α-CH acidity of alkyl-B(C 6 F 5 ) 2 compounds - the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry

• Published in: Chemical science (Cambridge) 2015-01, Vol.6 (1), p.816-825
• Main Authors: Moquist, Philip, Chen, Guo-Qiang, Mück-Lichtenfeld, Christian, Bussmann, Kathrin, Daniliuc, Constantin G, Kehr, Gerald, Erker, Gerhard
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c4sc01711k

Alkyl-B(C F ) boranes are markedly α-CH-acidic. Using DFT we have calculated the p -values of a series of examples. Typically, (C F ) B-CH [p (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this α-CH-B(C F ) acidity is in most cases not sufficient to allow for internal proton transfer in vicinal phosphane/borane frustrated Lewis pairs (FLPs). An exception is the slightly endergonic (+0.3 kcal mol ) tautomerization of the generated indane derived 1,3-P/B FLP to its zwitterionic borata-alkene/phosphonium isomer , which was successfully trapped by Piers' borane [HB(C F ) ] to yield the stable product . The pronounced α-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane -H C[double bond, length as m-dash]C(Me)CH[double bond, length as m-dash]CHB(C F ) undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR (R: phenyl, mesityl, or -butyl) reagents. α-CHB(C F ) carbanion ( borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.