Abstract
In this paper, we report the anisochrony of the fluorine atoms of a CHF2 group when linked to a pyrazole ring. The pyrazole is part of (4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazole also known as (4S,7R)-campho[2,3-c]pyrazole, which has two stereogenic centers. Gauge-Independent Atomic Orbital (GIAO)/Becke, 3-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,f) calculated 19F chemical shifts of the minimum energy conformations satisfactorily agree with the experimental data. The energy differences between minima need to consider solvent effects (continuum model) to be satisfactorily reproduced.
Highlights
Anisochrony in NMR is observed when a prochiral group is linked to a molecule possessing a stereogenic center
The fluorine atoms are diastereotopic, and two distinct signals were observed for each one
A natural bond orbital (NBO) analysis shows that the energetic difference between the conformations minima at 0◦ and 180◦ can be explained based on the stabilization due to the sum of the charge transfer between the lone pair of the pyridine-like nitrogen and the σ* C-H bond and between the σ N-N and the σ* C-F bonds
Summary
Anisochrony in NMR is observed when a prochiral group is linked to a molecule possessing a stereogenic center. In these conditions, the studied nuclei became diastereotopic [1,2,3,4]. The observation of diastereotopic signals for other nuclei have been reported less often, but, for instance 31 P [10,11,12,13,14,15,16,17,18] is much more common than for 15 N, where only one example has been described [19].
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