Carborane superacids

Reed and co-workers utilized the silylium carboranate salts to generate novel Bronstedt superacids based on carborane anions as conjugate bases by reaction of the salts with liquid HCl [Eq. (7)]. These carborane superacids are able to cleanly protonate Cgo and benzene at room temperature to yield and benzenium... 

Carborane Superacids H(CB11HR X.6). Recently, new carbon superacids, icosahedral carboranes H(CBnHR5X6) (where X = chlorine, bromine or iodine R = H, Me, Cl), have been described by Reed et al.34, whose conjugate base, the carborane anion (CBnHR5X6), is quite inert due to low nucleophilicity. 

Chiral bromohydrin derivatives reacted under acidic conditions with very high stereoselectivity (essentially stereospecific). This points to a mechanism involving a chiral bromonium ion as the reactive intermediate. The protonation of chloroethane by the carborane superacid H(CHBuClii) proceeds via a shared-proton intermediate that decays by HCl loss to form the carbocation-like ethyl carborane. This reacts with a second EtCl to form the Et2Cl+ cation. The transannular electrophilic bromination of a polycyclic system with two C=C in close proximity was studied by computational methods. The initial bromonium was found to rearrange into more stable carbocations through reaction with the nearby carbon—carbon double bond. 

Carboranes as anew class of weakly coordinating anions for strong electrophiles, oxidants, and superacids 98ACR133. 

AG values of deprotonation are as low as 249-250 kcal mol 1 for HS03F or HF associated with SbF5 or S03. The strongest superacid was found to be dodeca-fluorocarborane acid CBnF12H with a AG of 209 kcal mol 1 even suggesting that the dodeca(trifluoromethyl)carborane acid CBnlCF l H would be below the 200 kcal moL1 level ... 

Reed, C. A. 1998. Carboranes A new class of weakly coordinating anions for strong electrophiles, oxidants, and superacids. Acc. Chem. Res. 31 133-139. 

Scheme 33 Preparation of ctoso-borate Biytnsted superacid, 106, and a c/oio-carborane-based methylating reagent, 107 [94, 95]...

---