Chemistry Reference
In-Depth Information
O
O
Ar
Ar
R
1
R
1
X
CO
R
1
[M]
ArCHCH
2
O
Ar
R
1
Figure 10.1 Carbonylative coupling reactions.
materials. In addition, they also involve costs and waste generation that
could be avoided if the reactions could be conducted on the original arene or
heteroarene without exchange of H for halide. This goal of reactions using
C-H bonds has been investigated in many areas of organic chemistry in
recent years and much progress has been made.
2
The adaptation of such
reactions to carbonylation processes in conjunction with CO was perceived
as dicult owing to the potential of CO to reduce Pd(II) to Pd(0), but steady
progress has been made by many contributors over a number of years such
that suitable protocols now exist for these oxidative carbonylation reactions
for a variety of substrates. A selection of additives have been used to reox-
idize Pd(0) to Pd(II), a step that is normally achieved in conventional
coupling reactions by oxidative addition with aryl halide. These reactions
can be classified by the hybridization of the activated bond (sp, sp
2
and sp
3
)
and the supporting functionality of the substrate that contributes to dir-
ecting the reaction and thus providing specificity. In the following, progress
in this area is reviewed on this basis, illustrating the potential for further
developments of this chemistry.
3
Extending carbonylation from the aryl halide substrates described above
to less reactive C-H compounds represents a significant challenge owing to
binding of CO to the Pd(II) centre in competition with the weak interaction
with C-H. Also, p-backbonding to a carbonyl ligand may reduce the
nucleophilicity of the Pd centre for interaction with C-H, in addition to the
aforementioned potential of CO to promote the reduction of Pd(II) to Pd(0)
leading to catalyst decomposition. In order to maximize the eciency of the
transformation, many methods have been developed using a variety of
additives. Also, the choice of oxidant to return Pd(0) to Pd(II) is complex,
resulting in a number of different choices from different research groups.
Ultimately, it would be advantageous to use air (oxygen) as the terminal
oxidant, and attempts to achieve this continue (Figure 10.2).
Search WWH ::
Custom Search