Orbital interactions in cyclic molecules
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Reaction of endo-tricyclo[188.8.131.52²,⁴]octane (oct-6-ene), 2-methyl-endo-tricyclo[184.108.40.206²,⁴]octane (oct-6-ene) and exo-tricyclo[220.127.116.11²,⁴]octane (oct-6-ene) with selected electrophiles are examined. Electrophilic attack at the cyclopropane ring occurs with inversion and this has been rationalised from a consideration of HOMO/LUMO interactions, and in particular, the secondary orbital interactions of the LUMO of the electrophile with the sigma framework in the HOMO. Protonation of a cyclopropane occurs with substantial positive charge development in the transition state and this dictates that carbocation stabilities are qualitatively an important feature for understanding the reaction. For mercuration of a cyclopropane, the absence of significant charge development in the transition state is considered a consequence of poor overlap of the Hg 6s LUMO with the cyclopropyl HOMO orbitals and rearrangement is disfavoured. Where rearrangement is observed carbocation stability is important in dictating the reaction driving force. When attack at either a similarly substituted double bond or cyclopropyl ring is possible, the regiochemistry of electrophile attack lS rationalised from a consideration of cation stabilities. Proton attack occurs preferentially at a cyclopropyl ring while Br⁺ attack occurs at a double bond. However, for mercuric acetate attack, there is no significant cation stabilising preference for attack at either a double bond or a cyclopropyl ring, and the regiochemistry of electrophile attack follows from a consideration of the relative contributions of the pi and cyclopropyl orbitals to the HOMO.