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Issue 1, 2015
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α-CH acidity of alkyl–B(C6F5)2 compounds – the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry

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Abstract

Alkyl–B(C6F5)2 boranes are markedly α-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 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-H2C[double bond, length as m-dash]C(Me)CH[double bond, length as m-dash]CHB(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.

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

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Supplementary files

Article information


Submitted
10 Jun 2014
Accepted
26 Sep 2014
First published
26 Sep 2014

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2015,6, 816-825
Article type
Edge Article

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

P. Moquist, G. Chen, C. Mück-Lichtenfeld, K. Bussmann, C. G. Daniliuc, G. Kehr and G. Erker, Chem. Sci., 2015, 6, 816
DOI: 10.1039/C4SC01711K

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