Chemistry Reference
In-Depth Information
also the lifetime of such hydrides, which often underlies the regioselectivity
issues - whether the double bond shifts take place or not.
In fact, substantial insight into the functioning of such systems can be
gained from the catalytic oxidation of alcohols by Pd(II) complexes, actively
investigated because of its importance for the development of fuel cells and
in relation to biocatalysis.
PdH can directly bind O
2
to give hydroperoxide complexes Pd(OOH), thus
completely avoiding the Pd(0) state and the risk of deactivation of catalyst
due to the formation of Pd black.. Such complexes can take part in ligand
exchange to liberate hydrogen peroxide, which is rapidly decomposed.
305
The process was explicitly observed by Goldberg and co-workers using a
pincer PCP-Pd complex.
306,307
Apart from hydroperoxide, a hydroxyl com-
plex was observed, which is likely to be formed from facile decomposition of
the former (Scheme 9.101).
Similar observations were made by Stahl and co-workers using Pd complex
with electron-rich bulky monodentate IMes ligands.
308-310
In this case,
richer chemistry is involved and two pathways are competing: the Pd(0)
complex can either bind dioxygen to form an Z
2
-complex split by acid to
form hydroperoxide or it can undergo oxidative addition to acid to form
hydride, which reacts with dioxygen to form hydroperoxide (Scheme 9.102).
A few conclusions relevant in the context of oxidative regeneration of
Pd(II) species for oxidative Heck reactions can be drawn:
1. The PdH lifetime depends on ancillary ligands - electron-rich ligands
enhance the stability and such complexes may even reverse the re-
ductive deprotonation of PdH. Therefore, the reactions in the presence
of Pd complexes with such ligands should be and usually are air
P
t
Bu
2
P
t
Bu
2
P
t
Bu
2
O
2
PhH, rt
Pd
H
Pd
OOH
+
Pd
OH
P
t
Bu
2
P
t
Bu
2
P
t
Bu
2
Scheme 9.101 Binding of dioxygen by Pd hydride complex.
L
O
Pd
HOZ
O
O
2
L
L
L
Pd
ZO
Pd
OOH
+HOZ
L
L
L
O
2
-HOZ
ZO
Pd
H
Z=Ac,Bz
L = IMes
L
Scheme 9.102 Convergent pathways for scavenging of Pd(0) and PdH intermediates
by dioxygen.
Search WWH ::
Custom Search