Stabilizing Borinium Cations [X-B-X]+ through Conjugation and Hyperconjugation Effects.

Borinium, a two-coordinated boron cation, is a strong Lewis acid that was thought difficult to prepare due to a lower number of coordinating ligands and susceptibility toward nucleophilic reactions. Recently a dimesityl borinium cation (Mes2B+) with high thermal stability has been synthesized and characterized by Shoji and co-workers. These findings suggest that, despite being extremely electron-demanding, borinium cations might be stabilized with certain substituted function groups. In the present study, we have studied a series of the borinium cations [X-B-X]+ with different substituted groups using the NBO and the BLW methods. Our computations revealed that both π-conjugation and hyperconjugation effects can effectively stabilize substituted borinium cations [X-B-X]+. Substituents such as X = C═CH2 stabilize the borinium center through highly delocalized π-bonding, involving the formally "empty" boron p x and p y orbitals. We suggest the borinium cations [X-B-X]+ with X = cyc-N(CH)2 and especially X = C═CH2 as possible synthetic targets of novel borinium cations.