952208-17-2Relevant articles and documents
Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": Facile formation of phosphine-boranes and cationic phosphonium-boranes
Welch, Gregory C.,Cabrera, Lourdes,Chase, Preston A.,Hollink, Emily,Masuda, Jason D.,Wei, Pingrong,Stephan, Douglas W.
, p. 3407 - 3414 (2008/02/10)
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 [R 3P(C6F4)BF(C6F5) 2] and [R2PH(C6F4)BF(C 6F5)2]. These can be easily transformed into the cationic phosphonium-boranes [R3P(C6F 4)B(C6F5)2]+ and [R 2PH(C6F4)B(C6F5) 2]+ or into the neutral phosphino-boranes R 2P(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. The Royal Society of Chemistry 2007.