Regioselective Direct C-Alkenylation of Indoles.

The direct introduction of alkenyl groups into the indole framework avoiding its preliminary functionalization can be carried out using different synthetic strategies. Transition-metal complexes facilitate the C-H activation of indoles or alkenes allowing an efficient Csp2 - Csp2 bond formation. The hydroindolation of alkynes catalyzed by the same metal complexes or various acidic promoters can also be pursued for the alkenylation process. Conjugate addition of electron-poor alkenes and direct condensation of carbonyl derivatives with indoles are also of interest for this purpose. The regiochemical control can be exploited using the intrinsic C-3 reactivity of the indole ring. The introduction of a suitable directing group at the nitrogen atom allows the preparation of C-2 alkenylated derivatives by transition metal catalyzed reactions. This review collects the fundamental contributions in this field reported in literature during the last fifteen years.