Unimolecular Rearrangement Reactions in SilylpyrazolonesA Theoretical Approach

Abstract

Due to the easy migration of silyl groups, the chemistry of silylpyrazolones exhibits various unimolecular reaction channels not encountered in conventional pyrazolone chemistry. The unimolecular rearrangement process starting from the CH form of N-(trimethylsilyl)-pyrazolone (which is primarily formed in the intramolecular condensation reaction of silylhydrazones) is investigated by density functional (B3LYP) and ab initio (MP2) calculations. The transition states are discussed in detail. First, the organosilyl group migrates from the nitrogen atom to the oxygen atom (1,3-Si shift). The second and third steps consist of two consecutive 1,2-H shifts from the carbon to the nitrogen atom. The barriers for the hydrogen migrations determine the overall classical barrier. In comparison to the reactant CH form, the most stable isomer O-silylpyrazolone is lower in energy by 5 kcal mol - 1 . This finding is in agreement with the experimental result. The calculated structure of this isomer is compared with X-ray diffraction data. The OH and NH forms of N-(trimethylsilyl)-pyrazolone are slightly higher in energy than the CH form.