G. Verniest

Metalated 3-halo-1-azaallylic anions are important building blocks for the preparation of a wide variety of heterocyclic and highly functionalized compounds. A theoretical description of the structural properties of halogenated 1-azaallylic anions in vacuo and in tetrahydrofuran (THF) solution is presented to gain insight into their reactivity behavior. The configurational flexibility of fluorinated and chlorinated 1-azaallylic anions is examined, and it is shown that these anions have far less configurational flexibility as compared with nonhalogenated analogues, with a strong preference to occur as Z/anti isomers. In addition, the driving force for transmetalation, that is, the replacement of the lithium cations with K+, Cu+, ZnCl+, CuCl+, or MgBr+ is studied. To obtain reliable results, the structures were modeled in THF using the combined implicit/explicit solvent approach resulting in different coordination numbers for lithium in the Z/anti and E/anti isomers. Calculations on dimerization energies show that coordination with THF is energetically preferred over aggregation.

The hindered rotor scheme, originally developed for internal rotors in flexible chains (Van Speybroeck, V.; Van Neck, D.; Waroquier, M.; Wauters, S.; Saeys, M.; Marin, G. B. J. Phys. Chem. A 2000, 104, 10939), is extended to puckering motions in four-membered rings. The applicability of the approach is tested in a variety of heterocyclic compounds for which the partition function, entropy, and heat capacity are calculated. The entropy may be substantially altered by a correct description of the puckering mode. The equilibrium puckering angle ranges between 0° and 30° depending on the heterosubstitution X (CH2, O, S, NH, PH, CO, CS, CNH, CPH) in the four-membered ring.