Sterically hindered and completely arrested nitrogen inversion in pyrazolidines

Syntheses of trans-1,2-di-tert-butylpyrazolidine 1, d,l- and semi-meso-1,2-diisopropyl-3,5-dimethylpyrazolidines, 2a and 2b, respectively, have been developed. Activation parameters of the nitrogen inversion in 1 (ΔG≠ = 123 kJ mol−1 at 110 °C, ΔH≠ = 114 kJ mol−1, ΔS≠ = −15 J K−1 mol−1) have been determined. The steric veto of the nitrogen inversion in 2a has been confirmed. Chemical transformations of 1 have been studied, and the crystal structures of 2a·picrate and 2b·HCl determined.

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trans-1,2-Di-tert-butylpyrazolidine-(R)-(−)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphateC31H37N2O4PEe ⩾93% (according to 1H NMR)[α]D20=-11.2 (c 0.032, MeOH)Absolute configuration: unknown