Prominent Ligand Types in Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 30

2.315 Å

P

M

Natural Bite Angle

P

Figure 2.7 Natural bite angle.

R

R

P

M

Θ

P

R

R

Figure 2.8 Definition of pocket angle.

defined as the lowest energy P-M-P angle of the metal complex when steric
effects are not taken into consideration. 31 In a rough approximation, the
metal-preferred bite angle for square-planar complexes is 901, for tetrahedral
complexes 1091 and for trigonal bipyramidal complexes 1201. The more
closely the ligand natural bite angle matches the metal-preferred bite angle,
the better the ligand can stabilize the complex. Conversely, larger differences
between them lead to increased destabilization and, hence, differences in
reactivity. Steric bite angle effects are those which change the steric en-
vironment around the metal center by modification of the ligand backbone
while the substituents on the donor phosphorus atoms are held constant. In
this regard, the steric bite angle effect is independent of the steric effect
imposed by the groups on phosphorus (or other donor atom). The concept of
''pocket angle'' (Y) was introduced by Barron and co-workers as a method to
quantify steric bite angle effects. 32 As shown in Figure 2.8, the pocket angle
is the interior cone angle created by ligand at the metal center and provides a
measure of the interior space of the metal complex (Table 2.6).
Collectively, these ligand properties have served as the basis for the design
and synthesis of new ligands which continue to drive new reactivity in
homogeneous catalysis, especially in the area of Pd-catalyzed cross-coupling
reactions.

2.3 Modern Ligands in Cross-Coupling Reactions
Several classes of prominent supporting ligands have emerged that give rise
to highly active and selective catalysts that collectively make up the state of
the art in modern cross-coupling reactions. The following sections provide