Molecular orbital investigation of various reaction pathways in reaction of ketones with bromoform

Abstract

To elucidate the ring opening, nucleophilic reactions of dihaloepoxides the extensive calculations were done on a model system cyclohexanone-bromoform. In this reaction, the formation of dihaloepoxide is postulated as a key step that determines the distribution and stereochemistry of products. Every reaction scheme involves epoxide as a key intermediate (1). Three major products (2, 3, and 4) can be obtained, in principle, by three different competing reaction pathways. The calculations showed that all the pathways are exothermic. Reaction pathway 1 is most convenient, it does not include any intermediate, and its energy is not much affected by the polarity of the medium. In pathways 2 and 3, the calculations showed the intermediates having a largely carbocationic character on the spiro junction carbon atom. The step in which these intermediates are formed determines the reaction rate. Because of the polarity of intermediates in pathways 2 and 3, the base concentration and polarity of solvent determine the balance of reaction pathways and the product yield. (c) 2005 Wiley Periodicals, Inc.