Pd–Phosphine Precatalysts for Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 97

L 2 Pd (0 )

L 4 Pd(0)

N H 2
X

Pd

L

L n Pd(0)
n =1or2

L 2 PdX 2

X
X

L Pd Pd L

L

Pd
X

Figure 3.6 Generation of L n Pd(0) from precatalysts.

The proposed mechanism for the formation of the catalytically active
species is discussed in detail below for each individual class of precatalyst
type: L 4 Pd(0), L 2 Pd(0), L 2 PdX 2 , LPd(allyl)X, [LPdX] 2 and LPd(palladacycle)X.

3.4 L 4 Pd(0)
The widely used preformed Pd(0) catalyst (Ph 3 P) 4 Pd, nicknamed ''palladium
tetrakis'', is an air- and somewhat moisture-sensitive crystalline material. It
is easy to tell from the appearance of the complex whether or not de-
composition has started. If the material is exposed to the air, the color
changes from bright yellow to dark yellow and finally to orange-brown
(Figure 3.7). The catalyst is, however, produced in multi-kilogram quantities
with a reasonably good shelf-life when kept under inert conditions.
The preparation of (Ph 3 P) 4 Pd was originally reported as involving the use
of NH 2 NH 2 as a reducing reagent (Scheme 3.1). 21 This method requires
harsh reaction conditions such as high temperature (140 1C) in DMSO. In
addition, there are toxicity concerns regarding the use of hydrazine as a
reducing reagent in the process.
More recently, alternative patented methods were developed to make
(Ph 3 P) 4 Pd in multi-kilogram quantities under milder conditions without the
use of hydrazine. 22
There are a number of early examples where the preformed Pd(0) complex
(Ph 3 P) 4 Pd 23,24 has been used as catalyst with excellent results. The solution-
phase dissociation of (Ph 3 P) 4 Pd is shown in Scheme 3.2, where the resulting
(Ph 3 P) 3 Pd is in rapid equilibrium with (Ph 3 P) 2 Pd. 25
In the Heck reaction, an increase in reaction rate was observed on add-
ition of anions such as chloride in the form of TBAC (tetrabutylammonium
chloride), which was explained by the formation of a tricoordinate anionic
species [(Ph 3 P) 2 PdX] (16 electrons), which was proposed to be more reactive
than the neutral (Ph 3 P) 2 Pd species. 26 This theory was questioned more re-
cently, based on the evidence that dicoordinate species have been found
to be lower in energy than the tricoordinate species. 27 The most likely an-
ionic species present would be the dicoordinate 14-electron complex