The rearrangements and stereoisomerizations of four systems, vinylcyclopropane, 4-tert-butylvinylcyclopropane, 5-methylvinylcyclopropane, and 2,5-dimethylvinylcyclopropane, as well as a variety of deuterated derivatives and 1- and 2-methyl-, methoxy-, difluoro-, and amino-substituted species, were studied by density functional theory calculations using the B3LYP functional and the 6-31G basis set. Energies were evaluated with CASSCF(4, 4)/6-31G single point calculations. The major product is obtained by the si pathway. Structures on this path are essentially pure diradical in character. Higher energy diradical species and intermediates are responsible for the scrambling of the stereochemistry. The stereoselectivity of the reaction is increased by substituents which increase the relative energy of the species involved in competing stereoselectivities. The computed secondary kinetic isotope effects reproduce the experimental values reported in the literature.

The prototypical 1,2-cis-vinylcyclopropanecarbaldehyde-to-2,5-dihydrooxepin hetero-Cope-type rearrangement was studied by "exact" first-principle methods. The reaction pathway was examined. The reaction, as well activation energies, was calculated for the unimolecular transformation of vinylcyclopropanecarbaldehyde and various derivatives. The derivatives differ from vinylcyclopropanecarbaldehyde by replacement of the formyl (CH=O) by the thioformyl (CH=S) or formiminyl (CH=NH) group and, in part, by replacement of hydrogen atoms at the substituted carbon atoms of the cyclopropane ring by hydroxyl or formyl substituents. The experimental reaction parameters of vinylcyclopropanecarbaldehyde are surprisingly well reproduced by B3LYP/6-31G * density functional and MP2/6-31G * a b initio quantum theoretical calculations. Reactant and product are nearly isoenergetic, while the activation energy amounts to about 25 kcal/mol. In the case of the nitrogen and sulfur containing compounds the isomeric seven-membered ring structures are considerably favoured over the cyclopropanes. Due to a low calculated activation energy a rapid formation of the 2,5-dihydrothiepin is expected. Substitution of the hydrogen at the substituted ring carbon atoms in vinylcyclopropanecarbaldehyde by OH and CH=O also lowers the barrier and increases, in general, the exothermicity of the reaction. As shown by the reaction energies of isodesmic reactions both reactants and products are stabilized by substitution. However, the seven-memberd ring compounds are more strongly stabilized than the cyclopropanes. The OH group exerts a different effect depending on whether the linkage is geminal or vicinal to the C=X group. The difference is caused by hydrogen bond formation in geminal arrangements. The substituent effects in the cyclopropane series parallel those for the prototypical Cope-type and Claisen-type series. The particular feature of the cyclopropane series is the lower stability of the cyclopropanes relative to the corresponding open chain congeners. This is obviously due to the ring strain which overcompensates for attractive interactions between cyclopropane and the substituents. In consequence, the formation of seven-membered ring compounds proceeds more easily than the formation of the corresponding compounds in related Cope-type rearrangements.The [3,3]-sigmatropic Cope-type and Claisen-type rearrangements are useful and versatile reactions that have attracted paramount interest in the past years [']. The prototypical degenerate Cope rearrangement of 1,Shexadiene to 1,5-hexadiene, and the Claisen rearrangement of allylvinylether to 4-pentenal (eq. 1 in Scheme l), have been studied theoretically at a high level of ab initio quantum theoryc21 and, more recently, at the level of density functional the0ry[~1 [~]. The pericyclic reactions proceed over delocalized ("aromatic") transition structures in a concerted manner.According to eq. 2 of Scheme 1 the divinylcyclopropaneto-cycloheptadiene and vinylcyclopropanecarbaldehyde-todihydrooxepin rear...

The Brandi reaction is the transformation of spiro[cyclopropane-1,5Ј-isoxazolidines] into tetrahydropyridones under thermal conditions. According to calculations performed by the restricted and unrestricted density functional theory and post-Hartree−Fock single-and multireference methods of ab initio quantum chemistry, the reaction proceeds through two biradical intermediates. These intermediates result from the homolytic cleavage of the N−O bond of the isoxazolidine ring in the first step, and the homolytic cleavage of one of the C−C bonds of the spiro-fused cyclopropane in the second. The activation energy of the rate-determining first step of the parent reaction amounts to about 40 kcal mol −1 at the RDFT/ UDFT level of theory. This energy is not much higher than

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