Metal-catalyzed dehydrogenative synthesis of pyrroles and indoles from alcohols

This review is focused on the synthesis of various classes of pyrroles and indoles from alcohols as substrates or intermediates. Metal-induced dehydrogenation converts primary or secondary alcohols into the related carbonyl compounds (aldehydes or ketones), which can undergo in situ inter- or intramolecular attack by a carbon or nitrogen nucleophile to afford a new C–C or C–N bond, respectively. The formation of these bonds is the key step in the construction of the pyrrole and indole framework. Thus, pyrroles are obtained in good yields by dehydrogenative cyclization of β-hydroxyenamines, and dehydrogenative N-heterocyclization of amines with diols, High yields of pyrroles are also obtained by reacting 2-amino alcohols with alcohols or ketones. Relevant is also the synthesis of pyrroles by a three-component reaction. Indoles are formed in high yields by intramolecular cyclization of 2-amino- and 2-nitrophenylethyl alcohols, and 2-aminobenzyl alcohols. Moreover, indole derivatives are also conveniently obtained by reaction of glycols or alcohols with anilines or phenyl hydrazine, respectively.