Chemistry Reference
In-Depth Information
First generation of bulky NHCs:
Second generation of bulky NHCs:
Ph
Ph
Ph
Ph
NN
NN
NN
IPr
IMes
Ph
Ph
Ph
Ph
IP r*
NN
NN
SIPr
SIMes
NN
O
O
NN
NN
IPen t
n
n
IAd
IBox
5+n
Smal l NHCs:
NN
NN
NN
NN
NN
I
i
Pr
IMe
ITM
(S)I
t
Bu
(S) ICy
Figure 4.2 The most commonly used NHCs.
4.2.2 Routes to Free Carbenes and to Pd-NHC Complexes
There are several routes for the preparation of free carbenes. The most
common approach is via the deprotonation of the corresponding NHC salt
(Scheme 4.1).
15
However, they can also be synthesized by reduction of the
thione moiety with potassium or through the thermal decomposition of a
variety of adducts, including chloroform,
16
alcohols
17
and carbon dioxide.
18
Herrmann and co-workers were the first to report the use of a Pd-NHC
complex in catalysis. Complexes of the formula [PdI
2
(NHC)
2
] were success-
fully used in catalysis, notably in Heck coupling.
19
The strength of the Pd-
NHC bond was found to be particularly advantageous in these reactions and
the catalysts remained active even at high temperatures. Pd-NHC complexes
are generally prepared using three main routes (Scheme 4.2).
9d
First is the
substitution of a labile ligand using the free carbene [eqn (4.1)]. An inert
atmosphere is required in this case as the free carbene is both air and
moisture sensitive. The labile ligands can be, for example, dibenzylidenea-
cetone (dba), cyclooctadiene (COD), a phosphine or a bridging halide. The
advantages of this method are notably low temperatures and short reaction
times. The second method is the in situ generation of a free carbene starting
from the NHC salt. The ambient atmosphere can often be used in this case
as the NHC salt is typically air and moisture stable. The base used to
deprotonate the NHC salt can either be external [KOtBu for example, eqn
(4.2)] or linked to the palladium source [AcO in Pd(OAc)
2
for example, eqn
(4.3)]. Finally, transmetallation from Ag complexes can be applied [eqn (4.4)].
[Ag(NHC)X] can be formed from the NHC salt using routes (4.2) or (4.3) and
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