Use of ferrocenyl chelated palladacycles as catalysts for the Heck reaction

Article abstract of DOI:10.1007/s11172-010-0253-6

Two ferrocenyl palladacycles with bi-and tridentate (C,N) and (C,N,N) ligands were tested as possible catalysts for the Heck reaction. The latter complex efficiently catalyzed reactions of aryl halides with ethyl acrylate.

Full text of DOI:10.1007/s11172-010-0253-6

1
400
Russian Chemical Bulletin, International Edition, Vol. 59, No. 7, pp. 1400—1402, July, 2010
Use of ferrocenyl chelated palladacycles as catalysts for the Heck reaction
V. I. Sokolov, L. A. Bulygina, N. S. Khrushcheva, and N. S. Ikonnikov
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
2
8 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: stemos@ineos.ac.ru
Two ferrocenyl palladacycles with biꢀ and tridentate (C,N) and (C,N,N) ligands were
tested as possible catalysts for the Heck reaction. The latter complex efficiently catalyzed
reactions of aryl halides with ethyl acrylate.
Key words: the Heck reaction, ferrocenyl palladacycles, catalysts.
Formation of new carbon—carbon bonds is an essential
To find out how efficiently palladacycles 2 and 3 can
catalyze the Heck reaction, we carried out two series of
experiments involving a reaction of bromobenzene (4) with
ethyl acrylate (5) in the presence of only one potential
catalyst and different bases (Scheme 1, Table 1).
tool of organic synthesis. The most commonly used reacꢀ
tions of this type include the Heck and Suzuki—Miyaura
reactions. Like many others, they are catalyzed by palladiꢀ
um complexes; both inorganic salts and other derivatives
can serve as metal sources. Recently,1 palladacycles 1
prepared by cyclopalladation of ferrocenylazomethines
have been extensively tested as efficient catalysts for these
reactions.
—3
Scheme 1
In the present communication, we studied structurally
similar palladacycle 2 with a chelating ligand of the (C,N)
type. In compound 2, the electron donor is the N atom of
the tertiary amino group rather than the imino N atom as
in compound 1. In our previous detailed investigations of
the reactivities of such palladacycles, we have found that
their stoichiometric reactions with enones afford vinylferꢀ
rocenyl derivatives in high yields.4,5 Recently, we have
described the synthesis of fused bispalladacycle 3 with
a chelating ligand of the (C,N,N) type. This complex is
utterly inert to the reagents that successfully react with
bicyclic palladium derivatives containing (C,N) ligands.6
B = Et N, K CO , K PO , Cs CO , AcOK
3
2
3
3
4
2
3
Reagents and conditions: i. 2 or 3, DMF, 140 °C.
The Heck reactions were carried out under standard
conditions (DMF, 140 °C) except that no inert atmosphere
was employed. It turned out that both palladium comꢀ
plexes can catalyze the formation of ethyl cinnamate (6)
(see Table 1); however, the yields of the product are mostꢀ
ly low. One could expect that the yield of the product
would be increased in an inert atmosphere. To verify this
assumption, we conducted one experiment under argon,
achieving a 16% yield of ethyl cinnamate 6 (see Table 1,
entry 2).
The time of all the reactions was 7 h, whereupon the
reaction mixtures were worked up and analyzed on a gas
chromatograph to refine the yields of the products with
allowance for the unreacted starting bromobenzene (4)
(
see Table 1). One can see in Table 1 that noticeable yields
were obtained only in the presence of two bases (K CO
2
3
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1370—1372, July, 2010.
066ꢀ5285/10/5907ꢀ1400 © 2010 Springer Science+Business Media, Inc.
1

Ferrocenyl chelated palladacycles as catalysts
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
1401
Table 1. Catalytic reaction of bromobenzene (4) with ethyl acryꢀ
late (5) in the presence of bases*
the presence of K PO was slower than that in the presꢀ
3 4
ence of K CO , which may account for the highest yield
2
3
with K PO as a base (see Table 1, entries 1 and 2).
To confirm the stability of palladacycle 3 under our
experimental conditions, we heated it with a threefold
3
4
Entry
Base
Catalyst
Yield of 6
%)
Unreacted
(
C H Br (%)
6
5
excess of ethyl acrylate and K CO in DMF at 140 °C for
1
2
3
4
5
6
7
8
9
Et N
2
2
2
2
2
3
3
3
3
3
1.3
3.3 (16)**
23.0
4.7
3.5
3.8
0—
3.6
0—
<1.0
2.4
1.2
<1.0
3.2
2
3
3
1
2 h. According to TLC data, the final reaction mixture
K CO
2
3
K PO
contained only the starting compound 3.
3
4
Cs CO
Therefore, dimer 2 is an inefficient catalyst for the
Heck reaction under our experimental conditions. At the
same time, compound 3 is a promising catalyst, which
motivated us to examine its properties in more detail, priꢀ
marily in the presence of K CO since this base provided
2
3
KOAc
Et N
1.7
3
K CO
52.8
24.5
4.8
2
3
K PO
3
4
2
3
Cs CO
2
3
the highest yield of ethyl cinnamate 6 in our preliminary
experiments.
10
KOAc
18.8
1.6
*
The reaction time is 7 h; the reactions were not carried out in
A further series of experiments was intended to study
reactions of various aryl halides with ethyl acrylate (5) in
the presence of K CO (DMF, 140 °C) under argon
an inert atmosphere.
*The yield of compound 6 in an argon atmosphere is given in
*
2
3
parentheses.
(
Scheme 3, Table 2).
and K PO ) used in the Heck reaction. In addition, unlike
3
4
Scheme 3
palladacycle 3, the stability of which under various, inꢀ
6
cluding very drastic, conditions is well studied, dimer 2
was always used in reactions with vinyl ketones under mildꢀ
4
,5
er conditions (Scheme 2).
Scheme 2
Reagents and conditions: 3, K CO , DMF, 140 °C.
2
3
Compound
Ar
X
Br
I
Compound
10,15
11,16
Ar
X
I
Br
Br
4
8
9
,6
,13
,14
C H
4ꢀMeC H4
4ꢀMeOC H4
1ꢀNaphthyl
6
5
5
6
C H
6
6
4ꢀMeC H4 Br
12,17
6
It can be seen in Table 2 that the yields of ethyl cinꢀ
namates are sufficiently high, the conversion of the startꢀ
ing aryl halides being complete in 10 h. The exception is
1
ꢀbromonaphthalene (12) (Table 2, entry 6), which was
the sole reagent that was consumed incompletely over that
period of time. This is probably due to mutual steric hinꢀ
drances of the catalyst and aryl halide.
To sum up, we demonstrated that ferrocenyl palladaꢀ
cycle 3 is an efficient catalyst for the Heck reaction.
For the ferrocenyl palladacycles under study, two alterꢀ
native catalysis pathways can take place: associative,
without cleavage of the carbon—palladium σꢀbond, and
dissociative, when this bond breaks down and the cataꢀ
lytically active species contains no palladium—carbon
bond. The dissociative mechanism for palladacycle 3
can confidently be discarded. The sole possibility for
the metal atom in complex 3 to participate in catalysis is
its outerꢀsphere coordination with aryl halide and/or
the vinyl group of acrylate, which initiates the associaꢀ
tive mechanism with an increase in its coordination numꢀ
ber to 5—6.
Reagents and conditions: PhMe, Et N, 110 °C.
3
To study the stability of dimer 2 and its derivatives in
our experiments, we heated compound 2 with an excess of
ethyl acrylate in DMF in the presence of K CO or K PO
at 140 °C for 7 h. However, the expected product 7 was
not detected even in trace amounts, while the starting
dimer 2 decomposed completely even at 100 °C (TLC,
ethyl acetate as an eluent). Apparently, the Pd atom in
dimer 2 is rapidly replaced by ethyl acrylate in DMF and
the product decomposes on further heating. It should be
noted that visual resinification of the reaction mixture in
2
3
3
4

1402
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 7, July, 2010
Sokolov et al.
Table 2. Reactions of aryl halides 4 and 8—12 with ethyl
(see Table 1) and 1ꢀbromonaphthalene (12) (see Table 2, entry 6),
the yields of the products were determined by gas chromatograꢀ
phy. Since all the compounds obtained have been described earꢀ
acrylate (5) in the presence of palladacycle 3 and K CO3
2
under argon*
1
lier, we characterized them by comparing their H NMR spectra
with the literature data.
Entry
ArX
C H Br (4)
Product Yield (%)
1
2
3
4
5
6
6
13
14
15
16
17
41
77
90
63
46
18
The assistance by the Chembridge company is acꢀ
knowledged.
6
5
C H I (8)
6
5
4ꢀMeC H Br (9)
6
4
4ꢀMeC H I (10)
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 08ꢀ03ꢀ00169
and 08ꢀ03ꢀ92209 GFEN_a).
6
4
4ꢀMeOC H Br (11)
6
4
1ꢀBromonaphthalene (12)
*
The reaction time is 10 h.
References
Experimental
1
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(
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20 °C at a heating rate of 40 deg min–1, nitrogen as a carrier
2
gas). Catalysts 2 (see Ref. 7) and 3 (see Ref. 6) were prepared as
described earlier.
Catalytic reactions of ethyl acrylate (5) with aryl halides 4 and
8
—12 (general procedure). A mixture of an aryl halide (1 equiv.,
0
.04—0.06 mol), ethyl acrylate (5) (1.5 equiv.), a base (2 equiv.),
and a catalyst (0.002 equiv.) in DMF (6—10 mL) was heated
with stirring at 140 °C for 7—10 h (see Tables 1, 2). The reaction
mixture was diluted with water, and the product was extracted
with AcOEt. The extract was dried with Na SO and concenꢀ
2
4
trated. The residue was dissolved in AcOEt—hexane (1 : 3),
passed through a short column with silica gel, and concentrated.
For the reactions of ethyl acrylate (5) with bromobenzene (4)
Received January 28, 2010;
in revised form April 5, 2010