Pd(0)-Catalyzed Carboiodination: Early Developments and Recent Advances - New Trends in Cross-Coupling: Theory and Applications - page 281

Table 7.1 Roy and Hartwig's stoichiometric studies on reductive
elimination of aryl halides from ArPd(II)X dimers.

( o -Tol) 3 P

X

R 1 X

R 1 Pd

7 0 °C
C 6 D 6

Pd[P t Bu 3 ] 2

P t Bu 3

2

R 3 R 2

R 3 R 2

+P( o -tol) 3

R 1 = t Bu, R 2 =Me,R 3 =H
X=Cl:
X=Br:
X=I:

R 1 =R 2 =H,R 3 = t Bu
X=Cl:
X=Br:

7.10a-e

7.9a
7.9b
7.9c

7.9d
7.9e

X Yield of 7.10 (%) K eq
7.7a (X ¼ Cl) 70 9(3) 10 2
7.7b (X ¼ Br) 70 2.3(3) 10 3
7.7c (X ¼ I) 39 3.7(2) 10 5
7.7d (X ¼ Cl) 30 Not measured
7.7e (X ¼ Br) 75 3.3(6) 10 4
Adapted from Ref. 46. Copyright 2001 American Chemical Society.

t Bu 3 P

7.10b
+ 1/2 7.9b
+ Pd[P t Bu 3 ] 2

t Bu 3 P

X X X

Ar Ar Ar

-L
+L

P t Bu 3

7.9b

Pd

Pd Pd Pd

Pd Pd

Ar X

Ar X Ar X

L L L

Scheme 7.4 Hartwig's proposed mechanism of reductive elimination from ArPd(II)X
dimers involving ligand dissociation.
Adapted from Ref. 46. Copyright 2001 American Chemical Society.

species, which are formed from ligand substitution and subsequent reductive
elimination (Scheme 7.4). The authors also reasoned that the formation of
monomers and reductive elimination are faster than dissociation of P t Bu 3 .The
use of P t Bu 3 is crucial in this reaction and it is proposed that its steric influence
outweighs its high s-donating character, thus promoting the forward reaction.
This is contrary to the information presented in previous sections discussing
the factors that promote reductive elimination, which state that highly
s-donating ligands slow the rate of reductive elimination.
It was reasoned that reductive elimination from an ArPd(II)Cl species would
be faster than the corresponding bromide or iodide analogues if ground state
effects, such as carbon-halogen bond strengths, dominated. However, if bond
strengths and electronic properties of the metal-halogen bonds controlled the
rates of addition and elimination, reductive elimination from the ArPd(II)Cl
would be slower than the bromide or iodide equivalents. 47

7.4.3 Directly Observed Aryl Halide Reductive Elimination
from Monomeric Palladium(II) Complexes
Most organometallic reactions proceed via intermediates possessing a
free coordination site. Specifically, cross-coupling reactions with bulky