Recent Progress in the Synthesis of the Monocarba‐closo‐dodecaborate(−) Anions

Abstract

This Concept article focuses on the rapid growth in studies of the chemistry of the monocarba‐closo‐dodecaborate(−) anion (C1 carborane anion). As one of the most stable anions known, the C1 carborane anion has been useful for exploring the chemistry of highly reactive cations. On the other hand, development of novel functional molecules utilizing the unique properties of C1 carborane anion (e.g., σ‐aromaticity, rigid spherical skeleton) has progressed more slowly. The main reason for this is the relatively undeveloped state of synthetic chemistry in this area. Recent advances in the synthetic chemistry of C1 carborane anion are highlighted in this Concept article, focusing on cross‐coupling reactions at the carbon vertex, direct conversion of B−H bonds, and the synthesis of multivalent weakly coordinating anions. These progressions move this species beyond its well‐established role of highly stable “counter” monocharged anion.

Synthesis of closo-CB11H12– Salts Using Common Laboratory Reagents

“…Derivatives of the [CB 11 H 12 ] − anion, for example, H­[CB 11 H 6 Cl 6 ] will protonate arenes, such as benzene, to form crystalline superacids, such as [C 6 H 7 ]­[CB 11 H 6 Cl 6 ]. , Unlike for other conjugate bases, such as triflic acid, the acid form of the carborane will not undergo further reaction . The chemically stable and inert anion has applications in the formation of complex metal and phosphine anions, which can have use as catalysts in polymerization reactions and isomerization. ,, Other applications include catalysis in pericyclic reactions involving the lithium cation, stabilizing novel gold complexes, or in cancer therapy, such as boron neutron capture therapy (BNCT). ,− …”

Intermolecular Cyclization between Carboranylphosphines and Electron-Deficient Alkynes

“…Density functional theory calculations (DFT) were carried out with compound 3a on the B3LYP/6-31+G (d, p) level (see the Supporting Information). The highest occupied molecular orbital (HOMO) is mainly localized on the nido - o -carborane moiety, and the HOMO (−5.89 eV) is lower than the conventional carborane phosphorus-containing system . These results suggest that this kind of zwitterionic carborane-fused phosphonium salt is more stable than already reported phosphonium zwitterions.…”

Sonogashira coupling of the ethynyl monocarborane [CB11H11-12-CCH]−

“…Modern methods involve B-H activation of the upper belt positions B(2-6), while the positions B(7-12) can be derivatized by iodi-nation and subsequent transition metal-catalyzed cross coupling; both approaches have been summarized in recent reviews. [26][27][28][29] The synthesis of B( 12)-and B(7, 12)-alkynyl monocarboranes was pioneered by Finze and coworkers. Their method was based on Kumada coupling, which they first reported in 2008 and later improved by applying microwave heating (Fig.…”

“…Modern methods involve B–H activation of the upper belt positions B(2–6), while the positions B(7–12) can be derivatized by iodination and subsequent transition metal-catalyzed cross coupling; both approaches have been summarized in recent reviews. 26–29…”