Electron Pool Enrichment of Polyhedral Carboranes: A Key to Isomerization and Nucleophilicity

Carboranes are inorganic clusters exhibiting 3D aromaticity,
which
imparts exceptional thermal stability, low electrophilicity, and a
large HOMO–LUMO gap. The isomerization of carboranes at ambient
temperature remains, in many cases, a synthetic challenge. In this
work, we present a new method based on the reactivity of closo-1,2-C2B8H10 with various types
of carbenes. Such reactions yield arachno-shaped
carboranes, which undergo facile dihydrogen elimination at low temperatures,
accompanied by C–C bond cleavage and migration of the carbon
atoms. These compounds react in the reverse manner compared to common
neutral boranes, and their basicity is governed by the electronic
parameters of the coordinated carbene molecules. Treatment with hydrogen
chloride can thus leave the neutral carborane intact or protonate
it once or even twice. The doubly protonated carboranes immediately
undergo chlorination by the chloride anion. Thus, the carbene-carborane
adducts undergo hydrogen-induced redox changes.