Pd-II-Catalysed C-H Functionalisation of Indoles and Pyrroles Assisted by the Removable N-(2-Pyridyl)sulfonyl Group: C2-Alkenylation and Dehydrogenative Homocoupling

The easily installed and removed N-(2-pyridyl)sulfonyl group exerts complete C2 regiocontrol over the Pd-II-catalysed C-H alkenylation of indoles and pyrroles, affording the corresponding products in good isolated yields (typically >= 70%). A remarkable feature of this catalyst system is that it tolerates a wide variety of substituted alkenes, including conjugated electron-deficient alkenes, styrenes and 1,3-dienes, as well as conjugated 1,1- and 1,2-disubstituted olefins. The final reductive desulfonylation affords the C2-substituted, free-NH indoles and pyrroles in good yield. This N-(2-pyridyl)-sulfonyl-directing strategy has also been extended to the development of a protocol for the intermolecular, dehydrogenative homocoupling of indoles, providing 2,2'-biindoles. Mechanistic work based upon reactions with isotopically labelled starting materials and competitive kinetic studies of electronically varied substrates suggests a chelation-assisted electrophilic aromatic substitution palladation mechanism.