To further understand roles that Zn and DTBP played in this transformation, several control experiments were conducted. In the absence of either nickel catalyst or Zn, only trace decomposition of DTBP was observed at 50 °C over 16h. Moreover, we were able to correlate the stoichiometry of Zn and product yield based on GC analysis, indicating that Zn played a crucial role in the catalytic cycle rather than merely participating in a reductive event during reaction initiation (Scheme 2B). Three potential HAT reagents were independently considered: chlorine radicals (via oxidation of chloride from the nickel pre-catalyst); tert-butoxy radicals (via decomposition of DTBP); and methyl radicals (via β-scission of tert-butoxy radicals). Addition of different tetrabutylammonium halides salts, which has been shown in promoting HAT processes in metallaphotoredox reactions 8a,30 all led to inferior results, suggesting that chlorine radicals may not be the HAT agent in this reaction. (Scheme 2C) Moreover, the trace acetone generation in this reaction also excluded the possibility of methyl radicals being the major HAT agent (see supporting information for details). These observations support the notion that tert-butoxy radicals are the major HAT agent involved in the catalytic process. In summary, we report a method for nickel-catalyzed N-heterocycle C(sp 3 )-H functionalization using aldehydes as coupling reagents. We have demonstrated a highly cross-selective dehydrogenative coupling with a broad scope of aldehydes and N-heterocycles. An unusual combination of peroxide and zinc metal has been shown to be crucial for reaction success, with mechanistic investigations supporting the formation of radicals from both substrates generated from t-butoxy radical as the hydrogen atom transfer (HAT) agent. This method offers a strategy for exploring chemical space via a late-stage C(sp 3 )-H acylation approach. Further mechanistic study and investigation of the factors giving rise to the high cross-selectivity are currently underway.