sp2–sp3 diboranes: astounding structural variability and mild sources of nucleophilic boron for organic synthesis

Abstract

Despite the widespread use of organoborane reagents in organic synthesis and catalysis, a major challenge still remains: very few boron-centered nucleophiles exist for the direct construction of B–C bonds. Perhaps the most promising emerging solution to this problem is the use of sp²–sp³ diboranes, in which one boron atom of a conventional diborane(4) is quaternised by either a neutral or anionic nucleophile. These compounds, either isolated or generated in situ, serve as relatively mild and convenient sources of the boryl anion [BR₂]⁻ for use in organic synthesis and have already proven their efficacy in metal-free as well as metal-catalysed borylation reactions. This Feature article documents the history of sp²–sp³ diborane synthesis, their properties and surprising structural variability, and their burgeoning utility in organic synthesis.