Alkylation represents an important organic transformation in molecular science to develop privileged alkylated arenes and heteroarenes. Especially, the direct C−H bond alkylation using unactivated alkyl halides is a straightforward and attractive approach from both the step-economy and chemoselectivity perspectives. Substantial progress has been made in the direct alkylation using primary, secondary, and tertiary alkyl halides along with the methylation and fluoroalkylation. This Review broadly summarizes the transition-metal-catalyzed alkylations of C−H bonds on various arenes and heteroarenes with unactivated alkyl halides until October 2020. On the basis of the substrates utilized for alkylation, the Review is divided into two major sections: alkylation of arenes and alkylation of heteroarenes.

A mild and efficient nickel-catalyzed method for the chemo and regioselective coupling of unactivated alkyl chlorides with the C–H bond of indoles and pyrroles at 60 °C is described.

Nickel(II)-catalyzed intramolecular C(sp 2 )−H/C-(sp 3 )−H and C(sp 2 )−H/C(sp 2 )−H oxidative couplings in indoles are achieved via chelation assistance. These reactions provide access to biologically relevant fiveand six-membered substituted cyclopentaindolones, carbazolones, and indenoindolones in high yields and good chemoselectivity employing an air-stable and defined nickel catalyst, (bpy)Ni(OAc) 2 . The oxidative cyclizations proceeded either through a six-membered or an unconventional seven-membered nickelacycle. An extensive mechanistic investigation by experiments and theoretical calculations revealed a facile indole's C(2)−H nickelation and a rate-limiting reductive elimination process. This intramolecular oxidative cyclization operates via a probable Ni(II)/ Ni(III) pathway involving single-electron oxidation of nickel without the participation of a carbon-based radical.