Theoretical study of [2+1] cycloaddition of CO and CS to acetylenes forming cyclopropenones and cyclopropenethiones

The [2 + 1] cycloadditions of carbon monoxide and carbon monothioxide CX (X = O, S) to acetylenes (R1C drop CR2 with R-1 = H, OH and R-2 = CH3, OH, NH2, C6H5) have been studied at the B3LYP/6-311G(d,p) level. It has been shown that the reaction proceeds in two steps forming first an intermediate having the properties of both a carbene and a zwitterion followed by a ring closure leading to cyclopropenones or cyclopropenethiones. The solvent effect does not play an important role in the course of the cycloaddition. The estimation of the first vertical excitation energies by CIS and TD-B3LYP methods shows that the reactions likely take place in the ground state rather than in an excited state. All the studied cyclopropenones and cyclopropenethiones are aromatic as shown by their NICS values and confirmed by calculated and experimental NMR chemical shifts. Different reactivity criteria including HOMO coefficient, local softness, hardness, polarizability, and NICS are used to predict the site selectivity in all studied cases, and the NICS criterion seems to yield the best results among them.