Structure-constraint induced increase in Lewis acidity of tris(ortho-carboranyl)borane and selective complexation with Bestmann ylides

Abstract

The Lewis acidity of tris(ortho-carboranyl)borane has been slightly increased by mimicking the structural evolution from triarylborane to 9-aryl-9-borafluorene. The o-carborane-based analogue (C₂B₁₀H₁₀)₂B(C₂B₁₀H₁₁), obtained via salt elimination between LiC₂B₁₀H₁₁ and (C₂B₁₀H₁₀)₂BBr, has been fully characterized. Gutmann–Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies have confirmed the increase in Lewis acidity, which is attributable to structural constraint imposed by the CC-coupling between two carboranyl groups. Selective complexation of (C₂B₁₀H₁₀)₂B(C₂B₁₀H₁₁) with Bestmann ylides R₃PCCO (R = Ph, Cy) has been achieved, enabling further conversion into the zwitterionic phospholium salt through NHC-catalyzed proton transfer.