关键词: 烯酮, 环戊二烯, 环加成反应, 反应机理

Abstract: The cycloaddition reactions between ketene or substituted ketenes and cyclopentadiene have been studied by means of semiempirical AMImethod.The geometries of transition states and intermediates of each reaction path have been optimized by Berny technique, and the transition states were ascertained by frequency analysis.The results can be sumed up as follows:The reactions between ketene or methylketene and cyclopentadiene are concerted but nonsynchronous, taking place through twisted transitions states, with small charge transfer from cyclopentadiene to the ketenes.An analysis based on frontier molecular orbitals shows that the reaction mechanisms correspond to "2×[l + l]" description.Two types of orbital interactions are available in the vicinity of TS.One of them is the interaction of ketene LUMOwith cyclopentadiene HOMO, the other is that of ketene HOMOwith cyclopentadiene LUMO.Of these two interactions, the former is stronger than the latter, so that in TS, one ofthe C-Cbond is almost formed, while the distance of the other C-Cbond is rather large.The reac-ttions between chloromethyl-substituted ketene and cyclopentadiene proceed via a stepwise pathway.The stereoselectivity of the reactions observed through experiments are correctly predicted by the calculations and are analyzed in terms of electronic and steric effects of the substituents on the reacting ketene.

Key words: Ketene, Cylcopentadiene, Cylcoaddition reaction, Reaction mechanism