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

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

NH...pi interactions stabilize the most-hindered rotamer of the (S)-tyrosinato side group in bis[(S)-tyrosinato](diamine)cobalt(III) complexes: An NMR spectroscopic and DFT study

Conformational analysis based on an analysis of the vicinal alpha- and beta-proton coupling constants of (S)-tyrosinato ligands in diastereomers of a bis[(S)-tyrosinato](1,3-diaminopropane)cobalt(III) complex is used to calculate the mol fractions of the three most stable rotamers (t, g, h) of the (S)-tyrosinato ligand's side groups in D2O solution. The results of this conformational analysis indicate a population increase in the sterically least favorable rotamer h in diastereomers of Cl-molecular symmetry (complexes 5 and 6). The TOCSY spectrum of complex 6 in aqueous solution shows an exceptionally small chemical shift of one NH2 proton from the coordinated diamine, which is explained by an interligand NH center dot center dot center dot pi interaction. These findings demonstrate the persistence of this weak, noncovalent interaction in water solution. DFT calculations for complex 6, whose diamine ring is in a chair conformation, indicate that the complex with an h conformation in both its (S)-tyrosinato ligand side-residues, which yields the most frequent NH center dot center dot center dot pi interactions, represents an energy minimum. The fact that coordinated 1,3-diaminopropane in the examined complex 6 is in a chair conformation in aqueous solution is proved by the NMR analysis. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005