Sterically hindered nitrogen inversion in five-membered cyclic hydrazines

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].…”

3-Pyrazolines (2,3-dihydro-1H-pyrazoles): synthesis, reactivity, and physical and biological properties

“…5). 38 This result is very unexpected and the compounds were characterized by an NH band in the IR spectra and by two (30)(31)(32)(33) or three (29) signals in the 1 H NMR spectra due to the NH and the saturated CH at position 5. We have tried to repeat this work without success.…”

“…32 to prepare the sterically hindered 3-pyrazolines that were required to study their nitrogen inversion. 33,34 1,3-Difunctional compounds (β-dicarbonyls, β-ketoesters, and similar compounds) react with hydrazine to afford different types of pyrazoles. Amongst them are malononitriles that yield 3,5-diamino-1H-pyrazoles.…”

Aliphatic aldehydes in the synthesis of carbo- and heterocycles. Part I. Synthesis of three-, four-, and five-membered rings

“…Under analogous conditions, N,N′-dimethyl-and N,N′-diethylhydrazines gave rise, respectively, to 2,3,4,5-tetraalkyl-1,3,4-oxadiazolidines 108 and 109 as the major products. Presumably, the formation of different heterocycles in these reactions is determined by steric factors [123] Carbonyl group protection is an important problem of organic and medicinal chemistry. Numerous procedures have been developed for this purpose [126,127].…”