DOI: 10.1002/chem.200800657
Synthesis of New Hybrid Hydroquinone/Cobalt Schiff Base Catalysts:
Efficient Electron-Transfer Mediators in Aerobic Oxidation
Byron W. Purse, Lien-Hoa Tran, Julio Piera, Bjçrn kermark, and
[
a]
Jan-Erling Bäckvall*
Oxidation reactions are prevalent in academic and indus-
trial chemistry, and there is a great need for the continued
nium catalyst. This leads to a low-energy electron transfer
analogous to that occurring in biological systems (cf. the res-
piratory chain). This approach has been applied to a variety
of “biomimetic” aerobic oxidation reactions, including a
[1]
development of efficient catalytic methods. An attractive
oxidant is molecular oxygen, since it is inexpensive and en-
[2]
[9]
vironmentally benign, giving water as the only byproduct.
chloride-free Wacker oxidation, acetoxylation of alke-
[
9b,10]
[11]
Unfortunately, the reactivity of molecular oxygen is difficult
to control; it typically reacts under harsh conditions with
poor selectivity. Nature has solved this problem through the
use of electron-transport chains, which transfer electrons
nes,
Pd-catalyzed enallene carbo
ACHTREUNG
[
9b]
1,4-additions to conjugated dienes,
couplings, methane oxidation, and Ru-catalyzed oxida-
tion of alcohols and amines. Closely related biomimetic
coupled oxidations with hydrogen peroxide have also been
developed.
[12]
[13]
[14]
[15]
from a substrate to O in a stepwise fashion resulting in mild
2
[3]
[16]
conditions compatible with life.
In chemical catalytic oxidation reactions, a substrate-se-
lective redox catalyst, for example, a transition metal, re-
moves electrons from the substrate to give the oxidized
product. The resulting reduced form of the metal is reoxi-
dized, and it would be desirable to use molecular oxygen for
this reoxidation. Although direct reoxidation of the metal
In the course of our attempts to improve these reactions,
it occurred to us that the efficiency of the electron transfer
might be increased by covalently tethering two electron-
transfer mediators and in this way creating a hybrid redox
catalyst. A few molecules designed to test this approach
[17,18]
have been reported.
The first example, a hydroquinone/
[4–6]
by O has been reported in many cases,
this approach
cobalt–porphyrin hybrid, improved the rate of the Pd-cata-
lyzed aerobic 1,4-diacetoxylation of 1,3-cyclohexadiene, but
the stereoselectivity of the reaction was moderate and the
2
fails in other cases when electron transfer between the
metal and O is too slow to compete with the decomposition
2
[7]
[17]
of the reduced form of the metal complex.
catalyst was difficult to synthesize. The second example, a
To facilitate the electron transfer between the reduced
cobalt–salen type (H salen=N,N’-bis(salicylidene)ethylene-
2
substrate-selective catalyst and O various electron-transfer
mediators have been employed. A simple example of this
diamine) complex derived from 5-hydroxysalicylaldehyde,
had hydroquinone hydroxyl groups coordinated directly to
the cobalt, and this hampered its performance in Pd-cata-
2
principle is the Wacker process in which the redox couple
0
[18]
CuCl/CuCl facilitates electron transfer between Pd and
lyzed aerobic allylic acetoxylation.
2
[
8]
O2. In more recent examples, a quinone/hydroquinone
redox couple has been used in conjugation with an oxygen-
We now report on a second generation of this approach
that comprises cobalt salophens and salens with pendant hy-
droquinone groups (Scheme 1). These new hybrids offer
very efficient aerobic reoxidation of both palladium and
ruthenium catalysts in oxidative 1,4-diacetoxyation, in enal-
lene carbocyclization, and in alcohol oxidation. The oxidized
form of hydroquinone, that is, benzoquinone, is a very effi-
cient reoxidant for a variety of transition-metal catalysts, so
its inclusion in the hybrid design was a natural choice.
Cobalt Schiff base complexes were chosen as the oxygen-ac-
tivating component because of their demonstrated efficiency
activating
catalyst,
for
example,
cobalt–salophen
(
H salophen=N,N’-bis(salicylidene)-o-phenylenediamine),
2
to catalyze the aerobic reoxidation of a palladium or ruthe-
[
a] Dr. B. W. Purse, L.-H. Tran, Dr. J. Piera, Prof. B. kermark,
Prof. J.-E. Bäckvall
Department of Organic Chemistry
Stockholm University
1
06 91 Stockholm (Sweden)
Fax : (+46)-8-15-49-08
E-mail: jeb@organ.su.se
[9b,14]
in coupled aerobic oxidation,
and their simple and mod-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800657.
ular synthesis. Molecular modeling shows that, in this
7500
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 7500 – 7503