Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 56

Ph Cl

Cl
NMe 2

PPh 2
Me 2 N

Ph P Pd

(PhCN) 2 PdCl 2

MAP

Pd(MAP)Cl 2

Figure 2.42 Synthesis of Pd(MAP)Cl 2 showing the Pd-ipso-C interaction.

R 1

Cl

R 1

R 1

, Mg MgX (or Li)

Br

MgX (or Li) CuCl

ClP(R 4 ) 2

R 3 R 2

THF

P(R 4 ) 2

via

R 3 R 2

R 3 R 2

Figure 2.43 The improved ''benzyne route'' to substituted biarylphosphines.

R 3 = H
- Fixes conformation of R 2 P over bottom ring
- Enhances rate of reductive elimination

R = Bulky alkyl groups
- Enhance electron density increasing

the rate of oxidative addition

Larger size of R
- Enhances the rate of reductive
elimination
- Increases L 1 P(0)

R 3

P R 2

R 1 R 1

Bottom Aryl Ring
- Increases size of ligand
- Retards rate of oxidation by O 2
- Allows stabilizing Pd-arene interactions
- Enhances rate of reductive elimination

R 1 = H
- Prevents C − H activation of ligand
- Increases L 1 Pd(0)

R 2

Figure 2.44 Structural features of biarylphosphines.

Adapted from Ref. 1m.

with L 1 Pd(0) than L 2 Pd(0) for steric reasons. 160 The large size of these lig-
ands shifts the equilibrium in favor of the active L 1 Pd species over L 2 Pd, thus
increasing the rate of oxidative addition. The rate of oxidative addition is
further increased by the presence of electron-donating alkyl groups on
phosphorus. The rate of transmetallation is presumed to be similarly faster
with monoligated palladium species for steric reasons, 158f and reductive
elimination has also previously been shown to be faster from L 1 Pd species
than from the corresponding L 2 complexes. 161 The bulky nature of the alkyl
groups on phosphorus further enhances the rate of reductive elimination.
Theoretical studies have identified catalytic intermediates which feature