The possible structures of the deprotonated substrates and the S(N)2 substitution transition states of an I-2-mediated selective intramolecular dearomative oxidative coupling (IDOC) reaction of dimethyl (S)-2-(3-azido-3-(3-(2-((tert-butyldimethylsilyl)-oxy) ethyl)-1H-indol-2-yl) propyl) malonate under 3 conditions (M. Teng, W. Zi, D. Ma, Angew. Chem. Int. Ed. 2014, 53, 1814) are studied by DFT calculations, which shows that different lithium aggregates form between two functional groups of the deprotonated substrates under different conditions and determine the coupling regioselectivity. This mechanistic study provides a way of thinking to control the regioselectivity on similar substrates.