Chemistry Reference
In-Depth Information
11.2.2. Palladium Catalyst
Among the transition-metal catalysts developed for aerobic oxidation of alcohols, pal-
ladium-based complexes, in particular, have been investigated. In 1977, Schwaltz and
Blackburn reported the fi rst synthetically valuable, palladium-catalyzed aerobic oxida-
tion of alcohols that uses PdCl
2
with NaOAc [6]. Although the following 20 years had
seen few developments in this area, the palladium-catalyzed methods have received the
most attention over the recent years [7,8]. A generally accepted catalytic cycle is illus-
trated in Scheme 11.2. The cycle consists of the two separate processes: alcohol oxidation
and catalyst regeneration. In the alcohol oxidation process, palladium alkoxide is formed
after alcohol coordination, and then
-hydride elimination occurs to give a carbonyl
product. The resulting palladium hydride complex is converted to the corresponding
palladium hydroperoxo species by the reaction with molecular oxygen, and the subse-
quent ligand exchange regenerates the initial catalyst. There are two possible pathways
to give the Pd
II
hydroperoxide species: direct insertion of molecular oxygen to Pd
II
-
hydride bond or reductive elimination/peroxo formation/protonolysis sequence. The
feasibility of each pathway has been supported by experimental and theoretical studies.
In 1998, Uemura and coworkers reported the palladium-catalyzed aerobic oxidation
of alcohols, using a Pd(OAc)
2
, pyridine, and molecular sieve 3A system [8a,b]. Inspired
by the report, Sigman et al. and Stoltz and Ferreira independently disclosed the palla-
dium-catalyzed oxidative kinetic resolution of racemic alcohols, in which a naturally
occurring diamine, (
β
)-sparteine, serves as an effective chiral source (Scheme 11.3)
[9,10]. Sigman and coworkers employed the two reaction conditions: Pd(OAc)
2
in dichlo-
roethane (DCE) at 60°C and Pd(CH
3
CN)
2
Cl
2
at 70°C. On the other hand, Stoltz's
methods utilized Pd(nbd)Cl
2
with molecular sieves in toluene at 80°C. While there are
slight differences between the reaction conditions of the methods, both systems effi -
ciently resolve a range of benzylic alcohols with good to high
k
rel
values. An allylic
alcohol and an aliphatic alcohol also undergo resolution albeit with moderate
k
rel
values.
−
Alkoxide formation
X
HX
X
X
R
R
L
n
Pd
II
L
n
Pd
II
O
O
Alcohol binding
-Hydride elimination
β
H
R'
R'
OH
O
R
R'
R
R'
Alcohol oxidation process
L
n
Pd
II
X
H
L
n
Pd
II
X
2
Catalytic cycle
Catalyst regeneration process
O
2
H
2
O
2
HX
HX
L
n
Pd
II
X
OOH
Regeneration
L
n
Pd
0
L
n
Pd
II
O
O
O
2
HX
Scheme 11.2.
