?-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 ?-CH-acidic. Using DFT we have calculated the pKa-values of a series of examples. Typically, (C6F5)2B-CH3 [pKa (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this ?-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 ?-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane E-H2C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 C(Me)CHCHB(C6F5)2 undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR2 (R: phenyl, mesityl, or t-butyl) reagents. ?-CHB(C6F5)2 carbanion (i.e. borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.

SUBMITTER: Moquist P 

PROVIDER: S-EPMC5592807 | biostudies-other | 2015 Jan

REPOSITORIES: biostudies-other