Introduction to New Trends in Cross-Coupling - New Trends in Cross-Coupling: Theory and Applications - page 7

σ∗ σ∗

σ σ

E

π π

σ σ

L

X

L X

Pd

Pd

L

L

VB of L 2 Pd and Ar-X VB of concerted transition state of

oxidative addition

Figure 1.5 Valence bond (VB) representations for the two components L 2 Pd and Ar-X
and for a concerted transition state of the oxidative addition process.
Reproduced from Ref. 14.

[1,1 0 -bis(diphenylphosphino)ferrocene] under cationic conditions 8 and dnpf
[1,1 0 -bis(dinaphthylphosphino)ferrocene] in presence of a polar solvent and
TBAC (tetrabutylammonium chloride) additive 9 produce branched products
for electron-rich and electron-neutral olefins.
Since the original discoveries of cross-coupling reactions, there has been a
great deal of effort in this area to better understand the reaction mechanism,
where the role of the ligand is important. The electronic and steric nature of
the ligand (L) and the coordination number of Pd can significantly influence
two important steps of the cycle; oxidative addition and reductive elimin-
ation (Figure 1.5). 10 The role of ligands in the transmetallation step is not as
well understood; however, the groups of Hartwig, Amatore and Lloyd-Jones
have carried out some impressive work in the area of Suzuki-Miyaura
coupling. 11 The groups of Beletskaya 12 and Buchwald 13 have shown that
more electron-deficient ligands can increase the rate of C-N cross-coupling
reactions involving ureas and amides, respectively, likely reflecting an
increased rate of ''transmetallation'' (amide binding). 13 Oxidative addition
was considered to be the rate-limiting step, where the choice of the ligand is
important. For example, it is proposed that electron-rich ligands make the Pd
basic enough to do the oxidative addition of challenging aryl chlorides, while
with aryl iodides and bromides oxidative addition is relatively facile, even
with less electron-rich ligands such as Ph 3 P. Figure 1.5 shows the valence
bond (VB) representations for the two components L 2 Pd and Ar-X and for a
concerted, three-centered transition state of the oxidative addition process. 14
The energy (DG) required to excite one electron into the antibonding (s*)
orbital of the Ar-X bond decreases in the series Ar-Cl 4Ar-Br 4Ar-I.
The low reactivity of more challenging substrates such as unactivated
aryl chlorides was often attributed to the large bond dissociation energy of
the C-Cl bond (95 kcal mol 1 ) in comparison with Ar-Br (79 kcal mol 1 )or