Palladated Kaiser oxime resin as precatalyst for the Heck reaction in organic and aqueous media

Article abstract of DOI:10.1055/s-2006-951521

A particular Kaiser oxime resin derived palladacycle is an efficient precatalyst for the Heck reaction with aryl iodides and bromides in DMF and in aqueous solvents under relatively moderate temperatures (110 °C and 120 °C) and high TON up to 10⁵. Poisoning studies indicate that this polymer acts as a precatalyst. The polymer has been reused after recycling with low to moderate leaching of Pd(0). Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-951521

LETTER
2959
Palladated Kaiser Oxime Resin as Precatalyst for the Heck Reaction in
Organic and Aqueous Media
P
alladatedKaise
m
r
O
xime Resin for
i
the
H
ec
l
kRea
c
i
tion o Alacid, Carmen Nájera*
Departamento de Química Orgánica, Facultad de Ciencias and Instituto de Síntesis Orgánica, Universidad de Alicante, Apdo. 99,
03080 Alicante, Spain
Fax +34(965)903549; E-mail: cnajera@ua.es
Received 23 March 2006
Dedicated to Prof. Richard Heck for his pivotal contribution to palladium chemistry
vents under thermal or microwave irradiation.3m In spite
of the excellent efficiency of palladacycles 1 and 2 as pre-
catalysts in Heck reactions the use of polymer-supported
palladacycles has not been studied so far.
Abstract: A particular Kaiser oxime resin derived palladacycle is
an efficient precatalyst for the Heck reaction with aryl iodides and
bromides in DMF and in aqueous solvents under relatively moder-
ate temperatures (110 °C and 120 °C) and high TON up to 105. Poi-
soning studies indicate that this polymer acts as a precatalyst. The
polymer has been reused after recycling with low to moderate
leaching of Pd(0).
4-ClC6H4
N
OH
N OH
Key words: Heck reaction, palladacycles, polymers, alkenes, ary-
lation
Pd
Pd
N
Cl
)
2
HO
)
2
Cl
Cl
2
1
OH
Palladacycles appeared recently as very active catalysts
for C–C- and C–heteroatom-bond forming reactions.1 In
this respect oxime-derived palladacycles are considered
catalysts specially active in phosphane-free homo- and
cross-coupling reactions such as, Heck, Suzuki, Stille,
Ullmann, Cassar–Heck–Sonogashira, sila-Sonogashira,
Glaser, Hiyama, and alkoxycarbonylation reactions.2 We
have studied2,3 the scope of these types of complexes as
sources of highly active zero-valent palladium species4 ei-
ther in organic or in aqueous media. Concerning the Heck
vinylation reaction 4,4¢-dichlorobenzophenone oxime-de-
rived palladacycle 1 (Figure 1) has shown extremely high
activity as precatalyst with turnover number (TON) of up
to 1010 for the coupling of n-butyl acrylate and phenyl io-
dide and rates TOF (TON h–1) of 108 h–1 in DMF and with
Et3N as base.3e Reactions can be performed under air us-
ing aryl iodides, bromides and chlorides.3a,e Excellent
yields have been obtained when polybromobenzenes re-
acted with an excess of methyl acrylate3e and in the multi-
ple vinylation of polybromotribenzotriquinacenes and
fenestrindanes.5 4-Hydroxyacetophenone oxime derived
palladacycle 2 (Figure 1) is a very efficient precatalyst for
the mono- and diarylation of alkenes in aqueous solvents
under thermal or microwave conditions.3j–m Methoxylated
stilbenoids, which are chemoterapeutic agents for cancer
treatment, have steroselectively been prepared using com-
plex 2 in organic and aqueous solvents.3k The chemoselec-
tive arylation of acrolein diethyl acetal allows the
synthesis either of cinnamaldehyde derivatives using
K2CO3 as base or ethyl 3-arylpropanoates with (dicyclo-
hexyl)methylamine as base in organic and in aqueous sol-
N
OH
Pd
SS-O
)
2
PS
NO2
Cl
3
4
Figure 1
Corma’s group have anchored the phenolic group of com-
plex 2 to several solid supports such as silica gel, MCM-
41 silicate, polystyrene and ethyleneglycol dimethyl acry-
late polymer to afford complexes of the type 3, which
have been used for the Suzuki reaction in aqueous sol-
vents (Figure 1).6 Despite the observation of palladium
nanoparticles formation from decomposition of supported
palladacycles,4 there are no evidences on the catalytic ac-
tivity and recyclability of oxime-derived palladacycles in
the Heck reaction.
We describe here the straightforward preparation of a new
polymer-supported oxime-derived palladacycle derived
from Kaiser oxime resin 4 (Figure 1) and the studies about
its catalytic efficiency for the Heck reaction in organic
and aqueous media as well as leaching and recycling tests.
Kaiser oxime resin 4 (1% divinylbenzene cross-linked)
with an oxime loading of 1.1 mmol g–1 was treated with a
0.5 M methanolic solution of Li2PdCl4 at room tempera-
ture during one week, giving a new polymer 5 containing
an average of 0.12 mmol g–1 of Pd (determined by ICP-
OES). The related unsupported dimeric complex 6 was re-
gioselectively prepared under the same palladation condi-
tions from the corresponding 4-nitrobenzophenone oxime
(Figure 2). From its NMR spectra it was concluded that
the palladation took place in the phenyl group. Compari-
son of the IR spectra of complexes 5 and 6 led to conclude
that polymer 5 has a palladacycle structure.
SYNLETT 2006, No. 18, pp 2959–2964
0
3
.1
1
.2
0
0
6
Advanced online publication: 25.10.2006
DOI: 10.1055/s-2006-951521; Art ID: S08306ST
© Georg Thieme Verlag Stuttgart · New York

2960
E. Alacid, C. Nájera
LETTER
(
Heck reactions in water were carried out using dicyclo-
hexylmethylamine as base at 120 °C in a pressure tube.7
The arylation of tert-butyl acrylate with 4-chloroiodoben-
zene under these reaction conditions took place in shorter
reaction times employing the monomers 23l and 6 than
when the polymer 5 was employed (Table 1, entries 8–
10). In the arylation of 4-chlorostyrene, palladacycle 23k
showed lower efficiency than polymer 5, but higher
regioselectivity (Table 1, entries 11–13). From these ex-
periments can be concluded that palladated Kaiser oxime
resin 5 showed a good effciency for the Heck arylations in
DMF and H2O even comparable with the palladacycles 1,
2, and 6.
2
Cl
OH
N
OH
Pd
Pd
N
Cl
PS
NO2
NO2
5
6
Figure 2
Initial studies on the arylation of methyl acrylate with io-
dobenzene were performed using the polymeric pallada-
cycle 5 and also the dimeric complexes 1 and 6 in DMF at
110 °C and Et3N as base (Table 1, entries 1–3). These
couplings took place with similar yields in shorter reac-
tion times for the dimeric than for the polymeric complex.
The arylation of styrene with iodobenzene under the same
reaction conditions was less efficient using the polymeric
complex 5 than with palladacycle 1 (Table 1, entries 4 and
5). However, when dicyclohexylmethylamine was used
instead of Et3N the reaction time was lowered from 6
hours to 3.5 hours even using lower loading of Pd
(Table 1, entries 5 and 6). Under these reaction condi-
tions, the dimeric 6 and the polymeric 5 complexes be-
haved with a similar efficiency (Table 1, entries 6 and 7).
Recycling experiments for the arylation of acrylates with
aryl iodides in DMF with palladated Kaiser oxime resin 5
(0.1 mol% Pd loading) revealed that high yields (>90%)
were achieved in DMF after 8 cycles although increasing
the reaction time from 1.6 hours to 10 hours (Scheme 1).
A 0.4–5% leaching of Pd(0) in the final product was found
by ICP-OES analyses of the reaction mixture after filtra-
tion of the polymer and evaporation of the solvents during
several cycles.
Table 1 Heck Reaction Using Monomeric and Polymeric Complexes in DMF and in Watera
Entry ArI
Alkene
Cat.
Solvent
Base
Temp Time
Product
Yield (%)b
(mol% Pd)
(°C)
110
110
110
110
110
110
110
120
(h)
99c
CO2Me
1
2
3
4
5
6
7
8
PhI
CH2=CHCO2Me
1 (0.1)
5 (0.1)
6 (0.1)
1 (0.1)
5 (0.5)
5 (0.1)
6 (0.1)
2 (0.01)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
H2O
Et3N
1.2
Ph
Et3N
1.6
0.75
4
97 (92)
99
Et3N
CH2=CHPh
Et3N
98c,d
94e
Ph
Ph
Et3N
6
Cy2NMe
Cy2NMe
Cy2NMe
3.5
3.2
3
98f
96d
94g
t
4-ClC6H4I CH2=CHCO2t-Bu
CO2 Bu
4-ClC6H4
9
10
11
5 (0.01)
6 (0.01)
H2O
H2O
H2O
Cy2NMe
Cy2NMe
Cy2NMe
120
120
120
4
2
100 (88)
99
CH2=CH-4-ClC6H4 2 (0.01)
30
96 (79)h,i
4-ClC6H4
4-ClC6H4
12
13
5 (0.01)
6 (0.01)
H2O
H2O
Cy2NMe
Cy2NMe
120
120
23
12
96 (82)d
97f
a Reaction conditions: aryl iodide (1 mmol), alkene (1.5 mmol), cat. (see column), base (1.5 mmol), solvent (1.5 mL), temperature (see column).
The reactions in water were performed in a pressure tube.
b Determined by 1H NMR by using diphenylmethane as internal standard, in parenthesis yield after flash chromatography.
c Ref.3e
d a/b = 1:10 regioisomers ratio.
e a/b = 1:5 regioisomers ratio.
f a/b = 1:6 regioisomers ratio.
g Ref.3l
h Ref.3k
i a/b = 1:16 regioisomers ratio.
Synlett 2006, No. 18, 2959–2964 © Thieme Stuttgart · New York

LETTER
Palladated Kaiser Oxime Resin for the Heck Reaction
2961
5 (0.1 mol%)
5 (0.1 mol%)
OtBu
OMe
Ph
OMe
4-ClC6H4
OtBu
+
Ph
I
4-ClC6H4I
+
Cy2NMe, H2O,
120 °C
Et3N, DMF, 110 °C
O
O
O
O
1st cycle: 97%, 1.6 h
2nd cycle: 97%, 2.0 h
3rd cycle: 92%, 2.5 h
4th cycle: 97%, 3.0 h
5th cycle: 96%, 4.0 h
6th cycle: 94%, 4.5 h
7th cycle: 95%, 6.7 h
8th cycle: 90%, 10 h
1st cycle: 99%, 2.0 h
2nd cycle: 99%, 2.5 h
3rd cycle: 99%, 2.5 h
4th cycle: 93%, 2.0 h
5th cycle: 96%, 2.5 h
6th cycle: 98%, 2.5 h
7th cycle: 99%, 3.0 h
8th cycle: 98%, 3.0 h
Scheme 1 Recycling experiments in DMF
Scheme 2 Recycling experiments in H2O
In the case of the arylation in water the reaction time re- dicyclohexylmethylamine as base with 0.01 mol% of
mained between 2–3 hours during 8 cycles with yields be- precatalyst 5 took place in quantitative yield faster in
tween 93–99% (Scheme 2). The leaching of Pd(0) in the DMF than in H2O (Table 2, entries 1 and 3).
reaction mixture ranged from 2–2.7%. It can be deduced
However, with a lower 0.001 mol% Pd loading a faster
that the polymer showed an unusual high efficiency and
and efficient reaction occurred in water (Table 2, entries 2
stability, especially in water. Therefore, it seems that the
and 4). Under this low Pd loading TONs up to 105 were
polymeric complex 5 degradates more easily in DMF than
achieved. When tert-butyl acrylate was allowed to react
in water. On the other hand, XPS analyses revealed that in
with deactivated 4-methoxyiodobenzene the arylation
was faster in DMF than in H2O (Table 2, entries 5 and 6).
The arylation of acrylonitrile, methyl vinyl ketone and
N,N-dimethylacrylamide with iodobenzene also occurred
in a higher reaction rate in DMF (Table 2, entries 7–12).
However, acrylic acid was arylated with 4-fluoroiodoben-
zene to give the corresponding cinnamic acid with the
same efficiency under both reaction media (Table 2, en-
the new and reused complex 5 only Pd(II) was detected.
When these reactions were carried out in the presence of
Hg (300 equiv with respect to Pd) no reaction was ob-
served after five hours reaction time. All these results in-
dicate that the Pd(II) complex 5 is a precatalyst, being
Pd(0) nanoparticles formed by decomposition of the poly-
mer and present in the solution the active species.4
In order to study the scope in the applicability of this pal- tries 13 and 14). These results are similar to previously de-
ladated polymer 5 acting as precatalyst, several alkenes, scribed Heck reactions catalyzed by palladacycle 2 in
such as a,b-unsaturated carbonyl compounds and styrenes H2O.3l
were arylated with different aryl iodides and bromides in
DMF and in H2O (Table 2). The arylation of tert-butyl
acrylate with 4-chloroiodobenzene in the presence of
Table 2 Heck Reactions Using Palladated Kaiser Oxime Resin 5 in DMF and in Aqueous Solventsa
Entry ArI
Alkene
Pd (mol%) Solvent
Temp (°C) Time (h) Product
Yield (%)b
99
t
1
4-ClC6H4I
CH2=CHCO2t-Bu
0.01
DMF
110
2.5
36
CO2 Bu
4-ClC6H4
2
3
4
5
0.001
0.01
99
H2O
120
110
4
4
3
99 (88)
98
0.001
0.01
t
4-MeOC6H4I CH2=CHCO2t-Bu
DMF
99
CO2 Bu
4-MeOC6H4
6
7
0.01
1
H2O
120
110
120
110
120
110
120
110
14
96 (89)
96c
PhI
CH2=CHCN
DMF
H2O
2.5
CN
Ph
8
1
8
13
20
13
13
13
98 (81)d
79e
9
PhI
CH2=CHCOMe
CH2=CHCONMe2
CH2=CHCO2H
0.01
0.01
0.01
0.1
0.1
DMF
H2O
COMe
Ph
10
11
12
13
87 (82)e
97
PhI
DMF
H2O
CONMe2
Ph
96 (93)
99 (92)f
4-FC6H4I
DMF
CO2H
4-FC6H4
Synlett 2006, No. 18, 2959–2964 © Thieme Stuttgart · New York

2962
E. Alacid, C. Nájera
LETTER
Table 2 Heck Reactions Using Palladated Kaiser Oxime Resin 5 in DMF and in Aqueous Solventsa (continued)
Entry ArI
14
Alkene
Pd (mol%) Solvent
Temp (°C) Time (h) Product
Yield (%)b
99
0.1
H2O
120
110
13
14
15
4-FC6H4I
(E)-MeCH = CHCO2H 1
DMF
52 (45)g,h
CO2H
4-FC6H4
16
17
1
H2O
120
110
14
14
48 (36)g,i
81 (77)j
4-FC6H4I
(E)-PhCH=CHCN
CH2=CHCO2t-Bu
1
DMF
Ph
CN
4-FC6H4
4-ClC6H4
18
19
1
H2O
120
110
6
86 (74)k
97
t
4-ClC6H4Br
0.1
DMF
14
CO2 Bu
20
21
0.01
0.1
DMA–H2Ol 120
DMF 110
DMA–H2Ol 120
DMF 110
14
16
100
31
t
4-MeOC6H4Br CH2=CHCO2t-Bu
CO2 Bu
4-MeOC6H4
22
23
0.1
0.5
21
17
65
76
PhBr
CO2Me
CO2Me
CO2Me
Ph
CO2Me
24
25
0.5
DMA–H2Ol 120
DMF 110
17
24
72
4-ClC6H4Br
4-ClC6H4Br
(E)-MeCH=CHCO2Et 0.5
77c
CO2Et
4-ClC6H4
4-ClC6H4
26
27
0.5
DMA/H2Ol 120
DMF 110
DMA/H2Ol 120
23
13
69m (60)n
41o
CH2=CH-4-ClC6H4
0.01
4-ClC6H4
28
0.01
14
98p
a Reaction conditions: aryl iodide (1 mmol), alkene (1.5 mmol), complex 5 (see column), Cy2NMe (1.5 mmol), solvent (1.5 mL). A pressure
tube was used for the experiments in water.
b Determined by 1H NMR by using diphenylmethane as internal standard, in parenthesis yield after flash chromatography.
c Z/E = 1:5.
d Z/E = 1:4.
e 10% of 4-phenylbutan-2-one was also obtained.
f After recrystallization.
g Z/E = 1:24.
h 3-(4-Fluorophenyl)but-3-enoic acid (10%) and 4-(4-fluorophenyl)but-3-enoic acid (2%) were also obtained.
i 3-(4-Fluorophenyl)but-3-enoic acid (6%) and 4-(4-fluorophenyl)but-3-enoic acid (2%) were also obtained.
j Z/E = 1:10.
k Z/E = 1:19.
l Ratio of 4:1. TBAB (1 equiv) was added.
m Z/E = 1:7; 12% of ethyl 3-(4-chlorophenyl)but-3-enoate was obtained.
n Isolated yield for the E-diastereomer.
o a/b = 1:10 regioisomers ratio.
p a/b = 1:23 regioisomers ratio.
Then the arylations of b-substituted acrylic systems were When aryl bromides, such as 4-chloro- and 4-methoxy-
performed with 4-fluoroiodobenzene. In the case of cro- bromobenzene were used for the arylation of tert-butyl
tonic acid the b-arylation gave a moderate yield of (E)-3- acrylate in DMF, only the activated 4-chlorobromoben-
(4-fluorophenyl)but-2-enoic acid with high diastereose- zene afforded the corresponding cinnamate in good yield
lectivity. In addition isomeric 3-(4-fluorophenyl)but-3- (Table 2, entries 19 and 20). However, these processes
enoic acid and 4-(4-fluorophenyl)but-3-enoic acid in less could not be performed in neat H2O, whereas these aryla-
than 12% overall yield were obtained (Table 2, entries 15 tions took place in good yields by using aqueous N,N-di-
and 16). The same arylation was performed with cinna- methylacetamide (H2O–DMA
= 1:4) and tetra-n-
monitrile providing (E)-3-(4-fluorophenyl)-cinnamoni- butylammonium bromide (TBAB) as additive (Table 2,
trile with high diastereoselectively (Table 2, entries 17 entries 21 and 22). Methyl itaconate was regio- and dia-
and 18).
stereoselectively arylated with phenyl bromide to afford
Synlett 2006, No. 18, 2959–2964 © Thieme Stuttgart · New York

LETTER
Palladated Kaiser Oxime Resin for the Heck Reaction
2963
Table 3 Recycling Experiments with Palladated Kaiser Oxime Resin 5 in DMF and in Watera
Run
1
ArI
PhI
Alkene
Pd (mol%) Solvent
Temp (°C) Time (h)
Product
Yield (%)b
96c
95
99
99
CN
CH2=CHCN
1
DMF
DMF
DMF
DMF
110
110
110
110
2.5
7
Ph
2
CH2=CHCOCH3
CH2=CHCONMe2
CH2=CHCO2t-Bu
0.1
0.1
0.01
COMe
Ph
Ph
3
14
CONMe2
t
4
4-ClC6H4I
PhI
2.5
CO2 Bu
4-ClC6H4
1
2
3
4
CH2=CHCN
1
H2O
H2O
H2O
H2O
120
120
120
120
8
98d
92
99
99
CN
Ph
Ph
Ph
CH2=CHCOMe
CH2=CHCONMe2
CH2=CHCO2t-Bu
0.1
0.1
0.01
8.5
COMe
14
CONMe2
t
4-ClC6H4I
3
CO2 Bu
4-ClC6H4
a Reaction conditions: aryl iodide (1 mmol), alkene (1.5 mmol), complex 5 (see column), Cy2NMe (1.5 mmol), solvent (1.5 mL). A pressure
tube was used for the experiments in water. The polymer 5 was recovered by filtration after each run and dried under vacuum.
b Determined by 1H NMR by using diphenylmethane as internal standard.
c Z/E = 1:5.
d Z/E = 1:4.
the corresponding product in similar yields (Table 2, en-
tries 23 and 24).
Acknowledgment
We thank DGES of the Spanish Ministerio de Educación y Ciencia
(MEC) (grant: CTQ2004-00808/BQU), the Generalitat Valenciana
(grants: GRUPOS05/11) and the University of Alicante for financi-
al support. E.A. thanks MEC for a predoctoral fellowship.
In the case of ethyl crotonate the coupling with 4-chloro-
bromobenzene gave mainly the b-disubstituted product in
better Z/E diastereoselectivity in aqueous DMA than in
DMF (Table 2, entries 25 and 26). 4-Chlorophenylethyl-
ene was arylated with 4-chlorobromobenzene to give
higher yield and regioselectivity in aqueous DMA than in
DMF (Table 2, entries 27 and 28).
References
(1) (a) Herrmann, W. A.; Böhm, V. P. W.; Reisinger, C.-P. J.
Organomet. Chem. 1999, 576, 23. (b) Dupont, J.; Pfeffer,
M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917.
Recycling experiments were performed using recovered
polymer complex 5 in different Heck reactions in DMF
and in water (Table 3). Acrylonitrile, methyl vinyl ketone,
N,N-dimethylacrylamide and tert-butyl acrylate were ary-
lated with aryl iodides to give the corresponding products
in DMF or in water with similar yields and reaction times
as mentioned on Table 2. Polymer 5 was filtered off after
each run and used in a new experiment. Products were ob-
tained without contamination of the previous run prod-
ucts.
(c) Bedford, R. B. Chem. Commun. 2003, 1787.
(d) Singleton, J. T. Tetrahedron 2003, 1837. (e) van der
Boom, M.; Milstein, D. Chem. Rev. 2003, 103, 1759.
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(2) For an account, see: Alacid, E.; Alonso, D. A.; Botella, L.;
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2002, 2, 1823. (b) Alonso, D. A.; Nájera, C.; Pacheco, M. C.
J. Org. Chem. 2002, 67, 5588. (c) Botella, L.; Nájera, C.
Angew. Chem. Int. Ed. 2002, 41, 179. (d) Botella, L.;
Nájera, C. J. Organomet. Chem. 2002, 663, 46. (e) Alonso,
D. A.; Nájera, C.; Pacheco, M. C. Adv. Synth. Catal. 2002,
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Tetrahedron Lett. 2002, 43, 9365. (g) Alonso, D. A.;
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Chem. 2004, 69, 1615. (i) Alonso, D. A.; Botella, L.; Nájera,
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We can conclude that polymer 5 acts as a precursor of ac-
tive Pd(0) species for the Heck reaction of aryl iodides and
bromides either in organic or in aqueous solvents. Al-
though its efficiency is slightly lower than the related
dimeric palladacycles, it can be easily prepared and re-
used, at least during eight times, keeping an excellent cat-
alytic activity, especially in water. In addition, the
recovered polymer can be reused in different Heck reac-
tions to give pure products with the same yields than using
the unused polymer. However, low to moderate levels of
Pd leaching was always detected in the final products.
Further applications of this polymeric complex as precat-
alyst in carbon–carbon bond forming reactions are under
way in our group.
Synlett 2006, No. 18, 2959–2964 © Thieme Stuttgart · New York

2964
E. Alacid, C. Nájera
LETTER
(4) Carbapalladacycles decompose under different reaction
conditions to give Pd colloids, see for instance: (a) Yu, K.;
Sommer, W.; Richardson, J. M.; Weck, M.; Jones, C. W.
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(7) These reaction conditions were considered the most
appropriate when complex 2 was used as precatalyst. See ref.
3j–m.
Synlett 2006, No. 18, 2959–2964 © Thieme Stuttgart · New York