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α-CH acidity of alkyl–B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> compounds – the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry

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70

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80

References

2014

Year

Philip N. Moquist, Guo‐Qiang Chen, Christian Mück‐Lichtenfeld, Kathrin Bussmann, Constantin G. Daniliuc, Gerald Kehr, Gerhard Erker
Chemical Science

Abstract

Alkyl-B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> boranes are markedly α-CH-acidic. Using DFT we have calculated the p<i>K</i> <sub>a</sub>-values of a series of examples. Typically, (C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>B-CH<sub>3</sub> [p<i>K</i> <sub>a</sub> (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this α-CH-B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> 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<sup>-1</sup>) tautomerization of the <i>in situ</i> generated indane derived 1,3-P/B FLP <b>6</b> to its zwitterionic borata-alkene/phosphonium isomer <b>7</b>, which was successfully trapped by Piers' borane [HB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] to yield the stable product <b>8</b>. The pronounced α-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane <i>E</i>-H<sub>2</sub>C[double bond, length as m-dash]C(Me)CH[double bond, length as m-dash]CHB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR<sub>2</sub> (R: phenyl, mesityl, or <i>t</i>-butyl) reagents. α-CHB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> carbanion (<i>i.e.</i> borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.

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