Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts {di-μ-chlorobis(benzaldehydeoxime-6-C,N)dipalladium(II), di-μ-chlorobis(dimethylbenzylamine-6-C,N)dipalladium(II)} for the Heck reaction

Article abstract of DOI:10.1016/S0040-4039(00)01594-X

Amine and oxime palladacycles with an aryl-Pd covalent bond were found to be robust catalysts for the Heck reaction of aryl iodides, bromides and even chlorides. High TONs and TOFs were obtained with these palladacycles. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01594-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8981–8984
Aryl–Pd covalently bonded palladacycles, novel amino and
oxime catalysts
{
di-m-chlorobis(benzaldehydeoxime-6-C,N)dipalladium(II),
di-m-chlorobis(dimethylbenzylamine-6-C,N)dipalladium(II)}
†
for the Heck reaction
Suresh Iyer* and C. Ramesh
Organic Chemistry Synthesis Division, NCL, Pune 411 008, India
Received 24 July 2000; revised 8 September 2000; accepted 14 September 2000
Abstract
Amine and oxime palladacycles with an aryl–Pd covalent bond were found to be robust catalysts for
the Heck reaction of aryl iodides, bromides and even chlorides. High TONs and TOFs were obtained with
these palladacycles. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Heck reaction; amino palladacycle; oxime palladacycle; Lewis acid; tetrabutylammonium iodide.
1
We and others have reported several novel catalysts for the Heck reaction. Palladium
2
complexes have been the most commonly used catalysts with various phosphines as ligands.
Some of the more recent examples of efficient catalysts are novel cyclopalladated tris-o-
tolylphosphine, N-heterocyclic carbene palladium, tridentate aryl bisphosphine Pd(II) (PCP),
3–6
and ortho-palladated triaryl phosphites, all of which give very high TONs. The palladacycle
formed from Pd(OAc) and P(o-tol) was a better catalyst than the conventional ‘in situ catalyst’
2
3
produced from Pd(OAc) and P(o-tol) . A major drawback of the use of phosphine ligands in
2
3
such catalytic reactions is the oxidation of the phosphine to a phosphine oxide as well as
cleavage of the PꢀC bond, causing degradation of the ligand, reduction of the metal and
termination of the catalytic cycle. The palladacycles reportedly show extraordinary thermal
stability (140–180°C) and the PCP, triarylphosphite and carbene Pd complexes can be used in air
with slightly reduced efficiency.
*
Corresponding author.
NCL Communication No. 6596.
†
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01594-X

8982
In our study for novel catalysts for the Heck reaction, we decided to investigate the non
phosphino palladacycles from N,N-dimethylbenzylamine, benzaldoxime and benzophenone
oxime. These complexes have been previously reported and easily synthesized from readily
7
available N,N-dimethylbenzylamine, benzaldehyde oxime, benzophenone oxime and Li PdCl .
2
4
Such ligands are not as readily oxidizable as phosphines and the metal complexes with a
covalently bonded Pd to the aromatic ring, could be more stable and efficient catalysts (Scheme
1
).
Scheme 1. Preparation of palladacycle
Aryl iodides, bromides and electron deficient chlorides react with ethyl acrylate and styrene in
the presence of these Pd complexes as catalysts to give high yields of the substituted products
E-cinnamate and stilbene) with excellent turn over numbers. Table 1 shows the results of our
(
experiments with the various aryl halides and olefins in presence of these three palladacyles.
High TONs (72, 000–1, 45, 454) as well as high TOFs (1, 625–20, 780) can be obtained with aryl
iodides and bromides. The bromides and chlorides take longer reaction times for completion.
Electron withdrawing groups activate aryl chlorides. The use of a Lewis acid as a cocatalyst
(
ZnCl ), which would help labilize the halide, increased the reaction rate and the yield of the
2
reaction. The palladacycle 3 was an excellent catalyst and the reaction of bromobenzene with
ethyl acrylate and styrene gave 90 and 96% yields (TON 450) in 1 h (shorter reaction time
compared to the other catalysts). The TON for bromobenzene with the catalyst 1 (87, 000–89,
0
00) was higher than with the catalyst 2 (72, 000–78, 000).
In a typical experiment, 4-iodoanisole (11.7 g, 50 mmol), ethyl acrylate (10 g, 100 mmol),
K CO (8.28 g, 60 mmol) and 50 mL 1-methylpyrrolidinone, were taken in a round bottomed
2
3
flask and the Pd complex 1 (0.00027 g, 0.0005 mmol) was added as catalyst. The reaction
mixture was heated in an oil bath maintained at 150°C for 8 h followed by the usual extractive
work up and column chromatography over silica gel to give (E)-ethyl-4-methoxycinnamate
(
9.223 g, 88.3%, 44.77 mmol, TON 89, 540 (mol product per mol 1), TOF 11, 192 mol product
per mol 1 per hour).
When aluminium chloride was used as a cocatalyst, the reaction of 4-chlorotoluene, with
styrene, gave the substituted product (as E-isomer) in low yields. Bu NI was also used as
4
cocatalyst for the reaction of 4-chlorotoluene and 2-chloropyridine with ethyl acrylate and
styrene in the presence of palladacycle 3. The normal substitution products were obtained in
moderate yields.

8983
Table 1
Heck vinylation of aryl halides catalyzed by amine and oxime palladacycles
S. No. Reactant
mmol)
Olefin (mmol) Catalyst Product
Time (h) Yield TON
TOF
(
1
2
4-MeOPhI
50)
Ethylacrylate
(100)
1
Ethyl-4-methoxy-
cinnamate
8
88
89, 540 11, 190
(
3
1
2
48
90
76
453
226
4-MeOPhI
50)
Styrene (60)
4-Methoxy-
stilbene
76, 000
1, 740
(
2
3
2
48
6
7
92
97
96
1, 38, 666
485
1, 45, 454 20, 778
2, 888
90
3
4
PhI (50)
Ethylacrylate
Ethyl cinnamate
Ethyl cinnamate
(
100)
Ethylacrylate
100)
PhBr (50)
1
34
90
90, 000
2, 647
(
2
3
1
2
3
1
37
1
29
48
1
52
72, 000
437
86, 666
78, 000
483
1, 946
437
2, 988
240
89.8
86.6
52
96
51
5
6
7
PhBr (50)
Styrene (60)
Stilbene
483
4-ClPhNO2 Ethylacrylate
5) (10)
Ethyl-4-nitro-
cinnamate
31
287
(
2
1
30
42
60
71
300
70, 000
4-ClPhNO2 Styrene (60)
50)
4-Nitro-
stilbene
1, 666
(
2
2
40
24
69
78.5
350
350
8
9
0
4-ClPhCN
5)
Styrene (6)
4-Cyano-
stilbene
(
2-BrPy (5)
Ethylacrylate
(
2
Ethyl-3-(2-pyridyl)
acrylate
48
68
4, 090
10)
4-ClPhCH3 Styrene (4)
2)
a
1
3
3
4-Methylstilbene
12
24
18
28
(
b
a
Cocatalyst: AlCl ; temperature 130°C.
3
b
Cocatalyst: Bu NI; temperature 130°C.
4
We have thus described three novel non phosphino palladacycles as efficient catalysts for the
activation of aryl halides for reaction with olefins.
Acknowledgements
We thank the DST for a research grant and the CSIR for a SRF to C. Ramesh.
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