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2R
3
P
(PR
3
)
2
Pd + C
8
H
10
Pd
Otsuka 1990
2R
3
P
Pd(dba)
2
(PR
3
)
2
Pd + 2 dba
Hartwig 1995
2NaOH
MeOH-Toluene
2R
3
P
0-75
o
C
(PR
3
)
2
Pd + 2 NaBr
PdBr
2
Colacot 2010
Scheme 3.8 Various synthesis routes for L
2
Pd(0).
Until recently, (t-Bu
3
P)
2
Pd was the only commercially available L
2
Pd(0)
catalyst. There were initially two known methods for the preparation of this
catalyst. The first employed the highly volatile and unstable precursor Pd(Z
3
-
C
3
H
5
)(Z
5
-C
5
H
5
) (Scheme 3.8).
45
Later, a cinnamyl derivative, Pd(Z
3
-1-
PhC
3
H
4
)(Z
5
-C
5
H
5
), was reported as a precursor to generate the L
2
Pd(0)
in situ,
46,47
although it has not been demonstrated in a synthesis to isolate
the precatalyst. However, the reported synthesis of Pd(Z
3
-1-PhC
3
H
4
)(Z
5
-
C
5
H
5
) requires extreme cryogenic conditions.
The second method of preparation of (t-Bu
3
P)
2
Pd used the readily avail-
able Pd(dba)
2
as precursor (Scheme 3.8); however, the procedure involved
the recrystallization of the final compound with a large amount of solvent
under cryogenic conditions in order to remove the black color and dba from
the product.
44,48
Both of these methods seem to not be very practical and are therefore not
suited for a facile scale-up to multi-kilogram quantities.
It was not until 15 years later that a general and novel route was developed
by Johnson Matthey to make a series of L
2
Pd(0) catalysts.
22
Starting from the
readily available, inexpensive, air-stable precursor (COD)PdBr
2
, a stoichio-
metric amount of phosphine ligand was used in the presence of a Brønsted
base (e.g., alkali metal hydroxides) in a protic solvent to generate the desired
precatalysts. Employing this method, a number of L
2
Pd(0) catalysts could be
prepared in near quantitative yields on a large scale (Scheme 3.8).
Of special interest was the use of sterically bulky, electron-rich phosphines
such as t-Bu
3
P, Cy
3
P, (o-tol)
3
P, t-Bu
2
PhP, p-Me
2
NC
6
H
4
(t-Bu)
2
P and
t-Bu
2
(C
5
H
4
FeC
5
Ph
5
)P (Q-Phos), but the method was not limited to these
ligands alone (Figure 3.10).
The mechanism for the formation of the L
2
Pd(0) complexes was estab-
lished based on Scheme 3.9, where the intermediates 3 and 4 were isolated
and characterized by various methods including X-ray crystallography.
Generally, the L
2
Pd(0) (L
¼
tert-alkylphosphine) catalysts show higher ac-
tivities at elevated temperatures, to form presumably LPd(0) as the cataly-
tically active species.
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