Ligand-Free Ru-Catalyzed Direct sp³ C–H Alkylation of Fluorene Using Alcohols

Abstract

The sp³ C-H alkylation of 9<i>H</i>-fluorene using alcohol and a Ru catalyst via the borrowing hydrogen concept has been described. This reaction was catalyzed by the [Ru(<i>p</i>-cymene)Cl₂]₂ complex (3 mol %) and exhibited a broad reaction scope with different alcohols, allowing primary and secondary alcohols to be employed as nonhazardous and greener alkylating agents with the formation of environmentally benign water as a byproduct. A variety of 9<i>H</i>-fluorene underwent selective and exclusive mono-C9-alkylation with primary alcohols in good to excellent isolated yield (26 examples, 50-92% yield), whereas this reaction with secondary alcohols in the absence of any external oxidants furnished the tetrasubstituted alkene as the major product. Furthermore, a base-mediated C-H hydroxylation of the synthesized 9<i>H</i>-fluorene derivatives afforded 9<i>H</i>-hydroxy-functionalized quaternary fluorene derivatives in excellent yield.