Suzuki-Miyaura coupling with high turnover number using an N-acyl-N-heterocyclic carbene palladacycle precursor

Article abstract of DOI:10.1016/j.tetlet.2004.03.138

A simple N-acylimidazolium salt precursor to a NHC-complexed palladacyclic ligand gives high turnover numbers (10⁷) for Suzuki-Miyaura coupling and is applied to the preparation of biaryls used in the synthesis of coumarins. The results suggest that N-acyl-NHC derivatives can contribute to further expanding the rich chemistry of NHCs.

Full text of DOI:10.1016/j.tetlet.2004.03.138

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3849–3853
Suzuki–Miyaura coupling with high turnover number using
an N-acyl-N-heterocyclic carbene palladacycle precursorq
a
b,
*
Hector Palencia,a,b, Federico Garcia-Jimenez and James M. Takacs
*
aInstituto de Quimica/UNAM, Circ., Ext. S/N, Coyoacan, DF, 04510 Mexico
bDepartment of Chemistry, University of Nebraska––Lincoln, Lincoln, NE 68588-0304, USA
Received 2 March 2004; revised 19 March 2004; accepted 24 March 2004
Abstract—A simple N-acylimidazolium salt precursor to a NHC-complexed palladacyclic ligand gives high turnover numbers
(>107) for Suzuki–Miyaura coupling and is applied to the preparation of biaryls used in the synthesis of coumarins. The results
suggest that N-acyl-NHC derivatives can contribute to further expanding the rich chemistry of NHCs.
Ó 2004Elsevier Ltd. All rights reserved.
We are interested in palladium-catalyzed carbon–carbon
bond forming reactions,1 and in that regard, seek to
understand and exploit the unique features of novel
ligand systems to generate catalyst systems exhibiting
broad scope and high efficiency. Since their first report
by Arduengo,2 N-heterocyclic carbenes (NHCs) have
emerged as attractive alternatives to phosphine ligands
in a variety of catalytic reactions, including numerous
palladium-catalyzed reactions.3 Similarly, palladacyclic
complexes have recently generated significant interest
and such catalyst systems often exhibit excellent thermal
stability and high turnover numbers (TONs).4 Hybrid
systems can combine the advantages of different ligand
types to give new ligand systems with attractive fea-
tures,5 and in view of the successes of NHCs and pal-
ladacycles, we decided to synthesize ligands
incorporating both features.
been described, wherein the desired imidazoles were said
to be obtained via the reaction of an alcohol or phenol
with N,N-carbonyldiimidazole (CDI). For example, it
was reported that 1 was converted to N-aryl imidazole
2a upon treatment with CDI.6 While this procedure was
subsequently used with success in at least one other
published case,7 we found that treating 1,5-diisopropyl-
phenol (3) with CDI under the prescribed conditions, or
at substantially elevated temperatures, gave only the
carbamate 4. A similar observation was reported by
Fisher.8
While it wasn’t the derivative we originally intended,
having 4 in hand we decided to prepare the unsymmet-
rical N-acylimidazolium salt 6 via N-alkylation with
2-bromobenzyl bromide (5) (MeCN, 75 °C, 48 h); 6 is
obtained in 65% overall yield from phenol 3 (Scheme 1).
While many NHC structural motifs have been prepared,
N-acyl-NHC ligands have not been extensively exam-
ined.9 We reasoned that the 2-bromobenzyl group
should facilitate formation of the carbene-complexed
palladacycle 9, and furthermore, the carbonyl group
might be suitably disposed to act as a hemilabile
ligand,10 essentially forming a pincer-type ligand. Pincer
ligands themselves have attracted significant attention
in recent years due to their enhanced thermal stability
in Heck and related reactions.11
NHCs are typically prepared via deprotonation of an
imidazolium salt, their generation often carried out in
situ. Unsymmetrical NHC precursors are prepared via
N-alkylation of N-substituted imidazoles. At the time we
initiated our studies, an interesting approach to the
preparation of N-substituted imidazoles had recently
Keywords: N-Acyl-N-heterocyclic carbene; N-Heterocyclic carbene
ligand; NHC; Suzuki–Miyaura coupling; Coumarin synthesis; Biaryls;
Palladium catalyst; Catalysis.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.03.138
Two additional NHC precursors were prepared for
comparison. Aryl C–H insertion provides an alternative
route to chelated palladacycles, and therefore, the
N-benzyl derivative 7 was prepared in anticipation that
it too would serve as a precursor to 9. We included a
* Corresponding authors. Tel.: +1-402-472-6232; fax: +1-402-472-94-
02; e-mail: jtakacs1@unl.edu
0040-4039/$ - see front matter Ó 2004Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.03.138

3850
H. Palencia et al. / Tetrahedron Letters 45 (2004) 3849–3853
Me
precursor to a carbene-complexed palladacycle lacking
the carbonyl group for comparison as well. The known
imidazolium salt 8,12 prepared via N-alkylation of 2a,13
had previously been used in the palladium-catalyzed
preparation of oxyindoles from o-haloanilides and is
expected to form complex 10. (Scheme 2).
OH
CDI, CH2Cl2
reflux (5 h)
Me
Me
iPr
N
N
R
Me
2a (R = H)
b (R = Me)
1
OH
iPr
Pri
CDI, CH2Cl2
reflux (5 h)
Biaryls are building blocks and subunits present in many
natural products and have important applications,
including use as pharmaceuticals, agrochemicals, and
polymer constituents.14 A number of Csp2–Csp2 cross-
coupling methods have proven useful for the synthesis
of unsymmetrical biaryls;15 the Suzuki–Miyaura palla-
dium-catalyzed coupling of aryl halides or triflates with
boronic acids or esters is among the most powerful.16
Tertiary phosphines are the most commonly employed
ligands for such reactions, however, phosphines are
sensitive to air and moisture and suffer from competing
P–C bond cleavage at elevated temperatures.17 Thus,
NHC catalyst systems can exhibit significant advantages
in the reaction.18
4 (97%)
O
N
N
O
iPr
3
5, MeCN, reflux
48 h (65%)
iPr
O
Me
Br
Br
N
N
N
N
Br
X
Me
O
iPr
Me
6 (X = Br)
7 (X = H)
8
Scheme 1. Preparation of N-acyl-NHC precursor
derivatives.
6
and related
A brief study directed toward optimizing Suzuki–
Miyaura couplings using 6 is summarized in Table 1.
Two simple aryl halides substrates, methyl 4-bromo-
benzoate and 4-iodotoluene, were coupled with
phenylboronic acid using 1 mol % palladium (1:2
Pd2dba3/6) in dioxane. Base is needed for the in situ
formation of N-acyl carbene-complexed palladacycle 9
from its imidazolium salt precursor 6 and the nature of
that base is an important variable in such reaction.19
Several frequently used bases were screened, and in the
case of 4-iodotoluene, both NaOH (Table 1, entry 7)
and Cs2CO3 (entry 8) give high yields of the biaryl
product. With methyl 4-bromobenzoate, Cs2CO3 (entry
iPr
Me
O
N
N
N
Me
10
N
O
Me
iPr
Pd
Ln
Pd
Ln
9
Scheme 2. Presumed carbene-complexed palladacycles from 6, 7,
and 8.
Table 1. Optimizing Suzuki–Miyaura couplings using 6: The effect of the base and co-liganda
B(OH)2
R
R
0.005 [Pd2dba3], 0.01 6
1.1
+
base, dioxane,
80 ˚C, 20-24 h
11
X
Aryl halide
Co-ligand
Base
(%) Conversion
(%) Yieldb
c
1
4-MeO2CC6H4Br
4-MeO2CC6H4Br
4-MeO2CC6H4Br
2CC6H4Br
––
––
Ca(OH)2
15
81
5
31
2
K3PO4
3
––
––
NaOHc
Cs2CO3
Ca(OH)2
75
92
55
89
-MeO
44
5
4-MeC6H4I
4-MeC6H4I
––
––
26
57
15
21
d
6
K2CO3
NaOH
7
4-MeC6H4I
4-MeC6H4I
––
––
100
100
100
100
100
95
92
8
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
4-MeC6H4I
4-MeC6H4I
4-MeC6H4I
Ph2PH
(t-Bu)2PH
Cy3P
>99
>99
89e
10
11
a Unless indicated otherwise, all reactions are run in dioxane using 1 mol % palladium catalyst and co-ligand (if present) and 2 mol equiv of the
indicated base relative to aryl halide.
b Isolated yield based upon starting aryl halide; not corrected for recovered starting material.
c 1.5 equiv of base used.
d 10 mol % t-BuOK added initially to generate the NHC.
e Yield improved to 94% using K3PO4 in place of Cs2CO3.

H. Palencia et al. / Tetrahedron Letters 45 (2004) 3849–3853
3851
4) gives the best results, and it was used in all subsequent
studies.
catalyst, the conversion is complete and the isolated
yield, while no longer quantitative, remains high (97%)
(Table 2, entry 5). At 1 ppm catalyst (0.0001%) both the
degree of conversion (98%) and the isolated yield of
biphenyl (88%) drop somewhat (entry 6). This down-
ward trend continues at the 100 and 10 ppb levels; the
isolated yields drop off markedly (47% and 33%, entries
7 and 8). Benzene is detected as a side-product in these
latter reaction mixtures. Nonetheless, even with the drop
off in yield, the latter reactions proceed with turnover
numbers in the millions to tens of millions. We did not
attempt to further optimize the reaction conditions at
these low catalyst loadings, and thus it may be possible
to further improve their efficiency.
The addition of co-ligands can further stabilize and in-
crease the reactivity of palladium catalysts. For exam-
ple, Studer and co-workers found that secondary
phosphines are excellent co-ligands in the coupling
reactions of aryl chlorides,20 and adding Ph2PH (entry
10) or (tert-Bu)2PH (entry 11) improved the yield of
biphenyl 11 (R ¼ Me), rendering the reaction essentially
quantitative. The nature of the active species formed
from the combination of palladacycles with secondary
phosphines is not known, but it has been suggested that
the reaction of the secondary phosphine with an aryl
halide may form a tertiary phosphine in situ that is
important in the active catalyst.21 Tricyclohexylphos-
phine (PCy3) has been used in conjunction with a
palladacycle to give a very active catalyst for the Suzuki
coupling with aryl chlorides.22 However, adding PCy3 in
conjunction with 6 did not improve the yield (Table 1,
entry 11).
Imidazolium salt 7 could, via C–H insertion, also form
complex 9. At the 0.1% catalyst loading level, 7 per-
forms well and affords biphenyl in near quantitative
yield (entry 9). However, at 0.01% catalyst loading, both
the conversion (71%) and the isolated yield (49%) are
significantly lower (entry 10). NHC precursor 8 behaves
similarly (entries 11 and 12). It is expected to generate a
carbene-complexed palladacycle lacking the N-acyl
group (i.e., 10). These results suggest that the N-acyl
group improves the activity of the catalyst system, per-
haps due to the carbonyl functionality acting as a
hemilabile donor ligand.24
The data in Table 1 were obtained using 1% catalyst
loading. The search for coupling catalyst systems
exhibiting high efficiency (i.e., high turnover frequency)
and greater stability (i.e., high turnover number (TON))
is a very active area of research of late and good pro-
gress has been made.23 Table 2 summarizes the effect of
lower catalyst loading on the formation of biphenyl (11
R ¼ H) via the coupling of phenylboronic acid and
iodobenzene. Using 0.1% catalyst (entry 3) or 0.01%
catalyst (entry 4), biphenyl is formed quantitatively. The
NHC precursor 6 is an essential component to the suc-
cess of the coupling. Neither Pd2dba3 alone (i.e., omit-
ting both 6 and (tert-Bu)2PH, entry 1) nor Pd2dba3 plus
(tert-Bu)2PH (i.e., omitting 6, entry 2) were as efficient.
We briefly explored the catalyst scope at the 0.1%
loading level (Table 3). Aryl iodides generally give good
to excellent yields and showing good tolerance toward
steric hindrance on both the nucleophile as well as the
electrophile. For example, the 2-alkoxy-substituted aryl
iodide, 2-MOMOC6H4I, gives high yield of the coupled
product based upon its conversion (entry 9). Methyl
2-iodobenzoate couples efficiently with phenyl boronic
acid (93%, entry 1) as well as with a range of 2-methoxy-
substituted aryl boronic acids (81–97%, entries 2–7).
Some of these latter couplings have been reported using
Encouraged by the high yield obtained with 6 at 0.01%,
we continued to reduce the catalyst loading. At 0.001%
Table 2. Comparing NHC precursors and catalyst loading for the coupling of phenylboronic acid with iodobenzenea
NHC precursor
Catalyst (mol %)
Temperature (°C)
Conversion (%)
Yieldb (%)
TONc
1d
2e
––
––
6
0.1
0.1
95
95
80
86
77
81
7.7 ꢀ 102
8.1 ꢀ 102
103
3
0.1
0.01
95
100
100
100
98
>99
>99
97
4
6
160
160
160
160
160
95
104
5
6
0.001
0.0001
0.00001
0.000001
0.1
9.7 ꢀ 104
8.8 ꢀ 105
4.7 ꢀ 106
3.3 ꢀ 107
9.9 ꢀ 102
4.9 ꢀ 102
9.0 ꢀ 102
4.1 ꢀ 102
6f ;g
7f ;h
8f ;h
9
6
88
47
6
90
82
6
33
99
7
100
71
10
11
12
7
0.01
0.1
160
95
49
90
8
93
63
8
0.01
160
41
a Reaction conditions: iodobenzene (5 mmol, except as noted); phenylboronic acid (5.5–6.0 mmol); C2CO3 (10 mmol); the indicated percentage of
catalyst mixture [composition: 1.0 6; 0.5 Pd2dba3; 1.1 t-Bu2PH]; dioxane (6 mL); 48 h.
b Isolated yield based upon starting aryl halide; not corrected for recovered starting material.
c TON ¼ mmol biphenyl/mmol Pd.
d Catalyst ¼ Pd2dba3.
e Catalyst ¼ Pd2dba3 plus t-Bu2PH.
f 10 mmol of iodobenzene and 12 mmol of phenylboronic acid used.
g Average of two runs.
h Average of three runs.

3852
H. Palencia et al. / Tetrahedron Letters 45 (2004) 3849–3853
Table 3. Suzuki–Miyaura couplings using 6: Substrate scope when using 0.1% catalyst loading
R1
R3
0.05 mol% [P
d2dba3],
R3
R1
R4
R5
B(OH)2
0.1 mol% 6
R4
R5
+
R2
0.11 mol% (tBu)2PH,
Cs2CO3, dioxane
X
R2
R6
R6
12
Entry Aryl halide
Boronic acid
Tem-
pera-
ture
Time Con- Yield 12 R1
R2
R3
H
R4
R5
R6
(h)
ver- (%)
sion
(%)
(°C)
1
2-MeO2CC6H4I
PhB(OH)2
2-MeOC6H4B(OH)2
95
150
150
150
150
72
48
48
48
48
48
48
48
72
72
48
48
100
100
100
100
100
100
100
100
82
93
86
76
95
92
97
81
51
77
90
78
–
a
b
c
d
e
f
H
H
H
H
H
H
H
H
H
CO2Me
H
H
H
H
H
H
2
2-MeO2CC6H4I
2-MeO2CC6H4I
2CC6H4I
CO2Me OMe
CO2Me OMe
CO2Me OMe
H
H
3
2,4-(MeO)2C6H3B(OH)2
2,5-(MeO)2C6H3B(OH)2
2,3,4-(MeO)3C6H2B(OH)2
OMe
H
H
42-MeO
OMe
H
tBu
5
2-MeO2CC6H4I
2-MeO2CC6H4I
2-MeO2CC6H4I
2-MeO2CC6H4I
2-MOMOC6H4I
4-MeO2CC6H4I
CO2Me OMe OMe OMe
6
(5-tBu-2-MeO)C6H3B(OH)2 150
CO2Me OMe
CO2 Me OMe
CO2 Me OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
H
7
(5-F-2-MeO)C6H3B(OH)2
(5-Cl-2-MeO)C6H3B(OH)2
PhB(OH)2
150
150
95
g
h
i
F
8
Cl
H
9
OMOM
H
H
H
H
10
11
12
PhB(OH)2
95
100
79
j
CO2Me
CO2 Me
C(O)Me
H
H
H
H
4-MeO2CC6H4Br PhB(OH)2
4-MeC(O)C6H4Cl PhB(OH)2
150
150
j
H
30
k
H
traditional catalysts, for example Pd(PPh3)4.25 Using 6
at 50-fold lower catalyst loading, we obtain yields
comparable to those previously reported. The reaction
of methyl 4-bromobenzoate with phenylboronic acid is
sluggish but gives high yield of coupled product based
upon its conversion (entry 11). 4-Chloroacetophenone
fails to couple under the conditions employed (entry 12).
significant potential utility in catalysis. Further studies
are in progress.
Acknowledgements
Support from the University of Nebraska Research
Initiative and the NSF (CHE-0316825) is gratefully
acknowledged. The NMR spectrometers used were
purchased with shared instrument funds (NIH SIG-1-
510-RR-06307) and (NSF CHE-0091975, MRI-
0079750). We thank Prof. Kevin Shaughnessy for a
helpful discussion.
Coumarins are an important class of natural products,26
and the biaryls formed via the coupling of methyl 2-
iodobenzoate with 2-methoxy-substituted aryl boronic
acids are suitable precursors. To illustrate, several rep-
resentative biaryl products were treated with BBr3 to
give coumarin derivatives 13a–c in high yield (89–91%)
(Scheme 3).
O
References and notes
OMe
O
BBr3, CH2Cl2
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N-acyl-NHC-complexed palladacycle catalyst. In spite
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