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

Abstract

Alkyl–B(C₆F₅)₂ boranes are markedly α-CH-acidic. Using DFT we have calculated the pK_a-values of a series of examples. Typically, (C₆F₅)₂B–CH₃ [pK_a (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this α-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 α-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane E-H₂CC(Me)CHCHB(C₆F₅)₂ undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR₂ (R: phenyl, mesityl, or t-butyl) reagents. α-CHB(C₆F₅)₂ carbanion (i.e. borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.