Pd(0)-Catalyzed Carboiodination: Early Developments and Recent Advances - New Trends in Cross-Coupling: Theory and Applications - page 277

of products that can be made. New reactivity is derived from combining
rationally composed sets of catalytic steps and new catalysts. In 2010,
Lautens and Newman reported a synthesis of 2-bromoindoles via C-N
coupling that was only possible due to a reversible oxidative addition. 1 This
subsequently led to the discovery of a Pd(0)-catalyzed carboiodination/
cycloisomerization. 2 In the reaction pathway, an ArPd(II)I species undergoes
carbopalladation to an unactivated alkene. In the absence of syn-b-hydrogen
atoms, a strictly ligand-dependent carbon-iodine reductive elimination oc-
curs. The process retains the valuable iodine atom from the substrate, which
expedites further product derivatization. This atom-economical C-C bond-
forming reaction possesses a novel reaction mechanism in a very important
field and we consider that a comprehensive timeline of this chemistry will
help expand its utility and desirability.
The aim of this chapter is to put the origins of carbon-halogen reductive
elimination from transition metal complexes into perspective, while at the
same time highlighting the efforts towards developing the first Pd(0)-cata-
lyzed carboiodination. Additionally, contributions from other researchers in
this field will be highlighted and recent applications of this powerful
methodology will be discussed.
7.2 Classical Reactivity of Palladium Complexes
Palladium represents a reliable and versatile transition metal, known to have
great success in a wide array of catalytic reactions. Its tolerance towards di-
verse functional groups and harsh reaction conditions make it ideal for
everyday use in organic synthesis. Yet, despite these positive characteristics,
the relatively high cost of Pd compared to other transition metals, and also
diculties in separating the homogeneous catalyst from products, limit its
widespread application in some instances. Nonetheless, numerous industrial
processes, including the Wacker oxidation, have prevailed as some of the most
important industrial reactions which provide necessary chemical feed-
stocks. 3-7 The intense research efforts conducted in this field have provided
researchers with a strong groundwork on which modern reaction design and
development can be based.
Palladium is capable of performing a number of elementary steps (e.g.,
oxidative addition, transmetallation, carbopalladation and reductive elim-
ination), which can be combined into useful catalytic transformations. 8 The
relative rates of each elementary step can be readily influenced by modifying
the steric and electronic properties of L-type phosphorus or nitrogen-based
ligands, which will be a key focus in the following sections.
7.3 Oxidative Addition of Palladium to
Carbon-Halogen Bonds
Oxidative addition in a cross-coupling typically involves a reaction between a
Pd(0) complex and a polarized carbon-halogen bond, which undergoes