Synthesis of vicinal bromo-fluoro organic compounds using elemental fluorine

Abstract

The direct action of fluorine on bromine at −78° produces BrF which, without any isolation or purification, adds readily across various double bonds providing there is some hydrogen donor in the reaction mixture. Only trans addition of the elements of BrF was observed. When the reaction was applied to enons an easy elimination of HF can take place thus producing α-bromo enons. In our previous work we described the reaction of some olefins with IF, a reagent which was prepared in situ by the action of F 2 on I 2[1]. The literature deals with several other fluorohalogens, among them the three known bromo-fluoro compounds, BrF, BrF 3 and BrF 5, although only the last two are well characterized[2]. However, because of the instability and the high reactivity of these compounds, they have hardly been employed in organic chemistry[3]. We wish to describe here for the first time, an efficient and convenient method of synthesizing vicinal bromo-fluoro compounds using the primary source of the fluorine atoms, namely elemental fluorine itself[4]. When a mixture of F 2/N 2 is bubbled through a cold (−78°C), dilute solution of Br 2 (20 mmolar) in CFCl 3 (Freon) the bromine disappears and mainly BrF is produced. However, despite previous efforts, it seems that this compound cannot be isolated since it disproportionates easily to the very reactive BrF 3 and halogen on the aromatic ring can not be ascertained. The third product proved to be the corresponding bromo-ethoxy compound XV (15% yield). Prolonged chromatography or traces of HF from the reaction mixture results in the elimination of HF thus producing, quantitatively, 6-bromocumarin XVI[8]. 2-Bromo-3-fluoro cyclopentanone XVII (oil) if formed in 90% yield and isolated in almost pure form. As in the case of cumarin, XVII readily loses HF producing the 2-bromoclopentenone (XVIII)[9] in quantitative yield.