Metal-mediated allylation reaction in aqueous media

Abstract

Organic reactions in aqueous media have become more important. Many organic chemists have focused on using water as solvent because of its low cost and environmentally benign behavior. One of the most important reactions in organic synthesis is the Barbier-Grignard type reaction. Till now, several metals have already been exploited for their ability to mediate the allylation of carbonyl compounds. Some examples are Zn, Sn, In, etc. This project explored the Barbier-type allylation reaction using 5 different transition metals. They were iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo) and tungsten (W), with various reported promoting agents. Iron was found to be able to mediate the allylation reaction of carbonyl compounds in aqueous media. With the activation of fluoride salts, iron metal can effectively perform the reaction. Different fluoride salts had various activating ability to iron. Sodium fluoride (NaF) was the most effective promoter. The allylation of different carbonyl compounds with several allylic halides was surveyed. Although the reaction time of the iron-mediated allylation was comparatively longer than the other metals, the advantages of the present reaction are the low cost and toxicity of iron over the other metals. Mechanistic study was also carried out. The possible intermediates and the role of fluoride ions in the iron-mediated allylation reaction were studied. In the literature, it was reported that the intermediate of the tin-mediated allylation was diallyl-tin-dibromide. It was proposed that the iron-mediated allylation reaction followed a similar route. However, iron was paramagnetic and the intermediate could not be observed in 1H-NMR spectroscopy. Thus, complex (n3-C3H5)Fe(CO)3Br was synthesized to mimic the intermediate of the iron-mediated allylation. The results showed that (n3-C3H5)Fe(CO)3Br might not be suitable to explain fully the mechanism of the iron-mediated allylation reaction.