Iron(0), rhodium(I) and palladium(II) complexes with p-(N,N- dimethylaminophenyl) diphenylphosphine and the application of the palladium complex as a catalyst for the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/aoc.1801

The reaction of p-(N,N-dimethylaminophenyl)diphenylphosphine [PPh ₂(p-C₆H₄NMe₂)] with [Fe ₃(CO)₁₂], [Rh(CO)₂Cl]₂ and PdCl ₂ resulted in three new mononuclear complexes, {Fe(CO) ₄[I·¹-(P)-PPh₂(p-C ₆H₄NMe₂)]} (1a), trans-{Rh(CO) Cl[I·¹-(P)-PPh₂(p-C₆H ₄NMe₂)]₂} (2) and trans-{PdCl ₂[I·¹-(P)-PPh₂(p-C ₆H₄NMe₂)]₂} (3), respectively. A small amount of dinuclear nonmetal-metal bonded complex, {Fe₂(CO) ₈[μ-(P,N)-PPh₂(p-C₆H₄NMe ₂)]} (1b), was also isolated as a side product in the reaction of [Fe₃(CO)₁₂]. The complexes were characterized by elemental analyses, mass, IR, UV-vis, ¹H, ¹³C (except 1b) and ³¹P{¹H} NMR spectroscopy. The Pd complex 3 effectively catalyzes the Suzuki-Miyaura cross-coupling reactions of aryl halides with arylboronic acids in water-isopropanol (1:1) at room temperature. Excellent yields (up to 99% isolated yield) were achieved. The effects of different solvents, bases, catalyst quantities were also evaluated. Copyright

Full text of DOI:10.1002/aoc.1801

Full Paper
Received: 11 January 2011
Revised: 14 March 2011
Accepted: 19 March 2011
Published online in Wiley Online Library: 5 May 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1801
Iron(0), rhodium(I) and palladium(II) complexes
with p-(N,N-dimethylaminophenyl)
diphenylphosphine and the application of the
palladium complex as a catalyst for the
Suzuki–Miyaura cross-coupling reaction
∗
Chandan Sarmah, Malabika Borah and Pankaj Das
The reaction of p-(N,N-dimethylaminophenyl)diphenylphosphine [PPh2(p-C6H4NMe2)] with [Fe3(CO)12], [Rh(CO)2Cl]2 and
1
1
PdCl2 resulted in three new mononuclear complexes, {Fe(CO)4[η -(P)-PPh2(p-C6H4NMe2)]} (1a), trans-{Rh(CO)Cl[η -(P)-PPh2(p-
1
C6H4NMe2)]2} (2) and trans-{PdCl2[η -(P)-PPh2(p-C6H4NMe2)]2} (3), respectively. A small amount of dinuclear nonmetal-metal
bonded complex, {Fe2(CO)8[µ-(P,N)-PPh2(p-C6H4NMe2)]} (1b), was also isolated as a side product in the reaction of [Fe3(CO)12].
1
13
31
1
The complexes were characterized by elemental analyses, mass, IR, UV–vis, H, C (except 1b) and P{ H} NMR spectroscopy.
The Pd complex 3 effectively catalyzes the Suzuki–Miyaura cross-coupling reactions of aryl halides with arylboronic acids in
water–isopropanol (1 : 1) at room temperature. Excellent yields (up to 99% isolated yield) were achieved. The effects of different
solvents, bases, catalyst quantities were also evaluated. Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Keywords: aminophosphine ligand; metal complex; Suzuki-Miyaura reaction; aqueous solvent; room temperature
Introduction
been the most commonly employed ligands for this reaction.
Some of the most influential phosphine-based ligands used in
[
22,23]
During the last two decades, transition metal complexes bear-
ing phosphorous and nitrogen donor atoms occupying the
same ligand framework have received tremendous interest be-
cause of their structural diversity, unusual reactivity and catalytic
applications.[ Previous studies have demonstrated that metal
complexescontainingsuchligandscanbeexploitedascatalystsfor
a number of reactions, including rhodium-catalyzed hydrogena-
Suzuki–Miyaura reactions are simple tertiary,
hemilabile-
and other
type,[
24–26]
sterically crowded biphenyl-type
[27,28]
electron-rich phosphines.[
29,30]
In fact, there exist two reports in
which P,N-based ligands containing a p-phenyl backbone resulted
1,2]
[31,32]
indramaticperformancesincross-couplingreactions.
Thus,as
[
33,34]
part of our ongoing research into Suzuki–Miyaura reactions,
we explored the catalytic potential of the palladium–phosphine
complex 3 for the Suzuki–Miyaura reaction of aryl halides with
arylboronic acids. The effects of different solvents, bases and
catalyst quantities were also evaluated.
[
3–5]
tion and hydroformylation reactions,
palladium-catalyzed
and iron-catalyzed polymerization
Itisnowwellestablishedthattheligandbackbones
[
6–10]
cross-coupling reactions
reactions.[
11,12]
play a dominant role in the performance of such phosphine-based
catalysts. Until now, the major focus has been given to P,N donor
ligands that have flexible backbone unit, because the metal com- Experimental Section
plexes with such ligands often exhibit hemilabile behavior, i.e.
General Information
reversible decoordination and coordination of the weakly bonded
atom.[
2,13,14]
On the other hand, ligands possessing rigid and
The starting complex [Fe3(CO)12] was purchased from
Acros Chemicals. [Rh(CO)2Cl]2 and the ligand, p-(N,N-
dimethylaminophenyl)diphenylphosphine [PPh2(p-C6H4NMe2)],
werepurchasedfromAldrich. PdCl2 andothernecessarychemicals
were purchased from Rankem, India. The solvents used were of an-
alytical grade and distilled prior to utilization. Elemental analyses
were recorded by using an Elementar Vario EL III Carlo Erba 1108.
robust backbones have received relatively less attention. p-(N,N-
Dimethylaminophenyl)diphenylphosphine [PPh2(p-C6H4NMe2)],
is one of the least studied rigid backbone P,N-based phosphine
ligand. A literature survey revealed only a few complexes with this
[
15]
[16,17]
[18]
ligand, e.g. iron,
cyclometallated palladium,
rhodium,
copper[ and tungsten complexes.
19]
[20]
Thus, in order to extend
the scope of this ligand further, we report here the synthesis
and characterization of Fe(0), Rh(I) and Pd(II) complexes with
PPh2(p-C6H4NMe2).
∗
Correspondence to: Pankaj Das, Department of Chemistry, Dibrugarh Univer-
Palladium-catalyzed Suzuki–Miyaura cross-coupling reactions,
involving aryl halides with arylboronic acids in the presence of
a base, have emerged as one of the most versatile methods for
sity, Dibrugarh-786004, Assam, India. E-mail: pankajd29@yahoo.com
Department of Chemistry, Dibrugarh University, Dibrugarh-786004, Assam,
India
[
21]
the formation of C–C bonds. For many years, phosphines have
Appl. Organometal. Chem. 2011, 25, 552–558
Copyright ꢀc 2011 John Wiley & Sons, Ltd.

Iron(0), rhodium(I) and palladium(II) complexes with p-(N,N-dimethylaminophenyl)diphenylphosphine
+
+
+
3
3
[M + 2] , 627 [M − CH3 − 1] ; 474 [M − Fe(CO)4 + 1] ; 306
+
4
3
2
2
4
3
[PPh (p-C H NMe ) + 1] . IR (KBr): 2040 (s), 1959 (w), 1930 (m),
2
6
4
2
−
1
1
1
926 (w) cm , ν(CO). UV–vis (CHCl3): λmax(nm): 292, 244. H NMR
2
,3,4,6
7
1
1
(δ ppm): 7.27–7.67 (m, 12H, Ph, H
4.13 (s, 6H, CH3). P{ H} NMR (δ ppm): 68.18(s).
), 6.71–6.69 (m, 2H, Ph, H ),
2
2
31
1
P
5
6
7
6
7
1
Synthesis of {Rh(CO)Cl[η -(P)-PPh2(p-C6H4NMe2)]2} (2)
A solution of the PPh (p-C H NMe ) ligand (251 mg; 0.824 mmol)
2
6
4
2
8
in30 mlofCH2Cl2 wasaddeddropwisetoasolutionof[Rh(CO)2Cl]2
160 mg; 0.412 mmol) in 45 ml of CH2Cl2 for about 10 min. The
(
N
reaction mixture was stirred under a nitrogen atmosphere at
room temperature for 1 h. The color of the solution changed
from yellow to dark brown. The solvent was evaporated partially
in open atmosphere and, on treatment with 10 ml of hexane,
complex 2 was precipitated as a bright yellow solid which was
washed with hexane and dried in vacuo. Yield: 96%. Anal. calcd
H C
CH3
3
Scheme 1. Numbering of atom positions in phenyl rings.
IR spectra were recorded in KBr using a Shimadzu IR prestige-21
for C41H40N2OP2ClRh; C, 63.38%; H, 5.15%; N, 3.60%. Found: C,
−
1
1
FTIR spectrophotometer in the range 4000–250 cm . The
H
+
6
2.78%; H, 5.12%; N, 3.57%. MS-FAB m/z = 775 [M − 2] , 741 [M
and 3 P{ H} NMR spectra were recorded in CDCl3 operating at
1
1
+
+
−
Cl − 1] ; 712 [M − Cl − CO − 2] ; 304 [PPh2(p-C6H4NMe2) −
3
00.13 and 121.50 MHz, respectively, on a Bruker 300 MHz spec-
+
−1
1
] . IR (KBr): 1975 cm ν(CO). UV–vis (CH2Cl2): λmax (nm), 285.
1
3
trometer. The C NMR spectra were recorded on a Jeol JNM-ECS
1
3
H NMR (δ ppm): 7.64–7.70 (m, 8H, Ph, H ), 7.38–7.50 (m, 16H,
Ph, H ), 6.68–6.72 (m, 4H, Ph, H ), 2.98 (s, 12H, CH3). C NMR
1
13
4
00 MHz spectrometer operating at 100.52 MHz. The H and
NMR assignments were made with respect to the labeling chart
Scheme 1). The UV–vis spectra of the complexes were recorded
on a Shimadzu UV 1700. The FAB mass spectra were recorded on a
Jeol SX 102/DA-6000 mass spectrometer using argon/xenon (6 kV,
C
2
,4,6
7
13
8
1
2,6
(
(
(
δ ppm): 184.21 (CO), 153.48 (C ), 136.93 (C ), 133.13 (C ), 131.88
(
4
3
5
7
31
1
C ), 131.79 (C ), 128.33 (C ),111.67 (C ), 39.56 (CH3). P{ H}NMR
δ ppm): 42.83 (d, JRh−P = 178 Hz).
1
0 mA) as the FAB gas. The accelerating voltage was 10 kV and
the spectra were recorded at room temperature. The electrospray
mass spectra were recorded on a Thermo Finnigan LCQ Advantage
max ion trap mass spectrometer. The ion spray voltage was set at
1
Synthesis of the Complex {PdCl2-[η -(P)-PPh2(p-C6H4NMe2)]2}
(
3)
5
.3 kV and the capillary voltage was 34 V.
A solution of the ligand PPh2(p-C6H4NMe2) (345 mg; 1.13 mmol)
in 20 ml acetonitrile was added dropwise to a solution of PdCl2
(100 mg; 0.56 mmol) in 10 ml of acetonitrile. The resultant solution
1
Synthesis of {Fe(CO)4[η -(P)-PPh2(p-C6H4NMe2)]} (1a) and
{Fe2(CO)8[µ-(P,N)-PPh2(p-C6H4NMe2)]} (1b)
was refluxed under an N atmosphere for 3 h. A yellow-colored
2
compound was precipitated. After filtration and washing the
To a tetrahydrofuran solution of [Fe3(CO)12] (302 mg; 0.60 mmol),
PPh2(p-C6H4NMe2) (366 mg; 1.2 mmol) was added. The reaction
mixture was refluxed under nitrogen for 3 h, during which time
the color of the solution changed from green to dark brown.
After cooling, the reaction mixture was filtered and the solvent
was removed under reduced pressure to afford a dark brown
solid. The residue was dissolved in minimum amount of CH2Cl2
and then chromatographed on a silica gel column. Elution of
hexane–CH2Cl2 (80 : 20) gave complex 1a as a dark brown solid
and 1b as a yellow solid.
residue with acetonitrile, complex 3 was isolated as a bright
yellow solid. Yield: 86%. Anal. calcd for C40H40Cl2N2P2Pd: C, 60.97;
H, 5.11; N, 3.55. Found: C, 59.87; H, 5.09; N, 3.53. MS-ESI m/z = 665
+
+
[
M − C6H4NMe2] , 484 [M − {PPh2(p-C6H4NMe2)} + 1] , 448 [M
+
+
−
Cl − [PPh2(p-C6H4NMe2)] + 1] ; 306 [PPh2(p-C6H4NMe2) + 1] .
1 1
−
−1
IR (KBr): 433 cm , ν(Pd–P); 358 cm , ν(Pd–Cl). H NMR (δ ppm):
.60–7.67 (m, 8H, Ph, H ), 7.21–7.41 (m, 16H, H ), 6.67–6.69 (m,
H, H ), 2.98 (s, 12H, CH3). C NMR (δ ppm): 151.28 (C ), 136.73
C ) 134.56 (C ), 134.01 (C ), 129.93 (C ), 129.63(C ), 127.69(C ),
3
2,4,6
7
4
(
7
13
8
1
6
2
4
3
5
7
31
1
1
11.23(C ), 40.03 (CH3). P{ H}: 22.12 (s); UV-vis (CHCl3): λmax
(nm), 334, 400.
Complex 1a
Yield: 52%. Anal. calcd for C24H20NO4PFe: C, 60.89% (60.91); H,
4
.23%; N, 2.96%. Found: C, 60.16%; H, 4.21%; N, 2.93%. MS-ESI
General Information about Catalytic Experiments
+
+
+
m/z = 474 [M + 1] , 445 [M − CO] ; 418 [M − 2CO + 1] ;
+
The Suzuki–Miyaura cross-coupling reactions were carried out
under aerobic conditions. The progress of the reactions was
monitored by thin-layer chromatography using aluminum-coated
TLC plates (Merck) under UV light. The products were purified
by column chromatographic technique using silica gel (60–120
mesh). The various products separated were characterized by
3
06 [PPh2(p-C6H4NMe2) + 1] . IR (KBr): 2044(s), 1971(s), 1930(s)
−
1
1
cm , ν(CO). UV–vis (CHCl3): λmax(nm), 290, 243. H NMR (δ
ppm): 7.42–7.67 (m, 12H, Ph, H
2
,3,4,6
7
), 6.71–6.69 (m,2H,Ph,H ), 3.02
1
3
8
1
(s,6H,CH3). C NMR (δ ppm): 213.85 (CO), 151.68 (C ), 135.02 (C ),
6
2
4
3
5
1
33.03 (C ), 132.93 (C ), 130.51 (C ), 128.56 (C ), 128.46 (C ), 111.42
7
31
1
(C ), 40.31 (CH3); P{ H} NMR (δ ppm): 46.05(s).
1
melting point, mass spectroscopy and H NMR spectroscopy,
and compared with the authentic samples. Mass spectra of the
compounds were recorded in on a Jeol GCmate instrument in
EI mode. The melting points were determined using Buchi B450
Complex 1b
+
Yield: 12%. Anal. calcd for C28H20O8NPFe2: C, 52.41%; H, 3.12%; N,
2
.18%. Found: C, 51.91%; H, 3.10%; N, 2.21%. MS-ESI m/z = 643
melting point apparatus.
Appl. Organometal. Chem. 2011, 25, 552–558
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

C. Sarmah, M. Borah and P. Das
1
General Procedure for the Suzuki–Miyaura Reactions of Aryl
Halides using Complex 3 as Catalyst
carbonyl group in 1b. The H NMR data clearly show the coordina-
tion of the ligand through the N-atom, as the methyl proton has
shifted significantly downfield compared with the free ligand or
A 50 ml round-bottomed flask was charged with a mixture of
aryl halide (1 mmol), arylboronic acid (1.1 mmol), K2CO3 (3 mmol),
complex 3 (appropriate quantity) and solvent (6 ml). The mixture
was stirred at room temperature for the required time. After
completion, the mixture was diluted with water (20 ml) and
extracted with ether (3 × 20 ml). The combined extract was
washed with brine (3 × 20 ml) and dried over Na2SO4. After
evaporation of the solvent under reduced pressure, the residue
was chromatographed (silica gel, ethyl acetate–hexane, 1 : 9) to
obtain the desired products.
1
η -(P)-coordinated complex 1a. Compared with the complex 1a,
3
1
1
the P{ H} NMR of complex 1b shows a downfield shift of about
2
2 ppm, indicating bidentate nature of the ligand in complex 1b.
1
Synthesis and Characterization of {Rh(CO)Cl[η -(P)-PPh2(p-
C6H4NMe2)]2} (2)
The chlorobridged dimer, [Rh(CO) Cl] undergoes a bridge split-
ting reaction with four molar equivalents of the aminophos-
phine ligand, PPh (p-C H NMe ) in CH Cl to yield monocarbonyl
2
2,
2
6
4
2
2
2
1
complex {Rh(CO)Cl[η -(P)-PPh2(p-C6H4NMe2)]2} (2) (Scheme 2), in
which the two aminophosphine ligands are bonded to rhodium
center through phosphorous atoms. Elemental analyses and FAB-
mass spectra of the complex 2 are consistent with the above
formulation. In the mass spectra, complex 2 shows a low-intensity
molecular ion peak at m/z = 775 along with a moderate intense
peak due to M − Cl. The base peak appears at m/z = 714 (100%),
Results and Discussion
1
Synthesis and Characterization of {Fe(CO)4[η -(P)-PPh2(p-
C6H4NMe2)]} (1a) and {Fe2(CO)8[µ-(P,N)-PPh2(p-C6H4NMe2)]}
(1b)
+
2 6 4 2 2
which corresponds to the [Rh(PPh (p-C H NMe ) ] fragment,
The reaction of [Fe3(CO)12] with phosphine-based ligands to
synthesize mono- and binuclear complexes is well established
but, in the majority of cases, addition of a decarbonylat-
ing agent such as trimethylamine N-oxide is required.[
−
formed by the removal of CO and Cl ion from the complex. The
IR spectrum of complex 2 in KBr shows one strong terminal ν(CO)
35]
−1
band at 1975 cm , consistent with square planar Rh(I) carbonyl
In
3
1
this work, we have found that the reaction of [Fe3(CO)12]
with p-(N,N-dimethylaminophenyl)diphenylphosphine [PPh2(p-
C6H4NMe2)] in refluxing THF proceeded without the addition
of a decarbonylating agent to produce a mononuclear com-
complex. The P NMR spectra of the complex show one doublet
at δ 42.83 ppm (JRhP = 178 Hz). Theoretically there are two possi-
bilities that the two phosphorus atoms occupy either the mutual
trans or cis position. However, the presence of a single 31P NMR
signal and the high JRhP value indicate two phosphorous atoms
1
plex, {Fe(CO)4[η -P-PPh2(p-C6H4NMe2)]} (1a) as the major product
occupying a mutual trans position.[
43,44]
The H NMR spectra of the
1
along with a small amount of nonmetal–metal bonded binuclear
complex, {Fe2(CO)8[µ-P,N-PPh2(p-C6H4NMe2)]} (1b) as a minor
product (Scheme 2). The yields of the complexes were 52 and
complex 2 shows the aromatic protons as multiplets in the range δ
6.58–7.70 ppm. The methyl proton appears at δ 2.98 ppm, which
1
2% respectively. The elemental analyses and ESI-mass spectra of
is almost same as the free ligand, indicating that the NMe group
2
1
3
the complexes were in excellent agreement with the proposed
compositions. For example, the ESI-mass spectrum of complex
1
does not take part in bonding. The C NMR spectra show carbonyl
resonance at δ 184.21 ppm, methyl resonance at δ 39.56 ppm and
aromatic carbon resonances in the range 111–154 ppm.
a showed a high-intensity molecular ion peak at m/z = 474
(
100%) along with two fragment ion peaks at m/z = 445 and 418,
corresponding to the sequential loss of two CO ligands. The IR
1
Synthesis and Characterization of {PdCl2-[η -(P)-PPh2(p-
spectra of 1a in KBr exhibited three distinct terminal ν(CO) bands
at 2044s, 1971s and 1930s cm ; this pattern is consistent with
the other reported tetracarbonyl iron complexes.
ber and intensities of CO stretching frequencies are often used
to draw conclusions about axial and equatorial arrangement of
ligands in complexes of the type Fe(CO)4(phosphine).
based on the ν(CO) values of some reported Fe(CO)4(phosphine)-
type complexes,[
C6H4NMe2)]2} (3)
−
1
Treatment of PdCl2 with two equivalents of the ligand,
[
36,37]
The num-
PPh2(p-C6H4NMe2), in refluxing acetonirile under an N2 atmo-
1
sphere afforded the mononuclear complex, {PdCl2-[η -(P)-PPh2(p-
C6H4NMe2)]2} (3) (Scheme 2) as an air-stable solid. Similar to
complexes 1 and 2, the elemental analyses and mass spectra are in
good agreement with the proposed composition. The coordina-
tion of the aminophosphine ligand through phosphorus atom was
[
36,38]
Thus,
36,38–40]
we propose that the phosphine ligand
3
1
1
in complex 1a is axially located. The P{ H} NMR spectrum
3
1
1
confirmed by P{ H}-NMR spectroscopy, which showed a singlet
shows a singlet at δ 46.05 ppm. Compared with the free ligand
at δ 22.12 ppm and, compared with the free ligand this value had
(
δ −6.0 ppm),complex1ashowsadownfieldshiftofabout52 ppm
1
shifted 28 ppm downfield. The H NMR spectra show, in addition
indicates coordination of the aminophosphine ligand through the
to aromatic protons, a sharp singlet at δ 2.98 for methyl protons,
1
phosphorus atom. The H NMR spectra of the complex shows, in
addition to the aromatic proton, a strong singlet at δ 3.02 ppm,
which is almost the same as that of the free ligand, indicating
which is almost same of that of the free ligand, clearly indicating
that the amine group is not involved in coordination. The 13
C
1
3
NMR-spectra show characteristic resonances for the aromatic as
well as methyl carbon. The far-IR spectra of the complex shows
that the amine group is not involved in coordination. The C NMR
spectra of the complex 1a shows aromatic resonances in the range
δ 111–152 ppm, carbonyl resonance at δ 213.85 ppm and methyl
resonance at δ 40.31 ppm The ESI-mass spectrum of complex 1b
shows a very strong molecular ion peak at m/z = 643 (100%).
The infra-red spectra of the complex 1b show four terminal ν(CO)
bands in the range 2040–1926 cm , which is in good agreement
withtheinfra-reddataofthepreviouslyreportednonmetal–metal
bonded[Fe2(CO)8(µ-phosphine)].
−
1
a strong band at 358 cm , consistent with trans disposition of
[
45]
the two chlorides.
In general, two bands are expected for a
cis-isomer.[
45]
−
1
Suzuki–Miyaura Cross-coupling Reactions using Palladium
Complex 3 as Catalyst
[
41,42]
Theabsenceofanybandin
It is now well established that palladium complexes con-
taining phosphine-based ligands are excellent catalysts for
−
1
the range 1850–1700 cm suggests the absence of any bridging
wileyonlinelibrary.com/journal/aoc
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 552–558

Iron(0), rhodium(I) and palladium(II) complexes with p-(N,N-dimethylaminophenyl)diphenylphosphine
OC
PPh (p-C H NMe )
2 6 4 2
Rh
(
Me Np-C H )Ph P
Cl
2
6
4
2
2
1
/4 [Rh(CO) Cl]2 CH Cl , stir.
2
2 2
PPh (p-C H NMe )
2
6
4
2
rt, 1 h
PPh (p-C H NMe )
2
6
4
2
CO
1
/2[Fe (CO)12]
3
PPh (p-C H NMe )
+
Fe(CO)4
2
6
4
2
OC
Fe
(CO) Fe
4
THF, Reflux,
N atm. 3h
CO
2
CO
1
/2PdCl2 MeCN, reflux, 3 h
1a
1
b
Cl
PPh (p-C H NMe )
2
6
4
2
Pd
Cl
(
Me Np-C H )Ph P
2 6 4 2
3
Scheme 2. Synthesis of the complexes 1a, 1b, 2 and 3.
Complex 3
(
0.5 mol%)
Br
+
B(OH)2
Solvent,
O N
2
O N
2
K CO , rt, 10-24 h
2
3
Figure 1. Effects of solvents in the Suzuki–Miyaura cross-coupling reactions of 4-bromonitrobenzene with phenylboronic acid.
Suzuki–Miyaura cross-coupling reactions; thus we attempted to
use palladium complex 3 as a catalyst for the Suzuki–Miyaura
reactions. In the majority of cases with phosphine-based ligands,
an elevated temperature and/or use of an inert atmosphere is re-
results[47,48] than in situ-generated catalysts. We found that the
use of the pre-formed complex 3 gave a better result compared
with the in situ catalyst generated from PdCl2 and the aminophos-
phine ligand in a 1 : 2 molar ratio. Since the choice of solvents
and bases greatly influences the overall catalytic performances
in Suzuki–Miyaura reactions, several different solvents and bases
were examined and the results are represented in Figs 1 and 2. Our
results show that the reaction between 4-bromonitrobenzene and
phenylboronic acid proceeded well in both protic and aprotic sol-
vents,althoughsignificantvariationsinyieldswerenoticedinsome
cases. Almost quantitative product formations were obtained with
iso-propanol, ethanol, dichloromethane and dimethylformamide.
Water as a solvent gave only moderate conversion, but when it
[
29,46]
quired for effective catalytic performance.
To investigate the
effectiveness of complex 3 as a catalyst in the Suzuki–Miyaura re-
action, the reaction of 4-bromonitrobenzene with phenylboronic
acid was chosen as a model reaction using isopropanol as solvent,
K2CO3 asbaseand0.5 mol%complex3ascatalyst, andperforming
the reaction in aerobic conditions. The reaction completed within
24 h and 99% 4-nitrobiphenyl was isolated as the sole product.
Literature survey reveals that, in the Suzuki–Miyaura reaction, the
use of pre-formed complexes as catalysts often shows different
Appl. Organometal. Chem. 2011, 25, 552–558
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
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C. Sarmah, M. Borah and P. Das
Complex 3
0.5 mol%)
(
Br
+
B(OH)2
iPrOH-H O (1:1)
2
O N
2
O N
2
Base, rt, 10-16 h
Figure 2. Effects of bases in the Suzuki–Miyaura cross-coupling reactions of 4-bromonitrobenzene with phenylboronic acid.
i
was used as a co-solvent with PrOH or THF in 1 : 1 proportion,
quantitative product formation was achieved in a much shorter
time. For example, the reaction of 4-bromonitrobenzene with
properties of aryl bromides have little influence on the coupling
reactions, the nature of arylboronic acids has a substantial
influence on the overall performance of the catalyst. For example,
phenylboronic and 4-chlorophenylboronic acid were found to
be extremely efficient with 4-bromonitrobenzene to afford the
desired product in almost quantitative yield (Table 2, entries 1
and 5), while 3-nitrophenylboronic acid gave only 10% product
formation (entry 7).
i
phenylboronic acid in water– PrOH resulted in the quantitative
formation of coupling product in 10 h, whereas to achieve the
same conversion in neat PrOH required 24 h. Thus, for a base
optimization study water– PrOH (1 : 1) was used as the solvent.
i
i
Among bases, K2CO3 and LiOH were found to be the most ef-
fective bases whereas tert-butyl amine was found to be the least
effective (Fig. 2). Studies on optimization of catalyst quantity (Ta-
ble 1) revealed that the use of 0.5 mol% catalyst loading resulted
in completion of the reaction within 10 h (entry 1). However, on
decreasing the catalyst loading, the reaction time increased. A cat-
alyst loading of 0.06 mol% was essential to maintain quantitative
conversion of the product (entry 4). When the catalyst loading was
decreased to 0.02 mol% (entry 5), 65% product formation was ob-
tained in 24 h. A further decrease in catalyst loading to 0.01 mol%
resulted in only 40% product formation being achieved (entry 6).
To evaluate the scope and limitations of the current procedure,
the reactions of a wide array of electronically diverse aryl
bromides with arylboronic acids were examined using complex
Although coupling reactions with aryl bromides proceeded
i
smoothly in water– PrOH at room temperature to give the
desired coupling product in good to excellent yields, under
the same experimental conditions the reactions between 4-
chloronitrobenzene with phenylboronic acid produced only trace
amountofthecross-couplingproducts.Traditionally,arylchlorides
are less reactive than aryl bromides in Suzuki–Miyaura reac-
tions and generally require more drastic conditions and/or higher
catalyst loading. However, there exist a few reports in which
microwave heating[
49,50]
or conventional heating
[51,52]
method-
ologies were successfully used to obtain the coupling products
in good to excellent yields. Since only negligible amount (10%)
of cross-coupling product was obtained using 0.06 mol% of the
catalyst (Table 3, entry 1), we increased the catalyst quantity
to 0.5 mol% (entry 2); only a slight improvement in the cross-
coupling product formation was observed. However, entry 3 of
3
as the catalyst, and the results are shown in Table 2. It can
be seen from Table 2 that, in general, the aryl bromides with
electron-withdrawing substituents such as NO2, CHO, CO2Me and
COOH (entries 1–4) underwent the coupling reactions in nearly
quantitative yields (94–99%). The nonactivated aryl bromides
such as bromobenzene, 4-bromotolune and 4-bromoanisole)
also gave the coupling products in excellent yields (entries
–10). Interestingly, with a slightly extended reaction time, the
sterically demanding substrates such as 2-bromotolune and 2-
bromoanisole (entry 11 and 12) could also be coupled with
phenylboronic acid to give the desired product in reasonably
good yield. It is important to note that, although the electronic
i
i
Table 3 shows that the use of neat PrOH instead of water– PrOH
(1 : 1) improved the product formation significantly, and 58%
cross-coupling product was isolated. Hence, further a study with
(
i
aryl chlorides was performed in PrOH with 0.5 mol% of cata-
8
lyst. Besides 4-chloronitrobenzene, other electron-withdrawing
substituents such as, 4-chloroacetophenone (entry 4) and 4-
chlorobenzaldehyde (entry 5) gave only moderate yields. Our
results also showed that, under slightly extended reaction time,
nonactivated aryl chlorides such as 4-chlorobenzene and 4-
wileyonlinelibrary.com/journal/aoc
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 552–558

Iron(0), rhodium(I) and palladium(II) complexes with p-(N,N-dimethylaminophenyl)diphenylphosphine
Table 1. Effect of catalyst quantity on the Suzuki–Miyaura cross coupling reactions of 4-bromonitrobenzene with phenylboronic acida
Complex 3
Br
+
B(OH)2
iPrOH-H O (1:1)
2
O N
2
O N
2
K CO , rt, 10-48h
2
3
Entry
Catalyst (mol%)
Time (h)
Yield (%)b,c
1
2
3
4
5
6
0.50
0.25
0.12
0.06
0.02
0.01
10
13
16
24
36
48
99
99
99
99
65
40
a
Reaction conditions: 0.5 mmol 4-bromonitrobenzene, 0.55 mmol phenylboronic acid, 1.5 mmol base, solvent (6 ml). b Isolated yield. c Yields are of
average of two runs.
Table 2. Suzuki–Miyaura cross-coupling reactions of various aryl bromides with arylboronic acids using complex 3 as catalysta
Complex 3
(
0.06 mol%)
Br
+
B(OH)2
iPrOH-H O (1:1)
K CO , rt, 24-48 h
2
R
R'
R
R'
Yield (%)b,c
2
3
ꢁ
Entry
R
R
Time (h)
1
2
3
4
5
6
7
8
9
4-NO2
4-COMe
4-COOH
4-CHO
4-NO2
4-NO2
4-NO2
H
H
H
H
H
24
30
30
30
24
24
24
30
30
30
42
42
99
98
96
94
96
90
10
91
94
93
78
82
4-Cl
4-Me
3-NO2
H
4-Me
H
10
11
12
4-OMe
2-Me
H
H
2-OMe
H
a
Reaction conditions: 0.5 mmol aryl bromide, 0.55 mmol arylboronic acid, 1.5 mmol K2CO3, water– PrOH (6 ml). b Isolated yield. Yields are of average
i
c
of two runs.
Table 3. Suzuki–Miyaura cross-coupling reactions of aryl chlorides with phenylboronic acid using complex 3 as catalyst
Complex 3
Cl
+
B(OH)2
Solvent, K CO ,
2
3
R
rt, 24-36 h
R
Entry
R
Catalyst (mol%)
Solvent
Time
Yield (%)b,c
1
2
3
4
5
6
7
4-NO2
4-NO2
4-NO2
4-COMe
4-CHO
4-H
0.06
0.5
0.5
0.5
0.5
0.5
0.5
iPrOH–H2O (1 : 1)
iPrOH–H2O (1 : 1)
iPrOH
30
30
24
30
30
30
36
10
17
58
43
46
36
26
iPrOH
iPrOH
iPrOH
4-Me
iPrOH
a
Reaction conditions: 0.5 mmol aryl chloride, 0.55 mmol phenylboronic acid, 1.5 mmol K2CO3, solvent (6 ml). b Isolated yield. c Yields are of average
of two runs.
Appl. Organometal. Chem. 2011, 25, 552–558
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
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C. Sarmah, M. Borah and P. Das
chlorotoluene gave relatively poor yields (entries 6 and 7).
Although reduced yields of the coupling products were obtained
for aryl chlorides compared with aryl bromides, these results were
also significant, as we were able to use aryl chlorides as substrates
in the Suzuki–Miyaura reaction at room temperature.
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Insummary,weexploredthecoordinationabilityofanaminophos-
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were synthesized and characterized by different spectroscopic
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Acknowledgment
The Department of Science and Technology, New Delhi and the
University Grants Commission, New Delhi, are gratefully acknowl-
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services of SAIF (NEHU), Shillong and CDRI, Lucknow are also
acknowledged for NMR and mass analysis, respectively. We are
gratefult to Dr A. Thakur of Tezpur University for recording the
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Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 552–558