Nuclear magnetic double resonance studies of the dimethylcyclopropylcarbinyl cation. Measurement of the rotation barrier

Carbocations in Superacid Systems

“…[201][202][203] These studies have led to the conclusion that cyclopropylmethyl cations adopt bisected geometry and are static in nature with varying degrees of charge delocalization into the cyclopropane ring. The energy difference between bisected and eclipsed structures is estimated to be 13.7 kcal mol À1 (by temperature-dependent NMR studies) 204 and is quite close to the 12.3 kcal mol À1 energy obtained by molecular orbital calculations at the minimal basis set STO-3G. The methyl groups are nonequivalent and show a 1 H NMR shift difference of 0.54 ppm.…”

New Class of Glycoside Hydrolase Mechanism-Based Covalent Inhibitors: Glycosylation Transition State Conformations

“…In the case of covalent inhibitor 3, σ-bond participation requires a bisected geometry; however, the resultant cation likely remains in the original bisected geometry due to the high rotational barrier in bicyclobutonium ions. 30 We conclude covalent labeling by 2, relative to 3, involves a reaction coordinate that more closely matches that of the natural substrates.…”

“…Thus, we reason the enzyme binds 2 in a 2 H 3 half-chair with a resulting catalyzed formation of an E 3 allylic cation or allylic cation-like TS (rose box, Figure ), a species that is conformationally similar to the glycosylation TS ( 4 H 3 ). In the case of covalent inhibitor 3 , σ-bond participation requires a bisected geometry; however, the resultant cation likely remains in the original bisected geometry due to the high rotational barrier in bicyclobutonium ions . We conclude covalent labeling by 2 , relative to 3 , involves a reaction coordinate that more closely matches that of the natural substrates.…”

A mechanism-based inactivator of glycoside hydrolases involving formation of a transient non-classical carbocation

“…In particular, reactions that involve cyclopropyl s-bond participation are controlled by stringent geometric requirements 26,35,36 , and efficient participation of a cyclopropyl C-C s-bond requires the formation of a bisected geometry in the bicyclobutonium cation 35,37 . Indeed, in the case of the 2-cyclopropyl-2-propylium cation, this arrangement is 57 kJ mol À 1 lower in energy than the perpendicular conformation 38 . Because the third criterion requires the enzyme to catalyse its own inactivation, we were interested in comparing the extent of TS stabilization provided by the enzyme for its inactivation by 1 with that for the catalysed hydrolysis of a-D-galactopyranosides.…”