A quantum mechanistic insight into the chemo- and regio-selective [3 + 2]-cycloaddition reaction of aryl hetaryl thioketones with diazoalkanes and nitrile oxide derivatives.

In this quantum mechanistic study, density functional theory computations at the B3LYP hybrid level of theory, in addition to triple zeta basis set 6-311G (d, p), were utilized to investigate the chemoselectivities and regioselectivities of the [3 + 2] cycloaddition reaction of phenyl (2-thienyl) thioketone (B1) derivatives with nitrile oxide (B2) and diazopropane derivatives (B3). From the computations obtained, the reactions of nitrile oxide and diazopropane derivatives with phenyl (2-thienyl) thioketone proceed through an asynchronous one-step mechanism. The initial [3 + 2] cycloaddition reaction of B1 and B3 is followed by a nitrogen extrusion which is also highly asynchronous. Despite the steric and electronic effects of the substituent on the energetics, the reaction center is selectively observed at the thiocarbonyl site of B1. A study of the Parr functions at the different reaction sites in B1 indicates the addition of B2 and B3 via the atomic centers with the largest Mulliken atomic spin densities. These results show that the thiocarbonyl site is the most reactive center compared to the other ethylene groups on B1, irrespective of the three atom components used. The global electron density transfer results are in agreement with the selectivity and activation barriers observed in the reaction. Our results agree well with experimental observations.