To gain further insights into the reaction mechanism, a series of mechanistic experiments were carried out to illuminate various reaction pathways. Initially, UV-VIS spectroscopy was employed to analyze each building block and combinations. The emergence of a new absorption within the visible light range (~ 280-450 nm) was observed upon the mixture of aldehyde 22, amine 23, and boronic n-butylcatechol ester 119 in HFIP (Figure ), signaling the formation of an EDA complex. Furthermore, a radical clock experiment executed with cyclopropylmethyl boronic acid 120 afforded the ring-opening product 121 in good yield, verifying the formation of the alkyl radical from alkyl organoboron species (Figure ). Moreover, in the presence of 2,2,6,6tetramethylpiperidin-1-oxyl (TEMPO), the alkyl radical generated from the boron species under the standard reaction condition was intercepted by TEMPO, resulting in an adduct (127) that was isolated and characterized (Figure ). The reaction with cyclopropyl aldehyde 123 afforded cyclopropyl products 125 in moderate yield, accompanying a small amount of ring opening product 126 (4% yield). When (TMS)3SiH was introduced as an additive, the yield of the cyclopropyl ring opening product 126 increased to 28%. This suggests that the addition of (TMS)3SiH may facilitate a rapid quenching of the malonate carbon-centered radical via a hydrogen atom transfer (HAT) process (Figure ). These results suggested the generation of a-amino radicals from the iminium intermediate during the reaction. To gain further insight into the radical formation steps, a crossover experiment was conducted using aldehyde 22, amine 23, and N-benzylidene benzene sulfonamide 128, a known excellent radical acceptor. In the absence of aldehyde 22 and amine 23, the reaction of 128 with n-butyl boronic acid 24 and tert-butyl catechol afforded 18% of alkylated amine 129 (Figure ). In contrast, when aldehyde 22 and amine 23 were present, only product 25 was isolated, and 129 was not observed. This result suggests that this reaction proceeds through a confined radical pair recombination rather than a free radical addition process (Figure ). In this process, an a-amino radical, generated from the in situ formed iminium, reacts with the carbon-centered radical more rapidly than 128. Taking into account all the results obtained from the mechanistic studies, a proposed reaction mechanism is proposed in Figure . The reaction initiates with the formation of alkyl boronic tert-butylcatechol ester (131) and iminium intermediates from amine (12) and aldehyde (11). These two intermediates form an EDA complex 132 in HFIP. Under photoirradiation or thermal SET process, the EDA complex 132 undergoes a SET process, leading to the generation of alkyl radical (134) and a-amino radical (133). Concurrently, the loss of benzo [d][1,3,2]dioxaborol-2-ol (HOBcat) occurs, and finally, the amine product 135 is formed through a radical-radical cross-coupling process.