Zn(II)‐Stabilized Azo‐Anion Radical Catalyzed Sustainable C−C Bond Formation: Regioselective Alkylation of Fluorene, Oxindole, and Indoles

Abstract

AbstractHerein we report a sustainable approach for the alkylation of ketones, 9H‐fluorene, oxindole, and indole using alcohols as the alkylating agent catalyzed by a well‐defined air‐stable zinc catalyst (1 a) of a tridentate redox non‐innocent arylazo ligand, 2‐((4‐chlorophenyl)diazenyl)‐1,10‐phenanthroline (La). 2–3 mol % of 1 a efficiently produces substituted α‐alkylated ketones, 9‐alkylated fluorenes, C3‐alkylated oxindoles, and C3‐alkylated indoles in moderate to good isolated yields. In aerial condition, the formation of bis(indolyl)methane (BIMs) derivatives were observed when indoles were subjected to alkylation by primary alcohols. A few drug molecules containing BIMs were prepared in good isolated yields. The catalyst 1 a exhibited good chemoselectivity during the functionalization of fluorene and indole with oleyl alcohol and β‐citronellol. A few control experiments, including deuterium labeling experiments, performed to unveil the reaction mechanism indicate that the one‐electron reduced azo‐anion radical species [1 a]‐formed in situ, acts as the active catalyst. All the redox events occur at the redox‐active aryl‐azo ligand, which acts as the reservoir of hydrogen and electrons throughout the catalytic cycle, keeping the Zn(II)‐center as a template.