Improved synthesis of icosahedral carboranes containing exopolyhedral B-C and C-C bonds

Abstract

Carboranes are boron-rich molecular clusters possessing electronic characteristics that allow for orthogonal approaches to vertex-selective modifications. We report improved functionalization methods utilizing orthogonal chemistry to achieve efficient substitution at electron-rich B-vertices and electron-poor C-vertices of carborane. Functionalization of B-vertices with alkyl and (hetero)aryl groups using the corresponding Grignard reagents has been improved through the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand, resulting in reduced reaction times. Importantly, this method is tolerant towards alkyl-based Grignard reagents containing β-hydrogens. Furthermore, a transition metal-free approach to the substitution of carborane C-vertices with (hetero)aryl substrates has been developed under nucleophilic aromatic substitution (S_NAr) conditions. The selective substitution of carboranes afforded by these methods holds potential for the rational synthesis of heterofunctionalized boron clusters with substituents on both boron and carbon-based vertices.

Keywords: boron clusters; carborane; catalysis; cross-coupling; nucleophilic aromatic substitution.

Figure 1:

A) Previous substitution method for C-(hetero)arylation. B) Previous B-substitution method via Kumada coupling. C) Improved C- and B-vertex functionalization procedures.

Figure 2:

Reaction scheme for mono- and disubstitution of ortho-carborane. Ortho-carborane coupling products with GC-MS conversion and percent isolated yields in parentheses. See Table S2 in SI for exact reaction conditions.

Figure 3:

Reaction scheme for monocoupling of meta-carborane in the presence of B-Br bonds. Meta-carborane coupling products with GC-MS conversion and percent isolated yields in parentheses. See Table S2 in SI for exact reaction conditions.

Figure 4:

A) C-functionalization reaction scheme B) Products with GC-MS conversions and isolated yields in parentheses. C) Polyfunctionalized carboranes with associated single crystal X-ray structures obtained by extrapolating the general method in A. (*) indicates that conversion was determined through ¹H NMR instead of GC-MS. See Table S4 in SI for exact reaction conditions. ^a See SI for corresponding single crystal X-ray structures