Theoretical Study on the Reaction Mechanism for Synthesis of Quinolinones and Spiro Compounds from N -Arylpropynamides and Diselenides Mediated by (Dichloroiodo)benzene

The reaction mechanisms for the synthesis of quinolinones and spiro compounds from N-arylpropynamides (1a, R = H; 1b, R = OMe; 1c, R = F) and diselenides, mediated by (dichloroiodo)benzene (PhICl₂), which have been investigated using density functional theory (DFT) at the M06-2X/6-311G(d,p) level of theory. The in situ-generated PhSeCl electrophilically adds to the alkyne moiety of the substrates, initiating a cascade involving addition-dechlorination, spirocyclization, and subsequent divergent pathways. For substrate 1a (R = H), the key azaspiro[4.5] intermediate undergoes 1,2-carbon migration and deprotonation to yield quinolin-2-one as the major product. In contrast, for 1b (R = OMe), the spiro intermediate favors a demethylation pathway, leading to an azaspiro[4.5]trienone. For the fluoro-substituted substrate 1c, the pathway involves nucleophilic addition of water to the spiro intermediate, followed by HF elimination to form the spiro product. The calculated energy barriers and thermodynamic profiles rationalize the experimentally observed product selectivity, demonstrating that the para-substituent effectively controls the reaction outcome by steering the fate of a common spirocyclic cation intermediate.