Advances in C–C and C–X Coupling Using Palladium–NHeterocyclic Carbene (Pd–NHC) Complexes - New Trends in Cross-Coupling: Theory and Applications - page 149

[Pd(IPr*)(cin)Cl] ( 9 )
(1 mol% )
KOH, DME
RT - 65°C

R'

R R

R'

X + B(OH) 2

Ph
Ph

Ph
Ph

X=Cl,Br

O

O

NN

Ph

Ph Ph Ph

O

Pd

Cl

X=Cl:87% X=Br:95% X=Cl:93%

X=Br:99%

Ph

[Pd(IPr*)(cin)Cl]
9

N
O

N
N

X=Br:75% X = Br : 67% X = Br : 87%

Scheme 4.5 Preparation of tetra-ortho-substituted biaryls using [Pd(IPr*)(cin)Cl] (9).

NN

Cl Pd Cl

N

Cl

10

Figure 4.7 Structure of precatalyst 10.

from the NHC ligand. They concluded that flexible steric bulk was not the
only important factor in promoting the coupling and that electronic prop-
erties also played a crucial role.

4.3.1.1.2 Formation of sp 3 -sp 3 or sp 3 -sp 2 Bonds. Suzuki-Miyaura cross-
coupling is a very powerful reaction. Nevertheless, most of the bonds that
can be constructed by this methodology are sp 2 -sp 2 . The development of
precatalysts enabling the formation of sp 3 -sp 3 or sp 3 -sp 2 bonds is there-
fore highly important. 38 The main issue associated with such reactions is
that of b-hydrogen elimination. 39 The first report on the formation of an
sp 3 -sp 3 bond using an NHC system was reported by Caddick and co-work-
ers in 2004. 40 The system was comprised of Pd(dba) 2 (4 mol%) and
IPr HCl (8 mol%) in the presence of KOtBu and AgOTf (4 mol%) in THF
at 40 1C. The coupling of various alkyl bromides and alkyl-9-BBN reagents