Advances in C–C and C–X Coupling Using Palladium–NHeterocyclic Carbene (Pd–NHC) Complexes - New Trends in Cross-Coupling: Theory and Applications - page 175

Table 4.15 Reactivity of 45 in the microwave-promoted Hiyama reaction.

HO

Pd(OAc) 2 (0.1mol%)
45 (0.2 or 0.5 mol%)

X Si (OMe) 3

R

Ph

NN Ph

R +

NaOH aq ,1h
Microwave (80W, 120°C)
TBAB or not

Cl

X=Br,Cl

45

Entry X Product 45 (mol%) Additive Yield (%)

1 Br 0.2 - 95

MeO

2 Cl 0.2 TBAB 56 MeO

3 Br 0.2 - 86

4 Br 0.2 - 67

HOOC

5 Br 0.2 - 48

N

6 Br 0.2 TBAB 79

N

S

7 Br 0.2 TBAB 17

S

8 Br 0.5 TBAB 52

9 Cl 0.5 TBAB 53

N

10 Cl 0.5 TBAB 66

N

Notably, the deactivated 4-chloroanisole was successfully coupled with
phenyltrimethoxysilane in this case. Later, in 2013, the same group studied
the optimal ratio between Pd and NHC to promote the reaction. 105 A ratio of
1:5 was found to be beneficial in a number of examples.
In 2012, Lu and co-workers reported the use of [Pd(IPr)(Im)Cl 2 ](16a)in
Hiyama coupling. 106 Reaction was promoted by TBAF 3H 2 O using 1 mol%
of the Pd precatalyst at 120 1C in toluene. Mainly activated aryl chlorides
were coupled with arylsiloxanes with good overall yields (Scheme 4.32).
Yang and Wang prepared a series of dinuclear Pd-NHC precatalysts of
formula [[Pd(NHC)Cl 2 ] 2 (m-L)xCH 2 Cl 2 ] for Hiyama cross-coupling. 107 In this
formula, L is a linker being pyrazine or DABCO (Figure 4.21). The new sys-
tems were more active than the in situ systems described by Lee and Nolan 101