Oxime palladacycle-catalyzed Suzuki-Miyaura alkenylation of aryl, heteroaryl, benzyl, and allyl chlorides under microwave irradiation conditions

Article abstract of DOI:10.1002/adsc.201100019

A simple new protocol for the palladium-catalyzed Suzuki-Miyaura cross-coupling of organic chlorides under microwave irradiation is presented. Deactivated aryl and heteroaryl chlorides are efficiently cross-coupled with alkenylboronic acids and potassium alkenyltrifluoroborates using the 4,4′-dichlorobenzophenone oxime-derived palladacycle 1b as precatalyst in 0.1 to 0.5 mol% palladium loading, tris(tert-butyl)phosphonium tetrafluoroborate {[HP(t-Bu)₃]BF₄} as ligand, tetra-n-butylammonium hydroxide as cocatalyst, and potassium carbonate as base in N,N- dimethylformamide at 130°C under microwave irradiation conditions. Under these conditions, styrenes, stilbenes, and alkenylarenes are obtained in good to high yields, and with high regio- and diastereoselectivities in only 20 min. The reported protocol is also very efficient for the regioselective alkenylation of benzyl and allyl chlorides to afford allylarenes and 1,4-dienes. Copyright

Full text of DOI:10.1002/adsc.201100019

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DOI: 10.1002/adsc.201100019
Oxime Palladacycle-Catalyzed Suzuki–Miyaura Alkenylation of
Aryl, Heteroaryl, Benzyl, and Allyl Chlorides under Microwave
Irradiation Conditions
Josꢀ F. Cꢁvicos,a Diego A. Alonso,a,* and Carmen Nꢂjeraa,*
a
Departamento de Quꢁmica Orgꢂnica and Instituto de Sꢁntesis Orgꢂnica (ISO), Facultad de Ciencias, Universidad de
Alicante, Apartado 99, 03080, Alicante, Spain
Fax : (+34)-9-6590-3549; e-mail: diego.alonso@ua.es or cnajera@ua.es
Received: January 10, 2011; Revised: April 6, 2011; Published online: June 30, 2011
Dedicated to the memory of Professor Rafael Suau.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100019.
Abstract: A simple new protocol for the palladium-
catalyzed Suzuki–Miyaura cross-coupling of organic
chlorides under microwave irradiation is presented.
Deactivated aryl and heteroaryl chlorides are effi-
ciently cross-coupled with alkenylboronic acids and
potassium alkenyltrifluoroborates using the 4,4’-di-
chlorobenzophenone oxime-derived palladacycle 1b
as precatalyst in 0.1 to 0.5 mol% palladium loading,
tris(tert-butyl)phosphonium tetrafluoroborate {[HP-
under low catalyst loading conditions to deactivated
electrophiles such as aryl chlorides. Despite the over-
whelming impact of the Pd-catalyzed Suzuki synthesis
of biaryls in organic synthesis, the seemingly straight-
forward alkenylation of an aryl moiety has been, by
comparison, overlooked.[3] Furthermore, from the two
possible pair-wise combinations of aryl and alkenyl
units, the cross-coupling between an aryl halide (or
pseudohalide) and an alkenylboron nucleophile has
been much less studied,[4] particularly in the case of
the coupling of deactivated aryl chlorides.[5] Indeed,
the Suzuki alkenylation of these interesting substrates
has been scarcely considered, typically using high cat-
alyst loadings (1–3 mol% Pd) and trans-styrylboronic
acid as nucleophile.
ACHTUNGTRENNUNG(t-Bu)3]BF4} as ligand, tetra-n-butylammonium hy-
droxide as cocatalyst, and potassium carbonate as
base in N,N-dimethylformamide at 1308C under mi-
crowave irradiation conditions. Under these condi-
tions, styrenes, stilbenes, and alkenylarenes are ob-
tained in good to high yields, and with high regio-
and diastereoselectivities in only 20 min. The re-
ported protocol is also very efficient for the regiose-
lective alkenylation of benzyl and allyl chlorides to
afford allylarenes and 1,4-dienes.
Palladacycles[6] have been shown to be very effi-
cient precatalysts for the Suzuki cross-coupling reac-
tion.[7] In particular, we have recently demonstrated
the high catalytic activity of oxime palladacycles 1[8]
(Figure 1) in the Suzuki–Miyaura synthesis of biaryls
in organic[9] and aqueous solvents[10] using convention-
al heating or under microwave irradiation conditions.
With respect to the alkenylation of aryl halides, a
regio- and stereoselective synthesis of styrenes, stil-
benes, and alkenylbenzenes[11] has been carried out in
Keywords: alkenes; cross-coupling; homogeneous
catalysis; microwave chemistry; palladacycles;
Suzuki–Miyaura reaction
The Suzuki–Miyaura cross-coupling reaction consti-
tutes one of the most powerful carbon-carbon bond-
forming transformations.[1] In recent years various
modifications to this reaction have been made involv-
ing the nature of catalysts, solvents, bases, reaction
conditions, and synthetic techniques. Particularly in-
teresting aspected have resulted from advances in
new catalyst development[2] which have enabled an
expansion of the substrate scope of this reaction Figure 1. Oxime palladacycles.
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Josꢀ F. Cꢁvicos et al.
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our group by coupling potassium vinyl- and alkenyl- of the cine derivative 2a were obtained with other dif-
trifluoroborates with aryl bromides[12] and aryl chlor- ferent bulky and electron-rich phosphonium tetra-
ides[13] using oxime catalyst 1a in refluxing water and fluoroborates (Table 1, entries 2–5), although the iso-
DMF, respectively. Herein, we present a general pro- lated yields were in all cases significantly lower than
tocol for the Suzuki–Miyaura alkenylation of deacti- those obtained with [HP
vated aryl and heteroaryl chlorides with different Next, the catalytic activity of palladacycle 1b was
boron compounds using palladacycle 1b as catalyst compared with that of the 4-hydroxyacetophenone
under microwave irradiation conditions. oxime-derived palladacycle 1a (Figure 1) and other
The activity of palladacycle 1b was initially studied traditional Pd sources such as Pd(OAc)2, PdCl2, and
in the Suzuki–Miyaura alkenylation of deactivated Pd2A(dba)3 under the optimized loading conditions. As
ACHTUNGRTEN(NUNG t-Bu)3]BF4 as ligand.
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
aryl chlorides using 4-chloroanisole and trans-styryl- depicted in Table 1, entries 6–9, all these catalysts pre-
boronic acid as model substrates (Table 1). With this sented lower activity in the cross-coupling reaction
aim, the cross-coupling was performed using the reac- giving inferior yields than catalyst 1b.
tion conditions previously optimized for the arylation
Finally, we examined the effect on the yield and the
of these demanding electrophiles in our group,[9b] that selectivity of the boron source. Thus, we carried out
is, palladacycle 1b as catalyst in the presence of the coupling between 4-chloranisole and trans-2-phe-
tris[tert-butyl]phosphonium tetrafluoroborate – [HPACTHNUTRGNE(UNG t- nylvinylboronic acid pinacol ester, potassium trans-2-
– as ligand, tetra-n-butylammonium hy- phenylvinyltrifluoroborate,[16] and trans-2-phenylvinyl-
[14]
Bu)3]BF4
droxide (TBAOH) as additive, K2CO3 as base, in boronic acid MIDA ester[17] under similar reaction
DMF as solvent under microwave irradiation (40 W, conditions. As depicted in Table 1, entries 10–12, the
1308C) for 20 min. Under these conditions, the reaction only worked with the pinacol ester and the
Suzuki product 2a was obtained in a 71% isolated potassium trifluoroborate still affording lower yields
yield. However, a moderate regioselectivity was ob- than the corresponding boronic acid although with
served in the cross-coupling process and, in a 14% better selectivities in favour of the cine compound 2a.
yield, ipso-compound 3a was also obtained (Table 1,
To test the effectiveness of the catalytic system in
entry 1).[15] Compound 3a is generated through a the Pd-catalyzed alkenylation reaction, a range of de-
Heck-type addition-elimination mechanism where a activated aryl chlorides was examined using different
b-hydrogen transfers to the terminal carbon. On the alkenylboronic acids under the optimized reaction
other hand, almost perfect regioselectivities in favour conditions (Table 2). A very similar yield and regiose-
Table 1. Pd-catalyzed Suzuki alkenylation of 4-chloroanisole. Reaction conditions study.
Entry
X
Pd cat.
R1R2
Yield [%][a]
b/a[b]
2
1
2
3
4
5
6
7
8
(OH)2
(OH)2
(OH)2
(OH)2
(OH)2
(OH)2
(OH)2
(OH)2
(OH)2
1b
1b
1b
1b
1b
1a
U
85
22
45
46
25
9
34
48
54
57
67
<5
83/17
99/1
99/1
99/1
99/1
98/2
88/12
86/14
85/15
99/1
92/8
–
ACHTUNGTRENNUNG
[c]
–
–
–
[d]
[e]
9
10
11
12
[OC
F3K
N
1b
1b
1b
A
[a]
[b]
[c]
[d]
[e]
Determined by GC analysis over the crude reaction mixture.
Pd(OAc)2 (0.1 mol% Pd) was used as catalyst.
PdCl2 (0.1 mol% Pd) was used as catalyst.
Pd2A(dba)3 (0.1 mol% Pd) was used as catalyst.
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
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Oxime Palladacycle-Catalyzed Suzuki–Miyaura Alkenylation
Table 2. Suzuki alkenylation of aryl chlorides.
lectivity was observed when coupling 4-chloroanisole
and 4-chlorotoluene with trans-2-phenylvinylboronic
acid (Table 2, entries 1 and 2). 3,4-Dimethylchloro-
benzene was coupled with trans-2-phenylvinylboronic
acid in an excellent 94% isolated yield in the presence
of 0.25 mol% of palladacycle 1b, giving the corre-
sponding product with a good b/a regioselectivity (78/
22) (Table 2, entry 3). The regioselectivity of the pro-
cess was higher when using sterically hindered aryl
chlorides as demonstrated with the coupling of 2-
chlorotoluene
and
2,6-dimethylchlorotoluene
(Table 2, entries 4 and 5), as well as when 4-chloro-
phenol and 4-chloroaniline were used as electrophiles
(entries 6 and 7). As depicted in entry 8 for the cou-
pling of 3,5-dimethoxychlorobenzene, a successful vi-
nylation reaction required the employment of potassi-
um vinyltrifluoroborate as nucleophile,[18] affording
compound 2h in a 58% isolated yield. Good yields
(71–73%) were also observed when trans-1-hepten-1-
ylboronic acid and potassium trans-1-decen-1-yltri-
fluoroborate were employed as nucleophiles although,
regardless of the boron source employed, in moderate
regioselectivities, as depicted in Table 2, entries 9–11
for the coupling of 4-chloroanisole, 2,6-dimethylchlor-
obenzene, and 1-chloronaphthalene.
In order to test the effectiveness of the catalytic
system with heterocyclic chlorides, 2-chloropyridine,
2-chlorothiophene, and 2-chloroquinoline were sub-
mitted to the coupling with trans-2-phenylvinylboron-
ic acid under the optimized reaction conditions. As
shown in Table 2, entries 12–14, all three chlorides
stereospecifically afforded the corresponding adducts
2l–2n in excellent yields (84–95%) and very high b/a
regioselectivities (up to >99/1). Lower yields were
observed when the heterocyclic chlorides were cross-
coupled with b-alkyl-substituted alkenylboron nucleo-
philes as depicted in Table 2 entries 15 and 16, for the
coupling of 2-chlorothiophene and 2-chloroquinoline
with potassium trans-1-decen-1-yltrifluoroborate and
trans-1-hexen-1-ylboronic acid pinacol ester, which af-
forded regioselectively compounds 2o and 2p in 31
and 33% yield, respectively.
We next extended the substrate scope to allyl and
benzyl chlorides (Table 3). Oxime palladacycle 1b
(0.1 mol% Pd) in the presence of [HPACTHNURGTENUNG(t-Bu)3]BF4
(0.2 mol%) showed high catalytic activity towards the
alkenylation of allyl chlorides affording the corre-
sponding cross-coupled products 4 in high yields (63
to 95%) (Table 3, entries 1–6). As shown in entries 1–
4, (E)-(3-chloroprop-1-en-1-yl)benzene reacted with
alkenylboronic acids and potassium alkenyltrifluoro-
borates to give the corresponding 1,4-dienes in excel-
lent yields (91–95%), regio- and diastereoselectivities
with the exception of the vinylation reaction with po-
tassium vinyltrifluoroborate, that yielded compound
4b in a 63% isolated yield (Table 3, entry 2). In the
case of using crotyl chloride[19] as electrophile, good
[a]
Isolated yield for 2 and 3 after flash chromatography.
Determined by GC on the crude reaction.
0.5 mol% Pd and 1 mol% of [HPACHTUNTRGNEUNG(tBu)3]BF4 were used.
Reaction time: 40 min. Isolated as acetamide.
The potassium trifluoroborate derivative was used as
boron source.
The pinacol boronic ester derivative was used as boron
source.
[b]
[c]
[d]
[e]
[f]
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Josꢀ F. Cꢁvicos et al.
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Table 3. Suzuki cross-coupling reactions of allyl- and benzyl
chlorides.
lectivity, small amounts of the regioisomers 5e and 5f
were also identified.[19]
With respect to benzylic chlorides, 1-(chloro-
methyl)-4-vinylbenzene reacted with boronic acids
and potassium trifluoroborates yielding the corre-
sponding cross-coupled products 6 in good yields
(Table 3, entries 7–9). Thus, reaction with trans-2-phe-
nylvinylboronic acid afforded compound 6a in 86%
isolated yield (entry 7). On the other hand, a good
63% isolated yield was obtained when coupling with
potassium vinyltrifluoroborate (Table 3, entry 8), and
an 86% yield of compound 6c was achieved when
using potassium trans-1-decen-1-yltrifluoroborate as
nucleophile (Table 3, entry 9).
In summary, the catalyst formed in situ from readily
available and easily handled oxime palladacycle 1b
and tris(tert-butyl)phosphonium tetrafluoroborate,
shows very good activity in the Suzuki coupling of a
wide range of organic chlorides with alkenylboronic
acids or potassium alkenyltrifluoroborates under MW
irradiation conditions employing TBAOH as cocata-
lyst and DMF as solvent. Further studies addressing
the extension of this catalytic system to other palladi-
um-catalyzed transformations are currently underway.
Experimental Section
Typical Procedure for the Suzuki Coupling under
MW Irradiation Conditions
A fresh stock solution of catalyst 1b (30 mg, 0.0075 mmol) in
DMF (2.5 mL) and [HPACHTNUTRGNEUNG(t-Bu)3]BF4 (40 mg, 0.0375 mmol) in
DMF (2.5 mL) were prepared. A 10-mL microwave vessel
was charged with K2CO3 (1.5 mmol, 207 mg), TBAOH
(0.15 mmol,
(1.12 mmol, 166 mg), catalyst 1 (250 mL of the stock solu-
tion, 0.00075 mmol, 3 mg, 0.1 mol% Pd), [HP(t-Bu)3]BF4
120 mg),
trans-2-phenyvinylboronic
acid
AHCTUNGTRENNUNG
(250 mL of the stock solution, 0.00375 mmol, 4 mg), 4-chlor-
oanisole (0.75 mmol, 92 mL), and DMF (1.5 mL). The vessel
was sealed with a pressure lock and the mixture was heated
in air at 1308C by MW irradiation of 40 W for 20 min in a
CEM Discover MW reactor. After allowing the reaction
mixture to cool down to room temperature, it was filtered
through a pad of celite and poured into an excess of water
(5 mL) and extracted with Et2O (3ꢄ5 mL). The combined
organic phases were washed with water (3ꢄ5 mL), dried
(MgSO4), and evaporated. The obtained crude material was
purified by flash chromatography (hexane/AcOEt: 95/5).
The pure compound (E)-4-methoxystilbene (2a) was ob-
tained as a white solid; yield. 85%; mp 1358C (hexane); Rf
0.12 (hexane). IR (KBr): n=3387, 2958, 2924, 2878, 2859,
[a]
Isolated yield after flash chromatography.
Z/E: 40/60.
A 3% (GC conversion) of (E)-(3-methylpenta-1,4-dien-
1-yl)benzene (5e) was also observed.
[a]
[c]
[d]
Z/E: 16/84.
[e]
An 8% (GC conversion) of 1-(but-3-en-2-yl)naphthalene
(5f) was also obtained.
1459, 1378, 1067 cmÀ1 1H NMR (CDCl3, 300 MHz): d=
;
7.51–7.44 (m, 4H), 7.37–7.32 (t, 2H, J=7.34 Hz), 7.26–7.23
(t, 1H, J=6.1 Hz), 7.08, 6.96 (2d, 2H, J=16.4 Hz), 6.89 (d,
2H, J=8.9 Hz), 3.83 (s, 3H); 13C NMR (CDCl3, 75 MHz):
d=159.3, 137.6, 130.1, 128.6, 128.2, 127.7, 127.2, 126.6, 126.2,
114.1, 55.3; MS: m/z (%)=211 (M+ +1, 18%), 210 (M+,
yields were also obtained in the coupling with trans-2-
phenylvinylboronic acid (entry 5) and naphthalen-1-
ylboronic acid (entry 6), detecting some double bond
isomerization in the former case. Regarding regiose- 100), 209 (M+À1, 16), 195 (16), 167 (29), 166 (16), 165 (42),
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Oxime Palladacycle-Catalyzed Suzuki–Miyaura Alkenylation
151 (28), 136 (14), 134 (23), 105 (10), 104 (16), 76 (19), 62
(11), 50 (12).
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Financial support from the MICINN (Projects CTQ2007-
62771/BQU, CTQ2010-20387, and Consolider INGENIO
2010 CSD2007-00006), FEDER, from the Generalitat Va-
lenciana (Projects GV/2007/142, PROMETEO/2009/038),
and the University of Alicante is acknowledged.
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[18] Under the employed reaction conditions, no reaction
was observed when using 2,4,6-trivinylcyclotriborox-
ane-pyridine complex.
[19] Composition (1H NMR) of commercially available
crotyl chloride (95%, predominately trans): (E)-crotyl
chloride (82%), (Z)-crotyl chloride (15%), 3-chloro-1-
butene (3%).
Adv. Synth. Catal. 2011, 353, 1683 – 1687
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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