Chemistry Reference
In-Depth Information
O
O
O
n
-Bu
Pd(OAc)
2
, Et
3
N
Cl
O
n
-Bu
O
50 °C
O
5
5
5
.
2
Scheme 5.14
O
CO
2
Et
Pd(OAc)
2
, R
3
N
CO
2
Et
Cl
-CO
Br
Br
5.54
5.53
CN
(Ar
3
P)
2
Pd(OAc)
2
,
R
3
N
CO
2
Et
NC
5.55
Scheme 5.15
X
+ Ag
RPdL
RPdL
L
+ AgX
L
5.4
5.56
Scheme 5.16
5.1.3 Catalysts, Ligands and Reagents
Heck reactions are often carried out using palladium(0) complexes. Tetrakis(triphenylphosphine)palladium(0)
is frequently used, but does not allow the chemist to vary either the identity of the ligand, or the lig-
and:palladium ratio. A more convenient mixture is to use the air-stable palladium(0) dibenzylideneacetone
complex with the added ligands of choice. It is, however, not necessary to use a palladium(0) pre-catalyst.
Palladium(II) salts, especially the more soluble palladium(II) acetate, are often used, with added phosphines.
The palladium(II) salts are reduced to palladium(0)
in situ
.
For organic halide substrates, monodentate phosphines are usually preferred because bidentate phosphines
do not allow the partial dissociation step to generate the active site on palladium. However, in the presence
of a silver salt, bidentate phosphines can be used, as the ligand lost in step 2 of the mechanism will be the
halide (Scheme 5.16). Triflates may dissociate without such assistance (see Scheme 5.9), as this anion is a
poor ligand for palladium, also allowing the use of bidentate phosphines.
The most commonly used ligand is PPh
3
. The ratio of ligand to palladium is of great importance. Too much
ligand will inhibit the reaction (due to Le Chatelier's principle). If too little ligand is present, the catalyst may
decompose to give a palladium mirror. As iodide (if an aryl iodide is employed) and coordinating solvents
such as acetonitrile, are used, then the amount of phosphine can be reduced even to zero.
Several catalysts have been developed that allow efficient Heck reactions of reluctant substrates under
reasonable conditions and with high turnover numbers. One such catalyst is Herrmann's catalyst
5.58
,
16
a
palladacycle derived from (
o
-tol)
3
P
5.57
(Schemes 5.17 and 5.18). Palladium complexes with
t
-Bu
3
Palso
provide highly active catalysts (Scheme 5.19).
17
