Stereointegrity of Tröger's Base:  Gas-Chromatographic Determination of the Enantiomerization Barrier

Abstract

Tröger's base 1 (2,8-dimethyl-6 H,12 H -5,11-methanodibenzo[ b, f ][1,5]diazocine) undergoes enantiomerization in the gas and liquid phases. By enantioselective stopped-flow multidimensional gas chromatography, Eyring activation parameters of the enantiomerization barrier have been determined in the inert mobile gas phase (helium): Δ G ⧧ gas (298.15 K) = 112.8 ± 0.5 kJ mol - 1; Δ H ⧧ gas = 62.7 ± 0.3 kJ mol - 1; Δ S ⧧ gas = −168 ± 6 J (K mol) - 1 . An enantiomerization pathway proceeding via a degenerated retro-hetero-Diels−Alder ring opening or formation of a zwitterionic structure of 1 is proposed. By enantioselective dynamic gas chromatography, Eyring activation parameters have also been determined via computer-aided simulation of experimental interconversion peak profiles in the chiral stationary liquid phase: Δ G ⧧ liq (298.15 K) = 117.8 ± 0.5 kJ mol - 1; Δ H ⧧ liq = 48.9 kJ mol - 1; Δ S ⧧ liq = −231 ± 8 J (K mol) - 1 . Surprisingly, in the presence of the chiral stationary phase (CSP) Chirasil-β-Dex, required for enantiomer separation of 1, the enantiomerization barrier is higher than in the gas phase. The concept of the retention increment R ‘ has been applied to distinguish the enantiomerization barrier of 1 in the dissolved and complexed state of the stationary phase.