Pd(0)-Catalyzed Carboiodination: Early Developments and Recent Advances - New Trends in Cross-Coupling: Theory and Applications - page 283

k 2

Br

k 1
k - 1

P t Bu 3

Pd[P t Bu 3 ] Pd[P t Bu 3 ] 2 7.6b +

R 1

Scheme 7.6 Mechanism of reductive elimination involving ligand dissociation pro-
posed by Hartwig.

from chloride 7.12a was thermodynamically favored by a factor of 3000 over
that of bromide 7.12b, which was favored by a factor of 20 over iodide

7.12c. The authors tied this trend to the softness or nucleophilicity of the
halogen in the transition state for addition and elimination. 52,53 Similarly,
K eq for ortho-substituted arenes (7.12a-c) was larger than that of their non-
substituted analogs (7.12d-e). Interestingly, the kinetics were not in line
with the above-mentioned thermodynamics, as reductive elimination from
chloride 7.12a was slower than that from bromide 7.12b, despite being more
energetically favorable. They also claimed that the strength of the metal-
halogen bond is likely more important than the strength of the resulting
carbon-halogen bond. The observed rate constant [eqn (7.4)] is inversely
dependent on [ArX] and linearly dependent on [P t Bu 3 ], both with non-zero
y-intercepts. Based on these kinetic data, the authors proposed a mechanism
that involves reversible ArBr reductive elimination followed by trapping of
the of the Pd(0) intermediate by P t Bu 3 (Scheme 7.6).

B

k obs ¼ k 1 k 2 ½ P t Bu 3
k 1 k 2 ½ P t Bu 3

rate ¼ k obs ½ 7 : 12b (7 : 4)

These reports have shed light on the process of reductive elimination of
carbon-halogen bonds from Pd(II) metal centers, in addition to laying the
necessary groundwork needed for this rare elementary step to be used in new
catalytic processes.

7.4.4 Alkyl Halide Reductive Elimination from Transition
Metal Complexes
As highlighted in the preceding sections, the majority of carbon-halogen
bond reductive eliminations from late transition metals such as Pt(IV), 54
Pd(IV) and Pd(II) involve aryl halides. A few studies of reductive elimination
of alkyl halides via thermolysis involving Pt(IV) complexes have been re-
ported. 55-57 The Rh(III) species [(CH 3 CO)Rh(CO)I 3 ] has also been shown to
decompose through the loss of MeI. In the presence of a CO atmosphere, the
newly formed dicarbonyl complex [(CH 3 CO)Rh(CO) 2 I 3 ] reductively elimin-
ates to form acetyl iodide. 58
The first report of reductive elimination of alkyl halides from isolable
Rh(III) alkyl complexes 59,60 was from Frech and Milstein, who described the
reductive elimination of MeI from a bulky Rh(III) pincer complex in 2006
(Scheme 7.7). 61 By reacting nitrides 7.13 and 7.14 with an equimolar amount