Improved Synthesis of Icosahedral Carboranes Containing Exohedral B–c and C–c Bonds
We report improved 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 corresponding Grignard reagents has been improved by the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand. Using the developed catalytic conditions lead to cross-coupling transformations achieved within several hours. Importantly, this method is applicable towards alkyl-based Grignard reagents containing β-hydrogens. Additionally, transition metal-free substitution of (hetero)aryl substrates on C-vertices was developed under SNAr 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.