Pd(0)-Catalyzed Carboiodination: Early Developments and Recent Advances - New Trends in Cross-Coupling: Theory and Applications - page 280

PdCl 2 (20 mol%)
Cl 2
dioxane/H 2 O, 85 °C, 16 h

H

Cl

other ortho -chlorinated
products

N

N

N

N

H

Cl

7.7 7.8

Scheme 7.3 First example of a Pd-catalyzed ortho-chlorination of azobenzene by
Fahey.

their existence and exploiting their capacity in catalysis. 37 Carbon-halogen bond-
forming reactions involving high-valent Pd(IV) species have come to light as both
a useful and mechanistically interesting class of reactions. In 1970, Fahey re-
ported the first Pd-catalyzed directed ortho C-H chlorination of azobenzene 7.7
to form 7.8 using Cl 2 (Scheme 7.3). 38,39 Despite the low selectivities observed for
the monochlorinated product, this report established a novel and fascinating
proof-of-principle, which has developed into an active field of research.
Many stoichiometric and catalytic methods have been reported since
Fahey's pioneering work, including carbon-fluorine, 40 carbon-chlorine, 41
carbon-bromine 42 and carbon-iodine 43 bond formation. Many efforts in
this field are directed towards gaining a better mechanistic understanding of
the overall process. Nevertheless, we have elected to focus on reactions of
Pd(0/II) species and interested readers are directed to relevant reviews for a
discussion of higher valent palladium species. 44,45

7.4.2 Stoichiometric Aryl Halide Reductive Elimination from
Dimeric Palladium(II) Complexes
In 2001, Roy and Hartwig conducted an in-depth kinetic study regarding
stoichiometric carbon-halogen bond reductive elimination from dimeric
ArPd(II)X complexes 7.9a-e when reacted with excess P t Bu 3 (Table 7.1). 46
Prior to this seminal report, the oxidative addition of Pd(0) complexes to
aryl-halide bonds was thought to be irreversible. As a result of this study, the
authors rewrote the established rules for the kinetics and thermodynamics
of oxidative addition/reductive elimination.
In addition to a 10-fold increase in K eq values, dimers 7.9a-c containing o-Me-
substituted aryl ligands provided higher yields of their respective ArX products
7.10a-c than the complexes without ortho-substituted aryl ligands. Notably, each
successive change in halide ligand (i.e.,CltoBrorBrtoI)wasassociatedwith
an approximate 100-fold change in K eq . Excellent fit to a first-order appearance
of each product was obtained, in addition to observing both a positive first-
order dependence of 1/k obs versus 1/[P t Bu 3 ]andaninversedependenceofk obs on
[P(o-tol) 3 ], both with non-zero y-intercepts. All data were consistent with a
mechanism involving an irreversible formation of monomers from the dimeric