| Computational chemistry was used to elucidate mechanisms and phenomena in organic chemistry. (1) The rearrangement and the stereoisomerization of vinylcyclopropane and 1{dollar}spprime{dollar}-tert-butyl vinylcyclopropane were studied by DFT calculations in order to solve a long-lasting debate about the mechanism. The reaction follows a diradical mechanism with only one transition structure. The stereochemistry is explained by the involvement of high energy diradical species. The calculated secondary isotope effects reproduce the experimental data in the literature. (2) The Diels-Alder reactions of methyl cyclohexa-1,2-diene-1-carboxylate with furan was studied experimentally. All resulting products are endo adducts, although a mixture of two regioisomers is obtained. DFT computations on the reaction of propadiene with 1,3-butadiene and of 1,2-cyclohexadiene with butadiene and with furan provide an explanation for this experimental result. While there is a concerted mechanism available to these reactions, the stepwise mechanism is lower in energy. (3) Syn and anti 1,6:8,13-bismethano (14) annulenes were studied by various computational methods. It was possible to quantitatively calibrate the different methods used and elucidate the phenomena of aromaticity. (4) The {dollar}sp1{dollar}H-NMR spectra of isoporphycenato(II)palladium and (9) annulene anion show similar characteristic features. This similarity is further evidenced by a theoretical investigation of the conformational behavior of both molecules. (5) The conformational behavior of the newly discovered octapyrrolic macrocycles was studied by molecular mechanics. The lowest energy conformers of these molecules are chiral figure-eights. Theory was successful in explaining why some of these molecules are fluxional and others are rigid. (6) The deprotonation of the toluene radical cation was studied by ab initio methods. The theoretical kinetic isotope effects were obtained. They provide evidence that the microsomal oxidation of aromatic hydrocarbons by cytochrome P450 does not involve radical cations. |
