Chemistry Reference
In-Depth Information
other systems, b-hydride elimination proceeds via an associative pathway
where the ligand/counterion is partially coordinated to Pd(
II
), in other words,
a five-coordinate Pd species transition state.
A number of studies have investigated the mechanism by which the
catalyst is regenerated (D-A in Figure 4.18). Under the reaction conditions,
re-oxidation is of Pd(0) is generally not found to be rate determining. First,
this demonstrates the significant influence of the ligands, because as we
mentioned at the beginning, re-oxidation of Pd(0) species in the absence of
ligands is sluggish and aggregation of Pd(0) to Pd black dominates. In the
case of ligand-modulated systems, it has been found that unless the system
is mass transfer limited in O
2
, then the rate of alcohol oxidation is in-
dependent of O
2
pressure.
154,167
Due to facile re-oxidation under the reaction
conditions, determining the exact mechanism for this part of the catalytic
cycle is not straightforward. There are a number of possibilities, but arguably
the two most likely pathways proposed have been reductive elimination to
form Pd(0) (E) followed by oxidation with O
2
or direct hydrogen atom ab-
straction by molecular oxygen which bypasses the Pd(0) state (i.e. the path-
way involving G). There have been a number of studies which show that for
some systems a hydrogen atom abstraction pathway is possible,
168
but there
is now considerable evidence (both experimental and computational) to
support the theory that the mechanism proceeds via a reductive elimination
pathway and a Pd(0) state.
169
The Pd(0) intermediate reacts with O
2
to form
an Z
2
-peroxopalladium(
II
) complex (i.e. such as shown in F in Figure 4.18).
Stahl et al. managed to isolate such a complex from the direct reaction of a
Pd(0) complex, Pd(bathocuproine)(dba), with O
2
.
170
They also demonstrated
such peroxo species when NHC-Pd(0) complexes react with O
2
.
171
In this
study they further demonstrated that the addition of acetic acid resulted in
the formation of the hydroperoxopalladium(
II
) complex (i.e. such as shown
in H in Figure 4.18). Indeed, the role of acids and bases is crucial in this
catalytic cycle. It can be seen that at different stages of the catalytic cycle,
both Brønsted acid and Brønsted base are involved in key steps of the
catalytic cycle. As is probably evident from the examples we have given,
the use of a carboxylate salt such as acetate is common practice. Even in the
pyridine system, it is thought that acetate from Pd(OAc)
2
is responsible for
the generation of the alkoxide.
172
Although acetate is not strong enough to
deprotonate free alcohols, once the alcohol is coordinated to the metal
centre (B) it is activated and can be deprotonated by acetate. It can be seen in
Figure 4.18 that a Brønsted acid is required to complete the re-oxidation of
the catalyst. DFT studies on the oxygenation of an NHC-Pd
0
complex re-
vealed that the formation of the peroxopalladium species is reversible, which
would result in the re-formation of the Pd(0) species and further contribute
to catalyst deactivation by aggregation.
173
Consequently, it is important that
acid is present in order to protonate the peroxo species and lead to re-
generation of the catalyst. This was touched on earlier when we discussed
Waymouth and co-workers' studies with cationic complexes,
155
and it ap-
pears that the N,O-ligands with acidic groups are beneficial.
165,166
d
n
4
r
4
n
g
|
2
.
The
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