Gas–Phase Negative Ion Chemistry of Lewis Acid–Base Complexes

Document Type

Article

Publication Title

Journal of the American Chemical Society

Department

Chemistry

ISSN

0002-7863

Volume

120

Issue

40

DOI

10.1021/ja9804518

First Page

10511

Last Page

10522

Publication Date

9-1-1998

Abstract

The gas-phase negative ion chemistry of a series of Lewis acid−base complexes [Me2SBH3 (1), Me3NBH3 (2), Me3PBH3 (3), Me2SBF3 (4), Me2OBF3, Et3NBH3, and Et2OBF3] was investigated with use of the flowing afterglow triple-quadrupole technique. Ab initio MO calculations using the G2(MP2) and CBS-4 models were carried out for 1−4 and related species. The gas-phase reaction between OH- and complex 1 produces a stable carbanion, MeS(BH3)CH2- (1a), that does not isomerize under thermal conditions at room temperature to either of the lower energy borate isomers MeSCH2BH3- (1b) and CH3CH2SBH3- (1c). The structure of 1a was determined by tandem mass spectrometry and by ion/molecule reactions. The barriers for rearrangement of 1a to 1b and to 1c were calculated to be 29.3, and 27.6 kcal/mol, respectively, at the G2(MP2) level. The stable carbanions Me2N(BH3)CH2- (2a), Me2P(BH3)CH2- (3a), and MeS(BF3)CH2- (4a) were also generated by proton abstraction from the corresponding neutral complexes. The gas-phase acidities (ΔHacid) of 1−3 were determined from bracketing experiments to be 372.5 ± 2.0, 393.0 ± 2.0, and 374.5 ± 2.0 kcal/mol, respectively. Compared to their uncomplexed bases, the acidities of 1−3 are enhanced by 18−20 kcal/mol. The acidity enhancements were shown to be mainly due to the increased electron binding energies of the carbanions in the deprotonated complexes that result from electrostatic interactions with the strongly dipolar Lewis acid−base bonds. Enhanced reactivity of the Lewis acid−base complexes was also characterized. The complexes 1, 2, and Me2OBF3 undergo nucleophilic substitution at carbon with F- or NH2-, while no such reactions occur for the uncomplexed bases Me2S, Me3N, and Me2O. Similarly, facile β-elimination reactions occur between F- or OH- and the ethylated complexes Et3NBH3 and Et2OBF3, while the uncomplexed species are unreactive.

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