Abstract
The synthesis and characterization of the precursor isomers trans-4-(2-(pyridin-2-yl)vinylbenzaldehyde (I), trans-4-(2-(pyridin-4-yl)vinylbenzaldehyde (II), trans-4-(2-(pyridin-2-yl)vinylbenzoic acid (III) and (E)-4-(2-(pydridin-4-yl)vinylbenzoic acid (IV) are reported. These compounds were prepared in order to obtain trans-4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO (V). Compounds I and II were obtained by using a Knoevenagel reaction in the absence of a condensing agent and solvent. Oxidation of the aldehyde group using the Jones reagent afforded the corresponding acid forms III and IV. A condensation reaction with 4-amino-TEMPO using oxalyl chloride/DMF/CH2Cl2 provided the 4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO. Single crystals of compounds I, II and III were obtained and characterized by X-ray diffraction. Compound I belongs to space group P21/c, a = 12.6674(19) Å, b = 7.2173(11) Å, c = 11.5877(14) Å, β = 97.203(13)° and the asymmetric unit was Z = 4, whereas compound II was in the space group P21, with a = 3.85728(9) Å, b = 10.62375(19) Å, c = 12.8625(2) Å, β = 91.722 (2)° and the asymmetric unit was Z = 2. Compound III crystallized as single colorless needle crystals, belonging to the monoclinic system with space group P21, with Z = 2, with a = 3.89359(7) Å, b = 17.7014(3) Å, c = 8.04530(12) Å, β = 94.4030 (16)°. All compounds were completely characterized by IR, 1H-NMR, EI-MS and UV-Vis.
Highlights
Since the first stable radical compound was synthesized in 1961 [1] a series of stable radical structures, such as nitroxyl, phenoxyl, and hydrazyl have been successfully synthesized
We report the preparation of precursors to obtain low molecular weight styrylpyridine derivatives bearing tetramethylpiperidine 1-oxyl (TEMPO), and their structures obtained by X-ray crystallography analysis, together with their complete characterization
Solvent-free methods at temperatures of 120–140 °C were successfully used to synthesize different styrylpyridines and α,β-acrylonitriles as well as intermediates formed in the condensation reaction of methylpyridines and aromatic aldehyde compounds [53,54]
Summary
Since the first stable radical compound was synthesized in 1961 [1] a series of stable radical structures, such as nitroxyl, phenoxyl, and hydrazyl have been successfully synthesized. These compounds are stable radical compounds that have an electron spin in spite of being organic materials [2]. Nitrogen heterocyclic nitroxides and their diamagnetic derivatives (sterically hindered amine and hydroxylamine) are known as low-molecular-weight multifunctional antioxidants that can participate in one-electron redox processes. Molecules bearing nitroxides covalently linked to fluorophore have been used as fluorescence spin-label [13,14,15,16] or nuclear-localizing redox probes [17]
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