Thus, we propose a mechanism based on enantioselectivity. In Et2SiH2, which showed the best enantioselectivity, the steric hindrance was small and cobalt hydride formed very quickly, resulting in an increase in the concentration of the downstream intermediate. The increase in concentration facilitated radical chain reactions between the alkylcobalt(III) complex and carbon radicals that escape from the solvent cage. As a result, the alkyl cobalt (III) complex, which was initially a diastereomeric mixture (Co-C bond formation is nonselective), inclined toward the thermodynamically stable product. The stereochemistry of the alkyl cobalt (III) complex is reflected in the final product, which formed via one-electron oxidation and intramolecular nucleophilic substitution reactions. The radical chain reaction described here is supported by two findings: 1) the enantioselectivity of the asymmetric reaction with Et2SiH2 was markedly reduced (from 93% to 72%ee) when degassing (trifluoroethanol) was not performed; the free carbon radicals were most probably trapped by the oxygen molecules and inhibited the radical chain reaction. 2) the free energy difference between both diastereomers of alkylcobalt(III) complexes was 1.76 kcal/mol, as determined from DFT calculations; the free energy difference is consistent with the absolute configuration of product 2a and its enantioselectivity (93% ee) can be explained quantitatively. A CH- interaction between the aromatic rings of each ligand and the starting material in the stable alkylcobalt(III) complex structure is very probable. However, the rate of cobalt hydride formation is lower for bulky silyl hydrides. Under these conditions, radical chain reactions are less likely to occur, and the diastereomeric alkylcobalt(III) complexes form in an almost 1:1 ratio. In this case, enantioselective kinetic resolution occurred during the intramolecular nucleophilic displacement. This explanation of the enantioselectivity is supported by the following points: 1) unlike in the case of Et2SiH2, the enantioselectivity with (Me2SiH)2O was not reduced without degassing; 2) the energy difference between the two transition states in the intramolecular nucleophilic displacement was calculated to be 0.54 kcal/mol. The absolute configuration of the product was qualitatively consistent with its absolute configuration.