Reductive Cyclization of 2‐Nitro‐ and β‐Nitrostyrenes, 2‐Nitrobiphenyls, and 1‐Nitro‐1,3‐Dienes to Indoles, Carbazoles, and Pyrroles

This chapter presents a comprehensive review of reductive cyclizations of 2‐nitro‐ and β‐nitrostyrenes, 2‐nitrobiphenyls, and 1‐nitro‐1,3‐dienes to furnish indoles, carbazoles, and pyrroles, respectively, as well as heteroatom analogs thereof. Two variations of the reductive cyclization are discussed: the Cadogan–Sundberg reaction, which is mediated by ternary phosphorus compounds, and the Watanabe–Cenini–Söderberg reaction, which employs a palladium catalyst in the presence of carbon monoxide. A few closely related reductive cyclizations are also presented, as are comparisons with other cyclizations that form the nitrogen–carbon bond of indoles and derivatives.