Chirality of the trisubstituted nitrogen center – XRD, dynamic NMR, and DFT investigation of 1,2-dihydrobenzo[e][1,2,4]triazine derivatives

Molecular structures of two 1-aryl-2-benzyloxycarbonyl-1,2-dihydrobenzo[e][1,2,4]triazines 1 were established by single crystal XRD and compared to those of 4-benzyloxycarbonyl (2) and 4-benzyl (4) analogues. The structures revealed a highly pyramidalized asymmetric N(1) center stabilized by steric interactions in 1, but not in 2 and 3. Activation parameters for enantiomer interconversion were obtained by DNMR methods in C6D5Cl: ΔH‡ = 18.1(1) kcal mol−1, ΔS‡ = −0.6(1) cal mol−1 K−1 for Ar = Ph (1a) and ΔH‡ = 18.6(4) kcal mol−1, ΔS‡ = 5.2(6) cal mol−1 K−1 for Ar = 2-anisyl (1b). DFT computational investigation of the origin of the interconversion barrier on model compounds revealed steric destabilization of the inversion TS.