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OMe
PCy
2
i
-Pr
MeO
PCy
2
PCy
2
O
i
-Pr
i
-Pr
P
i
-Pr
i
-Pr
i
-PrO
i
-Pr
i
-Pr
t
-Bu
3
P
XPhos
BrettPhos
RuPhos
P(
t
-Bu
2
)
F
e
Ph
Ph
Ph
P
O
PPh
2
Ph
PPh
2
Ph
Ad
2
(
n
-Bu)P
Xantphos
Q-Phos
Figure 3.2
Selected electron-rich bulky ligands used in palladium-catalyzed coup-
ling reactions.
in situ-formed catalysts were not anticipated in the early days but were
identified during the last 5-10 years. Using the right catalyst at the optimum
loading, today one can significantly improve the overall eciency of a pro-
cess by reducing environmental waste, process time, and temperature, and
thus, take advantage of the process economics.
However, it could be possible to carry out some of the large-scale processes
successfully using the in situ-generated catalyst either using simple ligands
(e.g.,Ph
3
P) or by skillfully transferring the process from laboratory to pro-
duction with precise engineering control. Sometimes, the reaction solvent
used for the coupling reaction may not be suitable to preform the catalyst
precisely while prestirring the Pd precursor with a ligand under the coupling
conditions, thus leading to decreased catalytic e
ciency. The term ''pre-
catalyst'', however, refers to the isolated and purified L
n
PdX complex, pre-
pared by a separate process.
3.2 The 18-Electron Rule Versus Catalytic Activity and
Stability
The stability and reactivity of transition metal complexes can be predicted to
a certain extent using the ''18-electron rule''.
16,17
The valence shells of
transition metals consist of nine valence orbitals, which can accommodate
18 electrons. Combining the nine atomic orbitals with the ligand orbitals
creates nine molecular orbitals. These orbitals can be either metal-ligand
bonding or non-bonding. When the electron count of a metal complex adds
up to 18 electrons, it is said to have reached the same electronic configur-
ation as the noble gas in the same period, hence it is assumed to be a stable
complex. The 18-electron complexes are viewed as ''exchange inert'' and
ligand dissociation has to occur prior to any other reactions at the metal
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