Estudo das condiÃÃes experimentais para o Homo e Hetero-acoplamento 2-bromopiridinas utilizando um sistema eletroquÃmico

AUTOR(ES)

Tupolevck FlorÃncio

DATA DE PUBLICAÇÃO

2004

RESUMO

The homocoupling reaction of 2-bromomethylpyridines and 2- bromopyridine was investigated. NiBr2 catalyst was employed using an electrochemical process, DMF as solvent, Zn or Fe as sacrificial anode, NaI or tetraalkyl ammonium salts as supporting electrolyte, an undivided cell and ambient temperature (~26ÂC). The synergetic effect caused by supporting electrolyte and sacrificial anode was determined by the reactions following and indentifying reactive intermediaries with iodine. Reactions carried out with sacrificial anode of Zn indicates the presence of higher concentrations of organozinc. It was supposed a reductive transmetalation step, which was slowly consumed due a new transmetalation step, between organozinc and nickel complex (reverse transmetalation reaction). 2,2â-Bipyridine was formed in the absence of current dued a disproportionation step of the Ni intermediary complex, allowing the homocoupling process end. The organozinc transmetalation process was difficulted in the presence of tetraalkylammonium salts, what does not occur in the presence of NaI. Better results were obtained when Fe was used as sacrificial anode and NaI; corroborating literature results. The heterocoupling of halopyridines catalysed by Ni complex was also tested, by using the same electro-reductive method. The presence of brome in both reagents increased the reaction yields; it was observed that highest concentration catalyst or small addition of one of the reagents, do not interfer in the heterocoupling process as suggested in some works. The best hetero-coupling yields are near the theoretical results (50%).

ASSUNTO(S)

homocoupling quÃmica homo-acoplamento processo de transmetalaÃÃo quimica chemistry reductive transmetalation

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