Preparation of new buchwald-type secondary phosphine oxide ligands and applications in Suzuki-Miyaura reactions

Article abstract of DOI:10.1021/om101132t

Two air- and moisture-stable secondary phosphine oxides (SPOs), namely, 2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole (3a, R = H; 3b, R = OMe) were prepared and characterized. A tautomeric equilibrium exists between the secondary phosphine oxide 3a (or 3b) and its corresponding phosphinous acid 3a′ (or 3b′). The reaction of phosphinous acid 3a′ (or 3b′) with Pd(COD)Cl₂ (or PdBr₂) yielded the corresponding monopalladium complex bis(2-(tert-butylhydrophosphoryl)-1-(2-R- phenyl)-1H-imidazole)PdX₂ (5a, R = H, X = Cl; 5b, R = OMe, X = Br) in cis-form as well as dipalladium complex [(μ₂-2-(tert- butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole-N,P)Pd(I)X]₂ (6a, R = H, X = Cl; 6b, R = OMe, X = Br). The phosphinous acids 3a′ and 3b′ act as P,N-chelating ligands in the formation of 6a and 6b, which feature direct Pd(I)-Pd(I) bonds. Good to excellent reactivities were observed by applying these ligand-chelated palladium complexes in Suzuki-Miyaura coupling reactions.

Full text of DOI:10.1021/om101132t

Organometallics 2011, 30, 1139–1147 1139
DOI: 10.1021/om101132t
Preparation of New Buchwald-Type Secondary Phosphine Oxide Ligands
and Applications in Suzuki-Miyaura Reactions
Dan-Fu Hu, Chia-Ming Weng, and Fung-E Hong*
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40227,
Taiwan, Republic of China
Received December 2, 2010
Two air- and moisture-stable secondary phosphine oxides (SPOs), namely, 2-(tert-butylhydro-
phosphoryl)-1-(2-R-phenyl)-1H-imidazole (3a, R=H; 3b, R=OMe) were prepared and character-
ized. A tautomeric equilibrium exists between the secondary phosphine oxide 3a (or 3b) and its
corresponding phosphinous acid 3a0 (or 3b0). The reaction of phosphinous acid 3a0 (or 3b0) with
Pd(COD)Cl2 (or PdBr2) yielded the corresponding monopalladium complex bis(2-(tert-butylhydro-
phosphoryl)-1-(2-R-phenyl)-1H-imidazole)PdX2 (5a, R=H, X=Cl; 5b, R=OMe, X=Br) in cis-
form as well as dipalladium complex [(μ2-2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imida-
zole-N,P)Pd(I)X]2 (6a, R=H, X=Cl; 6b, R=OMe, X=Br). The phosphinous acids 3a0 and 3b0 act as
P,N-chelating ligands in the formation of 6a and 6b, which feature direct Pd(I)-Pd(I) bonds. Good to
excellent reactivities were observed by applying these ligand-chelated palladium complexes in
Suzuki-Miyaura coupling reactions.
1. Introduction
in the literature.3 Although recent applications of so-called
“eco-friendly” ligands, such as amines, imines, diazabutadie-
nes, oximes, and N-heterocyclic carbenes, are having merits
regarding low environmental impact and catalytic perfor-
mances,4 for many years, various kinds and shapes of organo-
phosphines and derivatives remain the most frequently em-
ployed ligands.5 Mainly, the electronically and sterically tunable
capacities of phosphines make this type of ligand one of the
most favorite choices among all ligands available.6
In general, the phosphorus atoms of organophosphine
ligands are rich in electron density and vulnerable to oxida-
tion. It is certainly not an agreeable character for long-term
storage of phosphine ligands. In addition, the oxidized
phosphine thereafter loses its coordination capability toward
soft atoms such as Pd(II) or Pd(0) in complexes and subse-
quently leads to a great reduction in its catalytic perfor-
mance. On the other hand, secondary phosphine oxides
The diverse forms of metal-catalyzed cross-coupling reac-
tions have probably been the most widely used methods for
the formation of C-X (X = C, N, O, S, etc.) bonds in
modern synthetic chemistry for the past few decades.1 Palla-
dium complexes have been extensively used as catalysts for
the coupling reactions in combination with numerous sup-
porting ligands, which acquire the desired reactivity and
selectivity. Among all the factors that affect the catalytic
performance of a transition metal complex catalyzed reac-
tion, a well-designed ligand is doubtless the most essential
one to the success of a catalytic reaction.2 Therefore, con-
siderable efforts have been made to search for more efficient
and affordable ligands. As a result, various ligands with
different shapes and functions had been explored and reported
*To whom correspondence should be addressed. Fax: 886-4-
22862547. E-mail: fehong@dragon.nchu.edu.tw.
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2011 American Chemical Society
Published on Web 02/02/2011
pubs.acs.org/Organometallics

1140 Organometallics, Vol. 30, No. 5, 2011
Hu et al.
Figure 1. Selected phosphinous acid-coordinated palladium and platinum complexes.
(SPOs) (R0R00PH (dO) are already in oxidized form and
therefore stable toward moisture and air. The equilibrium
existing between a secondary phosphine oxide (R0R00PH (dO)
and its tautomeric form, phosphinous acid (R0R00POH), is well
documented.7 Various structural forms of phosphinous acid
(PAs)-coordinated palladium or platinum complexes had
been reported as precatalysts in various C-C bond forma-
tion reactions (Figure 1).8
Recently, Buchwald-type phosphines, 20-R0-biphenyl-2-
yl-di-R00-phosphanes (R0=OMe, NMe2, etc.; R00=tBu, Ph,
Cy, etc.) are gaining a reputation as efficient ligands in various
Figure 2. Buchwald-type phosphines (I) and two bonding
modes (II and III) of phosphine-coordinated palladium com-
plexes.
palladium complex-catalyzed cross-coupling reactions.9 As
shown in Figure 2, one of the most distinguishing features
of Buchwald-type phosphine is the existence of a weak
interaction between its substituent of biaryl -R0 (with
electron-donating capacity) and the metal, besides the strong
P-M bonding, a brilliant application of the concept of
hemilabile ligand.10 This bonding mode provides flexibility
around the coordination sphere of the metal center.
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Lately, we have been interested in exploring a new type of
stable SPO ligand thus to enhance the catalytic perfor-
mance in coupling reactions. Our approach is thereby
through the modification on Buchwald-type phosphine
both by incorporating the notion of secondary phosphine
oxide on phosphorus site as well as replacing the featureless
phenyl ring by a more versatile heterocyclic ring, such as
imidazole.11 Presumably, it will bring about a new dimen-
sion of bonding capacity of the newly designed SPO li-
gands and hopefully improve their reactivities. Recently,
Ackermann demonstrated the potential of the application
of (N-aryl)azolyl-substituted secondary phosphine oxides,
a new type of modified SPOs, in palladium-catalyzed cross-
coupling reactions.12 Herein we report the preparation of
several modified Buchwald-type secondary phosphine oxi-
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their capacities as efficient mono- or bidentate ligands in
palladium-catalyzed cross-coupling reactions were inves-
tigated.
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Article
Organometallics, Vol. 30, No. 5, 2011 1141
Figure 3. ORTEP diagram of 3a. Hydrogen atoms are omitted
for clarity. Selected bond lengths (A) and angles (deg): P(1)-
O(1) 1.4742(11); P(1)-C(9) 1.8055(15); P(1)-C(10) 1.8250(15);
N(1)-C(7) 1.3751(19); N(1)-C(9) 1.3739(19); N(1)-C(1) 1.44-
07(17); C(9)-N(2) 1.3252(18); O(1)-P(1)-C(9) 114.74(7); O(1)-
P(1)-C(10) 114.06(7); C(9)-P(1)-C(10) 108.04(7); N(2)-
C(9)-N(1) 111.29(13); N(2)-C(9)-P(1) 121.03(12); N(1)-C(9)-
P(1) 127.51(11).
Figure 4. ORTEP diagram of 4. Hydrogen atoms are omitted
for clarity. Selected bond lengths (A) and angles (deg): P(1)-
O(1) 1.4819(12); P(1)-C(2) 1.8114(17); P(1)-C(9) 1.8118(16);
P(1)-C(10) 1.8267(19); O(1)-P(1)-C(2) 117.37(9); O(1)-
P(1)-C(9) 117.04(8); C(2)-P(1)-C(9) 89.92(7); O(1)-P(1)-
C(10) 114.03(9); C(2)-P(1)-C(10) 109.05(8); C(9)-P(1)-C(10)
106.69(8).
2. Results and Discussion
2.1. Preparation of Buchwald-Type Secondary Phosphine
Oxides (SPOs) 3a and 3b. First, the heterocyclic compound
1a (or 1b) was prepared in good yield by a Cu(I)-catalyzed
coupling reaction between imidazole and bromobenzene (or
2-bromoanilsole), which was then treated with BuLi and
P(tBu)Cl2 to give 2a (or 2b). Subsequently, acidic workup
gave 3a in 45% or 3b in 67% yield, respectively.13
in organic solvents and the ready reduction of the former
complex in the presence of reducing agents such as excess
phosphine and base.14 Unfortunately, attempts to crystal-
lize a 3a- or 3b-coordinated Pd(OAc)2 complex were not
successful. Therefore, unambiguous identification of the
composition of in situ 3b/Pd(OAc)2 is not possible. In
contrast to the palladium acetates, it is relatively conveni-
ent to grow crystals of palladium halide complexes in a not
strictly moisture-free environment. The composition of the
products including the ratio of ligand to metal could be
readily identified after structural determination by X-ray
diffraction methods. Thereby, the preparation of 3a- or 3b-
coordinated palladium halide complexes was pursued.
The reaction of phosphinous acid 3a (or 3b) with one
molar equivalent of either Pd(COD)Cl2 or PdBr2 at 60 °C for
a designated number of hours gave palladium complex 5a
(R=H, X=Cl) or 5b (R=OMe, X=Br) (Scheme 2). The
complex 5a (or 5b) was observed in cis-form of a bis-
phosphine ligands coordinated Pd(II)X2 complex. The chem-
ical shifts for 5a and 5b are 92.7 and 96.3 ppm, respectively,
The formation of compound 3a was confirmed by single-
crystal X-ray diffraction methods. The ORTEP diagram of
3a is depicted in Figure 3. A distinctly large coupling con-
stant, JP-H=482 Hz, between P(1) and its attached proton
strongly suggests the existence of a P-H bond. The presence
of a double bond between P(1) and O(1) is also evidenced by
its short bond length, 1.4742(11) (A). The geometric param-
eters of 3a surrounding the phosphorus atom indeed supports
the presence of a secondary phosphine oxide. Unexpectedly, a
cyclic phosphine oxide 4 was isolated as well during the
chromatographic process. The formation of 4 is presum-
ably caused by further deprotonation from aryl by excess
n-BuLi and then followed by cyclization and oxidation
thereafter. The crystal structure reveals that it is cyclic in
nature, with the phosphorus atom been oxidized (Figure 4).
2.3. Reaction of 3a or 3b with Palladium Salts. Generally,
the catalytic efficiency of a bis-tertiary phosphine ligand
coordinated Pd(OAc)2 complex in cis-form is superior to
that of bis-ligand-coordinated PdX2 (X=Cl, Br) complex in
cross-coupling reactions. Presumably this is due to its solubility
in 31P NMR. The distinctly large coupling constant (JP-H
)
between the phosphorus atom and its attached proton of 3a
(or 3b) no longer exists in 5a (or 5b), which corresponds to the
formation of a phosphinous acid form of the ligand. Inter-
estingly, the formation of 6a (R=H, X=Cl) or 6b (R=OMe,
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Gaumont, A.-C.; Denis, J.-M. Synlett 1995, 1168–1170.

1142 Organometallics, Vol. 30, No. 5, 2011
Hu et al.
Scheme 1
Scheme 2
structures of 5a (or 5b) and 6a (or 6b). We noticed that the
expected intramolecular hydrogen bonding between depro-
tonated and non-deprotonated secondary phosphine oxides
in cis-form of a bis-phosphine ligand coordinated palladium
complex was not detected.17 As shown, there are two molar
equivalents of 3a0 (or 3b0), each in its phosphinous acid form,
coordinated to palladium in 5a (or 5b). In 5a, the bond
lengths of P(1)-O(1) and P(2)-O(2) are 1.529(4) and
1.553(4) A, respectively. Both are typical single bonds.
Notably, two molar equivalents of 3a0 (or 3b0) ligands are
each in its R- and S-forms. Recently, asymmetric synthesis
using enantiopure SPOs as ligands has gained more and
more attention.18 Presumably, 3a0 (or 3b0) is a potential
candidate for asymmetric synthesis as a ligand after resolu-
tion of two enantiomers. In 5a, the palladium is situated in an
approximately square-planar environment. Similar structural
parameters are observed for 5b except the attached -OMe
group. The framework of 6a (or 6b) can be regarded as a bis-
3a0 (or 3b0)-bridged dipalladium complex. Both the phos-
phorus atom from phosphinous acid and the nitrogen atom
from imidazole of 3a0 (or 3b0) act as the coordinating sites, and
3a0 (or 3b0) can be regarded as an authentic P,N-chelating
ligand. It is particularly noteworthy that a nitrogen atom of
imidazole acts as a coordinating site as well. In a closely
related work, Beller did not mention the formation of a
complex with a Pd(I)-Pd(I) bond.19 Two molecules of 3a0
X = Br) was observed in the 31P NMR, accompanied with
some unidentified signals, by gradual conversion of the signal
to 104.0 or 107.9 ppm, respectively. Purification of 6a (or
6b) from the mixture containing 5a (or 5b) by a chro-
matographic process in silica was not achievable presum-
ably due to the fragility of the former. Fortunately, suitable
crystals were obtained for structural determination. It was
later confirmed that both 6a and 6b are unusual bimetallic
compounds with direct Pd(I)-Pd(I) bonds.15 For prolonged
reaction time during the formation of 5a or 5b, it was
accompanied with the formation of a new palladium complex,
with an even further downfield shifted signal at 124.1 and
125.7 ppm, respectively, for 7a or 7b, in the 31P NMR.16
All four compounds 5a, 5b, 6a, and 6b were crystallized in
CH2Cl2 solution, and their structures were determined by
single-crystal X-ray diffraction methods. The ORTEP dia-
grams of 5a, 5b, 6a, and 6b are shown in Figure 5-8,
respectively. The tautomeric shift of the secondary phos-
phine oxide 3a (or 3b) to its phosphinous acid form 3a0 (or
3b0) during the coordination is further evidenced by the
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Q.-F.; Sau, Y.-K.; Lo, S. M. F.; Sung, H. H. Y.; Williams, I. D.; Haynes,
R. K.; Leung, W. H. Inorg. Chem. 2004, 43, 4921–4926. (i) Jiang, X.; van
den Berg, M.; Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. Tetra-
hedron: Asymmetry 2004, 15, 2223–2229. (j) Dai, W.-M.; Yeung,
K. K. Y.; Leung, W. H.; Haynes, R. K. Tetrahedron: Asymmetry
2003, 14, 2821–2826.
(15) (a) Barder, T. E. J. Am. Chem. Soc. 2006, 128, 898–904.
(b) Kurosawa, H. J. Organomet. Chem. 2004, 689, 4511–4520. (c)
Murahashi, T.; Kurosawa, H. Coord. Chem. Rev. 2002, 231, 207–228.
(d) Dervisi, A.; Edwards, P. G.; Newman, P. D.; Tooze, R. P.; Coles,
S. J.; Hurthouse, M. B. Dalton Trans. 1998, 3771–3776.
(16) The 31P NMR signal shifts from 107.9 ppm of 6b to 125.7 ppm of
the new compound 7b. The structure of 7b was determined by single-
crystal X-ray diffraction methods. For details, see the Supporting
Information.
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Article
Organometallics, Vol. 30, No. 5, 2011 1143
Figure 5. ORTEP diagram of 5a. Hydrogen atoms are omitted
for clarity. Selected bond lengths (A) and angles (deg): Pd(1)-
P(1) 2.258(1); Pd(1)-P(2) 2.261(1); Pd(1)-Cl(2) 2.364(1);
Pd(1)-Cl(1) 2.388(1); P(1)-O(1) 1.529(4); P(2)-O(2) 1.553(4);
P(1)-Pd(1)-P(2) 92.11(5); P(1)-Pd(1)-Cl(1) 92.57(5); Cl(2)-
Pd(1)-Cl(1) 87.64(4); P(2)-Pd(1)-Cl(2) 87.61(5); O(1)-P(1)-
Pd(1) 117.58(16); O(2)-P(2)-Pd(1) 115.61(16).
Figure 7. ORTEP diagram of 6a. Hydrogen atoms are omitted
for clarity. Selected bond lengths (A) and angles (deg): Pd(1)-
N(2B) 2.093(4); Pd(1)-P(1A) 2.1917(13); Pd(1)-Cl(1) 2.4477(12);
Pd(1)-Pd(2) 2.5649(5); N(2B)-Pd(1)-P(1A) 176.88(11);
N(2B)-Pd(1)-Cl(1) 92.68(11); P(1A)-Pd(1)-Cl(1) 89.98(4);
N(2B)-Pd(1)-Pd(2) 91.52(11); P(1A)-Pd(1)-Pd(2) 85.63(3);
Cl(1)-Pd(1)-Pd(2) 171.24(3); N(2A)-Pd(2)-Pd(1) 92.08(11);
P(1B)-Pd(2)-Pd(1) 85.52(3); Cl(2)-Pd(2)-Pd(1) 167.62(3);
O(1A)-P(1A)-Pd(1) 109.87(15).
palladium atoms can be regarded as þ1, which is note-
worthy. A probable conformation with P,N-chelated
monopalladium complex by 3a was not observed. This
can be understood by the fact that the formation of a four-
membered ring is not preferable due to severe internal strain.
As for the structure of 6b, rather similar structural para-
meters are observed to those of 6a except the attached -OMe
group. To our best knowledge, these two compounds are the
first reported SPO-coordinated bimetallic complexes with
direct Pd(I)-Pd(I) bonds.
The formation of the dipalladium complex 6a (or 6b) with
a direct Pd(I)-Pd(I) bond is notable since the initial palla-
dium source is a Pd(II) salt. Barder also observed the
formation of a dipalladium complex with a direct Pd(I)-
Pd(I) bond from a reduction of a Buchwald-type phosphine
coordinated palladium dichloride complex.14a He proposed
a disproportionation mechanism for the dimeric compound,
which disintegrates into a Pd(II) plus a Pd(0) species through
the reduction process by PhB(OH)2. On the basis of Barder’s
assumption, it seems reasonable to postulate that the forma-
tion of 6 is first through the reduction of 5, followed by
addition of another molar equivalent of 5 and eventually
accompanied by elimination of two molar equivalents of 30 as
shown in Scheme 3. Conversely, the formation of an active
Pd(0) species might be possible through the disproportionation
Figure 6. ORTEP diagram of 5b. Hydrogen atoms are omitted
for clarity. Selected bond lengths (A) and angles (deg): Pd(1)-
P(1) 2.278(1); Pd(1)-P(2) 2.269(1); Pd(1)-Br(2) 2.499(1);
Pd(1)-Br(1) 2.509(1); P(1)-O(1) 1.521(2); P(2)-O(2) 1.565(3);
P(2)-Pd(1)-P(1) 93.38(4); P(2)-Pd(1)-Br(2) 87.04(3); P(1)-
Pd(1)-Br(1) 91.77(3); Br(2)-Pd(1)-Br(1) 88.07(2); O(1)-P(1)-
Pd(1) 116.74(11); O(2)-P(2)-Pd(1) 115.47(11).
(or 3b0) as chelating ligands, each in its R- or S-form, are
arranged in opposite formats, while two halide atoms are
located in terminal positions. Each palladium atom remains
in a nearly square-planar environment. In 6a, the bond
lengths of Pd(1)-Pd(2) and P(1A)-O(1A) are 2.5649(5)
and 1.608(4) A, respectively. Both are typical single bonds.
The bond angle of O(1A)-P(1A)-Pd(1) is 109.87(15)°,
which is a typical sp3 hybridization of the phosphorus atom
center. The five atoms Pd(1), Pd(2), N(2A), C(9A), and
P(1A) are not coplanar. Rather, they form a twisted penta-
gon such as cyclopentane. The formal charges of both of the

1144 Organometallics, Vol. 30, No. 5, 2011
Hu et al.
Figure 8. ORTEP diagram of 6b. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (deg): Pd(1)-N(3)
2.111(4); Pd(1)-P(1) 2.1997(11); Pd(1)-Br(1) 2.5466(5); Pd(1)-Pd(2) 2.5738(4); P(1)-O(2) 1.601(3); O(1)-C(9) 1.361(5); O(1)-C(10)
1.422(5); N(3)-Pd(1)-P(1) 173.82(10); N(3)-Pd(1)-Br(1) 92.48(10); P(1)-Pd(1)-Br(1) 93.63(3); N(3)-Pd(1)-Pd(2) 91.32(10);
P(1)-Pd(1)-Pd(2) 82.54(3); Br(1)-Pd(1)-Pd(2) 163.899(18); O(2)-P(1)-C(1) 106.83(16); O(2)-P(1)-C(11) 100.23(18); C(1)-
P(1)-C(11) 104.86(19); O(2)-P(1)-Pd(1) 112.13(12); C(1)-P(1)-Pd(1) 106.88(13); C(11)-P(1)-Pd(1) 124.60(14); C(9)-O(1)-C(10)
117.8(3).
Scheme 3
of 6 in solution. Presumably, it is this Pd(0) species that is
responsible mostly for the catalytic performance in the
cross-coupling reaction using 30 as the ligand. Recently,
several works on the preparation and structural determi-
nation of dipalladium complexes with a direct Pd(I)-Pd(I)
bond were reported. Most of the dipalladium complexes
contain a bridging ligand,20 which are often hyride, CO,
halides, isonitriles, olefins, and phosphines (chelating or non-
chelating) between the dinuclear palladium centers. Hartwig
also demonstrated that a dinuclear complex [Pd2(μ-Br)2-
(PtBu3)2] is a suitable precatalyst for amination of various
aryl halides.21 Recently, the discovery of a bimetallic
palladium(III) complex that can catalyze the formation of
carbon-heteroatom bonds adds a new facet to our under-
standing of the chemistry of one of the most widely used metals
in catalysis.22
3a- or 3b-coordinated Pd(OAc)2 complexes were not avail-
able, Pd(OAc)2 was chosen as the palladium source rather
than PdX2 (X = Cl, Br) due to its creditable catalytic
performance in most palladium-catalyzed cross-coupling
reactions. Several factors that affect the efficiency of the
reactions, including temperature, time, concentration,
bases, transition metal-ligand ratios, and solvent, had
been evaluated. First, the catalytic performance was found
to be better for 3b than 3a through a series of deliberate
experiments by using these two ligands.23 Presumably, the
enhanced reaction rate of 3b over 3a is due to the additional
-OMe group, which is consistent with Buchwald’s
observation.9a Since, the catalytic performance of 3b is
superior to 3a, the following optimizations of the reaction
conditions were all employing 3b rather than 3a. The
results for the Suzuki-Miyaura coupling reactions are
summarized in Tables 1-3.
2.3. Palladium-Catalyzed Suzuki-Miyaura Reactions Using
SPOs as Ligand. Palladium-catalyzed Suzuki-Miyaura cross-
coupling reactions of aryl halides and phenylboronic acid
were carried out by employing the newly made SPOs (3a or 3b)
as ligands. Although the X-ray determined crystal structures of
(23) The conversion rates are 40% and 60%, respectively, for em-
ploying 3a and 3b as the ligands under the following conditions:
4-bromotoulene (0.5 mmol), phenylboronic acid (0.6 mmol), NaOH
(1.5 mmol), THF (1 mL), Pd(OAc)2 (0.5 mol %), ligand (1 mol %), 60 °C,
2 h. Yield of the produced 4-phenyltoluene was determined by GC-MS
using undecane as internal standard.
(24) (a) Dubbaka, S. R.; Vogel, P. Org. Lett. 2004, 6, 95–98.
(b) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077–
1080. (c) Miyaura, N.; Yamada, K.; Suginome, H.; Suzuki, A. J. Am.
Chem. Soc. 1985, 107, 972–980.
(20) Murahashi, T.; Kurosawa, H. Coord. Chem. Rev. 2002, 231, 207.
(21) (a) Christmann, U.; Vilar, R.; White, A. J.; Williams, D. J. Chem.
Commun. 2004, 1294–1295. (b) Stambuli, J. P.; Kuwano, R.; Hartwig,
J. F. Angew. Chem., Int. Ed. 2002, 41, 4746.
(22) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302–309.

Article
Organometallics, Vol. 30, No. 5, 2011 1145
Table 1. Suzuki-Miyaura Coupling Reactions Employing Various Basesa
entry
base
NaOH
KOH
yield (%)b
1
2
3
4
5
6
7
8
80
>99
17
34
17
18
35
<1
<1
22
CsOH H2O
3
NaOt-Bu
KOt-Bu
Na2CO3
K2CO3
Cs2CO3
KF
9
10
11
12
CsF
K3PO4
K3PO4 nH2O
3
24
81
a Reaction conditions: 4-bromotoluene (0.5 mmol), phenylboronic acid (0.6 mmol), base (1.5 mmol), THF (1.0 mL), Pd(OAc)2 (1.0 mol %), 3b
(2.0 mol %), 60 °C, 1 h. b Yield was determined by GC-MS.
Table 2. Effect of Reaction Time and Pd/L Ratios on
Suzuki-Miyaura Reactiona
Table 3. Suzuki-Miyaura Coupling Reactions Employing
Various Aryl Halidesa
entry
Pd:L
time (h)
yield (%)a
1
2
3
4
5
6
7
1:2
1:2
1:2
1:2
1:1
1:2
1:3
1.0
1.0
0.5
15.0
0.25
0.25
0.25
>99
76b
98
12c
29
55
<5
a Reaction conditions the same as in the footnote of Table 1 except
using KOH as base. b Toluene as the solvent. c At room temperature.
Among all the influential factors that affect the catalytic
efficiency of the cross-coupling reactions, the role played
by base probably is the most unpredictable one.24 Thereby,
the impacts of employing different bases on the reaction
yields were first investigated, and the results are shown in
Table 1. Among all the bases screened, it was found that
potassium hydroxide in THF is the most effective (Table 1,
entry 2), while the other bases such as cesium hydroxide
and sodium and potassium tert-butoxide gave poor yields
a Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid
(0.6 mmol), KOH (1.5 mmol), THF (1.0 mL), Pd(OAc)2 (1.0 mol %), 3b
(2.0 mol %), 60 °C. b Yield of isolated product. c K2CO3 (1.5 mmol),
Pd(OAc)2 (2.0 mol %), 3b (4.0 mol %), toluene, 90 °C.
of product. However, the bases NaOH and K3PO4 nH2O
3
gave good yields of coupled product. We suspect that the
cation of the base could be one of the key factors in
Suzuki-Miyaura reactions with aryl bromides and using
the SPO 3b as ligand.
the results are presented in Table 3. The Suzuki-Miyaura
coupling reaction for electron-rich and electron-deficient
aryl bromides was carried out smoothly to give the corre-
sponding coupled products. Reactions of 4-bromoanisole
and 4-nitrobromobenzene with phenylboronic acid gave
67% and 83% yields, respectively (Table 3, entries 2 and 4).
The other heterocyclic substrate, 2-bromopyridine, gave
only 10% yield of product, which is presumably due to its
coordination with the palladium complex (Table 3, entry 8).
In addition, the aryl chlorides in the presence of potassium
carbonate as base forms a product (Table 3, entry 10).
2.4. Summary. Two new air- and moisture-stable second-
ary phosphine oxides containing the aryl-1H-imidazole
group were prepared. Applications of these ligands in
palladium-catalyzed Suzuki-Miyaura cross-coupling reac-
tions exhibited good to excellent efficiencies toward aryl bro-
mides. Only moderate performances were observed for aryl
chlorides. While the catalytic performances of theses SPOs are
compatible with their trialkyl or triaryl counterparts, these
In order to achieve optimized reactivity, we carried out
studies on variables such as solvent, temperature, and Pd:L
ratio. The results are presented in Table 2. It was found that
the combination of Pd:L (1:2) gave almost quantitative yield
of coupled product in THF for 1 h (Table 2, entry 1). When
the reaction was carried out using toluene as a solvent, it gave
76% yield, whereas reducing the reaction time to 15 min at
60 °C suppressed the corresponding yield. A better efficiency
was observed for the composition of 3b/Pd(OAc)2 in situ
equal to 2:1 (Table 2, entries 5-7). An attempt to run the
reaction at lower temperature gave poor results (Table 2,
entry 4). The optimal reaction condition is using 3b/Pd-
(OAc)2 (2:1) as the catalyst precursor in a solution of THF
(1.0 mL) and KOH (1.5 mmol) and under 60 °C for reacting
at least 1 h.
Employing the optimal reaction conditions, the scope of
this reaction using various aryl bromides was examined, and

1146 Organometallics, Vol. 30, No. 5, 2011
Hu et al.
compounds show an advantage over the other known
ligands, being stable toward air and moisture. The formation
of the dipalladium complex 6a or 6b with a direct Pd(I)-
Pd(I) bond is noteworthy. It is proposed that an active Pd(0)
species is responsible for the catalytic performance through
the disproportionation of 6a or 6b in solution.
acetate (2 ꢀ 5 mL). The combined organic layer was dried with
MgSO4, filtered, and concentrated. The residue was purified by
flash chromatography (ethyl acetate/methanol, 20:1) to afford a
white solid 3a in 45% yield (0.45 mmol, 111.0 mg).
Selected spectroscopic data for 3a: 1H NMR (CDCl3, δ/ppm)
7.703, 6.499 (d, 1H, P-H, 1JP-H=482 Hz), 7.605-7.585 (d, 2H),
7.532-7.472 (m, 3H), 7.354 (s, 1H), 7.309 (s, 1H), 1.137-1.093
(d, 9H, 3JP-H=17.6); 13C NMR (CDCl3, δ/ppm) 140.6, 139.3
(d, C-P, 1JP-C=203), 136.8, 130.7, 130.6, 129.4, 129.1, 125.9,
125.0, 121.4, 33.0, 32.2 (d, C-P, 1JP-C=118); 31P NMR (CDCl3,
δ/ppm) 32.0 (d, P-H, 1JP-H=482 Hz); MS (EI, m/z) 248.0 [M]þ.
3.4. Preparation of 2-(tert-Butylhydrophosphoryl)-1-(2-meth-
oxyphenyl)-1H-imidazole (3b). Unexpectedly, the procedures for
the preparation of 3a did not work for 3b. Therefore, another
reaction pathway was pursued as follows.
3. Experimental Section
3.1. General Procedures. All reactions were carried out under
a nitrogen atmosphere using standard Schlenk techniques or in a
nitrogen-flushed glovebox. Freshly distilled solvents were used.
All processes of separations of the products were performed by
centrifugal thin layer chromatography (CTLC, Chromatotron,
Harrison model 8924) or column chromatography. GC-MS
analysis was performed on an Agilent 5890 gas chromatograph
(Restek Rtx-5MS fused silica capillary column: 30 m, 0.25 mm,
0.5 μm) with an Agilent 5972 mass selective detector. Routine 1H
NMR spectra were recorded on a Varian-400 spectrometer at
400.00 MHz. The chemical shifts are reported in ppm relative to
internal standard TMS (δ=0.0). 31P and 13C NMR spectra were
recorded at 121.44 and 75.46 MHz, respectively. The chemical
shifts for the former and the latter are reported in ppm relative to
internal standards H3PO4 (δ = 0.0) and CHCl3 (δ = 77) or
CH2Cl2 (δ=53), respectively. Mass spectra were recorded on a
JEOL JMS-SX/SX 102A GC/MS/MS spectrometer. Electro-
spray ionization high-resolution mass spectra (ESI-HRMS)
were recorded on a Finnigan/Thermo Quest Mat 95 XL mass
spectrometer.
3.2. Synthesis and Characterization of 1-Phenyl-1H-imidazole
(1a) and 1-(2-Methoxyphenyl)-1H-imidazole (1b). Into a 250 mL
round-bottomed flask were placed copper iodide (1.14 g,
6.0 mmol), 1,10-phenanthroline (1.08 g, 6.0 mmol), tripotassium
phosphate (19.10 g, 90.0 mmol), imidazole (2.86 g, 42.0 mmol),
and N,N-dimethylformamide (50 mL). After stirring at 25 °C for
a few minutes, 30.0 mmol of bromobenzene (3.16 mL) was
added. The solution was then heated and stirred at 120 °C for
48 h. After cooling to 25 °C, the solution was diluted with ethyl
acetate (20 mL), filtered through silica gel, and washed with
ethyl acetate. The organic layer was washed twice with brine
(40 mL), then dried with MgSO4, filtered, and concentrated. The
residue was purified by flash chromatography (hexane/ethyl
acetate, 3:1) to afford 1a in 78% yield (3.37 g, 23.40 mmol).
Similar procedures were carried out for the preparation of 1b
except for the replacement of bromobenzene by 2-bromoanisole
(3.74 mL, 30 mmol). The yield of 1b is 85% (4.43 g, 24.89 mmol).
Selected spectroscopic data for 1a: 1H NMR (CDCl3, δ/ppm)
7.86 (s, 1H, NdCH-N), 7.51-7.47 (t, 2H, Ar), 7.40-7.36 (d, 3H,
Ar), 7.29 (s, 1H, N-CHdC), 7.21 (s, 1H, CdCH-N); 13C NMR
(CDCl3, δ/ppm) 136.4, 134.9, 129.8, 129.2, 126.8, 120.8, 120.7,
117.6; MS (EI, m/z) 144.1 [M]þ.
To a N2-flushed 100 mL round-bottomed flask, charged with
1.0 mmol of (tBu)PCl2 (0.318 g) and 10 mL of hexane, was
gradually added 2.0 mmol of Et2NH (207 μL) at 0 °C. The
suspension was then slowly warmed to 25 °C and stirred for 3 h.
After filtration, the solvent was removed under reduced pressure
at 0 °C before 10 mL of THF was added. Precaution against
exposure to air had been taken. To another flask containing a
solution of 2-(1H-imidazol-1-yl)anisole (0.174 g, 1.0 mmol) with
10 mL of THF was added 1.1 mmol of n-BuLi (0.44 mL, 2.5 M in
hexane) at -78 °C for 1 h and then was warmed to 0 °C.
Subsequently, the solution in the latter flask was slowly trans-
ferred to the former flask at 0 °C. The mixed solution was then
heated to 60 °C for 2 h. After cooling to 0 °C, the reaction was
quenched by water (2 mL). Subsequently, the resulting solution
was washed with HCl (3.0 M, 2 mL) and NaOH (3.0 M, 2.5 mL),
and then the aqueous layer was extracted twice with ethyl
acetate (2 ꢀ 5 mL). The combined organic solution was dried
over MgSO4. Finally, the residue was purified through column
chromatography (1:10 to 20:1 ethyl acetate/hexane to ethyl
acetate/methanol) to afford white power 3b in 67% yield (0.67
mmol, 186 mg).
Selected spectroscopic data for 3b: 1H NMR (CDCl3, δ/ppm)
7.55, 6.36 (d, 1H, P-H, 1JP-H=476 Hz), 7.47-7.43 (m, 2H, Ar),
7.35, 7.19 (s, 2H, CdC), 7.08-7.03 (m, 2H, Ar), 3.82 (s, 3H,
3
OMe), 1.15, 1.11 (d, 9H, t-Bu, JP-CC3H9=16 Hz); 13C NMR
(CDCl3, δ/ppm) 153.8, 140.5, 139.2, 130.7, 130.2, 130.0, 128.6,
125.6, 120.5, 111.6, 55.6, 32.74, 32.0, 23.65; 31P NMR (CDCl3,
δ/ppm) 29.5 (d, P-H, 1JP-H=476 Hz); MS (EI, m/z) 278.1 [M]þ.
3.5. Formations of 5a and 6a from the Reaction of 3a with
Pd(COD)Cl2. Into a N2-flushed 20 cm3 Schlenk tube were
placed Pd(COD)Cl2 (57 mg, 0.2 mmol), 3a (99 mg, 0.4 mmol),
and THF (1.0 mL). The solution was stirred at 60 °C for 40 min.
An alternative way is to stir at 25 °C for several hours in
dichloromethane (1.0 mL). Recrystallization from dichloro-
methane gave light yellow 5a in 86% yield (0.176 mmol, 116 mg).
For a prolonged reaction time, the formation of 6a was
observed by gradual conversion of the 31P NMR signal from
92.7 ppm of 5a to 104.0 ppm for 6a. Purification of 6a from a
mixture containing 5a and some unidentified products by the
chromatographic process was not achievable probably due to
the fragility of the former. 1H and 13C NMR spectra of the
mixture exhibit too many signals that are uninformative for
structural assignment of 6a.
Selected spectroscopic data for 1b: 1H NMR (CDCl3, δ/ppm)
7.76 (s, 1H, NdCH-N), 7.35-7.30 (t, 1H, Ar), 7.24-7.22 (d, 1H,
Ar), 7.19 (s, 1H, N-CHdC), 7.14 (s, 1H, CdCH-N), 7.04-6.98
(m, 2H, Ar); 13C NMR (CDCl3, δ/ppm) 152.1, 137.3, 128.5, 128.4,
126.1, 125.0, 120.6, 119.8, 112.0, 55.3; MS (EI, m/z) 174.0 [M]þ.
3.3. Preparation of 2-(tert-Butylhydrophosphoryl)-1-phenyl-
1H-imidazole (3a). Into a N2-flushed 100 mL round-bottomed
flask were placed 1.0 mmol of 1a (0.144 g) and 10 mL of THF.
The solution was cooled to -78 °C before 1.1 mmol of n-BuLi
(0.44 mL, 2.5 M in hexane) was added. The suspension was
slowly warmed to 0 °C and stirred for another 30 min. Subse-
quently, the solution was slowly transferred to another flask,
which contained 1.0 mmol of tert-butyldichlorophosphine
(0.159 g) in 10 mL of THF at 0 °C. The reaction mixture was
heated gradually from 0 to 60 °C and stirred for another 2 h.
After completion, the solution was diluted with 5 mL of ethyl
acetate at 0 °C and quenched with NH4Clsolution(10 mL, 1.0 M).
After stirring for a few minutes, the organic layer was
separated and the aqueous layer was extracted twice with ethyl
Selected spectroscopic data for 5a: 1H NMR (CDCl3,δ/ppm)
7.904 (s, 2H), 7.506 (m, 6H), 7.378 (m, 4H), 7.278 (s, 2H), 7.155
(s, 2H), 1.321, 1.265 (d, 18H, J = 22.4 Hz); 13C NMR (CDCl3,
δ/ppm) 144.08, 143.60, 136.80, 129.236, 129.00, 128.60, 127.93,
125.84, 125.48, 24.70; 31P NMR (CDCl3,δ/ppm) 92.7; MS
(APCI, m/z) 670.7 [M]þ.
3.6. Formation of 5b and 6b from the Reaction of 3b with
PdBr2. Similar steps to those shown in section 3.5 for the
reaction of 3b with PdBr2 had been proceeded. A N2-flushed
20 cm3 Schlenk tube contained a mixture of PdBr2 (53.0 mg,
0.2 mmol), 3b (110.0 mg, 0.4 mmol), and THF (1.0 mL). The
mixture was stirred at 25 °C for 1 h until the unsoluble powder

Article
Organometallics, Vol. 30, No. 5, 2011 1147
PdBr2 was consumed. Recrystallization from dichloromethane
gave light yellow 5b in 73% yield (0.146 mmol, 120.0 mg). Similar
to the previous case, for prolonged reaction times, the forma-
tion of 6b was observed by the 31P NMR signal at 107.9 ppm
besides the 96.3 ppm of 5b. Complex 6b is vulnerable to
chromatographic processes; therefore, its purification from a
mixture was not actualized. Too many signals were observed
in teh 1H and 13C NMR spectra for the mixture, which caused
the structural assignment of 6b to not be credible.
washed with brine and dried with anhydrous magnesium sulfate.
The dried organic layer was concentrated in vacuo. The residue was
purified by column chromatography to give the desired product.
3.8. X-ray Crystallographic Studies. Suitable crystals of 3a, 4,
5a, 5b, 6a, and 6b were sealed in thin-walled glass capillaries
under a nitrogen atmosphere and mounted on a Bruker AXS
SMART 1000 diffractometer. Intensity data were collected in
1350 frames with increasing ω (width of 0.3° per frame). The
absorption correction was based on the symmetry equivalent
reflections using the SADABS program. The space group deter-
mination was based on a check of the Laue symmetry and syste-
matic absences and was confirmed using the structure solution.
The structure was solved by direct methods using the SHEL-
XTL package.25 All non-H atoms were located from successive
Fourier maps, and hydrogen atoms were refined using a riding
model. Anisotropic thermal parameters were used for all non-H
atoms, and fixed isotropic parameters were used for H atoms.26
Crystallographic data for compounds 3a, 4, 5a, 5b, 6a, and 6b
are available from the Supporting Information.
Selected spectroscopic data for 5b: 1H NMR (CD2Cl2, δ/ppm)
7.524 (s, 2H), 7.396 (s, 2H), 7.256 (s, 2H), 7.187 (s, 2H),
7.003-6.982 (d, 2H, J=8.4 Hz), 6.701 (s, 2H), 3.811 (s, 6H),
1.336-1.207 (d, 18H); 13C NMR (CD2Cl2, δ/ppm) 154.7, 131.0,
126.9, 125.8, 123.4, 119.7, 112.3, 56.3, 28.3; 31P NMR (CD2Cl2,
δ/ppm) 96.3; MS (ESI, m/z) 821.2 [M]þ.
3.7. General Procedure for the Suzuki-Miyaura Cross-
Coupling Reactions. Suzuki-Miyaura cross-coupling reactions
were performed according to the following procedures. The four
reactants, Pd(OAc)2, 3b, boronic acid, and base, were placed in a
suitable oven-dried Schlenk flask. It was evacuated for 0.5 h and
backfilled with nitrogen gas before adding solvent and aryl
halide through a rubber septum. The aryl halides being solids
at room temperature were added prior to the evacuation/backfill
cycle. The flask was sealed with a rubber septum, and the solution
was stirred at the required temperature for the designated
number of hours. Then, the reaction mixture was diluted with
ethyl acetate (3 mL) and the cooled solution poured into a
separatory funnel. The mixture was washed with aqueous
NaOH (1.0 M, 5 mL), and the aqueous layer was extracted
with ethyl acetate (2ꢀ5 mL). The combined organic layer were
Acknowledgment. We thank the National Science
Council of the ROC (Grant NSC 98-2113-M-005-006-
MY3) for financial support.
Supporting Information Available: Crystallographic data for
the structural analysis have been deposited with the Cambridge
Crystallographic Data Center, CCDC no. 759451, 759450, 759453,
793730, 759452, and 759454 for compounds 3a, 4, 5a, 5b, 6a,
and 6b, respectively. Copies of this information may be
obtained free of charge from the Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: þ44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.
cam.ac.uk). This material is available free of charge via the
Internet at http://pubs.acs.org.
(25) Sheldrick, G. M. SHELXTL PLUS User’s Manual, Revision
4.1; Nicolet XRD Corporation: Madison, WI, 1991.
(26) The hydrogen atoms were riding on carbons or oxygens in their
idealized positions and held fixed with C-H distances of 0.96 A.