Gas-phase basicity and acidity trends in alpha,beta-unsaturated amines, phosphines, and arsines

The acidity and basicity trends in the series of alpha,beta-unsaturated ami nes, 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 alpha,beta-unsaturated amines, phosphines, and arsin es are less basic but significantly more acidic than the corresponding satu rated analogues. However, while both vinyl- and ethynylamine protonate pref erentially at the beta-carbon atom, vinyl- and ethynylphosphine are phospho rus bases in the gas-phase. Arsines resemble closely the corresponding phos phines, although protonation at the C-alpha atom competes with protonation at the heteroatom. The enhanced acidity of unsaturated compounds can be att ributed 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 bw relative basi city of these unsaturated compounds results from a destabilization of the p rotonated species due to unfavorable interactions of the XH3+ group With th e C-C pi-system. Protonation at the beta-carbon is strongly favored for ami nes but unfavorable for phosphines and arsines, because the carbocation for med 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.