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, NH 2, C6H5) have been studied at the B3LYP/6-311G(d,p) level. It has been show n that the reaction proceeds in two steps forming first an intermediate hav ing the properties of both a carbene and a zwitterion followed by a ring cl osure leading to cyclopropenones or cyclopropenethiones. The solvent effect does not play an important role in the course of the cycloaddition. The es timation of the first vertical excitation energies by CIS and TD-B3LYP meth ods shows that the reactions likely take place in the ground state rather t han in an excited state. All the studied cyclopropenones and cyclopropeneth iones are aromatic as shown by their NICS values and confirmed by calculate d and experimental NMR chemical shifts. Different reactivity criteria inclu ding HOMO coefficient, local softness, hardness, polarizability, and NICS a re used to predict the site selectivity in all studied cases, and the NICS criterion seems to yield the best results among them.