An efficient Knoevenagel route using green chemistry conditions was applied for the synthesis of halogen- and cyano- substituted pyridinevinylene compounds. Absorption and fluorescence emission spectra of these conjugated compounds were recorded and compared in order to evaluate the effect of substituents on the electronic properties of pyridinevinylene compounds. The substituents studied were terminal Cl and F, two or three aromatic rings, as well as a cyano group attached to a C=C double bond. The compounds synthesized are: (E)-2-(4-fluorostyryl)pyridine, (E)-2-(4-chlorostyryl)pyridine, (E)-4-(4-chlorostyryl)pyridine, 2,3-diphenylacrylonitrile, 3-phenyl-2-(pyridin-2-yl)acrylonitrile, 3-phenyl-2-(pyridin-3-yl)acrylonitrile, 2-phenyl-3-(pyridin-2-yl)acrylonitrile, 3,3′-(1,4-phenylene)bis(2-phenylacrylonitrile), 3,3′-(1,4-phenylene)bis(2-(pyridin-2-yl)acrylonitrile), and 3,3′-(1,4-phenylene)bis(2-(pyridin-3-yl)acrylonitrile). The solvent-free method used in this work allows obtaining each compound by controlling the reaction temperature. The compounds were characterized by infrared spectroscopy and 1H-NMR spectroscopy.

Abstract:The compound 1-(p-fluorophenyl)-2-( -pyridyl)ethanol and its corresponding dehydration compound 1-(pfluorophenyl)-2-( -pyridyl)ethene were obtained from the Knoevenagel condensation reaction between 2-methylpyridine with p-fluorobenzaldehyde. The X-ray structure determined for 1-(p-fluorophenyl)-2-( -pyridyl)ethanol reveals that the compound crystallizes in the monoclinic system space group, P2 1 /n, containing four molecules in each crystal unit cell [a = 5.3664 (15) (2)°. The molecular structure shows the p-fluorophenyl ring attached to the double bond and located trans to the pyridine ring. The crystallography data give evidence that the intermediary compound is actually the alcohol just before the dehydration process that yields the trans double bond of the 1-(p-fluorophenyl)-2-( -pyridyl)ethene.

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