A palladacyclic azobenzene derivative as a catalyst for carbon-carbon bond formation reactions

Article abstract of DOI:10.1007/s11172-012-0277-1

A bis-chelated palladacycle in which the Pd atom is [(C, N)(C, N)]-tetracoordinated to two azobenzene ligands proved to be a moderately active catalyst for the Suzuki and Heck reactions. A voltammetric study revealed that the two-electron oxidation of this complex is accounted for by the irreversible Pd^II/Pd^IV transition.

Full text of DOI:10.1007/s11172-012-0277-1

Russian Chemical Bulletin, International Edition, Vol. 61, No. 10, pp. 1998—2000, October, 2012
1998
Brief Communications
A palladacyclic azobenzene derivative as a catalyst for
carbon—carbon bond formation reactions
L. A. Bulygina, N. S. Khrushcheva, S. M. Peregudova, and V. I. Sokolov
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. Eꢀmail: stemos@ineos.ac.ru
A bisꢀchelated palladacycle in which the Pd atom is [(C, N)(C, N)]ꢀtetracoordinated to
two azobenzene ligands proved to be a moderately active catalyst for the Suzuki and Heck
reactions. A voltammetric study revealed that the twoꢀelectron oxidation of this complex is
accounted for by the irreversible PdII/PdIV transition.
Key words: azobenzene, palladacycles, catalysis, the Heck reaction, the Suzuki reaction,
voltammetry.
Our recent investigations1—4 have revealed that not
plex can follow the associative pattern involving a PdIV
intermediate.5
For this reason, we found it
interesting to study the catalytꢀ
ic activity and stability of a fourꢀ
coordinate palladium complex
in catalytic reactions. Bisꢀcheꢀ
only chelated palladacycles (1) with bidentate (C, N)ꢀ
type coordination but also bisꢀchelated palladacycles (2)
with tridentate (C, N, N)ꢀtype coordination are efficient
catalysts for the Heck and Suzuki reactions. The high staꢀ
bility and almost complete inertness of complex 2 in
stoichiometric reactions suggest that catalysis by this comꢀ
lated palladacycle 3 seems to be
such a complex. This is an easiꢀ
ly accessible azobenzene derivꢀ
ative with the [(C, N)(C, N)]ꢀ
type of chelation.
This complex can be prepared in two ways and has a
synꢀconfiguration (Xꢀray diffraction data).6—9 We syntheꢀ
sized complex 3 as described earlier7 (Scheme 1).
Palladacyclic complex 3 was tested for catalytic propꢀ
erties in a number of the Suzuki and Heck reactions
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1980—1982, October, 2012.
1066ꢀ5285/12/6110ꢀ1998 © 2012 Springer Science+Business Media, Inc.

Catalytic palladacycle of azobenzene
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
1999
Scheme 1
ammetry (CV). The CV curves of complex 3 show two reꢀ
duction peaks (Fig. 1, a) and one oxidation peak (Fig. 1, b).
The oneꢀelectron reduction peaks are reversible: the reꢀ
verse sweep of the potential reveals the corresponding oxiꢀ
dation peaks equal in height. The halfꢀwave potentials of
the reduction peaks are –0.93 and –1.32 V (vs. sat. CE).
The reduction of complex 3 occurs by successive addition
of electrons to either azobenzene ligand, which gives rise
to stable (on the CV time scale) radical anions and then
radical dianions. The oxidation peak is irreversible; its poꢀ
tential is 0.79 V (vs. sat. CE). No reverseꢀscan peaks were
observed with an increase in the potential scan rate to
1 V s–1. Obviously, the oxidation peak reflects the irreꢀ
versible PdII/PdIV transition.
Reagents and conditions: a: Hg(OAc)2, MeOH, reflux, LiCl;
b: NaI, Me2CO; c: Pd2(DBA)3•C6H6, C6H6.
One can conclude that the oxidation of complex 3
produces highly reactive unstable PdIV derivatives that
seem to decompose in subsequent chemical reactions into
palladium and azobenzene. Thus, our electrochemical data
provide evidence against the associative mechanism of caꢀ
talysis involving a PdIV intermediate for reactions cataꢀ
lyzed by complex 3.
(Scheme 2, Table 1) and proved to be moderately active.
Unlike palladium complexes 1 and 2, which ensure good
yields of products in the Suzuki reaction even at room
temperature (1) or at 55 C (2),2 complex 3 is ineffective
under such mild conditions (see Table 1, entry 1). Only at
elevated temperatures does it have a catalytic effect (see
Table 1, entries 2, 3). High temperatures are also required
for the Heck reactions catalyzed by complex 3; the yields
of products are low (entries 4, 5). Moreover, the thermal
stability of complex 3 proved to be not very high: the
reactions at elevated temperatures produced a small
amount of palladium black.
I/A
20
a
0
–20
–40
Scheme 2
–1
0
E/V
Reagents and conditions: a: PhB(OH)2 (8), 3, K2CO3, DMF,
120 C; b: ethyl acrylate (11), 3, DMF, 140 C.
I/A
b
Ar = Ph (6, 9, 12), 4ꢀMeOC6H4 (7, 10, 13)
To find out whether the catalysis by palladium comꢀ
plex 3 follows the associative mechanism involving a PdIV
intermediate, we studied this complex by cyclic voltꢀ
20
10
0
Table 1. The Suzuki and Heck reactions catalyzed by palladacyꢀ
clic complex 3
Entry ArBr Product
Reaction
partner
T/C
Yield
(%)
1
2
3
4
5
6
6
7
6
7
9
9
10
12
13
PhB(OH)2
PhB(OH)2
PhB(OH)2
Ethyl acrylate 140
Ethyl acrylate 140
55
120
120
No product
70
74
58
27
0.5
1.0
E/V
Рис. 1. Cyclic voltammograms: reduction (a) and oxidation (b)
of palladium complex 3.

2000
Russ.Chem.Bull., Int.Ed., Vol. 61, No. 10, October, 2012
Bulygina et al.
Experimental
References
1
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H NMR spectra were recorded on a Bruker instrument
(300 MHz). Catalyst 3 was synthesized as described earlier.7
Electrochemical studies were carried out in a threeꢀelectrode
undivided cell under dry argon with 0.1 M Bu4NPF6 in acetoꢀ
nitrile as a supporting electrolyte. The potentials of a working
glassy carbon rod electrode (face area 2 mm2) were measured
versus an aqueous saturated calomel electrode (sat. CE), with
a platinum plate as an auxiliary electrode. Voltammograms were
recorded on an IPCꢀPro M potentiostat. The scan rate was
200 mV s–1; the concentration of palladium complex 3 was
1•10–3 mol L–1
.
Reactions of phenylboronic acid with aryl bromides (the Suꢀ
zuki reaction). Phenylboronic acid (3 mmol), aryl bromide
(2 mmol), and K2CO3 (4 mmol) were added with stirring to
DMF (8 mL). The mixture was stirred for 5 min and then comꢀ
plex 3 (0.5 mol.%) was added as a catalyst. The reaction mixture
was stirred at 120 or 55 C for 5 h and diluted with water. The
product was extracted with diethyl ether. The extract was dried
with Na2SO4 and concentrated. The residue was chromatoꢀ
graphed on SiO2 with hexane—CHCl3 (9 : 1) as an eluent.
Reactions of ethyl acrylate with aryl bromides (the Heck reacꢀ
tion). Aryl bromide (9.5 mmol), ethyl acrylate (14.3 mmol), and
K2CO3 (19 mmol) were added with stirring to DMF (8 mL). The
mixture was stirred for 5 min and then complex 3 (0.5 mol.%)
was added as a catalyst. The reaction mixture was stirred at
140 C for 7 h and diluted with water. The product was extracted
with AcOEt. The extract was dried with Na2SO4 and conꢀ
centrated. The residue was chromatographed on SiO2 with hexꢀ
ane—AcOEt (20 : 1) as an eluent.
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tov, Dokl. Akad. Nauk SSSR, 1976, 228, 367 [Dokl. Chem.
(Engl. Transl.), 1976].
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 11ꢀ03ꢀ00252).
Received April 3, 2012;
in revised form September 19, 2012