Pd–Phosphine Precatalysts for Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 117

RO

NHC

Pd

Cl

( A )

Pd NHC

Ph

Cl + Ph OR

Cl

+ RR

Cl

N

N
Pd

Cl

2R-M

R N

RNHC

( B )

Pd NHC

+2M-Cl

Cl Cl

NHC

Pd

Ar

N N Ar

NHC =

Scheme 3.29 Generation of NHC-Pd(0) from cinnamyl- (A) and PEPPSI (B)

precatalysts.

Pd(I) dimer, Pd(II)(R-allyl)Cl and Pd(II) palladacycles. Although the focus of
this chapter is on phosphine-containing precatalysts, the NHC (N-hetero-
cyclic carbene)-based catalysts such as the palladium cinnamyl complexes
developed by Nolan and co-workers 97-99 and the PEPPSI complexes intro-
duced by Organ and co-workers 100,101 are noteworthy. Both the cinnamyl and
the PEPPSI type of precatalyst also generate the proposed catalytically active
species LPd(0). The proposed activation pathways are shown in Scheme 3.29.
In the case of the cinnamyl complexes, an added base such as KOt-Am (tert-
amylate) or KOH is suggested to activate the complex by nucleophilic attack
on the cinnamyl moiety (A). The PEPPSI complexes are activated by initial
transmetallation with an organometallic reagent, followed by reductive
elimination of R-R (B). The pyridine ligand is a labile ligand that easily
dissociates to generate LPd(0).
However the phosphine-containing Pd(I) dimer, Pd(II)(R-allyl)Cl and Pd(II)
palladacycles catalysts all activate by different pathways. This is discussed in
more detail in the following sections.

3.7.1 [(t-Bu 3 P)Pd(l-Br)] 2
The first synthesis of the palladium(I) dimer [(t-Bu 3 P)Pd(m-Br)] 2 that was
demonstrated by Mingos and co-workers gave a very poor yield, 102 although
some improvements were subsequently made by the same group
(Scheme 3.30). 103 Colacot et al. recently developed a high-yielding patented
route to the dimer, and it is now a readily commercially available

precatalyst. 104
Initially, this complex was only investigated in reactions with a selected
number of reagents, namely CO, H 2 , nitriles, alkenes and alkynes. 103 The
complex is air sensitive and it has been shown to react with aerial oxygen to
form a second dimeric complex by insertion of O 2 into the Pd(I)-Pd(I) dimer
and subsequent C-O bond formation (Scheme 3.31). 105