title: SN2 regioselectivity in the esterification of 5- and 7-membered azacycloalkane quaternary salts: a DFT study to reveal the transition state ring conformation prevailing over the ground state ring strain.
abstract: The nucleophilic esterification of 5- and 7-membered N-phenylcyclic ammonium salts resulted in distinctive regioselectivity, despite their comparable ring strain in the ground states relative to the corresponding cyclopentane and cycloheptane (both 25.9 kJ mol(-1)). The former underwent a selective ring-opening reaction, while the latter predominantly underwent ring-emitting with concurrent ring-opening reactions. A DFT study of the model compounds revealed that the regioselection in the 5- and 7-membered azacycloalkane quaternary salts is plausibly directed by the transition state ring conformation, and not by the ground state ring strain. Remarkably, at the ring-opening transition state, the 5-membered cyclic skeletal structure expands toward the unstrained and thus less frustrated 6-membered cyclohexane conformation. On the other hand, the 7-membered counterpart expands at the ring-opening transition state toward the more frustrated 8-membered cyclooctane conformation to promote the alternative ring-emitting process.
authors.affiliation: Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan. ytezuka@o.cc.titech.ac.jp skawauchi@polymer.titech.ac.jp.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Kimura
authors.forename: Susumu
authors.initials: S
authors.lastname: Kawauchi
authors.forename: Takuya
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Tezuka