Pd(N,N-dimethyl β-alaninate)2 as a high-turnover-number, phosphine-free catalyst for the suzuki reaction

Article abstract of DOI:10.1055/s-2007-965883

A novel crystalline, air- and moisture-stable Pd salt, Pd(N,N-dimethyl β-alaninate)₂, was synthesized. This salt was demonstrated to constitute a low-priced, phosphine-free, yet high-turnover-number (TON = 10 ⁴) catalyst for the Suzuki reactions of various aryl bromides and iodides under mild and simple reaction conditions. Moreover, it was observed that the presence of an olefin could dramatically hamper the Suzuki reaction. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-965883

PAPER
393
Pd(N,N-Dimethyl b-alaninate)2 as a High-Turnover-Number, Phosphine-Free
Catalyst for the Suzuki Reaction
Phosphine-Free
C
a
i
talyst
w
n
ith
H
igh-Turnove
C
r-Number for
S
u
ui
, Tian Qin, Jia-Rui Wang, Lei Liu,* Qing-Xiang Guo*
Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. of China
E-mail: leiliu@ustc.edu; E-mail: qxguo@ustc.edu.cn
Received 25 August 2006; revised 12 October 2006
studies the Pd/ligand loading was higher than 1 mol% and
Abstract: A novel crystalline, air- and moisture-stable Pd salt,
Pd(N,N-dimethyl b-alaninate)2, was synthesized. This salt was
demonstrated to constitute a low-priced, phosphine-free, yet high-
turnover-number (TON = 104) catalyst for the Suzuki reactions of
therefore, the corresponding catalyst could not be consid-
ered as a HTC. Due to these two problems, further studies
on the use of non-phosphine ligands in the Suzuki reaction
various aryl bromides and iodides under mild and simple reaction are still highly warranted, where the key challenge is how
conditions. Moreover, it was observed that the presence of an olefin
to achieve high turnover numbers without employing any
could dramatically hamper the Suzuki reaction.
expensive ligand.
Key words: palladium, Suzuki reaction, phosphine-free ligand,
Herein we wish to report that a well-defined Pd(II) salt of
high-turnover-number catalyst
N,N-dimethyl b-alaninate is an inexpensive, yet highly ef-
ficient catalyst for the Suzuki reaction. This work is part
of our continuing efforts to investigate how to use more
The Pd-catalyzed cross coupling of aryl and alkenyl ha-
economically competitive metals (e.g. Fe, Co, Ni, and Cu)
lides with organoboronic acids is known as the Suzuki re-
and/or ligands (in particular, phosphine-free ligands) to
action.1 This reaction, normally performed with 1–5
accomplish the traditional Pd-catalyzed reactions.14 Previ-
mol% of Pd along with phosphine ligands, has become
ously we reported that the combination of Pd(OAc)2 and
one of the most versatile methods for the C–C bond for-
mation in organic synthesis. The Suzuki reaction has been
the N,N-dimethyl b-alanine ligand constituted an efficient
catalytic system for the Heck reaction of aryl bromides.15
successfully utilized in the synthesis of a wide variety of
After the study was completed for some time, it was dis-
valuable compounds including pharmaceuticals, natural
covered unexpectedly that some nice single crystals were
products, and functional organic materials. Despite these
produced in the mother liquor left of the mixture of
successes, large-scale industrial applications of the Suzu-
Pd(OAc)2 and N,N-dimethyl b-alanine. Through X-ray
measurements it was determined that the crystal corre-
sponded to Pd(N,N-dimethyl b-alaninate)2 (Figure 1).
Subsequently we were pleased to find that crystalline
Pd(N,N-dimethyl b-alaninate)2 could be readily prepared
in large quantities as an air-stable and moisture-stable pal-
ki reaction are still limited mainly due to the following
two problems.2 First, Pd is expensive and contamination
of the product by Pd has to be tightly controlled. Second,
and perhaps more limiting is that many phosphine ligands
are more expensive than Pd and are not pleasant to work
with, because they are toxic, air-sensitive, and difficult to
ladium salt.
recycle. Accordingly, one of the current challenges in the
field is the development of high-turnover-number cata-
lysts (HTC)3 that utilize inexpensive and non-phosphine
ligands.
To meet the challenge, a number of phosphine-free cata-
lysts have been examined for the Suzuki reaction. Previ-
ously studied ligands included heterocyclic carbenes,4
oxazolines,5 oximines,6 imines,7 diazabutadienes,8 simple
amines,9 bispyridines,10 hydrazones,11 pyrazoles,12 and
surfactants.13 Good to excellent yields were usually re-
ported for these ligands in the Suzuki reaction, which
strongly suggested the promising application of the non-
phosphine ligands in the Pd catalysis. Nonetheless, two
problems have to be pointed out for the previously report-
ed phosphine-free ligands: (1) most of these phosphine-
free ligands are not commercially available and some are
cumbersome to synthesize; and (2) in most of the above
Figure 1 Crystal structure of Pd(N,N-dimethyl b-alaninate)2
SYNTHESIS 2007, No. 3, pp 0393–0399
x
x
.
x
x
.2
0
0
7
Advanced online publication: 12.01.2007
DOI: 10.1055/s-2007-965883; Art ID: F13106SS
© Georg Thieme Verlag Stuttgart · New York

394
X. Cui et al.
PAPER
With Pd(N,N-dimethyl b-alaninate)2 in hand, we next ex- and electron-poor (activated) aryl bromides could be effi-
amined whether this well-defined salt could be used di- ciently converted to the desirable products in high yields
rectly, without any other additive, to catalyze the Suzuki (entries 1–16). A highly sterically crowded substrate
reactions. To begin our study, we performed the Suzuki could also be tolerated (entry 17). The turnover numbers
reaction between 4-bromotoluene and phenylboronic acid in most of the cases were about 104, demonstrating that
with 0.1 mol% of Pd(N,N-dimethyl b-alaninate)2 using Pd(N,N-dimethyl b-alaninate)2 was a high-turnover-num-
DMF as the solvent as previously described.15 It was ber catalyst. Besides aryl bromides, aryl iodides could
found that the reaction proceeded smoothly at 100 °C in a also be successfully transformed to the desired Suzuki
high yield without any inert gas protection (Table 1, entry coupling products under the same conditions (entries 18–
1). However, when the temperature was dropped to 50 °C, 21). Nonetheless, Pd(N,N-dimethyl b-alaninate)2 was not
the yield decreased dramatically (entry 2). We then dis- active enough to handle an aryl chloride (entries 22,23).
covered that by adding water to the reaction we could ob-
tain a high yield at 50 °C and even at room temperature
The above data demonstrated Pd(N,N-dimethyl b-alani-
nate)2 to be a generally applicable, high-turnover-number
(entries 3 and 4). By changing the base from K2CO3 to
catalyst for the Suzuki reaction. This strongly supported
K3PO4 we further increased the yield from 89% to 98%
our previous proposition that N,N-dimethyl b-alanine was
a catalytically active ligand in the combination of
(entries 5–7). An additional improvement was then made
when the solvent was changed to EtOH–H2O, which was
more convenient to handle than DMF–H2O. Using K3PO4
Pd(OAc)2 and N,N-dimethyl b-alanine that could effi-
ciently catalyze the Heck reaction.15 At this point we not-
as the base in EtOH–H2O, the Suzuki reaction could be
ed an interesting yet puzzling behavior concerning these
complete within one hour at 50 °C with an excellent yield
Pd-catalyzed reactions. That is, when the same Pd catalyst
(99%), even when the catalyst loading was dropped to
was utilized, the Suzuki reaction could be accomplished
0.01 mol% (entry 9).
under much milder conditions than the Heck reaction. For
Using the optimized reaction conditions (EtOH–H2O/ our own Pd(N,N-dimethyl b-alaninate)2 catalyst, the Heck
K3PO4, 50 °C, 1 h, in air), we next examined the applica- reaction had to be performed at about 130 °C for over ten
tion of Pd(N,N-dimethyl b-alaninate)2 to the cross cou- hours with 0.1 mol% of catalyst, whereas the Suzuki reac-
pling of a variety of aryl halides with several arylboronic tion could be carried out at 50 °C in an hour with only 0.01
acids under 0.01 mol% catalyst loading (Table 2). The re- mol% of catalyst (Scheme 1). Similar behavior has been
sults indicated that Pd(N,N-dimethyl b-alaninate)2 truly reported in many other studies, where the Heck reaction
constituted an operationally simple, low-priced, yet high- was always achieved at a higher temperature, in a longer
ly efficient catalyst system for the Suzuki reaction of reaction time, and/or with a higher loading of the catalyst
many aryl bromides. Both the electron-rich (deactivated) as compared to the Suzuki reaction.16
Table 1 Suzuki Reaction between 4-Bromotoluene and Phenylboronic Acida
Br
+
B(OH)2 Pd complex
base, solvent
Entry
Catalyst (mol%)
Base
Temp (°C)
Time (h)
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
9
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.01
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
K3PO4
KF
100
50
50
r.t.
50
50
50
50
50
13
10
3
DMF
96
10
89
85
5
DMF
DMF–H2Oc
DMF–H2Oc
DMF–H2Oc
DMF–H2Oc
DMF–H2Oc
EtOH–H2Od
EtOH–H2Od
10
5
1
98
96
95
99
3
K2CO3
K3PO4
1
1
a General conditions: 4-bromotoluene (5 mmol), phenylboronic acid (7.5 mmol), catalyst = Pd(N,N-dimethyl b-alaninate)2, base (10 mmol),
solvent (15 mL), in air.
b GC yields.
c DMF (7.5 mL) and H2O (7.5 mL).
d EtOH (7.5 mL) and H2O (7.5 mL).
Synthesis 2007, No. 3, 393–399 © Thieme Stuttgart · New York

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A High-Turnover-Number Phosphine-Free Catalyst for Suzuki Reaction
395
Table 2 Suzuki Reactions Catalyzed by the Pd(N,N-Dimethyl b-alaninate)2 Salta
Pd complex (0.01%)
B(OH)2
+
X
Ar
Ar
R
K3PO4, EtOH–H2O (1:1)
50 °C, in air, 1 h
R
Entry
1
Aryl halide
Boronic acid
Product
Yield (%)b
98
Br
B(OH)2
2
3
99
96
Br
B(OH)2
B(OH)2
Br
MeO
MeO
Me2N
MeO
Br
4
5
6
7
8
98
97
98
96
91
B(OH)2
MeO
B(OH)2
B(OH)2
Br
Me2N
Br
MeOC
MeOC
Me2N
MeOC
Br
B(OH)2
MeOC
MeO
B(OH)2
Br
OMe
Me2N
9
10
11
99
96
98
Br
B(OH)2
B(OH)2
B(OH)2
MeOC
MeOC
OMe
OMe
MeOC
MeO
O2N
MeO
MeOC
MeO
Br
Br
O2N
O2N
OMe
12
97
Br
B(OH)2
O2N
13
14
15
16
17
99
97
88
90
92
Br
B(OH)2
NC
NC
HO
NC
NC
HO
Br
Br
B(OH)2
OMe
MeO
B(OH)2
B(OH)2
B(OH)2
Br
Br
Br
18
99
B(OH)2
I
MeOC
MeOC
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Table 2 Suzuki Reactions Catalyzed by the Pd(N,N-Dimethyl b-alaninate)2 Salta (continued)
Pd complex (0.01%)
B(OH)2
+
X
Ar
Ar
R
K3PO4, EtOH–H2O (1:1)
50 °C, in air, 1 h
R
Entry
19
Aryl halide
Boronic acid
Product
Yield (%)b
B(OH)2
97
I
20
94
B(OH)2
I
MeO
MeO
MeO
I
21
22
23
97
B(OH)2
MeO
<20
<20
B(OH)2
Cl
B(OH)2
B(OH)2
Cl
O2N
O2N
24
25
94
97
Br
Br
O
B(OH)2
O
a Reaction conditions: aryl halide (5 mmol), arylboronic acid (7.5 mmol), Pd(N,N-dimethyl b-alaninate)2 (0.0005 mmol), K3PO4 (10 mmol),
EtOH (7.5 mL), H2O (7.5 mL), in air.
b Isolated yield.
Pd complex (0.1 mol%)
we never observed any Heck-coupling product under any
of the above reaction conditions.
R
+
Ar Br
R
Ar
NMP, K2CO3, 130 °C, 10 h
yield: 80–100%
Using the above method we have also examined the ef-
fects of (E)-1,2-diphenylethene, triphenylethene, and
1,1,2,2-tetraphenylethene on the Suzuki reaction between
4-bromotoluene and phenylboronic acid. It was found
again that the Suzuki reaction could not proceed well even
when a sub-stoichiometric amount of olefin was present.
To explain the observations, we propose the following
mechanism (Scheme 2). First, a Pd(0) complex I with the
ligand (N,N-dimethyl b-alanine) is generated from the re-
action of Pd(II) with solvents or substrates.17 Second, ox-
idative addition of aryl halide to I produces complex II,
which undergoes transmetalation with the boronic acid to
generate complex III. Finally, reductive elimination of III
produces the desired coupling product and I. The reasons
that addition of an olefin dramatically hampered the Suzu-
ki reaction are two-fold: (1) both I and II could form com-
plexes with the olefin that stops the catalytic cycle, and (2)
only a small portion of Pd(II)L2 is converted to the cata-
lytically active Pd(0) complex I and therefore, a small
amount of olefin is sufficient to slow down the turnover
frequency. Thus, our experiments not only indicate that
the Suzuki reaction was much more facile than the Heck
reaction, but also demonstrate that the presence of an ole-
fin can retard the Suzuki reaction.
B(OH)2
Pd complex (0.01 mol%)
+
Ar Br
Ar
R
EtOH/H2O, K3PO4, 50 °C, 1 h
R
yield: 90–100%
Scheme 1 Comparing the reactivity of Heck and Suzuki reactions.
Pd complex = Pd(N,N-dimethyl b-alaninate)2.
To further elucidate the different reactivity between the
Heck and Suzuki reactions, we performed the following
control experiments where a certain amount of alkene was
added to the Suzuki reaction mixture of 4-bromotoluene
and phenylboronic acid (Table 3). It was found that when
1 equivalent (or 100 mol%) of styrene was added to the
Suzuki reaction mixture (Pd = 0.1 mol%), almost no 4-
methylbiphenyl could be detected after one hour at 50 °C.
Similarly, almost no 4-methylbiphenyl could be detected
when 0.1 equivalent of styrene was added. After a further
decrease of styrene to 0.01 equivalent (or 1 mol%), we
started to observe the desired Suzuki reaction product (i.e.
4-methylbiphenyl) after one hour, albeit the yield was low
(50%). Next we reduced the styrene loading to 10–3, 10–4,
10–5, and 10–6 equivalents, where the corresponding yield
of the desired 4-methylbiphenyl product increased dra-
matically from 65% to 96%. It is worth mentioning that
Synthesis 2007, No. 3, 393–399 © Thieme Stuttgart · New York

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A High-Turnover-Number Phosphine-Free Catalyst for Suzuki Reaction
397
Table 3 Suzuki Reaction in the Presence of Alkenesa
Pd complex (0.1 mol%)
B(OH)2
Br
+
K3PO4, alkene
EtOH–H2O, 50 °C, 1 h
Ph
Ph
Ph
Added alkene (equiv)
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Yield (%) of
4-methylbiphenyl
Yield (%) of
4-methylbiphenyl
Yield (%) of
4-methylbiphenyl
Yield (%) of
4-methylbiphenyl
b
b
1
0
0
0
20
42
50
74
78
92
–
–
b
0.1
27
75
95
98
99
99
–
0.01
0.001
10–4
10–5
10–6
50
65
74
92
96
45
85
90
95
98
a Reaction conditions: 4-bromotoluene (5 mmol), phenylboronic acid (7.5 mmol), Pd complex = Pd(N,N-dimethyl b-alaninate)2, base (10
mmol), EtOH (7.5 mL), H2O (7.5 mL), in air. GC yields.
b The alkene was not fully soluble under these conditions.
R'
action conditions. Furthermore, we observed that the
presence of an olefin could dramatically hamper the Suzu-
ki reaction. This finding may provide interesting mecha-
nistic insights into the lower reactivity of the Heck
reaction as compared to the Suzuki reaction.
O
R
O
Pd(0)
X
N
(I)
R
Pd(II)L2
All chemicals were purchased from Acros. NMR spectra were re-
corded on a Bruker AV300 spectrometer in CDCl3 using TMS as in-
ternal standard.
R
N
O
Pd
R
N
O
O
Pd
X
O
Pd(N,N-Dimethyl b-Alaninate)2
(II)
A mixture of N,N-dimethyl-b-alanine (2 equiv) and K2PdCl4 (1
equiv) was stirred in H2O at r.t. for 10 min. The solution was adjust-
ed to pH 8 with aq 10% NaOH whereupon a precipitate was formed.
The yellow solid obtained was collected by filtration and dried un-
der vacuum; yield: 72%.
(III)
R'
R'
XB(OH)2
B(OH)2
1H NMR (300 MHz, CDCl3): d = 2.45 (t, J = 5.25 Hz, 4 H), 2.56 (m,
O
O
4 H), 2.57 (s, 12 H).
O
+
13C NMR (75 MHz, CDCl3): d = 34.5, 49.0, 60.1, 176.0.
Ph
N
O
Pd
N
Pd
(I)
(0)
Ph
Suzuki Reaction; General Procedure
To a mixture of aryl halide (5 mmol), arylboronic acid (7.5 mmol),
Pd(N,N-dimethyl b-alaninate)2 (0.0005 mmol), and K3PO4 (10
mmol) were added H2O (7.5 mL) and EtOH (7.5 mL). The mixture
was stirred at 50 °C in air for 1 h. The resulting mixture was cooled
to r.t. and extracted with Et2O. The Et2O layer was separated, dried,
and concentrated. The residue was purified by chromatography
(hexane–EtOAc) to afford the desired product (Table 2).
R
N
Pd
R
N
Pd
X
+ Ph
X–
O
+
O
O
O
(II)
Ph
Scheme 2 Proposed mechanism for the Suzuki reaction catalyzed
by Pd(N,N-dimethyl b-alaninate)2
4-Methylbiphenyl18
To summarize, in the present study we have synthesized a
novel crystalline, air-stable, and moisture-stable Pd salt,
Pd(N,N-dimethyl b-alaninate)2. This salt was demonstrat-
ed to constitute a low-priced, phosphine-free, yet high-
turnover-number catalyst for the Suzuki reactions of vari-
ous aryl bromides and iodides under mild and simple re-
1H NMR: d = 2.38 (s, 3 H), 7.24 (d, J = 8.1 Hz, 2 H), 7.32 (m, 1 H),
7.41 (t, J = 7.5 Hz, 2 H), 7.48 (d, J = 8.1 Hz, 2 H), 7.56 (d, J = 7.5
Hz, 2 H).
13C NMR: d = 21.22, 127.13, 128.84, 129.61, 137.14, 138.52,
141.32.
Synthesis 2007, No. 3, 393–399 © Thieme Stuttgart · New York

398
X. Cui et al.
PAPER
13C NMR: d = 127.19, 127.49, 127.65, 128.96, 140.26, 140.84.
4-Methoxybiphenyl19
1H NMR: d = 3.84 (s, 3 H), 6.97 (m, 2 H), 7.30 (d, J = 7.2 Hz,1 H),
7.40 (t, J = 7.5 Hz, 2 H), 7.53 (m, 4 H).
2,6-Dimethylbiphenyl24
1H NMR: d = 2.03 (s, 6 H), 7.08–7.16 (m, 5 H), 7.34 (m, 1 H), 7.35–
7.44 (m, 2 H).
13C NMR: d = 55.48, 114.39, 126.82, 126.90, 128.31, 128.88,
133.95, 141.00, 159.35.
13C NMR: d = 20.98, 126.73, 127.15, 127.41, 128.54, 129.16,
136.18, 141.23, 142.00.
4-Dimethylaminobiphenyl6
1H NMR: d = 3.00 (s, 3 H), 6.83 (s, 2 H), 7.30 (d, J = 7.2 Hz, 1 H),
7.40 (t, J = 7.5 Hz, 2 H), 7.53 (m, 4 H).
2-Methylbiphenyl25
1H NMR: d = 2.27 (s, 3 H), 7.23–7.26 (m, 4 H), 7.30–7.35 (m, 3 H),
7.38–7.41 (m, 2 H).
13C NMR: d = 40.56, 112.87, 126.05, 126.32, 127.74, 128.74,
129.26, 141.29, 150.02.
13C NMR: d = 20.58, 125.89, 126.88, 127.37, 128.19, 129.32,
129.92, 130.43, 135.44, 142.08, 142.12.
4-Acetylbiphenyl20
1H NMR: d = 2.64 (s, 3 H), 7.42 (m, 1 H), 7.47 (m, 2 H), 7.63 (m, 2
H), 7.68 (m, 2 H), 8.03 (m, 2 H).
2-Methoxybiphenyl25
1H NMR: d = 3.80 (s, 3 H), 7.97–7.02 (m, 2 H), 7.29–7.34 (m, 3 H),
7.40 (m, 2 H), 7.52 (d, J = 6.9 Hz, 2 H).
13C NMR: d = 26.66, 127.24, 127.30, 128.29, 128.96, 129.01,
135.93, 139.90, 145.78, 197.70.
13C NMR: d = 55.58, 111.35, 120.93, 126.98, 128.06, 128.70,
129.68, 130.83, 130.97, 138.66, 156.56.
4-Methoxy-4¢-dimethylaminobiphenyl21
1H NMR: d = 2.99 (s, 6 H), 3.83 (s, 3 H), 6.83 (m, 2 H), 6.94 (d,
J = 8.7 Hz, 2 H), 7.46 (m, 4 H).
Acknowledgment
13C NMR: d = 40.81, 55.45, 113.12, 114.27, 127.44, 129.38, 134.10,
149.73, 158.43.
We thank NSFC (No. 20332020 and 20472079) for financial sup-
port.
4-Acetylamino-4¢-methoxybiphenyl22
1H NMR: d = 2.62 (s, 3 H), 3.86 (s, 3 H), 7.00 (d, J = 8.7 Hz, 2 H),
7.57 (d, J = 8.7 Hz, 2 H), 7.64 (d, J = 8.5 Hz, 2 H), 8.00 (d, J = 8.4
Hz, 2 H).
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13C NMR: d = 26.67, 55.47, 114.53, 126.69, 128.45, 129.04, 132.34,
135.42, 145.45, 160.05, 197.75.
4-Methoxy-4¢-nitrobiphenyl22
1H NMR: d = 3.87 (s, 3 H), 7.01 (d, J = 8.7 Hz, 2 H), 7.58 (d, J = 8.7
Hz, 2 H), 7.68 (d, J = 8.7 Hz, 2 H), 8.26 (d, J = 8.7 Hz, 2 H).
13C NMR: d = 55.55, 114.76, 124.25, 127.18, 128.68, 131.20,
146.70, 147.33, 160.61.
4-Nitrobiphenyl23
1H NMR: d = 7.44–7.52 (m, 3 H), 7.61 (m, 2 H), 7.73 (d, J = 8.9 Hz,
2 H), 8.29 (d, J = 8.9 Hz, 2 H).
13C NMR: d = 124.14, 127.43, 127.83, 129.00, 129.22, 138.80,
147.65.
4-Cyanobiphenyl9
1H NMR: d = 7.44–7.48 (m, 3 H), 7.60 (m, 2 H), 7.67–7.75 (m, 4 H).
13C NMR: d = 110.89, 118.99, 127.24, 127.73, 128.71, 129.15,
132.61, 139.14, 145.64.
4-Cyano-4¢-methoxybiphenyl22
1H NMR: d = 3.86 (s, 3 H), 7.01 (d, J = 8.7 Hz, 2 H), 7.53 (d, J = 8.6
Hz, 2 H), 7.63 (d, J = 8.2 Hz, 2 H), 7.69 (d, J = 8.2 Hz, 2 H).
13C NMR: d = 55.34, 109.97, 114.51, 119.09, 127.00, 128.30,
131.33, 132.50, 145.08, 160.18.
4-Hydroxybiphenyl7
1H NMR: d = 4.81 (s, 1 H), 6.90 (d, J = 8.7 Hz, 2 H), 7.26 (m, 1 H),
7.44 (m, 2 H), 7.50 (d, J = 8.6 Hz, 2 H), 7.55 (d, J = 8.2 Hz, 2 H).
13C NMR: d = 115.79, 126.87, 128.54, 128.88, 134.17, 140.90,
155.20.
p-Terphenyl18
(8) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3,
1H NMR: d = 7.34 (m, 2 H), 7.46 (m, 4 H), 7.63–7.67 (m, 8 H).
1077.
Synthesis 2007, No. 3, 393–399 © Thieme Stuttgart · New York

PAPER
A High-Turnover-Number Phosphine-Free Catalyst for Suzuki Reaction
399
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Synthesis 2007, No. 3, 393–399 © Thieme Stuttgart · New York