Asymmetrical nonbridgehead nitrogen—XXIV

The stereodynamics of 1,2-dipropyldiaziridines

N-alkylated trans-diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2-di-n-propyldiaziridine 1, 1-isopropyl-2-n-propyldiaziridine 2, and 1,2-diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil-beta-Dex with high separation factors alpha (1-isopropyl-2-n-propyldiaziridine: 1.18; 1, 2-diisopropyldiaziridine: 1.24; 100 degrees C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150 degrees C, and peak coalescence at elevated temperatures. The enantiomerization barriers DeltaG(double dagger) and activation parameters DeltaH(double dagger) and DeltaS(double dagger) were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1-isopropyl-2-n-propyldiaziridine and 1,2-diisopropyldiaziridine exhibit similar high interconversion barriers DeltaG(double dagger) (100 degrees C) of 128.3 +/- 0.4 kJ mol(-1) and 129.8 +/- 0.4 kJ mol(-1), respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process.

Aza-β3-Cyclohexapeptides: Pseudopeptidic Macrocycles with Interesting Conformational and Configurational Properties Slow Pyramidal Nitrogen Inversion in 24-Membered Rings!

Among pseudopeptidic foldamers, aza-beta3-peptides have the unique property to possess nitrogen stereocenters instead of carbon stereocenters. As the result of pyramidal inversion at N(alpha)-atoms along the backbone, they behave as a set of C8-based secondary structures in equilibrium. This structural modulation is exploited here to prepare 24-membered macrocycles with great efficiency. Both crystal structures and spectroscopic data establish that aza-beta3-cyclohexapeptides adopt a highly organized conformation where the relative configuration of chiral nitrogen atoms is alternated. This makes them an interesting scaffold as the stereocontrol occurs spontaneously through the cyclization. These compounds reveal an unprecedented slow pyramidal nitrogen inversion in macrocycles. Pyramidal ground state stabilization, hindered rotation, steric crowding, and H-bond cooperativity are proposed to participate in this striking phenomenon. The equilibrium between invertomers of aza-beta3-cyclohexapeptides is reminiscent of the interchange between the two chair forms of cyclohexane.