Chemistry Reference
In-Depth Information
R
R
R
NN
R
R
P
[M]
[M]
Coordination
sphere
Coordination
sphere
Figure 4.4
Steric environment of NHCs versus phosphines.
the steric effect of NHCs. Another tool, developed by Cavallo and co-workers,
which allows the assessment of the bulk of an NHC ligand, is the percent
buried volume (%V
bur
).
25
The latter can be calculated from crystal structure
data using the software SambVca.
26
Recently, Cavallo and co-workers ex-
panded the utility of this software by developing the ''steric maps.''
24,27
This
helps in visualizing the steric environment of an NHC ligand around the
metal centre.
4.3 Pd-NHC Complexes in Catalysis
A cross-coupling reaction consists of joining two organic fragments together
through the formation of a new bond, using an organometallic catalyst. For
decades, palladium has proven to be the metal of choice for such transfor-
mations.
1a
Historically, phosphines have been the dominant choice of lig-
ands in palladium-catalysed cross-coupling reactions.
7
However, NHCs are
nowadays ecient challengers, owing to the specific properties of the Pd-
NHC bond.
8,9
The purpose of this chapter is to discuss the main recent
advances in C-C and C-X coupling using Pd-NHC complexes, especially
since the awarding of the 2010 Nobel Prize in Chemistry. Each family of
cross-coupling is treated separately and the content of each topic is outlined
in the corresponding introductions.
4.3.1 Reactions Through Transmetallation Mechanisms
Most cross-coupling reactions require a transmetallating agent, because the
use of such reagents very often renders the coupling more ecient and also
more selective. The accepted reaction mechanism for these transformations
is very similar
9d
and is depicted in Scheme 4.3.
First, the palladium precatalyst is reduced to its active Pd(0) form, then the
oxidative addition of the organic halide (X = Cl, Br, I) or pseudohalide (X =
OTs, OTf, for example) takes place. In the latter, the diculty of the coupling
increases in the order R-I
o
R-Br
o
R-Cl, which is due to the increasing R-
X bond dissociation enthalpies.
28
Because organochlorides are more abun-
dant and generally cheaper than organobromides and -iodides, they are the
most desirable substrates to use. However, they are also the most dicult to
activate, which is why current efforts in cross-coupling chemistry are dedi-
cated to improving their reactivity. The second part of the catalytic cycle is
the transmetallation step. This is where the second organic fragment is
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