Kinetic study on reactions of orto and para substituted arylmethylsulfide oxidation. Substituent effect.

KINEN, C.O.; ROSSI, L.I.; VELASCO, M.I.; HOYOS DE ROSSI, R. M.

Los Cocos, Córdoba, Argentina

Congreso; 9a Conferencia Latinoamericana de Físico-Química Orgánica; 2007

Transition metals and their organometallics complexes play a central role in Organic synthesis, due to their participation as catalyst in many and variable reactions. [1] In previous studies, we have presented an oxidation method of sulfides to sulfoxides that shown a high degree of chemioselectivity respect to diverse functional groups. [2] Only a few examples of kinetic studies of this reactions catalyzed by transitions metals are found in the literature. [3] On these bases and our interest in a better understanding of the reaction mechanism, we present the kinetic study of the sulfoxidation reaction. The influence of the different substituents on the reaction rate of a series of orto y para-substituted arylmethylsulfides was studied. The investigation was carried out with the following reactants: 4-(methylthio)-benzaldehide, 4-(methylthio)-phenylisothiocyanate, 1-metoxi-4-(methylthio)-benzene, 4-bromothioanisole, metil-4-nitrophenylsulfide, phenylmethylsulfide, 4-(methylthio)-benzonitrile y 4-(methylthio)-benzoic acid, 4-(methylthio)-acethophenone, 2-(methylthio)-benzaldehide, 2-(methylthio)-benzoic acid and 2-(methylthio)-phenylisothiocyanate. Cooper bromide or iron bromide was used as catalyst and Nitric acid as oxidant agent. The reactions were carried out using acetonitrile as solvent at 25ºC with magnetic stirring. All these reactions were followed by UV-Visible spectroscopy and gas chromatography. The products were identified by 1H NMR, 13C NMR and GC-Mass against authentic sample. The first studied substrate was 4-(metiltio)benzaldehído. Different reaction conditions were tested modifying the catalyst, oxidant and molar relation substrate: oxidant. Of all the tried conditions, the most adapted for the accomplishment of this study was: FeBr3 5%, HNO3 13%, acetonitrile as solvent and magnetic stirring. All the other substrates were oxidized in identical conditions in order to get comparable results. Results shown that the electronic nature of substituent wouldn’t have a significant effect on the reaction rate, whereas the diminution in the values of the rate observed in orto- substituted substrates against para-substituted, could be attributed to the steric effect. On the other hand, reactions of substrates with substituents capable to coordinate with metallic centers shown inhibition with cooper bromide but were catalyzed by iron bromide. This effect could be due to the greater ability of cooper to coordinate sulfides. substituent wouldn’t have a significant effect on the reaction rate, whereas the diminution in the values of the rate observed in orto- substituted substrates against para-substituted, could be attributed to the steric effect. On the other hand, reactions of substrates with substituents capable to coordinate with metallic centers shown inhibition with cooper bromide but were catalyzed by iron bromide. This effect could be due to the greater ability of cooper to coordinate sulfides.