Metal-catalyzed synthesis of five-membered ring N-heterocycles. A recent update

The review deals with the metal-catalyzed synthetic methods used for the construction of 5-membered nitrogen-containing aromatic and nonaromatic heterocycles with one to three nitrogen atoms, published in the years 2008–2017. The methodology is based on the metalcatalyzed activation of the alkyl C( sp 3 )–H, alkenyl C( sp 2 )–H, and terminal alkyne C( sp )–H bonds of appropriate precursors in order to perform intermolecular and intramolecular reactions. These bond forming reactions include C( sp 3 )–C( sp 2 ), C( sp 2 )–C( sp 2 ), and C( sp 2 )–C( sp ) coupling, hydroamination at C( sp 2 )–H and C( sp )–H bonds, and oxidative amination of alkyl C( sp 3 )–H, alkenyl and arene C( sp 2 )–H, and terminal alkyne C( sp )–H bonds. Two further bond forming methods are intramolecular nitrenoid–arene C( sp 2 ) and cycloaddition reactions. Transition metals are systematically used in these synthetic methods, and only in a few cases Lewis acids are used as catalysts. In general, the metal-catalyzed cyclizations leading to five-membered heterocycles are very efficient and particularly useful in the synthesis of natural and unnatural biologically active products.