Indoles via Palladium‐Catalyzed Cyclization

Abstract

The palladium-catalyzed assembly of the functionalized pyrrole nucleus on a benzenoid scaffold is a widley used synthetic tool for the preparation of indole derivatives. This construction can be categorized into four main types: (1) cyclization of alkynes; (2) cyclization of alkenes; (3) cyclization via C-vinyl reactions; and (4) cyclization via N − arylation or N − vinylation reactions. The first approach is the most versatile in terms of range of the added functional groups and of the bonds that can be created in the construction of the pyrrole ring. This method is based on the utilization of precursors containing nitrogen nucleophiles and carbon-carbon triple bonds. The nitrogen nucleophile and alkyne moiety may be part of the same molecule or belong to two different molecules. Some of the most general cyclizations of indoles are summarized. Alkene-based cyclizations to give indoles are also summarized. Cyclization to indoles via arene vinylation has limited synthetic scope. Indoles can be prepared via cyclizations proceeding through N-arylation and N-vinylation reactions based on pioneering work. In general, only synthetic procedures where palladium catalysis is involved in the pryrrole ring construction event are discussed in this chapter. Keywords: Indoles; Palladium catalysts; Pyrroles; Cyclization; Vinylation; Alkenes; Alkynes; Copper; Catalysts; Substituted indoles; Mechanisms; Method comparisons; Experimental procedures