Completion of Crystallographic Data for the Series of 4-Halogenated-1H-pyrazoles: Crystal Structure Determination of 4-Iodo-1H-pyrazole and Spectroscopic Comparison

Abstract

Prior to 2021, 4-bromo-1H-pyrazole (published in 1999) was the only structurally characterized 4-halogenated-1H-pyrazole in the Cambridge Crystallographic Data Center (CCDC). The structures of 4-chloro-1H-pyrazole and 4-fluoro-1H-pyrazole were published in 2021 and 2023, respectively. Herein, we report the crystal structure for 4-iodo-1H-pyrazole, completing the crystallographic data for the series of 4-halogenated-1H-pyrazoles. The bromo and chloro analogs are isostructural, forming trimeric H-bonding motifs, whereas the fluoro and iodo analogs form non-isostructural catemers. We also compare the experimental and theoretical (by DFT calculations) IR and 1H NMR spectroscopic data of the four halogenated 4-X-pzH compounds and unsubstituted pyrazole (pzH). An explanation is offered for some counterintuitive structural, infrared, and 1H-NMR spectroscopic data.

Recent Advances in the Synthesis of Pyrazole Derivatives: A Review

“…Pyrazole, as an aromatic heterocycle with π-electron excess, displays distinct reactivity patterns in organic chemistry. Nucleophilic attacks are favored at positions 3 and 5, while electrophilic substitution reactions preferentially take place at position 4 [10] (Figure 2). Unsubstituted pyrazole can be represented in three distinct tautomeric forms, as illustrated in Figure 3.…”

“…Reactions 2023, 4, FOR PEER REVIEW 10 Huang and Katzenellenbogen [23] successfully obtained a series of 4-alkyl-1,3,5-triarylpyrazoles 44 in a regioselective manner via the oxidation of pyrazolines 43. These pyrazolines were initially prepared via a cyclocondensation reaction between phenyl and 4methoxyphenylhydrazine and chalcones 41, followed by alkylation at the C-4 position of the pyrazoline ring in 42 (see Scheme 15).…”