AN ABINITIO STUDY OF THE CYCLIZATION AND REARRANGEMENT OF VINYLKETENE, IMIDOYLKETENE, AND FORMYLKETENE

The 1,4-cyclization was studied in three model systems; formylketene, imidoylketene, and vinylketene, using ab initio MO methods. Structures of stationary points on the reaction potential energy surfaces were located at the HF/6-31G level. Relative energies, estimated at the MP4SDQ/6-31G level together with zero-point contributions, are as follows (in kcal/mol): formylketene (-17) → TS(4) → oxetone (0); imidoylketene (-10) →TS(15) → azetinone (0); and vinylketene (0) →TS(31) cyclobutenone (0). The ring-opening of cyclobutenone is energetically more favorable than that of cyclobutene. The electronic reorganization accompanying the ring-closure of substituted ketenes was also examined using a localized orbital analysis. The process is found to be similar to the 1,5-cyclization of vinylazide and imidoylazide. Finally, the ketoketene–ketoketene rearrangement, including migration of a hydrogen in the formylketene, was considered. This 1,3-hydrogen shift is calculated to be a quite difficult chemical process requiring a large activation energy (even larger than that in formic acid). © 1990, American Chemical Society. All rights reserved.