Substituent effects on decarbonylation: Theoretical study of the interconversion of 1,2-bisketenes, cyclopropenones and alkynes

The structures and energies of the stationary points for decarbonylation of substituted 1,2-bisketenes 14 forming cyclopropenones 1 which further decarbonylate to alkynes 16 have been calculated by ab initio and hybrid B3LYP methods. At the HF/6-31G* level ketenylcarbene intermediates 15 are formed from 14 but at the MP2/6-31G* and B3LYP levels the conversion of 14 to 1 becomes a one-step process. Decarbonylation of bisketenes 14 is favored by electronegative substituents, but decarbonylation of cyclopropenones 1 to alkynes 16 is favored by electropositive substituents, in agreement with experimental results. The stabilization of cyclopropenones by π-donor substituents (NH₂, OH and F) explains why OR and F, but not Me₃Si, substituted cyclopropenones have been observed as products from bisketene photolyses.