Kinetic Study of the Oxidation Reactions of orto and para-Substituted Arylmethylsulfides?

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

Los Cócos, Córdoba, Argentina.

Conferencia; CLAFQO-9, 9º Conferencia Latinoamericana de Físicoquímica Orgánica. 9th Latin American Conference on Physical Organic Chemistry; 2007

CLAFQO

Transition metals and their organometallic complexes play a central role in Organic Synthesis due to their participation as catalysts in numerous and variable reactions.^[1] In previous studies,^[2] we have presented a sulfoxidation method that showed a high degree of chemoselectivity whit respect to diverse functional groups.

In order to gain a better understanding of the reaction mechanism, we carried out a kinetic study of the sulfoxidation reaction. Only a few examples of kinetic studies of these reactions catalysed by transition metals are found in the literature.^[3]

The influence of the different substituents on the reaction rate of a series of orto and para-substituted arylmethylsulfides was studied. The substrates used were Me-S-Ph-X, where X = 4-NCS; 4-OMe; 4-CHO; 4-Br; 4-NO₂; H; 4-CN; 4-COOH; 4-CO-Me; 2-CHO; 2-COOH; 2-NCS. CuBr₂ or FeBr₃ were used as catalysts and HNO₃ as oxidant agent. The reactions were performed in homogeneous solution using acetonitrile as solvent at 25ºC with magnetic stirring. All reactions were followed by UV-Vis spectroscopy and/or gas chromatography and were conducted in duplicate. The products were identified by ¹H NMR, ¹³C NMR and/or GC-Mass and found to be identical to authentic samples.

Different reaction conditions were tested modifying the catalyst, oxidant and molar ratio substrate : oxidant in order to get reaction rates within the time limit of our techniques. The most appropriate for this study was FeBr₃ 5%, HNO₃ 13%, acetonitrile as solvent and magnetic stirring. Results show that the electronic nature of the substituent does not have a significant effect on the reaction rate, whereas the decrease in the rate observed in orto-substituted compared with para-substituted compounds, could be attributed to the steric effect.

On the other hand, reactions of substrates with substituents capable of coordinating with metallic centers showed inhibition with CuBr₂ but were catalysed by FeBr₃. This effect may be attributed to the greater ability of copper to coordinate sulfides.

[1] Organometallics in Organic Synthesis, Chem. Rev. 2000, 100 (8). Thematic Issue.

[2] C.O. Kinen, L.I. Rossi*, R. Hoyos de Rossi*. Applied Catalysis A: General, 2006, 312, 120.

[3] a- V.K. Sivasubramanian, M. Ganesan, S. S. Rajagopal, R. Ramaraj. J. Org. Chem. 2002, 67, 1506. b-  T.K. Ganesan, S. Rajagopal, J.B. Bharathy, A.I.M. Sheriff. J. Org. Chem. 1998, 63, 21.