Keith Fagnou

Azines, diazines or thiazole N-oxides are highly reactive substrates in palladium-catalyzed direct arylation reaction. For these reactions, the results are inconsistent with an SEAr reaction pathway and may best fit with a concerted metalation-deprotonation-like (CMD) mechanism.

A domino palladium-catalyzed Heck-intermolecular direct arylation reaction has been developed, giving access to a variety of dihydrobenzofurans, indolines, and oxindoles. A variety of sulfur-containing heterocycles such as thiazoles, thiophenes, and benzothiophene can be employed as the direct arylation coupling partner in yields up to 99%.

The industrially important coupling of aromatic compounds has generally required differential prefunctionalization of the arene coupling partners with a halide and an electropositive group. Here we report that palladium, in conjunction with a copper oxidant, can catalyze the cross-coupling of N -acetylindoles and benzenes in high yield and high regioselectivity across a range of indoles without recourse to activating groups. These reactions are completely selective for arene cross-coupling, with no products arising from indole or benzene homo-coupling detected by spectroscopic analysis. This efficient reactivity should be useful in the design of other oxidative arene cross-couplings as well.

While the use of π-deficient azine halides in palladium catalyzed cross-coupling reactions is common, the use of π-electron deficient azine organometallics has been less intensively examined. In recent years, important advances have been made that are beginning to address this deficiency and need. The purpose of this tutorial review is to highlight and discuss the innovations that facilitate the synthesis of azine-containing biaryls with a focus on the pyridine structural motif. Given the number of important compounds which exhibit azine-heterobiaryls and the wide use of cross-coupling methods in their synthesis, this review should be of interest among synthetic organic chemists and organometallic chemists alike.