GENERATION OF ALKYL CARBANIONS IN THE GAS-PHASE

The scope and limitations of collision-induced decarboxylation of carboxylate negative ions as a method for generating gas-phase carbanions are described. Simple alkyl carbanions can be observed as fragments from a variety of aliphatic carboxylates, provided only that the carbanion has a lifetime toward electron detachment greater than ~25 µs. Several heretofore unknown alkyl carbanions have been observed, including the neopentyl anion, the 2- and 3-methylbutyl anions, cyclopentylmethyl anion, and cyclohexylmethyl anion, as well as cyclic and polycyclic carbanions such as the 1- and 2-methylcyclopropyl anions and the 1-bicyclopentyl anion. An empirical correlation between the electron affinities of primary alkyl radicals (RCH2) and the analogous alkoxyl radicals (RO) is described that correctly predicts the stability and detectability of the corresponding primary carbanion fragments (RCH2)- from collision-induced decarboxylation. A number of α-substituted methyl anions and β-substituted ethyl anions have been formed by this method; both familiar and novel examples of these types of carbanions are descr;ibed. The competing higher energy fragmentations and low-energy reactions of collisionally activated carboxylate ions are also discussed, including β-radical cleavages, remote-site fragmentations, and intramolecular isomerizations that occur prior to dissociation. © 1990, American Chemical Society. All rights reserved.