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Gas-Phase Basicity and Acidity Trends in α,β-Unsaturated Amines, Phosphines, and Arsines

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journal contribution
posted on 01.05.1999, 00:00 by Otilia Mó, Manuel Yáñez, Michèle Decouzon, Jean-François Gal, Pierre-Charles Maria, Jean-Claude Guillemin
The acidity and basicity trends in the series of α,β-unsaturated amines, phosphines, and arsines were analyzed through the use of G2 ab initio calculations and the examination of experimental data obtained by means of FT-ICR techniques. The α,β-unsaturated amines, phosphines, and arsines are less basic but significantly more acidic than the corresponding saturated analogues. However, while both vinyl- and ethynylamine protonate preferentially at the β-carbon atom, vinyl- and ethynylphosphine are phosphorus bases in the gas-phase. Arsines resemble closely the corresponding phosphines, although protonation at the Cα atom competes with protonation at the heteroatom. The enhanced acidity of unsaturated compounds can be attributed essentially to a stabilization of the anions because of a favorable interaction of the XH- group with the C−C multiple bonds. This stabilizing effect is maximum for amines and minimum for arsines. The low relative basicity of these unsaturated compounds results from a destabilization of the protonated species due to unfavorable interactions of the XH3+ group with the C−C π-system. Protonation at the β-carbon is strongly favored for amines but unfavorable for phosphines and arsines, because the carbocation formed is much less stabilized when the heteroatom of the XH2 group belongs to the second or the third-row than when it is a first-row atom.