Ligand-assisted nickel catalysis enabling sp3 C–H alkylation of 9H-fluorene with alcohols

Abstract

Herein we report a nickel-catalyzed sp³ C–H alkylation protocol of 9H-fluorene using cheap and abundant alcohols as the source of alkyl groups. The developed method spans a host of primary, secondary, aliphatic and alicyclic alcohols as the alkylation partner and exhibits chemoselectivity during alkylation. The alkylated products were synthesized in moderate to high yields that can rival those of a handful number of other metal-catalyzed processes. One of the important intermediates during this alkylation reaction is 9-alkylidenefluorene, which undergoes hydrogenation to realize the final product. The process is ligand-dominated and follows a radical pathway, which is distinguishably different from all prior approaches to alkylate fluorene. To showcase that hydrogen is stored in the ligand backbone, we isolated a crucial intermediate where the hydrazo-form of the ligand was prominent and used such a molecule to hydrogenate 9-alkylidenefluorene.