A DFT STUDY ON THE VINYLCYCLOPROPANECARBALDEHYDE-TO-2,5-DIHYDROOXEPINHETERO-COPE-TYPE REARRANGEMENT AND ON RELATED REACTIONS

The prototypical 1,2-cis-vinylcyclopropanecarbaldehyde-to -2,5-dihydro oxepin hetero-Cope-type rearrangement was studied by ''exact'' first-p rinciple methods. The reaction pathway was examined. The reaction, as well activation energies, was calculated for the unimolecular transfor mation of vinylcyclopropanecarbaldehyde and various derivatives I The derivatives differ from vinylcyclopropanecarbaldehyde by replacement o f the formyl (CH=O) by the thioformyl (CH=S) or formiminyl (CH=NH) gro up and, in part, by replacement of hydrogen atoms at the substituted c arbon atoms of the cyclopropane ring by hydroxyl or formyl substituent s. The experimental reaction parameters of vinylcyclopropanecarbaldehy de are surprisingly well reproduced by B3LYP/6-31G density functional and MP2/6-31G ab initio quantum theoretical calculations. Reactant a nd product are nearly isoenergetic, while the activation energy amount s to about 25 kcal/mol. In the case of the nitrogen and sulfur contain ing compounds the isomeric seven-membered ring structures are consider ably favoured over the cyclopropanes. Due to a low calculated activati on energy a rapid formation of the 2,5-dihydrothiepin is expected. Sub stitution of the hydrogen at the substituted ring carbon atoms in viny lcyclopropanecarbaldehyde by OH Emd CH=O also lowers the barrier and i ncreases, in general, the exothermicity of the reaction. As shown by t he reaction energies of isodesmic reactions both reactants and product s are stabilized by substitution. However, the seven-memberd ring comp ounds are more strongly stabilized than the cyclopropanes. The OH grou p exerts a different effect depending on whether the linkage is gemina l or vicinal to the C=X group, The difference is caused by hydrogen bo nd formation in geminal arrangements. The substituent effects in the c yclopropane series parallel those for the prototypical Cope-type and C laisen-type series. The particular feature of the cyclopropane series is the lower stability of the cyclopropanes relative to the correspond ing open chain congeners. This is obviously due to the ring strain whi ch overcompensates for attractive interactions between cyclopropane an d the substituents. In consequence, the formation of seven-membered ri ng compounds proceeds more easily than the formation of the correspond ing compounds in related Cope-type rearrangements.