Next, to shed light on the mechanism of this transformation, a series of control experiments were performed (Scheme 3C). First, radical trapping experiments with 2,2,6,6,tetramethylpiperidine-1-oxyl (TEMPO) led to the TEMPO-trapped product 71 in 30% yield, while inhibiting the desired phosphonylation process. Additionally, a radical clock experiment with 72 as the starting material led to the ring-opening desaturated product 73, which further supported the radical nature of this method. These experimental results strongly support the intermediacy of -aminoalkyl radical. The deuterium labeling experiments with 74-D as the starting material provided compelling evidence for the 1,5-HAT mechanism. Among the two regio-isomers isolated, 21-1-D showed complete deuterium retention, while 21-2-D showed deuterium transfer. These results indicated that different regio-isomers came from different rotamers of the starting material, while proving that iminium isomerization is not operable in this transformation. Scheme 1. Scope of HAT-induced -C(sp 3 )-H phosphonylation of aliphatic amines. [a] Determined by isolated yields. [b] Determined by 1 H and 31 P NMR spectroscopy of the crude reaction mixtures.