Preparation of cobalt-containing bulky monodentate phosphines with electron-withdrawing/donating substituents on bridged arylethynyl and their applications in Suzuki coupling reactions

Article abstract of DOI:10.1016/j.tet.2008.04.117

Several cobalt-containing bulky monodentate phosphines (μ-PPh₂CH₂PPh₂)Co₂(CO)₄(μ,η-(^tBu)₂PC{triple bond, long}C(C₆H₄R)) (4a: R=H; 4b: R=p-F; 4cp: R=p-CF

Full text of DOI:10.1016/j.tet.2008.04.117

Tetrahedron 64 (2008) 6221–6229
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journal homepage: www.elsevier.com/locate/tet
Preparation of cobalt-containing bulky monodentate phosphines
with electron-withdrawing/donating substituents on bridged
arylethynyl and their applications in Suzuki coupling reactions
*
Bo-Yuan Shiu, Po-Cheng Huang, Yi-Luen Huang, Fung-E. Hong
Department of Chemistry, National Chung Hsing University, Taichung 40227, Taiwan
a r t i c l e i n f o
a b s t r a c t
Several cobalt-containing bulky monodentate phosphines (m-PPh2CH2PPh2)Co2(CO)4(m,h
-(tBu)2PC^C
Article history:
Received 1 February 2008
Received in revised form 28 April 2008
Accepted 29 April 2008
(C6H4R)) (4a: R¼H; 4b: R¼p-F; 4cp: R¼p-CF3; 4cm: R¼m-CF3; 4d: R¼p-OMe) were prepared from the
reactions of (tBu)2PC^C(R–C6H4) (2a: R¼H; 2b: R¼p-F; 2cp: R¼p-CF3; 2cm: R¼m-CF3; 2d: R¼p-OMe)
with Co2(CO)6(
m-PPh2CH2PPh2) 3. Further reactions of 4a, 4b, 4cp, 4cm, and 4d with Pd(OAc)2 yielded
Available online 1 May 2008
unique palladium complexes
(m-PPh2CH2PPh2)Co2(CO)4(m,h k m-OAc) (5a:
-(tBu)2PC^C(C6H3R)- C1)Pd(
R¼H; 5b: R¼p-F; 5cp: R¼p-CF3; 5cm: R¼m-CF3; 5d: R¼p-OMe, respectively). The strong electron-
withdrawing substituents, –F and –CF3, assist the ortho-metalation process during the formation of 5b,
5cp, and 5cm. The more positively charged palladium center in 5b (or 5cp, 5cm) enhances the proba-
bility for PhB(OH)ꢀ3 to attack the metal center and the rate of reduction thereafter. DFT studies on the
charges of these palladium centers support this assumption. The enhancement of the reaction rates of
the Suzuki–Miyaura cross-coupling reactions using 5b, 5cp, and 5cm is thereby attributed to this effect.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Suzuki–Miyaura cross-coupling reaction
DFT methods
ortho-Metalation
Phosphine ligand
Transmetalation
Electron-withdrawing substituent
1. Introduction
experimental5 and theoretical6 methods, and as a result are quite
well understood. In contrast, transmetalation is a process that is
The organic reactions catalyzed by transition metal complexes
have been a widely studied subject vital to the modern synthetic
chemistry.1 Among them, the palladium-catalyzed cross-coupling
reactions used for the formation of carbon–carbon bonds are
probably the most well-known.2 Ever since its disclosure, the pal-
ladium-catalyzed Suzuki–Miyaura reaction has been a focus of
extensive investigation by both experimental and theoretical
means.3 To get a clear picture of the reaction mechanism, an in-
sightful discernment on the reaction pathways is essential. In the
generally accepted simplified mechanism proposed by Suzuki2a an
unsaturated Pd(0)Ln (n¼1 and 2 for monodentate and bidentate
ligands, respectively) species serves as the catalyst precursor. Three
major parts of the mechanism are: (1) oxidative addition; (2)
transmetalation; and (3) reductive elimination. Each major part
consists of several consecutive elementary steps. The first part
involves the addition of an aryl halide to Pd(0)Ln. During the second
part, the halide is exchanged for another aryl group supplied by an
arylboronic acid. In the third part, a biaryl product is released from
the Pd(II) complex giving back Pd(0)Ln. Oxidative addition and
reductive elimination processes are frequently observed in transi-
tion metal-catalyzed reactions.4 Thereby, the basic components of
these two processes have been extensively studied by both
more complicated and less understood. Obviously, the experi-
mental data for the transmetalation process are difficult to accu-
mulate due to the complexity encountered in characterizing the
mechanistically relevant intermediates.7
It has been demonstrated repeatedly that a well-chosen ligand
is crucial to the success of a palladium-catalyzed Suzuki reaction.3b
Thereby, considerable efforts have been devoted to find more effi-
cient ligands. For many years, organophosphines of various kinds
and shapes and their derivatives have been the most frequently
employed ligands. 1a,3b,8 Nevertheless, a limited number of transi-
tion metal-containing phosphines have also been reported.9,10 Our
previous studies had demonstrated that a new category of metal-
containing phosphines
(m-PPh2CH2PPh2)Co2(CO)4(m,h
-R1C^CR2)
{R1,R2: P(tBu)2, P(iPr)2, PCy2, PPh2, Ph, py} could be prepared
handily according to the processes described elsewhere (Diagram
1).11 This new class of phosphines naturally led to the ligands of
bulky character, which is obviously beneficial to the reductive
elimination process in catalytic cycle. Their roles as legitimate and
R1
C C
R2
t
i
R1, R2: P( Bu)2, P( Pr)2, PCy2, PPh2, Ph, py, H
(OC)2Co
Ph2P
Co(CO)2
PPh2
C
H2
* Corresponding author.
Diagram 1. A generalized structure for cobalt-containing phosphines.
E-mail address: fehong@dragon.nchu.edu.tw (F.-E. Hong).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.04.117

6222
B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
R
R
1. n-BuLi, -78 °C, 1 hr
2. ClP( Bu)2, THF, r.t., 24h
t
C
C P( Bu)2
C
C H
t
1a: R = H
2a: R = H
1b: R = p-F
1cp: R = p-CF3
2b: R = p-F
2cp: R = p-CF3
1cm: R = m-CF3
1d: R = p-OMe
2cm: R = m-CF3
2d: R = p-OMe
t
( Bu)2
(CO)2
P
Co
P
O
C
Ph2
[O]
C
(CO)2
R
P
Co
Ph2
4aO: R = H
t
4cmO: R = m-CF3
4dO: R = p-OMe
P( Bu)2
+ Co2(CO)6(dppm) 3
C
C
R
t
THF, 60 °C
( Bu)2
(OC)2Co
Co(CO)2
PPh2
(CO)2
P
Ph2P
Co
C
P
O
+ Pd(OAc)2
C
Ph2
Pd
C CH3
H2
O
C
4a: R = H
P
Co
Ph2
4b: R = p-F
5a: R = H
(CO)2
4cp: R = p-CF3
5b: R = p-F
5cp: R = p-CF3
4cm: R = m-CF3
4d: R = p-OMe
R
5cm: R = m-CF3
5d: R = p-OMe
Scheme 1. Syntheses of cobalt-containing monodentate phosphines 4a–d and palladium complexes 5a–d. Here and throughout the article letters a, b, c, and d in abbreviations
correspond to R¼H, F, CF3, and OMe, respectively; while p and m stand for para- and meta-position, respectively. The phosphine oxides are designated by O.
active ligands in palladium-catalyzed cross-coupling reactions have
been carefully evaluated.11,12
4cm: R¼m-CF3; 4d: R¼p-OMe) were synthesized according to the
procedures shown in Scheme 1. Firstly, the alkynyl phosphines
[(tBu)2PC^C(C6H4R)] (2b: R¼p-F; 2cp: R¼p-CF3; 2cm: R¼m-CF3;
2d: R¼p-OMe) were prepared from the corresponding terminal
alkynes HC^C(C6H4R) (1b: R¼p-F; 1cp: R¼p-CF3; 1cm: R¼m-CF3;
1d: R¼p-OMe), respectively, via deprotonation at ꢀ78 ꢁC for 1 h
followed by quenching with ClP(tBu)2 at 25 ꢁC for 24 h. Then, 2b,
An organometallic version of 2-(di-tert-butylphosphino)bi-
phenyl,13 {[( -(tBu)2PC^C(C6H5))]}
m-PPh2CH2PPh2)Co2(CO)4][(m,h
4a, was synthesized according to Scheme 1. Firstly, alkynyl phos-
phine, [(tBu)2PC^C(C6H5)] 2a, was prepared bya modified literature
procedure.14 Then, treatmentofa dppm-bridged dicobalt compound
[Co2(CO)6(
THF at 60 ꢁC afforded 4a. A unique palladium complex, {[(
PPh2CH2PPh2)Co2(CO)4][( -OAc)}
-(tBu)2PC^C(C6H4R)- C1)]Pd(
m
-P,P–PPh2CH2PPh2)] 3 with 1 molar equivalent of 2a in
2cp, 2cm, and 2d were allowed to react with Co2(CO)6(m-
m-
PPh2CH2PPh2) 3 at 60 ꢁC for 48 h to yield 4b, 4cp, 4cm, and 4d,
respectively, as reddish-brown solids. Either during the crystalli-
zation or during the chromatographic separation, compounds 4cm
m,
h
k
m
5a, was prepared from the reaction of the cobalt-containing bulky
phosphine 4a with Pd(OAc)2 (Scheme 1). As disclosed by its crystal
structure, 5a is a result of an ortho-metalation process accompanied
by the release of 1 equiv of acetic acid.12g,13,15 The Suzuki–Miyaura
cross-coupling reactions between an aryl halide and phenylboronic
acid were carried out with either isolated and purified 5a or 5a
prepared in situ from 4a and Pd(OAc)2.15 In any case, the Pd(II) cat-
alyst precursor is probably reduced to an active Pd(0) catalyst first by
PhB(OH)ꢀ3 generated from phenylboronic acid in basic medium.16 In
addition to the Suzuki reaction, complex 5a was also employed as
a catalyst precursor12g for the amination of bromobenzene by
morpholine demonstrating acceptable results.
and 4d have been partially oxidized to pink solids {[(
m-
PPh2CH2PPh2)Co2(CO)4][(
m,h
-(tBu)2(O])PC^C(C6H4R))] (4cmO:
R¼m-CF3; 4dO: R¼p-OMe). Compounds 4b, 4cp, 4cmO, 4d, and
4dO were characterized by spectroscopic methods such as 1H, 13C,
and 31P NMR as well as elemental analysis and mass spectrometry.
In 13C NMR, the quartet signals of –CF3 are weak for both 4cm and
4cmO. The assignment of the signal is referred to the more dis-
cernible pattern in 5cm. Their molecular structures were de-
termined by single-crystal X-ray diffraction methods (Table 1). The
ORTEP diagrams are depicted in Figures 1–5.
It was speculated that the positive charge of the palladium
center could be increased by introduction of a proper electron-
withdrawing substituent into the arylethyne unit of 5. Thereby, the
reaction rate between the anionic PhB(OH)ꢀ3 and the more posi-
tively charged palladium center shall be significantly increased as
well as the rate of reduction process followed. It shall lead to the
increase of the whole reaction rate thereafter. We were interested
in examining the validity of this assumption. In this study, the
preparation of several substituted analogs of phosphine 4a bearing
electron-withdrawing/donating groups such as –F, –CF3, and –OMe
at the designated positions of the phenyl group as well as their
applications in palladium-catalyzed Suzuki cross-coupling re-
actions are presented.
2.2. Preparation of palladium complexes {[(
Co2(CO)4][( -OAc)} (5b: R[p-F;
-(tBu)2PC^C(C6H3R)- C1)]Pd(
5cp: R[p-CF3; 5cm: R[m-CF3; 5d: R[p-OMe)
m-PPh2CH2PPh2)-
m,h
k
m
Several palladium complexes {[(
m
-PPh2CH2PPh2)Co2(CO)4]-
[(m
,h
-(tBu)2PC^C(C6H3R)- C1)]Pd(
k
m-OAc)} (5b: R¼p-F; 5cp: R¼p-
CF3; 5cm: R¼m-CF3; 5d: R¼p-OMe) related to 5a were prepared
from the reactions of their corresponding cobalt-containing bulky
phosphines (4b: R¼p-F; 4cp: R¼p-CF3; 4cm: R¼m-CF3; 4d: R¼p-
OMe) with Pd(OAc)2. The reactions carried out in THF at 25 ꢁC for
1 h furnished reddish-brown solids in about 55% yield. All com-
pounds were characterized by spectroscopic methods. In addition,
the molecular structure of 5cm depicted in Figure
6 was
determined by a single-crystal X-ray diffraction method (Table 1).
The molecular structure of 5cm revealed the formation of a Pd–Caryl
bond via the ortho-metalation process. The process occurred on
C(8) rather than C(4) due to the latter position being shielded by
the nearby CF3 substituent. The acetate ligand coordinates to the
palladium center in a chelating mode. Judging from the similarities
of 1H and 31P NMR spectra (Table 2), the rest of complexes
are expected to possess the structural features analogous to those
of 5cm.
2. Results and discussion
2.1. Preparation of cobalt-containing phosphines {[(
m-PPh2-
CH2PPh2)Co2(CO)4][(
m,h
-(tBu)2PC^C(C6H4R))]} (4b: R[p-F;
4cp: R[p-CF3; 4cm: R[m-CF3; 4d: R[p-OMe)
A number of cobalt-containing phosphines {[(m-PPh2CH2PPh2)-
Co2(CO)4][(
m,
h
-(tBu)2PC^C(C6H4R))] (4b: R¼p-F; 4cp: R¼p-CF3;

B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
6223
Table 1
Crystal data of 4b, 4cp, 4cmO, 4d, 4dO, and 5cm
Compound
4b
4cp
4cmO
4d
4dO
5cm
Formula
Formula weight
Crystal system
Space group
a(Å)
C
45H44Co2FO4P3$1/5CH2Cl2
C46H44Co2F3O4P3
928.58
Triclinic
Pꢀ1
C46H44Co2F3O5P3
944.58
Monoclinic
P21/n
11.3617(13)
21.439(2)
19.242(2)
d
C46H47Co2O5P3
890.61
Monoclinic
P21/c
11.7610(8)
18.4205(12)
21.2621(14)
d
C46H47Co2O6P3
906.605
Triclinic
Pꢀ1
C48H46Co2F3O6P3Pd$3/2Cd
1111.03
Triclinic
Pꢀ1
895.56
Monoclinic
P21
12.0836(10)
19.4019(16)
19.3272(15)
d
11.997(5)
17.865(7)
22.706(9)
66.915(8)
81.074(7)
87.751(8)
4422(3)
2
12.0443(13)
17.8356(18)
22.293(2)
67.346(2)
82.616(2)
89.279(2)
4379.3(8)
2
12.6101(9)
13.5008(9)
16.6265(11)
79.4820(10)
86.810(2)
69.2030(10)
2601.6(3)
2
b(Å)
c(Å)
a
b
(ꢁ)
(ꢁ)
101.345(2)
d
106.724(2)
d
104.8740(10)
d
g
(ꢁ)
V(Å3)
4442.6(6)
4
1.339
4488.7(9)
4
1.398
4452.0(5)
4
1.329
Z
Dc (Mg/m3)
1.395
1.375
1.418
l
m
q
(Mo K
(mmꢀ1
Range(ꢁ)
a
)
) (Å)
0.71073
0.923
2.01–26.02
16,431
1027
0.71073
0.913
1.85–26.45
16,928
1099
0.0689
0.71073
0.902
1.46–26.07
8836
532
0.0367
0.71073
0.896
1.48–26.03
8726
505
0.0303
0.71073
0.914
1.86–26.06
17,076
1027
0.0597
0.71073
1.119
1.64–26.06
10,140
573
Observed reflections(F>4s(F))
No. of refined parameters
R1 for significant
reflectionsa
0.0398
0.0558
wR2 for significant
reflectionsb
0.0705
1.061
0.1688
0.980
0.0857
0.999
0.1053
0.999
0.1370
0.945
0.1652
0.813
GoFc
a
R1¼j
S
(jF0jꢀjFcj)/ jF0jj.
S
b
c
wR2¼{
S
w(F20ꢀF2c)2/
S
w(Fo2)2}1/2; w¼0.0250, 0.1100, 0.0500, 0.0900, 0.0750, and 0.1638 for crystal data of 4b, 4cp, 4cmO, 4d, 4dO, and 5cm, respectively.
GoF¼[
S
w(F20ꢀFc2)2/(NrflnsꢀNparams)]1/2
.
d
1/4 n-Hexane was added without hydrogen.
2.3. Structural comparison of 4b, 4cp, and 4d
parameters of 4b, 4cp, and 4d are given in Table 3 for comparison
(Diagram 2). As shown in Table 2, these frameworks are rather
similar. Again, it shows that the alkyne-bridging dicobalt frame-
work Co2C2 core is a rigid entity.
The structures of 4b, 4cp, and 4d reveal that each molecule
contains a pseudo-tetrahedral Co2C2 core, which is typical for an
alkyne-bridged dicobalt complex. In addition, the dppm acts as
a bridging ligand toward both cobalt atoms. Both substituents of
the alkyne are bent away from the dicobalt moiety, as predicted by
Dewar–Chatt–Duncanson model, and reside on the same side.17 In
this regard, 4b, 4cp, and 4d can be considered as authentic metal-
containing monodentate phosphine ligands. The variations of
structural parameters for all compounds are small, except for the
–F, –CF3, and –OMe substituents on phenyl ring. Selected structural
2.4. Application of 4/Pd(OAc)2 in Suzuki reactions
As known, the performance of a palladium-catalyzed carbon–
carbon cross-coupling reaction is governed by a number of factors
such as the palladium source employed, ligand, base, solvent,
reaction temperature, etc.18 Furthermore, the ratio of ligand to
palladium is also crucial since various bonding modes are possible
for different types of ligands. Here, Suzuki coupling reactions were
Figure 1. ORTEP drawing of 4b.
Figure 2. ORTEP drawing of 4cp.

6224
B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
Figure 5. ORTEP drawing of 4dO.
Figure 3. ORTEP drawing of 4cmO.
the active species derived from 4b–d/Pd(OAc)2 are similar to that of
4a/Pd(OAc)2. As reported, monophosphine-coordinated complexes
Pd(0)L (L¼phosphine) might be the catalytically active species.8d,19
Based on this postulation, the reduction of 5 by PhB(OH)ꢀ3 is pro-
posed (Scheme 3)16,20 resulting in a low-coordinated Pd(0) com-
plex, 6, as an active species. It is followed by the oxidative addition
of ArX and the formation of a three-coordinate Pd(II) complex, 7.21
In the presence of excess 4, the concentration of the active species 6
will be reduced due to the formation of a multi-ligand palladium
complex, thereby diminishing the catalytic efficiency (entries 1 and
2). On the contrary, the yield is rather low in the absence of
carried out by employing the newly made cobalt-containing
phosphine ligands 4a, 4b, 4cp, 4cm, and 4d-modified palladium
complexes (Tables 4–6). In most cases, the reactions were carried
out with 1.0 mmol of 2-bromothiophene, 1.5-fold excess of phe-
nylboronic acid, 3-fold excess of a base in 1.0 mL solvent, and
1.0 mol % of 4/Pd(OAc)2 at 60 ꢁC (Scheme 2). Initially, KF was chosen
as the base due to its good performance in most of the Suzuki
reactions.
As shown in Table 4, the best yield was obtained with the 4b/
Pd(OAc)2 ratio being 1:1 rather than with the 2:1 ratio generally
giving the best yield in the case of an organophosphine ligand
(entries 1 and 3). Yet, it is consistent with our previous observations
for 4a/Pd(OAc)2 as a catalyst precursor.11b Therefore, these obser-
vations naturally lead to the conclusion that the bonding modes of
Figure 4. ORTEP drawing of 4d.
Figure 6. ORTEP drawing of 5cm.

B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
6225
Table 2
CH3
CH3
(13)
1H and 31P NMR of 5a, 5b, 5cp, 5cm, and 5da
H3C
R
C
CH3
CH3
CH3
5a
5b
5cp
5cm
5d
(6)
4b: R = F
(1)
31P NMR (
[P(tBu)2]
[dppm]
d
/ppm)
(3)
(2) C
P
C
(1)
(9)
91.6
38.8
91.4
38.6
91.5
38.7
91.4
38.5
91.5
38.5
(3)
C
(1)
F
F
(2)
4cp: R=
C
F
(OC)2Co
Co(CO)2
(1)
PPh2
(46)
1H NMR (
[OAc]
d
/ppm)
(1)
(2)
2.1
2.0
2.0
2.0
2.0
(45)
Ph2P
(3)
(2)
C
4d: R = OCH3
a
in CD2Cl2.
H2
(5)
phosphine ligand 4 (entry 6). It implies that the phosphine ligand
plays an important role in promoting the catalytic efficiency.
As known, the base selection is essential to the success of
a palladium-catalyzed Suzuki reaction.22 Therefore, the effects of
the various bases used in the reactions were examined (Table 5).
Excellent yields were observed with NaOH and K2CO3 (entries 2
and 3). The catalytic efficiency is also good with KF as a base (entry
1). The yields were lower when CsF and Cs2CO3 were employed
under the same reaction condition (entries 7 and 8).
As shown in Table 5, K2CO3 and NaOH are equally efficient in the
reaction. To differentiate their activities, a lower reaction temper-
ature and shorter reaction hours were used (Table 6). It is obvious
that NaOH is a better choice (entry 1 vs 2). The reaction that
stopped after 1 h was still very efficient (entry 4) and even that run
at 40 ꢁC for 3 h provided an acceptable result (entry 5).
Diagram 2. Generalized structure for 4b, 4cp, and 4d.
4b, 4cp, 4cm, and 4d were employed in the palladium-catalyzed
Suzuki reaction under the above conditions. As shown in Table 9,
the catalytic efficiency is better in the case of an electron-with-
drawing substituent placed on the arylethyne unit (entries 2 and 3).
On the contrary, low catalytic efficiency was observed in the case of
the electron-donating substituent (entry 4).
For comparison, the same ligands were employed in the reaction
with p-bromobenzaldehyde as an aryl halide (Scheme 4). The trend
shown in Table 9 was observed here as well. Moreover, the effect of
the electron-withdrawing substituent is clearly demonstrated. As
shown inTable 10, the catalytic efficiency was the highest in the case
Table 4
Dependence of the yield of the Suzuki–Miyaura reaction on the 4b/Pd(OAc)2 ratioa
The influence of the solvent used was evaluated as well
(Table 7). The reaction conditions were the same as above. The best
yield was observed with toluene as a solvent (entry 5). Un-
expectedly, a poor result was observed with 1,4-dioxane (entry 3).
The Suzuki–Miyaura reaction between 2-bromothiophene and
phenylboronic acid employing 4b with various Pd sources in the
NaOH/toluene system was carried out at 40 ꢁC for 3 h (Table 8). As
shown, excellent yield was obtained with Pd(OAc)2 as a palladium
source (entry 1). Interestingly, low yield was obtained with
PdCl2(CH3CN)2 (entry 5).
Entry
Ligand/Pd
Yieldb (%)
1
2
3
4
5
6
2.0/1.0
1.5/1.0
1.0/1.0
0.5/1.0
0.1/1.0
0.0/1.0
32
75
93
84
54
10
a
Reaction conditions: 1.0 mmol of 2-bromothiophene, 1.5 mmol of phenylboronic
acid, 3.0 mmol of KF, 1 mL THF, 60 ꢁC, 24 h.
b
Average of two runs; isolated yield.
The optimized conditions for the 4b-assisted palladium-cata-
lyzed Suzuki reaction emerged from summing up the above studies,
namely, Pd(OAc)2 as a palladium source, NaOH as a base, toluene as
a solvent, 40 ꢁC, and 3 h reaction time. For comparison, ligands 4a,
Table 5
Dependence of the yield of the Suzuki–Miyaura reaction on basea
Entry
Base
Yieldb (%)
1
2
3
4
5
6
7
8
KF
93
>99
>99
88
78
30
Table 3
K2CO3
NaOH
K3PO4
KOH
Na2CO3
Cs2CO3
CsF
Comparison for selected structural parameters of 4b, 4cp, and 4d
4b
4cp
4d
1.357(3)
Bond length (Å)
C(1)–C(2)
1.352(5)
1.366(7)
16
0
Co(1)–C(1)
Co(1)–C(2)
Co(2)–C(1)
Co(2)–C(2)
Co(1)–Co(2)
Co(1)–P(2)
Co(2)–P(3)
P(1)–C(1)
P(2)–C(45)
P(3)–C(45)
F(1)–C(6)
C(6)–C(46)
C(6)–O(5)
1.986(4)
1.967(4)
2.007(4)
1.942(4)
2.4689(7)
2.2328(12)
2.2268(12)
1.816(4)
1.810(4)
1.851(4)
1.358(5)
d
2.004(5)
1.934(4)
2.011(5)
1.976(5)
2.4623(12)
2.2276(17)
2.2457(17)
1.809(5)
1.840(5)
1.838(5)
d
2.0188(18)
1.9549(18)
1.9860(18)
1.9752(18)
2.4907(4)
2.2290(5)
2.2392(5)
1.8155(19)
1.8323(17)
1.8331(18)
d
a
Reaction conditions: 1.0 mmol of 2-bromothiophene, 1.5 mmol of phenylboronic
acid, 3.0 mmol of base, L/Pd¼1:1 mol %, 1 mL THF, 60 ꢁC, 24 h.
b
Average of two runs; isolated yield.
Table 6
Suzuki–Miyaura reaction with K2CO3 versus NaOHa
Entry
Temp. (ꢁC)
Base
Time (h)
Yieldb (%)
1
2
3
4
5
60
60
60
60
40
K2CO3
NaOH
NaOH
NaOH
NaOH
6
6
3
1
3
74
>99
>99
>99
82
1.496(8)
d
d
d
1.372(2)
Bond angle (ꢁ)
C(2)–Co(1)–C(1)
C(2)–Co(2)–C(1)
C(2)–C(1)–P(1)
C(1)–C(2)–C(3)
Co(1)–C(2)–Co(2)
Co(1)–C(1)–Co(2)
C(13)–P(1)–C(9)
P(2)–C(45)–P(3)
P(2)–Co(1)–Co(2)
P(3)–Co(2)–Co(1)
40.01(14)
40.00(14)
132.4(3)
40.55(19)
40.07(19)
131.8(4)
136.0(4)
78.06(17)
75.66(16)
109.7(3)
106.8(3)
94.05(4)
97.93(5)
39.89(7)
40.06(7)
132.29(15)
138.46(17)
78.65(7)
a
Reaction conditions same as in footnote of Table 5.
Average of two runs; isolated yield.
b
136.9(4)
78.33(15)
76.38(14)
110.1(2)
108.79(19)
98.52(4)
93.63(4)
S
76.91(6)
S
Br
+ 1.5
1 mol % 4/Pd(OAc)2
3 equiv. base, 1 mL solvent
109.78(11)
109.55(9)
92.110(16)
99.563(15)
B(OH)2
Scheme 2. Suzuki–Miyaura cross-coupling reaction of 2-bromothiophene with
phenylboronic acid in the presence of 4/Pd(OAc)2.

6226
B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
t
t
( Bu)2
(CO)2
P
( Bu)2
t
(CO)2
( Bu)2
P
(CO)2
Co
P
Co
P
Ar
P
C
C
C
Co
Ph2
P
O
O
Ph2
Pd
+ B(OH)3Ph-
Reduction
Pd
C
+ ArX
Ph2
Pd
C CH3
C
(CO)2
X
P
P
Co
C
(CO)2
Oxidative
Addition
Co
P
Ph2
Ph2
Co
(CO)2
Ph2
R
R
R
6
5
7
Pd(II) complex
Pd(0) complex
Pd(II) complex
Scheme 3. Proposed reduction and oxidative addition steps in 5-assisted Suzuki–Miyaura cross-coupling reaction.
of the strongest electron-withdrawing substituent, –CF3, placed on
the arylethyne unit (entry 3). The worst catalytic performance was
observed in the case of the electron-donating group, –OMe (entry 4).
It shows that the electronic property of the substituent on the aryl-
ethyne unit indeed influences the reaction rate and yield.
5b and 5cp, respectively. Since the state-of-the-art density func-
tional theory (DFT) method has proven to be a useful tool in pro-
viding reliable results in the studies of transition metal-mediated
catalytic reactions, it was used to obtain information concerning
the charges of the palladium centers in 5.23 This method at the
B3LYP level was utilized to examine the validity of the postulation.
To make the computations feasible, the actual reaction participants
5 were simplified to model compounds, 50 according to Diagram 3.
The results of Natural Population Analysis (NPA) of the palla-
dium center charges in 50 are listed in Table 11. The positive charges
of the palladium centers decrease in the following order:
5c0>5b0>5d0>5a0>5e0. It is in accordance with the electron-with-
drawing/donating capacities of the substituents. Obviously, the
analysis of the palladium center charges in 5 validates the as-
sumption that an electron-withdrawing substituent on the aryl-
ethyne unit would facilitate the attack of PhB(OH)ꢀ3 on metal,
thereby enhancing the reaction rate.
2.5. DFT studies on the charges of palladium centers in 5
As postulated, the positive charge of the palladium center could
be increased by placing an electron-withdrawing substituent on
the arylethyne unit of 5a. Thereby, the rate of the reduction process
from Pd(II) to Pd(0) by the anionic PhB(OH)ꢀ3 will be enhanced. It
could lead to the increase of the overall reaction rate. The positive
charge of the palladium center in 5b or 5cp is expected to be higher
than that in 5a. It is attributed to the presence of strong electron-
withdrawing substituents, –F and –CF3, in the arylethyne units of
Table 7
Dependence of the Suzuki–Miyaura reaction on solventa
3. Summary
Entry
Solvent
Yieldb (%)
1
2
3
4
5
THF
MeOH
1,4-Dioxane
CH3CN
Toluene
82
72
15
32
>99
We have demonstrated that placing electron-withdrawing
substituents on the arylethyne part of 5 leads to a rate increase in
Table 10
Suzuki–Miyaura coupling reaction between p-bromobenzaldehyde and phenyl-
a
Reaction conditions: NaOH as a base, 40 ꢁC, 3 h, the rest is the same as in foot-
boronic acid with various ligandsa
note of Table 5.
Yieldb (%)
b
Entry
Ligand
Average of two runs; isolated yield.
1
2
3
4
5
4a
4b
4cp
4cm
4d
55
75
90
66
32
Table 8
Suzuki–Miyaura reaction with various palladium sourcesa
Entry
Palladium source
Yieldb (%)
a
1
2
3
4
5
Pd(OAc)2
>99
83
26
43
9
Reaction conditions: 1.0 mmol of p-bromobenzaldehyde, 1.5 mmol of phenyl-
boronic acid, and 3.0 mmol of NaOH, L/Pd¼1:1 mol %, 1 mL of toluene, 40 ꢁC, 3 h.
Pd(COD)Cl2
[(h
3-C3H5)PdCl]2
b
Average of two runs; isolated yield.
Pd2(dba)3
Pd(CH3CN)2Cl2
Me
Me
a
Reaction conditions are the same as in footnote of Table 7 except for the palla-
P
dium sources.
O
O
b
HC
HC
5a': R=H
5b': R=p-F
5c': R=p-CF3
5d': R=p-OMe
5e': R=p-Me
Average of two runs; isolated yield.
Pd
C CH3
Table 9
Suzuki–Miyaura reaction with various ligandsa
Entry
Ligand
Yieldb (%)
R
1
2
3
4
5
4a
4b
4cp
4cm
4d
93
>99
>99
95
Diagram 3. Simplified model compounds of 50.
Table 11
The Natural Population Analysis (NPA) of the palladium center charges in 50
74
a
Reaction conditions are the same as in footnote of Table 7 except for the ligand
Model compound
Charge
5a0
5b0
5c0
5d0
5e0
used.
0.487
0.493
0.496
0.489
0.486
b
Average of two runs; isolated yield.
O
C
O
H C
1 mol % 4/Pd(OAc)2
3 equiv. base, 1 mL solvent
+ 1.5
B(OH)2
H
Br
Scheme 4. Suzuki–Miyaura cross-coupling reaction of p-bromobenzaldehyde with phenylboronic acid in the presence of 4/Pd(OAc)2.

B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
6227
the Suzuki coupling reaction. It is attributed to the enhancement of
the Pd(II) to Pd(0) reduction rate. The more and more positively
charged palladium centers on 5b, 5cm, and 5cp enhance the
probability for the anion PhB(OH)ꢀ3 to attack resulting in the ac-
celeration of the reduction rate. The assumption is supported by the
Natural Population Analysis (NPA) from DFT calculations.
(q, 1C, JC–F¼31.7 Hz, –CF3), 122.16–146.80 (30C, arene), 30.83, (s,
6C, –C(CH3)3), 35.10 (d, 2C, JP–C¼22.78 Hz, –C(CH3)3), 32.31 (t, 1C,
JP–C¼21.2 Hz, –CH2–). Elemental analysis calculated for
C46H44Co2F3O4P3: C, 59.48; H, 4.96. Found: C, 58.13; H, 5.19.
Compound 4cmO. 1H NMR (CDCl3,
arene), 3.44–3.30 (m, JP–H¼8.0 Hz, 1H, dppm), 1.35 (d, JP–H¼13.2 Hz,
18H, tBu); 31P NMR (CDCl3,
/ppm): 35.52 (s, 2P, dppm), 59.01 (s, 1P,
P(tBu)2); 13C NMR (CD2Cl2,
/ppm): 127.12 (q, 1C, JC–F¼31.7 Hz,
d/ppm): 7.00–7.78 (m, 24H,
d
4. Experimental
d
–CF3), 122.65–145.31 (30C, arene), 28.34, (s, 6C, –C(CH3)3), 38.42 (d,
2C, JP–C¼63.15 Hz, –C(CH3)3), 33.77 (t, 1C, JP–C¼21.2 Hz, –CH2–).
Elemental analysis calculated for C46H44Co2F3O5P3: C, 58.44; H,
4.87. Found: C, 57.51; H, 4.95. MS (FAB): m/z¼945.0 [M]þ.
4.1. General information
All manipulations were carried out under a dry nitrogen at-
mosphere. Solvents including deuterated solvents were purified
before use. Most separations were performed by Centrifugal Thin
Layer Chromatography (CTLC, Chromatotron, Harrison model
8924). The 1H and 31P NMR spectra were recorded on a Varian-400
spectrometer at 400.44 and 162.10 MHz, respectively; 13C NMR
spectra were recorded on a Varian VXR-300S spectrometer at
75.43 MHz. Chemical shifts are reported in parts per million rela-
tive to the residual proton signals of deuterated CHCl3 or CH2Cl2.
Mass spectra were recorded on a JOEL JMS-SX/SX 102A GC/MS/MS
spectrometer. Elemental analyses were obtained on a Heraeus
CHN-O-S-Rapid instrument.
Compound 4d. 1H NMR (CD2Cl2,
d/ppm): 6.67–7.30 (24H, arene),
3.80 (s, 3H, –OMe), 3.21–3.17 (m, 1H, dppm), 3.42–3.39 (m, 1H,
t
dppm), 1.24 (d, JP–H¼10.8 Hz, 18H, Bu); 31P NMR (CDCl3,
d
/ppm):
/ppm):
36.65 (s, 2P, dppm), 44.74 (s, 1P, P(tBu)2); 13C NMR (CD2Cl2,
d
127.94–132.89 (30C, arene), 30.56 (d, JP–C¼14.1 Hz, 6C, –C(CH3)3),
34.79 (d, JP–C¼23.1 Hz, 2C, –C(CH3)3), 32.04, (t, JP–C¼21.2 Hz, 1C,
–CH2–). Elemental analysis calculated for C46H47Co2O5P3: C, 62.03;
H, 5.32. Found: C, 61.25; H, 4.88. MS (FAB): m/z¼891.0 [M]þ.
4.3. Synthesis of {[(
PC^C(C6H3R)-
C1)]Pd(
R[p-CF3; 5cm: R[m-CF3; 5d: R[p-OMe)
m
-PPh2CH2PPh2)Co2(CO)4][(
m,h
-(tBu)2-
k
m-OAc)} (5a: R[H; 5b: R[p-F; 5cp:
4.2. Synthesis of {[(m-PPh2CH2PPh2)Co2(CO)4(m,h
-(tBu)2PC^C-
(C6H4R))]} (4a: R[H; 4b: R[p-F; 4cp: R[p-CF3; 4cm: R[m-
CF3; 4d: R[p-OMe)
A 100 mL round-bottom flask equipped with a magnetic stirrer
was charged with 1.00 mmol of 4a (0.860 g), 1 molar equiv of
Pd(OAc)2 (1.0 mmol, 0.223 g), and 15 mL THF. The solution was
stirred at 25 ꢁC for 1 h before the solvent was removed under
reduced pressure. The residue was further separated by CTLC. A
reddish-brown band eluted with CH2Cl2/ethyl acetate (10:1)
Dicobalt octacarbonyl Co2(CO)8 (2.00 mmol, 0.684 g), dppm
(2.00 mmol, 0.769 g), and THF (15 mL) were placed in a 100 mL
round-bottom flask charged with a magnetic stirrer. The solution
was stirred at 60 ꢁC for 6 h giving rise to a yellow compound
was identified as {[(
m-PPh2CH2PPh2)Co2(CO)4][(m,h
-(tBu)2PC^C-
[Co2(CO)6(
m
-P,P-dppm)] 3. Without further separation, the reaction
(C6H4)- -OAc)} 5a. The yield was 50% (1.00 mmol, 1.025 g).
k
C1)]Pd(
m
flask was charged with one equiv of [(tBu)2PC^C(C6H4R)] (2a: R¼H,
0.493 g; 2b: R¼p-F, 0.529 g; 2cp: R¼p-CF3, 0.629 g; 2cm: R¼m-CF3,
0.629 g; 2d: R¼p-OMe, 0.553 g) in THF (15 mL). Subsequently, the
solution was stirred at 60 ꢁC for 48 h before the solvent was
removed under reduced pressure. The residue was further sepa-
rated by CTLC. A reddish-brown band eluted with CH2Cl2/ethyl
Similar procedures were used for preparing 5b, 5cp, 5cm, and 5d.
The yields for 5b, 5cp, 5cm, and 5d were 45%, 50%, 50%, and 58%,
respectively.
Compound 5a. 1H NMR (CD2Cl2,
d/ppm): 7.54–6.92 (24H, arene),
3.30 (t, JP–H¼10.2 Hz, 2H, CH2), 1.53 (d, JP–H¼16.0 Hz, 18H, tBu); 31
P
NMR (CD2Cl2, d
/ppm): 38.8 (s, 2P, dppm), 91.6 (s, 1P, P(tBu)2).
acetate (1:1) was identified as {[(
m
-PPh2CH2PPh2)Co2(CO)4-
Compound 5b. 1H NMR (CD2Cl2,
d/ppm): 7.36–6.53 (23H, arene),
(m,
h
-(tBu)2PC^C(C6H4R))]} (2a: R¼H; 2b: R¼p-F; 2cp: R¼p-CF3;
3.63 (t, JP–H¼10.4 Hz, 2H, CH2), 1.50 (d, JP–H¼14.0 Hz, 18H, tBu); 31
P
2cm: R¼p-CF3; 2d: R¼p-OMe). The yields were 62% (1.24 mmol,
1.067 g) for 4a, 65% (1.30 mmol, 1.141 g) for 4b, 55% (1.10 mmol,
1.021 g) for 4cp, 50% (1.00 mmol, 0.928 g) for 4cm, and 59%
(1.18 mmol, 1.050 g) for 4d.
NMR (CD2Cl2, d
/ppm): 38.56 (s, 2P, dppm), 91.37 (s, 1P, P(tBu)2).
Compound 5cp. 1H NMR (CD2Cl2,
d/ppm): 7.36–7.02 (23H, arene),
3.63 (t, JP–H¼10.4 Hz, 2H, CH2), 1.51 (d, JP–H¼14.4 Hz, 18H, tBu); 31
P
NMR (CD2Cl2, d
/ppm): 38.7 (s, 2P, dppm), 91.5 (s, 1P, P(tBu)2).
Compound 4b. 1H NMR (CD2Cl2,
d/ppm): 6.85–7.34 (24H, arene),
Compound 5cm. 1H NMR (CD2Cl2,
d
/ppm): 7.68 (d, JH–H¼7.6 Hz,
3.24–3.16 (m, 1H, dppm), 3.39–3.31 (m, 1H, dppm), 1.26
1H, arene), 7.36–7.16 (m, 23H, arene), 3.63 (t, JP–H¼10.4 Hz, 2H,
t
(d, JP–H¼11.2 Hz, 18H, Bu); 31P NMR (CDCl3,
d
/ppm): 36.88 (s, 2P,
CH2), 1.49 (d, JP–H¼14.4 Hz, 18H, tBu), 2.00 (s, 3H, CH3CO2ꢀ); 31P NMR
dppm), 45.20 (s, 1P, P(tBu)2); 13C NMR (CD2Cl2,
d/ppm): 128.01–
(CD2Cl2,
(CD2Cl2,
d
/ppm): 38.5 (s, 2P, dppm), 91.4 (s, 1P, P(tBu)2); 13C NMR
132.78 (30C, arene), 30.62, 30.49 (d, 6C, –C(CH3)3, JP–C¼13.1 Hz),
34.81 (d, JP–C¼22.2 Hz, 2C, –C(CH3)3), 31.96 (t, JP–C¼21.2 Hz, 1C,
–CH2–). Elemental analysis calculated for C45H44Co2FO4P3: C, 61.52;
H, 5.05. Found: C, 61.25; H, 5.31. MS (FAB): m/z¼879.0 [M]þ.
d/ppm): 127.12 (q, 1C, JC–F¼31.7 Hz, –CF3), 121.31–158.99
(30C, arene), 30.40, (s, 6C, –C(CH3)3), 39.15 (d, 2C, JP–C¼21.9 Hz,
–C(CH3)3), 41.21 (t, 1C, JP–C¼20.5 Hz, –CH2–), 23.87 (s, 1C, CH3COꢀ2 ),
188.42 (s, 1C, CH3COꢀ2 ). Anal. Calcd: C, 52.78; H, 4.21. Found: C,
51.70; H, 4.64. MS (FAB): m/z¼980 [Mꢀ4CO]þ.
Compound 4cp. 1H NMR (CD2Cl2,
d/ppm): 6.97–7.36 (m, 24H,
arene), 3.20–3.17 (m, 1H, dppm), 3.33–3.30 (m, 1H, dppm), 1.25
Compound 5d. 1H NMR (CD2Cl2,
d/ppm): 7.35–6.41 (24H, arene),
t
(d, JP–H¼11.2 Hz, 18H, Bu); 31P NMR (CDCl3,
d
/ppm): 36.39 (s, 2P,
3.65 (t, JP–H¼10.0 Hz, 2H, CH2), 1.51 (d, JP–H¼14.0 Hz, 18H, tBu); 31
P
dppm), 44.94 (s, 1P, P(tBu)2); 13C NMR (CD2Cl2,
d/ppm): 150.08
NMR (CD2Cl2, d
/ppm): 38.5 (s, 2P, dppm), 91.5 (s, 1P, P(tBu)2).
(s, 1C, –CF3), 128.09–132.82 (30C, arene), 30.59, (d, JP–C¼13.1 Hz,
6C, –C(CH3)3), 34.85 (d, JP–C¼23.1 Hz, 2C, –C(CH3)3), 32.00 (t, 1C,
JP–C¼21.2 Hz, –CH2–). Elemental analysis calculated for
4.4. General procedure for the Suzuki
cross-coupling reactions
C
46H44Co2F3O4P3: C, 59.60; H, 4.78. Found: C, 60.33; H, 4.90. MS
(FAB): m/z¼929.0 [M]þ.
Suzuki cross-coupling reactions using 4a–d were performed
according to the following general procedure. Compounds 4a–d
(0.01 mmol), phenylboronic acid (0.183 g, 1.50 mmol), and NaOH
(0.120 g, 3.00 mmol) were placed into a 20 mL Schlenk flask. The
flask was evacuated and backfilled with nitrogen before adding
Compound 4cm. 1H NMR (CDCl3,
d/ppm): 6.96–7.45 (m, 24H,
arene), 3.15 (q, J¼12 Hz, 1H, dppm), 3.29 (q, J¼12 Hz, 1H, dppm),
t
1.26 (d, JP–H¼11.2 Hz, 18H, Bu); 31P NMR (CDCl3,
d
/ppm): 36.41 (s,
2P, dppm), 44.65 (s, 1P, P(tBu)2); 13C NMR (CD2Cl2,
d/ppm): 127.12

6228
B.-Y. Shiu et al. / Tetrahedron 64 (2008) 6221–6229
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solution was stirred at 40 ꢁC for 3 h. Subsequently, excess amount of
water was added and the product was extracted with ether
(3ꢂ20 mL). The combined organic extracts were dried over anhy-
drous MgSO4 and concentrated under vacuum. The crude residue
was purified by flash chromatography on silica gel.
4. Metal-Catalyzed Cross-coupling Reactions; Diederich, F., Stang, P. J., Eds.;
Wiley-VCH: Weinheim, Germany, 1998.
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4.5. X-ray crystallographic studies
Suitable crystals of 4b, 4cp, 4cmO, 4d, 4dO, and 5cm were
sealed in thin-walled glass capillaries under nitrogen atmosphere
and mounted on a Bruker AXS SMART 1000 diffractometer. In-
tensity data were collected in 1350 frames with increasing
u (width
of 0.3ꢁ per frame). The absorption correction was based on the
symmetry equivalent reflections using SADABS program. The space
group determination was based on a check of the Laue symmetry
and systematic absences, and was confirmed using the structure
solution. The structure was solved by direct methods using
a SHELXTL package.24 All non-H atoms were located from succes-
sive 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.25
Crystallographic data for compounds 4b, 4cp, 4cmO, 4d, 4dO, and
5cm are summarized in Table 1.
9. For general reviews, see: (a) Delacroix, O.; Gladysz, J. A. Chem. Commun. 2003,
665; (b) Salzer, A. Coord. Chem. Rev. 2003, 242, 59.
10. (a) Jesen, J. F.; Johannsen, M. Org. Lett. 2003, 5, 3025; (b) See Refs. 8g,h,k,l.
11. (a) Hong, F.-E.; Chang, Y.-C.; Chang, R.-E.; Chen, S.-C.; Ko, B.-T. Organometallics
2002, 21, 961; (b) Chang, Y.-C.; Hong, F.-E. Organometallics 2005, 24, 5686.
12. (a) Hong, F.-E.; Ho, Y.-J.; Chang, Y.-C.; Huang, Y.-L. J. Organomet. Chem. 2005,
690, 1249; (b) Chang, C.-P.; Huang, Y.-L.; Hong, F.-E. Tetrahedron 2005, 61, 3835;
(c) Hong, F.-E.; Chang, Y.-C.; Chang, C.-P.; Huang, Y.-L. Dalton Trans. 2004, 157;
(d) Hong, F.-E.; Ho, Y.-J.; Chang, Y.-C.; Lai, Y.-C. Tetrahedron 2004, 60, 2639; (e)
Hong, F.-E.; Chang, C.-P.; Chang, Y.-C. Dalton Trans. 2003, 3892; (f) Hong, F.-E.;
Lai, Y.-C.; Ho, Y.-J.; Chang, Y.-C. J. Organomet. Chem. 2003, 688, 161; (g) Lee, J.-C.;
Wang, M.-G.; Hong, F.-E. Eur. J. Inorg. Chem. 2005, 5011.
5. Computational methods
All calculations were carried out using the Gaussian 03 package,
in which the tight criterion (10ꢀ8 hartree) is the default for the SCF
convergence.26 The molecular geometries were fully optimized
with the hybrid B3LYP-DFT method under C1 symmetry, in which
the Becke three parameter exchange functional27 and the
Lee–Yang–Parr correlation functional28 were used. The LANL2DZ
13. Buchwald, S. L.; Zim, D. Org. Lett. 2003, 5, 2413.
14. O’Connor, T. J.; Patel, H. A. Can. J. Chem. 1971, 49, 2706.
including the double-z basis sets for the valence and outermost
15. Gan, Y.-H.; Lee, J.-C.; Hong, F.-E. Polyhedron 2006, 25, 3555.
16. Bedford, R. B.; Hazelwood, S. L.; Horton, P. N.; Hursthouse, M. B. Dalton Trans.
2003, 4164.
17. (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71; (b) Chatt, J.; Duncanson, L. A.
J. Chem. Soc. 1953, 2939.
18. (a) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.; Clapp, A. R.; Uyeda,
H. T.; Deschamps, J. R.; Lassman, M. E.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127,
6744; (b) Braga, A. A. C.; Morgon, N. H.; Ujaque, G.; Maseras, F. J. Am. Chem. Soc.
2005, 127, 9298.
19. Yin, J.; Rainka, M. P.; Zhang, X. X.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124,
1162.
core orbitals combined with pseudo-potential was used for Pd,29,30
and 6-31G(d) basis sets for the other atoms. All the stationary
points found were characterized via harmonic vibrational fre-
quency analysis as minima (number of imaginary frequency
Nimag¼0). Natural Population Analysis (NPA)31 was performed with
NBO 5.0 software32 using B3LYP/LANL2DZ as the level. Stability
analysis33 has been performed to determine if the Kohn–Sham (KS)
solutions are stable with respect to variations, which break spin
and spatial symmetries.
20. Bedford, R. B.; Cazin, C. S. Chem. Commun. 2001, 1540.
21. (a) Stambuli, J. P.; Bu¨hl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 9346; (b)
Beare, N. A.; Hartwig, J. F. J. Org. Chem. 2002, 67, 541; (c) Stambuli, J. P.; Stauffer,
S. R.; Shaughnessy, K. H.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 2677; (d)
Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295; (e) Aranoys, A.; Old, D. W.;
Kiyomory, A.; Wolfe, J. P.; Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1999,
121, 4369.
22. (a) Miyaura, N.; Yamada, K.; Suginome, H.; Suzuki, A. J. Am. Chem. Soc. 1985, 107,
972; (b) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077; (c)
Dubbaka, S. R.; Vogel, P. Org. Lett. 2004, 6, 95.
23. (a) Pople, J. A. Angew. Chem., Int. Ed. 1999, 38, 1894; (b) Hehre, W. J.; Radom, L.;
Schleyer, P. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley-Interscience:
New York, NY, 1986; (c) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density
Functional Theory; Wiley-VCH: Weinheim, 2000; (d) Ziegler, T. Chem. Rev. 1991,
91, 651; (e) Davidson, E. R. Chem. Rev. 2002, 100, 351.
6. Supplementary material
Crystallographic data for the structural analysis have been
deposited at the Cambridge Crystallographic Data Center, CCDC
nos. 653734, 653735, 666136, 653736, 653737, and 666137 for
compound 4b, 4cp, 4cmO, 4d, 4dO, and 5cm, 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 website: http://
www.ccdc.cam.ac.uk).
24. Sheldrick, G. M. SHELXTL PLUS User’s Manual. Revision 4.1; Nicolet XRD
Corporation: Madison, Wisconsin, USA, 1991.
25. The hydrogen atoms were ride on carbons or oxygens in their idealized
positions and held fixed with the C–H distances of 0.96 Å.
Acknowledgements
26. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox,
We thank the National Science Council of the R.O.C. (Grant NSC
95-2113-M-005-015-MY3) for financial support.
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