Classically closo-carborane anions, particularly
[HCB11H11]− and [HCB9H9]−, and their derivatives have primarily
been used as weakly coordinating anions to isolate reactive intermediates,
platforms for stoichiometric and catalytic functionalization, counteranions
for simple Lewis acid catalysis, and components of materials like
liquid crystals. The aim of this article is to educate the reader
on the contemporary nonclassical applications of these anions. Specifically,
this review will cover new directions in main group catalysis utilized
to achieve some of the most challenging catalytic reactions such as
C–F, C–H, and C–C functionalizations that are
difficult or impossible to realize with transition metals. In addition,
the review will cover the utilization of the clusters as dianionic
C σ-bound ligands for coordination chemistry, ligand substituents
for coordination chemistry and advanced catalyst design, and covalently
bound spectator substituents to stabilize radicals. Furthermore, their
applications as solution-based and solid-state electrolytes for Li,
Na, and Mg batteries will be discussed.
In this feature article we cover new directions in the fundamental and applied chemistry of the closo-carborane anions [HCB11H11]−1 and [HCB9H9]−1, including energy storage applications, ionic liquids, anionic carborane fused heterocycles/radicals, ligand substituents, and ligands for catalysis and coordination chemistry.
Here we report the surprising discovery that high-energy vinyl carbocations can be generated under strongly basic conditions, and that they engage in intramolecular sp3 C–H insertion reactions through the catalysis of weakly coordinating anion salts. This approach relies on the unconventional combination of lithium hexamethyldisilazide base and the commercially available catalyst, triphenylmethylium tetrakis(pentafluorophenyl)-borate. These reagents form a catalytically active lithium species that enables the application of vinyl cation C–H insertion reactions to heteroatom-containing substrates.
The syntheses of unsymmetrical N-heterocyclic carbenes (NHCs) that contain a single N-bound icosahedral carborane anion substituent are reported. Both anionic C-2 and doubly deprotonated dianionic C-2/C-5 NHC lithium complexes are isolated. The latter species is formed selectively, which reveals a surprising directing effect conveyed by icosahedral carborane anion substituents.
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