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

The Lewis acidity of tris(-carboranyl)borane has been slightly increased by mimicking the structural evolution from triarylborane to 9-aryl-9-borafluorene. The -carborane-based analogue (CBH)B(CBH), obtained salt elimination between LiCBH and (CBH)BBr, 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 (CBH)B(CBH) with Bestmann ylides RPCCO (R = Ph, Cy) has been achieved, enabling further conversion into the zwitterionic phospholium salt through NHC-catalyzed proton transfer.