Chemistry Reference
In-Depth Information
However, N-heterocyclic carbenes (NHCs) are nowadays honourable com-
petitors, due to an interesting range of properties including reactivity, sta-
bility and tunability.
8
In this chapter, we focus on the advances made in C-C and C-X coupling
using palladium-N-heterocyclic carbene (Pd-NHC) complexes, especially
since the 2010 Nobel Prize award. After touching upon the fundamental
chemistry behind NHCs and Pd-NHC complexes, we survey the use of these
fascinating systems in cross-coupling chemistry.
9
The content of each topic
varies depending on how much knowledge is already available in the lit-
erature. Readers who require more information can consult the cited
references.
4.2 N-Heterocyclic Carbenes and Pd-NHC
Complexes
4.2.1 The Birth of a New Class of Ligands
Until recently, NHCs were long considered as laboratory curiosities
10
or
phosphine mimics
11
and many years of studies were necessary to realise their
real potential as an indisputable powerful class of ligands. The first metal
complexes bearing NHC ligands were reported separately by
¨
fele
12
and
Wanzlick and Schoenherr
13
in 1968.
¨
fele prepared the pentacarbonyl(1,3-
dihydro-1,3-dimethyl-2H-imidazol-2-ylidene)-chromium complex 1 (Figure 4.1)
by sublimation of the chromium salt [HIMe][CrH(CO)
5
]. Wanzlick and
Sch
¨
nherr synthesized bis(1,3-diphenylimidazolo)mercury diperchlorate (2)
from the carbene salt and [Hg(OAc)
2
] in DMSO (Figure 4.1).
The seminal breakthrough occurred in 1991, when Arduengo et al. re-
ported the preparation of the first isolable free NHC, IAd (Figure 4.2).
14
The
discovery that carbenes could be isolated and handled under an inert at-
mosphere consequently gave rise to considerable interest in their coordin-
ation chemistry. Since then, a wide range of NHCs have been prepared and
their complexes with nearly all transition metals have been synthesized.
8c
Over the last few decades, NHCs have been modified by varying their skeletal
structure and also the nature of the groups attached to it. Despite the fact
that many NHCs have been prepared throughout the years, the most ecient
and utilized congeners are the five-membered ring imidazolylidenes and
imidazolinylidenes illustrated in Figure 4.2.
2+
Ph
Ph
N
N
N
N
[
ClO
4
]
2
-
H
g
C
r
(CO)
5
N
Ph
N
Ph
1
2
Figure 4.1 First metal-NHC complexes reported by
¨
fele and Wanzlick and
Schoenherr.
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