Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R3P(C6F4)BF(C6F5)(2)] and [R2PH(C6F4)BF(C6F5)(2)]. These can be easily transformed into the cationic phosphonium-boranes [R3P(C6F4)B(C6F5)(2)](+) and [R2PH(C6F4)B(C6F5)(2)](+) or into the neutral phosphino-boranes R2P(C6F4)B(C6F5)(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C6F5)(3), while the acidity of the phosphine-boranes is diminished.