The importance of reactions for introducing functional groups site-selectively to heterocyclic compounds has been increased from the past to the present. According to these demands, reactions using pyridinium salts have been developed to date. However, in the site-selective functionalization of heteroarenes, disadvantages, such as multiple pre-functionalization steps, unnecessary procedures, and less atom-economic reactions, are increasingly drawing attention. This thesis describes two reactions about the site-selective C−H functionalization of heteroarenes via one-pot reaction strategies in order to overcome these limitations. In Chapter 1, the regioselective functionalization reaction is introduced by using heteroarene N-oxides, which were discarded as waste in the previous research, enabled by a traceless nucleophile strategy. Also, in the workup step of sequential process, it explains how to enable a new reaction at the same time as the end of previous reaction. Chapter 2 introduces the more efficient merging of two successive reactions in one-pot to develop the remote C−H functionalization away from original functional group. In this process, it introduces cascade reactions initiated by the selective cleavage of N−O bond without external requirements by controlling the properties of the leaving group.