alpha-CH acidity of alkyl-B(C6F5)(2) compounds - the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry

Alkyl-B(C6F5)(2) boranes are markedly alpha-CH-acidic. Using DFT we have calculated the pK(a)-values of a series of examples. Typically, (C6F5)(2)B-CH3 [pK(a) (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this alpha-CH-B(C6F5)(2) 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(-1)) tautomerization of the in situ generated indane derived 1,3-P/B FLP 6 to its zwitterionic borata-alkene/phosphonium isomer 7, which was successfully trapped by Piers' borane [HB(C6F5)(2)] to yield the stable product 8. The pronounced alpha-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane E-H2C=C(Me)CH=CHB(C6F5)(2) undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR2 (R: phenyl, mesityl, or t-butyl) reagents. alpha-CHB(C6F5)(2) carbanion (i.e. borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.