Pd(0)-Catalyzed Carboiodination: Early Developments and Recent Advances - New Trends in Cross-Coupling: Theory and Applications - page 307

Table 7.5 Effects of ligand sterics and electronics on C-I reductive
elimination.

L

L

L

-L Pd

Pd I

Pd I

I

L

O

O

O

7.53 7.50a 7.54

reductive
elimination

I

O

7.52a

DG z (kcal mol 1 )

Entry Ligand (%)

P t Bu 3 1 24.9

FcP t Bu 2 2 23.7

P i Pr 3 3 26.3

P(OMe) 3 a 4 26.9

PMe 3 a 5 36.9

PH 3 a 6 30.5

a Resting state is a Pd II L 2 species.

Δ G ‡ = 11.9 kcal/mol L

O

β -H elimination

Pd

7.57

I

Pd(P t Bu 3 ) 2

I

I

G ‡ Δ = 29.6 kcal/mol

O

O

7.56

reductive elimination

7.55

O

7.58

Scheme 7.29 Comparison of b-hydride elimination and reductive elimination path-
ways. Adapted from Ref. 100 with permission.

7.7.4 Competing b-Hydride Elimination
Experimental results demonstrate that when monosubstituted olefin 7.55 is
subjected to the standard carbohalogenation conditions, only products such
as 7.57, resulting from a Heck reaction, are observed (Scheme 7.29). The
competing b-hydride elimination pathway was calculated and compared
with the desired carbohalogenation reaction. The computations show that
b-hydride elimination is an extremely facile process with a transition-state
energy of 11.9 kcal mol 1 . The barrier for carbon-halogen reductive elim-
ination, on the other hand, was calculated to be 29.6 kcal mol 1 . Therefore,
avoiding b-hydride elimination is a challenging goal that will require the
development of more specialized catalyst systems.