Ion-dipole complex formation from deprotonated phenol fatty acid esters evidenced by using gas-phase labeling combined with tandem mass spectrometry

The behavior of para-hydroxy-benzyl and hydroxy-phenylethyl fatty acid esters and methoxy derivatives toward the NH₃/NH⁻₂ system was investigated. Under these negative ion chemical ionization (NICI) conditions, proton abstraction takes place mainly at the more acidic site (i.e., phenol); however, this reaction is not entirely regioselective. Using NICI-ND₃ conditions, both isomeric phenoxide and enolate molecular species are produced in competition from these phenol esters. Their respective low-energy collision-activated dissociation spectra are studied, and they strongly differ, showing that these molecular species are not convertible to a common structure. Analysis of specific fragmentations of the OD-enolate parent species labeled by ND₃ in the gas phase, indicates that by charge-promoted cleavage, isomerization into an ion—dipole intermediate takes place prior to dissociation. This complex, containing a ketene moiety, isomerizes into different isomeric forms via two consecutive proton transfers: the first, which is very exothermic, is irreversible in contrast to the second, less exothermic reaction, which occurs via a reversible process. It is evidenced by the loss of labeling at phenol or enolizable sites in the fragment ions. Such a stepwise process does not take place from the phenoxide parent ion, which preferentially yields a very stable carboxylate ion. A thermochemical approach, using estimated acidity values, yields a rationalization of the observed reactivities of the various substrates studied.