Recently, BX3 (X= F, Cl, Br) promoted transition-metal-free reactions have gotten a wide acceptance to synthesis organoboron compounds. In this context, Yang and Li very recently reported a direct intramolecular aminoboration of allenes as a means for the synthesis of heterocyclic borane compounds. In addition to this, a catalyst free annulative thioboration of unfunctionalized olefins was also reported in 2017 by Chun-Hua Yang group. In this study, a DFT calculation was performed to examine the mechanisms of these two BCl3 promoted experimentally tested reactions focusing on elucidating the detailed cyclization mechanisms. Our calculation results suggest that the BCl3 induced C=C double bond activation path is highly favorable than the BCl3---SH interaction activation path for the thioboration of unfunctionalized olefins. In other words, the mechanism of the thioboration of unfunctionalized olefins involves 5-exo-trig cyclization and intramolecular deprotonation steps. On the other hand, 6-endo-trig cyclization and 1,3-H transfer steps are found to be kinetically unfavorable. Furthermore, the calculation results reveal that gem-dimethyl substituents on the substrate have less influence on the activation energies of both 5-exo-trig cyclization and intramolecular deprotonation steps. The calculation results also reveal that the thioboration of unfunctionalized internal alkenes may be not feasible, consistent to the experimental observation. Our calculation results are in good agreement with the experimental observation. Key words: Organoboron compounds, Aminoboration, Thioboration, Gaussian, Mechanism, Density Functional Theory, Computational study and Cyclization