A practical palladium (Pd)-catalyzed approach enables the synthesis of phenazines by reductive cyclization of 2-nitro-N-phenylanilines using either carbon monoxide (CO) gas or phenyl formate as an in situ CO source. The method avoids harsh reagents and minimizes byproducts. Differences in selectivity and reaction behavior between CO and its surrogate are highlighted.

Abstract

Phenazines are a diverse class of nitrogen-containing heterocycles with a wide range of chemical structures and biological applications. We have developed a synthetic route for phenazines through a palladium-catalyzed reductive cyclization of 2-nitro-N-phenylanilines using either gaseous carbon monoxide or phenyl formate as an in situ CO source as efficient and cheap reductants. Our protocol offers a practical alternative to existing methods, achieving good yields without relying on large amounts of strongly reducing or oxidizing agents. Moreover, it minimizes the formation of unwanted byproducts, which is a common drawback in traditional phenazine synthesis. The key advantages of this protocol include the use of a simple Pd-catalyst in the presence of 1,10-phenanthroline (Phen), an inexpensive and commercially available ligand. These features make our method very convenient for phenazine synthesis. An uncommon inverse correlation between catalyst loading and product selectivity, as well as key differences between the use of gaseous CO and its surrogate, will also be discussed.