Unveiling the molecular mechanisms of the cycloaddition reactions of aryl hetaryl thioketones and C,N-disubstituted nitrilimines.

Many synthetic routes to constructing biologically active heterocyclic compounds are made feasible through the (3 + 2) cycloaddition (32CA) reactions. Due to a large number of possible combinations of several heteroatoms from either the three-atom components (TACs) or the ethylene derivatives, the potential of the 32CA reactions in heterocyclic syntheses is versatile. Herein, the cycloaddition reaction of thiophene-2-carbothialdehyde derivatives and C,N-disubstituted nitrilimines have been studied through density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. In the present study, a one-step 32CA and two-step (4 + 3) cycloaddition (43CA) reaction mechanisms involved in TACs reactions and ethylene derivative have been investigated. In all reactions considered, the one-step 32CA cycloaddition is preferred over the two-step 43CA. The TAC chemoselectively adds across the thiocarbonyl group present in the ethylene derivative in a 32CA fashion to form the corresponding cycloadduct. Analysis of the electrophilic [Formula: see text] and nucleophilic [Formula: see text] Parr functions at the various reaction centers in the ethylene derivative show that the TAC adds across the atomic centers with the largest Parr functions, which is in total agreement with the experimental observation. The selectivities observed in the titled reactions are kinetically controlled.