Recent developments in the chemistry of antiaromatic boroles

First isolated in 1969, most progress in the chemistry of the antiaromatic 4 pi electron borole system has been made during the 1980s. However, besides the fundamental aspects of the electronic structure and reactivity, boroles have not encountered serious research efforts for a rather long timeframe. This is somewhat surprising given the fact that boroles feature a unique combination of antiaromaticity, strong electrophilicity and unusual electronic properties. It was not until 2008 that interest was resparked. Since then, tremendous progress has been achieved in this area, particularly with respect to synthetic access, structural characterization and reactivity. Various differently substituted borole derivatives have been successfully isolated and characterized both in solution and in the solid state, which provided a more thorough understanding of the structure/reactivity relationship. This feature article is intended to provide a general overview on the electronic structure and the consequences of antiaromaticity on the inherent properties of these highly reactive species. The different synthetic methodologies to generate boroles and their divergent reactivity patterns will be described in great detail, which will emphasize their high potential and relevance in modern chemistry.