To gain further insight into the mechanism of this 1,5-HAT borylation reaction, we performed a series of mechanistic investigations. First, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added to standard conditions, and the reaction yielded 16% of the corresponding TEMPO-trapped product 3 as a white solid, instead of the desired borylation product (Scheme 4a). Second, we designed and managed to synthesize the remote cyclopropyl substrate 1ad, and the reaction of 1ad in standard conditions gave the ring-opened terminal borylation product in 32% yield (Scheme 4b). These two experiments indicated the involvement of carbon radical intermediates in this borylation reaction. In recent days, the Chechik group reported a new type of radical scavenger, namely CHANT (Scheme 5), to detect short-lived radical intermediates. The radical trapping product in their method are terminal alkenes which are bench-stable and can be analyzed by mass spectrometry (MS). To further testify the 1,5-HAT process, we chose CHANT as radical trapping reagent, and carried out the reaction in standard conditions. The trapped radicals were then analyzed by electrospray ionization (ESI) HRMS (Scheme 5, see Supporting Information for details). MS peak at m/z 337.2849 indicates the trapped radical intermediates, whereas it cannot distinguish the nitrogen-centered radical R1 or the carbon-centered radical R2, since they are isomers. In order to distinguish them, the tandem mass spectrometry was analyzed at m/z 337, the two low-intensity peaks F1 (m/z 237.1961) and F2 (m/z 181.1336) which contain two nitrogen atoms, could only be attributed to the trapped R1, the fragments showed that the acyl group RCO was dissociated. The most strong-intensity peaks F3 (m/z 264.1957) is a stable acylium ions, and F4 (m/z 236.2008) is F3 losing CO.