1,3,2-Diazaborolyl Anions – From Laboratory Curiosities to Versatile Reagents in Synthesis

Diazaborolyl anions with coordinatively unsaturated boron centers are exceedingly powerful in their nature as nucleophiles, Brønsted bases, or reducing agents. Lithium-, magnesium-, and zinc-diazaborolyls find use as excellent transfer reagents of the borolyl group to s-and p-block elements, transition metals, and lanthanides. Cross couplings with bromoarenes or acyl chlorides affording 2-aryl-diazaboroles and the related acyl boranes were elegantly carried out by use of the more selective zinc diazaborolyls. Particular highlights in this chemistry are the syntheses of stable bis(diazaborolyl) metal(II)-radicals of gallium, indium, and thallium as well as 1,2-bis(diazaborolidinyl)diphosphenyl radical anions. Moreover, stability is imparted to the first isolable digermavinylidene by two geminal diazaborolyl substituents.

Lithium diazaborolyls and related species serve as transfer reagents of the borolyl anion to a large variety of electrophiles, giving rise to stable Ga^II, In^II, Tl^II radicals as well as digermavinylidines. Novel diphosphenes and dibismuthenes owe their stability to the bulky and electron-releasing boryl units. Diazaborolyls of late transition metals such as Mn, Re, Fe, Co are also accessible.

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