. 20 mL volume) in the
presence of Teflon-coated magnetic stirrer bar (3 mm ꢁ 10 mm).
Toluene and DME were distilled from sodium benzophenone ketyl
3. Conclusion
In summary, the new air-stable ligand 1, which was synthesized
in high yield, generate a very active and broadly useful Pd catalyst

X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
103
Table 5
Palladium-catalyzed amination of aryl chloride and octylamine.a
Entry
1
ArCl
Amine
Product
Time [h]
24
Yield (%)b
OctylNH2
24
43
33
2
3
OctylNH2
OctylNH2
24
24
4
OctylNH2
OctylNH2
24
24
23
40
5
a
t
ArCl (1 mmol), amine (1.2 mmol), BuONa (1.4 mmol), DME (4.0 mL), at 120 ꢀC under N2 for the indicated time.
Yield of isolated product.
b
under nitrogen. Most commercially available amines were used as
mixture was warmed to room temperature and stirred for addi-
tional 12 h and the LiCl that formed was removed by filtration over
a pad of Celite under Schlenk conditions. The resulting filtrate was
treated dropwise with water. The organic layer was then separated
from the aqueous layer, dried over MgSO4, and filtered. Removal of
the solvent provided the white solid, which was dissolved in 10 mL
of CH2Cl2. After filtration the clear filtrate was dropped into hexane
(100 mL, vigorously stirred). The white precipitate that formed was
separated via suction filtration. Removal of the volatiles in vacuo
afforded 2 as a white solid (2.76 g, 82%). 1H NMR (400 MHz, CDCl3)
received. Some amines may require distillation depending on the
t
conditions. BuONa were purchased from Fluka. Silica gel (Merck,
70e230 and 230e400 mesh) was used for column chromatog-
raphy. 1H NMR spectra were recorded on a Mercury-Plus (400 MHz
or 600 MHz) spectrometer. Spectra were referenced internally to
the residual proton resonance in CDCl3
ramethylsilane (TMS, 0.00 ppm) as the internal standard. Chem-
ical shifts ( ) were reported as part per million (ppm) in scale
downfield from TMS. 13C NMR spectra were referenced to CDCl3 (
(d 7.26 ppm), or with tet-
d
d
d
d
77.0 ppm, the middle peak). 31P NMR spectra were referenced to
85% H3PO4 externally. Coupling constants (J) were reported in Hertz
(Hz). Mass spectra (EI-MS) were recorded on an HP 5989B Mass
Spectrometer. The products described in GC yield were accorded to
the authentic samples/dodecane calibration standard from
Agilent 6890 GC system. All yields reported refer to isolated yield of
compounds estimated to be greater than 95% purity as determined
by capillary gas chromatography (GC) or 1H NMR. Compounds
described in the literature were characterized by comparison
of their 1H, and/or 13C NMR spectra to the previously reported
data. 2-Mesitylindene was prepared according to the literature
procedure [25].
d
7.69 (d, J ¼ 8.4 Hz, 1H), 7.48 (d, J ¼ 8.4 Hz, 1H), 7.36e7.32 (m, 1H),
7.26e7.22 (m, 1H), 6.90 (s, 2H), 3.56 (s, 2H), 2.27 (s, 3H), 2.14 (s, 6H),
1.84e1.14 (m, 22H); 13C NMR (100 MHz, CDCl3)
161.3, 147.2, 143.2,
d
136.7, 136.3, 135.2, 134.5, 128.0, 126.2, 124.1, 123.5, 122.2, 44.1, 34.6,
32.4, 30.9, 27.3, 27.2, 26.2, 21.1, 20.9; 31P{1H} NMR (243 MHz,
CDCl3):
d
ꢂ15.17; MS (EI): 430.11(Mþ); Anal. Calcd for C30H39P: C,
83.68, H, 9.13; Found: C, 83.60, H, 9.24.
4.3. Preparation of [(2-mesitylindenyl)dicyclohexyl-phosphine]
PdCl2 (2)
To a stirred solution of PdCl2(CH3CN)2 in dry CH2Cl2 was added 1
(100 mg, 0.23 mmol) at room temperature. The reaction mixture
was stirred for 1 h, and then the volatiles were removed, leaving 2
as a bright yellow solid (113 mg, 95%). 1H NMR (400 MHz, CDCl3)
4.2. Preparation of (2-mesitylindenyl)dicyclohexylphosphine (1)
In a 100 mL flask 2-mesitylindene (1.83 g, 7.8 mmol) was dis-
solved in Et2O (50 mL) under an argon atmosphere. The mixture
was cooled to ꢂ78 ꢀC, and nBuLi (3.2 mL, 2.5 M solution in hexane,
8.0 mmol) was added. The solution was stirred for 30 min at ꢂ78 ꢀC
and then for 12 h at ambient temperature. Then the mixture was
cooled to ꢂ60 ꢀC and Cy2PCl (1.8 g, 7.8 mmol) was added. The
d
7.32 (b, 4H), 7.19 (b, 4H), 6.81 (b, 4H), 3.53 (s, 4H), 2.22e1.07 (m,
62H); 13C NMR (100 MHz, CD2Cl3)
156.2, 147.0, 141.6, 136.9, 134.8,
134.0, 127.5, 125.4, 124.9, 124.2, 122.3, 46.5, 29.5, 27.0, 26.0, 22.6,
21.8, 20.4; 31P{1H} NMR (243 MHz, CDCl3):
20.34; Anal. Calcd for
C60H78Cl2P2Pd: C, 69.39, H, 7.57; Found: C, 69.52, H, 7.60.
d
d

104
X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
Table 6
Palladium-catalyzed amination of aryl bromide.a
Entry
1
ArX
Amine
Product
Time [h]
24
Yield (%)b
98
2
24
83
3
OctylNH2
24
35
4
5
OctylNH2
24
24
<5
<5
6
24
<5
a
t
ArBr (1 mmol), amine (1.2 mmol), BuONa (1.4 mmol), DME (4.0 mL), at 120 ꢀC under N2.
Yield of isolated product.
b
4.4. General procedures for reaction condition screenings
dodecane (170 mg, 1.0 mmol, internal standard) and water were
added. The organic layer was subjected to GC analysis. The GC yield
obtained was previously calibrated by authentic sample/dodecane
calibration curve. Purified by column chromatography (hexane/
ethyl acetate as eluent) to afford the desired product.
Pd source (0.010 mmol), phosphine ligand (1) (8.6 mg,
t
0.020 mmol) and BuONa (134 mg, 1.4 mmol) were loaded into
a Schlenk tube equipped with a Teflon-coated magnetic stir bar. The
tube was evacuated and flushed with nitrogen for three times.
Chlorobenzene (112 mg, 1.0 mmol), aniline (111 mg, 1.2 mmol) and
solvent (4.0 mL) were loaded into the tube. The tube was then
placed into a preheated oil bath and stirred for the time period as
indicated in Table 2. After completion of reaction, the reaction tube
was allowed to cool to room temperature. Ethyl acetate (10 mL),
4.5. General procedures for palladium-catalyzed amination of aryl
halides
Pd(dba)2/1-catalyzed amination of aryl halides: An oven-dried
Schlenk tube equipped with a magnetic stirring bar was charged

X. Hao et al. / Journal of Organometallic Chemistry 706-707 (2012) 99e105
105
isotropic thermal parameters. The structures were refined on F2 by
full-matrix least-squares methods using the SHELXTL-97 program
package [28]. The crystal data and structural refinements details are
listed in Table 7.
Table 7
Crystal data and structure refinements for 2.
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C60H78Cl2P2Pd
1038.46
298(2)
Monoclinic
P2(1)/n
12.6537(9)
16.6557(12)
12.9308(9)
90
97.1040(10)
90
2704.3(3)
4, 1.275
0.538
1096
Acknowledgments
b (Å)
c (Å)
We thank the National Natural Science Foundation of China
(nos. 21072071) and the Science Foundation of Central China
Normal University for financial support.
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
Volume (Å3)
Z, Dcalcd (mg mꢂ3
)
Appendix. Supplementary data
Absorption coefficient (mmꢂ1
F(000)
)
Crystal size (mm3)
0.16 ꢁ 0.12 ꢁ 0.10
2.00e28.34
33025
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2012.02.007.
q
range (ꢀ)
Reflections collected
Independent reflections
6689 [Rint ¼ 0.0482]
Completeness to
q (%)
99.1
References
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.9482 and 0.9189
6689/0/298
0.947
0.0333, 0.0829
0.0445, 0.0860
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Crystals of 2 for X-ray diffraction were obtained by recrystalli-
zation of the pure product from dichloromethane/hexane layers.
Crystallographic data was collected on a Bruker SMART CCD area-
detector diffractometer with graphite-monochromated Mo
radiation (
Ka
l
¼ 0.71073 Å). Diffraction measurements were made at
room temperature. An absorption correction by SADABS was applied
to the intensity data. The structures were solved by Patterson
method. The remaining non-hydrogen atoms were determined from
the successive difference Fourier syntheses. All non-hydrogen atoms
were refined anisotropically except those mentioned otherwise. The
hydrogen atoms were generated geometrically and refined with