Borylenes BR are a fascinating class of reactive intermediates that have remained rather mysterious to chemists for decades, despite their fundamental importance associated with the close electronic relationship to the well-known carbenoids CR_2. However, in stark contrast to the latter species, conclusive evidence for the existence of free borylenes or a free borylene mechanism has not yet been obtained. Only recently, borylenes have been successfully incorporated into the ligand sphere of transition metals, thus enabling the isolation and structural characterization of stable borylene complexes. While these achievements are an important step towards an improved understanding of the electronic properties of borylenes, all attempts to generate free borylenes selectively have been consistently hampered by their high reactivity. Various more-or-less successful trapping approaches have been developed to substantiate their existence, some of which subsequently having been disputed.
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