Advances in C–C and C–X Coupling Using Palladium–NHeterocyclic Carbene (Pd–NHC) Complexes - New Trends in Cross-Coupling: Theory and Applications - page 192

R R

NN

R R

tBu

[Pd(IPr**)(pyridinyl)Cl 2 ]( 20 )
(3 mol%)

Ph Cl Pd Cl

I

+ Ph

N

Cs 2 CO 3
DMF : H 2 O (3:1)
90°C, 1 h

R=

H

27%

[Pd(IPr**)(pyridinyl)Cl 2 ]

tBu

20

Scheme 4.42 Alkynylation reaction using the very bulky IPr** ligand.

Ph

NN

N

NN

N

R

Pd Cl R

I Pd I

Br Pd Br

N

67a:R=CH 2 Ph
67b :R= i Pr
67c:R=CH 2 CH 2 OH
67d :R=C 4 H 9
67e:R=allyl

68a:R=F
68b :R=Cl
68c:R=Br
68d :R=Me

N

N

Fe

N

Cl

666

Figure 4.31 Structures of some precatalysts active in alkynylation reactions.

R'

X

67e (1 mol% ) 1) Solvent r emo val

R

R + H R' R' R

Ar

K 3 PO 4
DMSO, 100°C, 1 h

2) A rH
TF A, 50 °C , 3 h

X=Br,I

Ph

F Ph

F 3 C

F

X=I:89%
X=Br:65%

X = Br: 6 9% X=Br:87% X=Br:76% X=Br:73%

Scheme 4.43 Tandem Sonogashira-hydroarylation reaction using 67e.

reaction (Scheme 4.43). 140 Finally, Ghosh and co-workers recently described
the synthesis of a series of abnormal PEPPSI-type precatalysts for copper- and
amine-free coupling under air (68,Figure4.31). 141 However, only iodides and
activated bromides could be used as coupling partners (Table 4.23).

4.3.4 Direct Arylation
Direct arylation has recently attracted a lot of attention for the construction
of (het)aryl-(het)aryl bonds. 142 Selectivity issues can be encountered, but the