α-CH acidity of alkyl-B(CF) compounds - the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry.

Alkyl-B(CF) boranes are markedly α-CH-acidic. Using DFT we have calculated the p -values of a series of examples. Typically, (CF)B-CH [p (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this α-CH-B(CF) 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(CF)] to yield the stable product . The pronounced α-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane -HC[double bond, length as m-dash]C(Me)CH[double bond, length as m-dash]CHB(CF) undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR (R: phenyl, mesityl, or -butyl) reagents. α-CHB(CF) carbanion ( borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.