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 I produces a stable carbanion, MeS(BH3)CH2- (1a), that does not isomerize under thermal conditions at room temperature to either of the lower energy berate isomers MeSCH2BH3- (Ib) and CH3CH2SBH3- (Ic). The structure of la was determined by tandem mass spectrometry and by ion/molecule reactions. The barriers for rearrangement of la to Ib and to Ic 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 (Delta H-acid) 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 beta-elimination reactions occur between F- or OH- and the ethylated complexes Et3NBH3 and Et2OBF3, while the uncomplexed species are unreactive.

Bis zu 20 kcal mol−1 werden die Gasphasen‐CH‐Aciditäten von Dimethylsulfid, Trimethylamin und Trimethylphosphan durch Boran‐Komplexierung am Heteroatom erhöht. Deprotonierung der flüchtigen Lewis‐Säure‐Base‐Komplexe in der Gasphase erzeugt dipolstabilisierte Carbanionen (siehe unten), die sich nicht in die stabileren Borat‐Isomere umlagern. Die Komplexierung erhöht zudem die Reaktivität von Methyl‐ und Ethylsubstituenten gegenüber Substitutions‐ und Eliminierungsreaktionen.magnified image

Borane complexation at the heteroatom increases the gas-phase CH acidities of dimethyl sulfide, trimethylamine, and trimethylphosphane by up to 20 kcal mol−1. Deprotonation of the volatile Lewis acid–base complexes in the gas phase produces dipole-stabilized carbanions (see below) that do not rearrange to the more stable borate isomers. The complexation also enhances the reactivity of pendant methyl and ethyl groups toward substitution and elimination reactions.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHydride binding energies of boranesDerek B. Workman and Robert R. SquiresCite this: Inorg. Chem. 1988, 27, 11, 1846–1848Publication Date (Print):June 1, 1988Publication History Published online1 May 2002Published inissue 1 June 1988https://pubs.acs.org/doi/10.1021/ic00284a003https://doi.org/10.1021/ic00284a003research-articleACS PublicationsRequest reuse permissionsArticle Views215Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Abstract7, and BH3CN‐ and CO2 as a convenient reference H‐‐acceptor compound are observed.