Pd–Phosphine Precatalysts for Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 102

1) Dissociative pathway
L
Pd
L

Ar
Pd
L

(ArX)
Pd
L

-L

+ Ar-X

Pd
L

X

oxidative
addition

loss of ligand

2) Substrate-assisted ligand displacement pathway
L
Pd
L

(ArX)
Pd
L

Ar
Pd
L

+A r-X

-L

X

oxidative
addition

loss of ligand

Scheme 3.7 Dissociation of L 2 Pd(0) to LPd(0).

0 h 2 h 24 h 72 h

Figure 3.9 Decomposition of commercially available (t-Bu 3 P) 2 Pd in air.

precatalyst activation pathway may have a profound influence on the out-
come of the reaction and rate. However, more experimental studies are re-
quired to understand the ''active catalytic'' species in the cycle. For example,
the bulky (o-tol) 3 P is not a good ligand for Ar-Cl coupling, suggesting that
the electronic properties play an equally important role.

3.5.2 Synthesis, Properties and Applications of L 2 Pd(0)
Complexes
The first examples of an isolated L 2 Pd(0) catalyst to be used in cross-coupling
reactions were reported by Dai and Fu, who employed (t-Bu 3 P) 2 Pd in Negishi
couplings. 44
The new-generation Pd(0) catalyst (t-Bu 3 P) 2 Pd was introduced as a white,
crystalline, air-stable solid by Dai and Fu; 44 however, it was later found to
decompose within 1-2 days when kept under an aerobic atmosphere (see
Figure 3.9). It is believed to be more stable in pure crystalline form than the
widely used (Ph 3 P) 4 Pd. This white solid can be stored intact for longer per-
iods under an inert atmosphere. In certain reactions, significant decreases
in activities with side reactions or no activities are observed when the
orange, grey, or black materials are used.