Nitrogen chirality via the sterical veto of N inversion

A theoretical study of inversion barriers and NMR chemical shifts of 3‐pyrazolines (2,3‐dihydro‐1H‐pyrazoles)

“…The next step was the calculation of energies and NMR chemical shifts of six known compounds ( 2 – 7 ) (Scheme 5). Compounds 2 and 3 are described in reference [9], without data for inversion barriers, and compounds 4 to 7 in references [11,41].…”

“…Despite that all of the minima were calculated, only the most stable, trans and trans‐trans , are provided in Table 5. Spectra for compounds 2 , 9 , 10 , and 11 were recorded at 60 MHz [9,38] while those of the remaining compounds were recorded at 300 or 400 MHz [11,41].…”

“…Conformational information about the blue compounds is available for cyclopentane [2,3], cyclopentene [4], pyrrolidine [2,5], pyrrolines [6], and pyrazolidines [7–11].…”

“…In what concerns nitrogen inversion, it has been reported and reviewed for pyrrolidines [12–14], of the two pyrrolines, only the 3‐pyrroline has been reported [15], and the barrier in pyrazolidines has been studied by several authors including ourselves [8,9,11,16]. Finally, the 3‐pyrazolines are the object of the present paper.…”

Five‐Membered Heterocycles: 1,2‐Azoles. Part 1. Pyrazoles

“…Finally, the resolution of compounds 425 and 426, whose chirality resides exclusively in the nitrogen atoms, has been reported by Kostyanovsky [411]. For previous studies on chiral nitrogen atoms in pyrazole reduced derivatives due to hindered inversion see Reference [412].…”