Palladium-catalyzed γ-arylation of α,β-unsaturated esters from silyl ketene acetals

Article abstract of DOI:10.1002/anie.201002328

(Figure Presented) Smarty cat: A method for the palladiumcatalyzed γ-arylation of α,β-unsaturated esters via silyl ketene acetals in the absence of fluoride has been developed. The coupling proceeds with electron-rich and electron-poor aryl bromides and vinyl bromides in high yields with a high tolerance for other functional groups.

Full text of DOI:10.1002/anie.201002328

Angewandte
Chemie
DOI: 10.1002/anie.201002328
Arylation Reactions
Palladium-Catalyzed g-Arylation of a,b-Unsaturated Esters from Silyl
Ketene Acetals**
David S. Huang and John F Hartwig*
The palladium-catalyzed coupling of enolates with aryl and
vinyl electrophiles has become a useful method for the
construction of carbon–carbon bonds.[1] Although many
palladium complexes have been identified that catalyze the
coupling of enolates of ketones, esters, amides, and aldehydes
with haloarenes, the vinylogous coupling of dienolates of a,b-
unsaturated carbonyl compounds with haloarenes (g-aryla-
tion) has been less studied. g-Aryl a,b-unsaturated carbonyl
compounds are useful building blocks because they can be
further functionalized at the carbonyl or olefinic positions.[2]
To develop a catalyst for the g-arylation of a,b-unsatu-
rated carbonyl compounds, one must address several issues,
including the regioselectivity for forming products from a-, b-,
or g-arylation, the selectivity for monoarylation versus diary-
lation, and the potential condensation of the reactive
products. The selectivity for formation of products from a-
or g-arylation versus b-arylation depends on whether an
enolate complex forms by transmetalation with the arylpalla-
dium halide intermediate, as occurs during the a-arylation of
carbonyl compounds, or whether it forms by insertion of the
a,b-unsaturated carbonyl compound into the metal–aryl
bond, as occurs during a Heck reaction. The selectivity for
products from a- versus g-arylation likely depends on the
stability and reactivity of the series of isomeric arylpalladium
dienolate intermediates that result from transmetalation.
Despite these obstacles, a few g-arylation reactions of a,b-
unsaturated ketones and aldehydes have been reported.
Terao et al. reported the palladium-catalyzed g-arylation of
unfuntionalized a,b-unsaturated aldehydes and ketones.[3]
Martin and Buchwald reported the g-arylation of similar a-
substituted a,b-unsaturated aldehydes.[4] Varseev and Maier
later reported a sequential g-arylation and dehydrogenation
for the synthesis of substituted tetralones.[5] Finally, Hyde and
Buchwald reported the g-arylation of g-substituted a,b- or
b,g-unsaturated ketones[6] and lactones[7] with aryl chlorides
and bromides in moderate to good yield.
low to moderate yields.[8] Moreover, the previous couplings of
a,b-unsaturated ketones and aldehydes have required ele-
vated temperatures and substrates that contain substituents at
either the a or g position of the carbonyl compound for high
yield.
Herein, we report the g-arylation and g-vinylation of a,b-
unsaturated esters in high yield with broad scope by coupling
aryl, heteroaryl, and vinyl halides with the corresponding
silicon dienolates. These reactions occur in high yield with a
variety of esters, including those lacking substituents at the a
and g positions, and with aromatic and vinylic electrophiles
that contain potentially reactive functional groups. Moreover,
these reactions occur without typical fluoride additives to
activate silicon enolates.
Our studies of the coupling of haloarenes with the
dienolates of a,b-unsaturated esters began with alkali metal
dienolates of a,b-unsaturated esters. However, the reaction of
bromobenzene with the alkali metal dienolate of methyl
trans-2-hexenoate did not form the g-aryl a,b-unsaturated
esters in significant yield. Thus, we investigated reactions of
silicon enolates of a,b-unsaturated esters. These studies were
based on our prior work on the coupling of aryl halides with
silyl enol ethers and silyl ketene acetals.[9–11]
Our initial studies on the coupling of silyl ketene acetals
derived from a,b-unsaturated esters focused on the model
palladium-catalyzed coupling of bromobenzene with the
trimethylsilyl ketene acetal of methyl-2-hexenoate (1a).
Most previous a-arylations of silyl ketene acetals or silyl
enol ethers were conducted with fluoride additives, such as
ZnF2, CsF, MgF2, Bu3SnF, or CuF2. In contrast to these
previous studies, the coupling of silyl ketene acetal 1a
occurred without a fluoride additive. Instead, the reaction
formed the g-aryl E-product 2 in high yield in the presence of
inexpensive zinc chloride. Previously reported coupling
reactions of aryl halides with silicon enolates without a
fluoride additive have been limited to the a-arylation and
vinylation of silicon enolates in the presence of thallium
acetate[11] or to intramolecular a-vinylations.[9]
Despite these advances, the coupling of enolates of a,b-
unsaturated esters is underdeveloped. The g-arylation of
acyclic esters has been limited to reactions of tin enolates in
A summary of the effect of the silyl group and additive on
the coupling of bromobenzene with the silyl ketene acetals of
methyl hexenoate is shown in Table 1. The g-arylation
product was formed in the highest yields with the triethylsilyl
(TES) ketene acetal (1b; Table 1, entry 3). The coupling
between 1b and bromobenzene in the presence of ZnF2
occurred in low yield (Table 1, entry 4), perhaps owing to
the poor solubility of ZnF2 in tetrahydrofuran. Reactions of
1b with bromobenzene in the presence of ZnBr2 and ZnI2
(Table 1, entries 5 and 6) formed the g-arylation product 2 in
lower yield than the same reaction in the presence of ZnCl2.
The coupling between 1b and bromobenzene with catalytic
[*] D. S. Huang, Prof. Dr. J. F. Hartwig
Department of Chemistry, University of Illinois Urbana-Champaign
Urbana, Il 61801 (USA)
Fax: (+1)217-244-8024
E-mail: jhartwig@illinois.edu
Homepage: http://www.scs.uiuc.edu/hartwig/
[**] We thank the NIH (GM-58108). for support of this work and
Johnson-Matthey for gifts of palladium.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002328.
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Table 1: Effects of the silyl group and zinc halides on the g-arylation of
silyl ketene acetals with bromobenzene.[a]
Table 2: Aryl and vinyl bromide scope of the g-arylation of 1b.[a]
Entry
Acetal
Additive
t [h]
Yield [%][b]
Entry
[Pd] cat.[b]
Yield [%][c]
1
2
3
4
5
6
1b
1a
1b
1b
1b
1b
1b
1c
none
ZnCl2
ZnCl2
ZnF2
ZnBr2
ZnI2
24
2
4
24
4
25
12
12
<5
82
87
11
81
<5
83
64
1
PhBr
PhBr
PhBr
4-tBu-C6H4Br
3,5-Me2-C6H3Br
2-MeC6H4Br
2-PhC6H4Br
2,6-Me2C6H3Br
2,6-Me2C6H3Br
2,6-Me2C6H3Br
1-bromonaphthalene
2-bromonaphthalene
2-OMe-C6H4Br
3-OMe-C6H4Br
4-OMe-C6H4Br
4-NH2-C6H4Br
4-MeN(H)-C6H4Br
4-NMe2-C6H4Br
4-CF3-C6H4Br
A
B
C
A
A
A
A
A
B
C
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
82
77
77
82
78
83
78
96
83
95
82
90
83
81
83
74
80
83
77
91
91
81
82
58
84
89
76
58[i]
69[j]
64
51
34
2[d]
3[e]
4
5
6
7
7[c]
8
ZnCl2
ZnCl2
8[f]
9[g]
10
11
12
13
14
15
16[f,h]
17[h]
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
[a] Reaction conditions: acetal (0.26 mmol), PhBr (0.20 mmol), additive
(0.30 mmol), [Pd(dba)2] (0.010 mmol), PtBu3 (0.01 mmol), THF
(0.8 mL). [b] Yields were determined by GC analysis with dodecane as
an internal standard (average of two runs). [c] 0.2 equiv ZnCl2
amounts of ZnCl2 occurred in yields similar to those of
reactions with stoichiometric ZnCl2, although the reaction
was slower (Table 1, entry 7). Ester 2 was not formed from the
reaction of 1b with bromobenzene in the absence of ZnCl2
(Table 1, entry 1).
4-F-C6H4Br
4-Cl-C6H4Br
4-CN-C6H4Br
Several palladium catalyst systems for the coupling of silyl
ketone acetal 1b with bromobenzene were also explored;
ester 2 did not form in a detectable amounts from the
coupling of 1b with bromobenzene in the presence of catalyst
systems containing X-phos, dppe, dppf, Xantphos, rac-
BINAP, PCy3, or PPh3 ligands. Ester 2 formed in the presence
of [Pd(dba)2] and Qphos, but the yield was lower than that
from the reaction conducted with a catalyst containing PtBu3.
The coupling of silyl ketene acetal 1b with chlorobenzene
and iodobenzene was also attempted. The coupling product 2
was not detected from the reaction with chlorobenzene, and
the g-aryl a,b-unsaturated ester 2 was formed in only 29%
yield from the coupling of 1b with iodobenzene after
24 hours.
The scope of the g-arylation with a variety of aryl
bromides is shown in Table 2. g-Aryl a,b-unsaturated esters
were obtained in high yields from reactions with both
electron-rich and electron-poor aryl bromides. The reaction
proceeded in high yield with aryl bromides containing a single
ortho substituent (Table 2, entries 6 and 7); higher catalyst
loadings were required for more sterically hindered aryl
bromides (Table 2, entry 8), but the reaction of 2,6-dimethyl-
bromobenzene occurred in high yield. The reaction tolerated
a variety of functional groups that could be used for further
elaboration, such as a chloro, nitro, cyano, acyl, and a
phenacyl group (Table 2, entries 21–25). The coupling also
4-NO2-C6H4Br
4-C(O)Me-C6H4Br
4-C(O)Et-C6H4Br
4-CO2Me-C6H4Br
3-bromothiophene
3-bromopyridine·BEt3
6-bromoquinoline
5-bromoindole
2-bromoindene
1-Br-2-Me-1-propene
[a] Reaction conditions: 1b (1.3 mmol), aryl or vinyl halide (1.0 mmol),
ZnCl2 (1.5 mmol), [Pd] cat. (0.010 mmol), THF (2 mL). [b] [Pd] cat.: A:
[Pd(dba)2], PtBu3 (1:1); B: [Pd(PtBu3)2]; C: [{P(tBu3)Pd(m-Br)}2]. [c] Yield
of isolated product (average of two runs). [d] 0.4% [Pd] cat. [e] 0.1% [Pd]
cat. [f] 5% [Pd] cat. [g] 2% [Pd] cat. [h] 2.6 equiv 1b, 3.0 equiv ZnCl2.
[i] Isolated as uncoordinated pyridine. [j] Product was isolated in 85%
purity.
entry 30). The coupling of 1b with the BEt3 adduct of 3-
bromopyridine occurred faster and in comparable yields to
the reactions of the free pyridine. The use of a borane to
inhibit coordination of a pyridine to the catalyst and to
À
promote carbon halogen bond cleavage and subsequent
reductive elimination has been previously reported for the
amidation of aryl halides.[12]
À
proceeded with aryl bromides that contained acidic N H
groups (Table 2, entries 16, 17, and 30).
The coupling also occurred with vinyl bromides; g-vinyl-
a,b-unsaturated esters were isolated in moderate yields from
the reactions of 2-bromoindene and 1-bromo-2-methyl-1-
propene (Table 2, entries 31 and 32).
The isolated complexes [Pd(PtBu3)2] and [{(PtBu3)Pd(m-
Br)}2] were particularly active for the coupling of silyl ketene
acetal 1b with aryl bromides. For example, the coupling of 1b
Avariety of heteroaryl bromides also successfully coupled
with silyl ketene acetal 1b, including the coupling of 1b with
bromothiophene, bromopyridine, and bromoquinoline
(Table 2, entries 27–29), and the coupling of 1b with 5-
À
bromo indole containing an unprotected N H bond (Table 2,
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with bromobenzene proceeded in high yield in the presence of
just 0.4% [Pd(PtBu3)2] (Table 2, entry 2) or 0.2%
[{(PtBu3)Pd(m-Br)}2] (Table 2, entry 3). The coupling between
silyl ketene acetal 1b and a larger 10 mmol scale of
bromobenzene in the presence of 0.2% [{(PtBu3)Pd(m-Br)}2]
gave the coupled product ester 2 in 69% yield.
The coupling of the more sterically demanding substrates
also proceeded with lower catalyst loadings of [Pd(PtBu3)2] or
[{(PtBu3)Pd(m-Br)}2] than of the combination of [Pd(dba)2]
and PtBu3 (Table 2, entries 9 and 10). Thus, [Pd(PtBu3)2] or
[{(PtBu3)Pd(m-Br)}2] could used in place of [Pd(dba)2] and
PtBu3 in most cases when a single-component catalyst is
preferred.
The scope of the process with different silyl ketene acetals
is shown in Table 3. The reaction tolerates substitution at the
a, b, g, or d positions of the acetal. Products from reactions of
silyl ketene acetals containing substituents at the a or
Scheme 1. Potential mechanism for the coupling of 1b with PhBr.
TES=triethylsilyl.
acidity of the zinc halide facilitates transmetalation of the
enolate from silicon to palladium, we conducted the coupling
of enolate 1b with bromobenzene in the presence of
BF3·THF. The reaction with BF3·THF in place of ZnCl2
proceeded to full conversion, albeit over a longer reaction
time and in diminished yield (Table 4, entry 3). To assess
whether the zinc chloride served as a source of halide,[14] the
coupling of enolate 1b with bromobenzene was performed in
the presence of zinc triflate. This reaction occurred in a yield
Table 3: g-Arylation of substituted silyl ketene acetals.[a]
Entry
1[c,d]
R1
R2
H
R3
H
R4
Yield [%][b]
Me
Me
62 (E)
10 (Z)
57 (E)
5 (Z)
75
2
Me
H
Me
H
Table 4: Effect of different additives on the coupling of 1b with
bromobenzene.[a]
3[d]
4
Me
iPr
Me
H
H
H
H
H
68
[a] Reaction conditions: acetal (1.5 mmol), PhBr (1.0 mmol), ZnCl2
(1.5 mmol), [Pd(dba)2] (0.010 mmol), PtBu3 (0.01 mmol), THF (2 mL).
[b] Yield of isolated product (average of two runs). [c] 2.2 equiv silyl
ketene acetal, 3.0 equiv ZnCl2. [d] 2% [Pd(dba)2], 2% PtBu3.
Entry
Additive
t [h]
Yield [%][b]
1
2
3
ZnCl2
Zn(OTf)2
BF3·THF
N(octyl)3MeCl
N(octyl)3MeCl
4
4
48
8
87
95
39
83
45
4
b positions were isolated as a mixture of olefin geometries
(Table 3, entries 1 and 2). The coupling of a silyl ketene acetal
containing two substituents at the g position required
increased catalyst loading, but occurred in 75% yield
(Table 3, entry 3). Finally a substrate substituted in the
d position reacted in similar yields. Under our reaction
conditions, no product was observed from the g-arylation of
the silyl ketene acetals derived from a,b-unsaturated lactones.
A potential mechanism for the palladium-catalyzed
coupling of 1b with aryl bromides is shown in Scheme 1.
This catalytic cycle is analogous to that proposed for the
palladium-catalyzed a-arylation of carbonyl compounds. In
this cycle, the zinc chloride would facilitate the formation of
the palladium dienolate complex (4 and 5), and reductive
elimination from a palladium dienolate complex would
generate the desired product. Reductive elimination from
an h3-palladium dienolate complex (4) has been reported by
Kurosawa and co-workers,[13] but the zinc chloride induced
transmetalation of a silyl ketene acetal or silyl enol ether with
the palladium aryl halide complex has not been studied.
To understand the promoting effect of zinc chloride, we
studied reactions containing additives that possess Lewis
acidic properties or chloride ions. To determine if the Lewis
5[c]
24
[a] Reaction conditions: 11 (0.030 mmol), 1b (0.13 mmol), additive
(0.15 mmol), THF (0.8 mL). [b] Yields were determined by GC analysis
with dodecane as an internal standard (average of two runs). [c] Reaction
performed in the presence of 4 equiv 4-ClC6H4Br.
that was comparable to that from the reaction in the presence
of zinc chloride (Table 4. entry 2). Because the yield and
reaction times for the coupling in the presence of zinc triflate
were comparable to those of reactions in the presence of zinc
chloride, we conclude that the ZnCl2 is not simply a source of
halide; instead the Lewis acidic property likely helps labilize
the halide on palladium.[15]
At the same time, a genuine source of halide without
Lewis acid did promote the coupling process. The reaction of
1b with bromobenzene catalyzed by [Pd(dba)2] and PtBu3 in
the presence of methyltrioctylammonium chloride occurred
in high yield (Table 4, entry 4), although the rate was slower
than that for reactions conducted with zinc chloride. Because
ammonium halides are not Lewis acidic, it appears that the g-
arylation process with the silyl enolates can be accelerated by
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5759

Communications
either a Lewis acid or a Lewis base. The palladium-catalyzed
product is formed by reductive elimination from a palladium
dienolate complex. Further studies are required to determine
the precise mechanism for activation of silyl ketene acetal, but
the formation of this dienolate complex is promoted by the
mildly acidic zinc chloride. Further studies on reactions of
dienolates and efforts to develop stereoselective transforma-
tions of these species are in progress.
coupling of 1b with bromobenzene in the presence of
methyltrioctylammonium chloride and zinc chloride together
required long times and did not occur in high yield (Table 4,
entry 5); thus, we conclude that the ammonium chloride and
zinc chloride promote the coupling of silyl ketene acetal and
bromobenzene through different mechanisms.
To determine which portion of the catalytic cycle is
affected by the zinc and ammonium additives, reactions of
enolate 1b were performed with an arylpalladium bromide Experimental Section
General procedure for the g-arylation of silyl ketene acetals catalyzed
complex that is closely related to the complexes that would be
intermediates in the coupling process. Arylpalladium halide
complexes of P(1-Ad)tBu2 (1-Ad = 1-adamantyl) are more
easily isolated than those containing PtBu3, and the combi-
nation of [Pd(dba)2] and P(1-Ad)tBu2 catalyzed the formation
of 2 in 86% yield. Thus, we studied complex 6 containing P(1-
Ad)tBu2. The reaction of silyl ketene acetal 1b with complex 6
was faster in the presence of zinc chloride than in the absence
of any additive (Table 5 entries 1 and 2). When the reaction
between 6 and 1b was performed in the presence of an excess
by PtBu3 and [Pd(dba)2] (Table 2): PtBu3 (2.0 mg, 0.010 mmol; in a 1
dram vial) was placed inside a drybox under either an argon or
nitrogen atmosphere. Bromobenzene (157 mg, 1.00 mmol) was
added, followed by triethyl(1-methoxyhexa-1,3-dienyloxy)silane
(346 mg, 1.30 mmol). Zinc chloride (204 mg, 1.50 mmol) was then
added, followed by [Pd(dba)2] (5.8 mg, 0.010 mmol). A Teflon-coated
stirrer bar was then added, followed by 2 mL of THF. The vial was
closed with a cap that was lined with a PTFE/silicone septum and
removed from the drybox. The reaction mixture was stirred at room
temperature until GC analysis showed that full consumption of the
aryl bromide had occurred. Upon completion, the reaction mixture
was diluted with 30 mL of ethyl acetate (EtOAc) and washed three
times with saturated NaHCO3. An additional 10 mL of EtOAc was
added to the combined aqueous washes for back-extraction. The
combined organic solutions were washed once with brine and dried
with anhydrous MgSO4. The suspension was filtered through a plug of
Celite. Volatile materials were removed by rotary evaporation. The
crude mixture was then purified by column chromatography on silica
gel (acetone/pentane) to give the g-aryl a,b-unsaturated ester.
Table 5: Stoichiometric reactions between arylpalladium halide specides
with 1b.[a]
Received: April 20, 2010
Published online: July 6, 2010
Entry
Additive
t
Yield 2 [%][b]
1
2
none
ZnCl2
ZnCl2
30 h
27
65
82
69
Keywords: cross-coupling · enolates · homogeneous catalysis ·
10 min
10 min
30 min
.
3[c]
4
palladium · a,b-unsaturated compounds
N(octyl)4Cl
[a] Reaction conditions: 6 (0.030 mmol), 1b (0.13 mmol), additive
(0.15 mmol), THF (0.8 mL). [b] Yields were determined by GC analysis
with dodecane as an internal standard (average of two runs). [c] Reaction
performed in the presence of 4 equiv 4-ClC6H4Br.
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that bound to palladium) to trap the palladium(0) product,
the g-arylation product 2 was formed in yields that were
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Angewandte
Chemie
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