Geminal interactions at the termini of polyenes

Abstract

The study describes a comprehensive investigation of the geminal union between the groups methylene, ethylene, cyclopropane and the polyenes ethylene, butadiene, hexatriene, allyl, pentadienyl, heptatrienyl (the latter three as cation and anion). The interactions are assessed with the frontier molecular orbital method within the framework of simple perturbation theory. It is concluded that geminal interactions with methylene fall in the groups: cyclohexadienyl cation (strong interaction), cyclopentadiene (some interaction), cyclobutenyl anion, cyclooctatrienyl anion, cyclopropene, cyclohexadienyl anion (little, if any, net interaction), cycloheptatriene, cyclobutenyl cation, cyclooctatrienyl cation (deconjugated by way of a homoaromatic interaction). For ethylene the geminal interaction groupings are found to be: benzyl cation and anion (strongly conjugated), trifulvene, pentafulvene, heptafulvene (some interaction), methylenecyclobutenyl anion and methylenecyclooctatrienyl anion (little, if any, net interaction), methylenecyclobutenyl cation, methylenecyclooctatrienyl cation (deconjugated by way of a homoaromatic interaction). Whereas for cyclopropane the geminal interactions are grouped as: spiro[2,5]octadienyl cation (strongly conjugated), spiro[2,4]- heptadiene (some interaction), spiro[2,3]hexenyl anion, spiro[2,7]decatrienyl anion, spiro[2,2]pentene, spiro[2,5]octadienyl anion (little, if any, net interaction), spiro[2,3]hexenyl cation, spiro[2,7]decatrienyl cation, spiro[2,6]nonatriene (deconjugated by way of a homoaromatic interaction). Where possible, analogies are drawn between homoaromatic (vicinal) union and the geminal interactions. Ringcurrent effects are found to be quite important In cyclopentadiene (through methylene hyperconjugation) and cycloheptatriene (through a 1-6 homoaromatic interaction), but much less so in the fulvenes. It is concluded that little correlation may be expected between the resonance energies, ring currents and other criteria of aromaticity in non-alternant hydrocarbons.