Ion—Molecule Reactions of Simple Amines with Carbonyl Compounds in the Gas Phase. An Experimental and Theoretical Study of Enamine Formation

Abstract

Ion–molecule reactions of mono- and diamines with acetone and pentan-3-one were investigated under chemical ionization, using the carbonyl compound as reagent gas. To check the reactivity of different plasma ions, the reactions of selected ions with neutral butylamine were carried out under low pressure in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. All the primary monoamines gave rise to the nucleophilic addition–elimination reaction product, formed by the reaction of the protonated ketone dimer with a neutral amine. Protonated ketone monomers gave rise only to protonated amines; no addition–elimination products were observed. The structure of the nucleophilic addition–elimination product ion was independent of the structure of the amine but depended considerably on the structure of the ketone. Comparison of the collision-induced dissociation mass spectra of the product ions with those of authentic protonated imines showed that, with acetone as reagent gas, only protonated imines were formed. However, when the size and branching of the ketone increased, enamine formation became clearly more favourable. The formation of protonated amines and enamines must take place through different mechanisms because theoretical calculations show that a high energy barrier is separating them from each other, making isomerization improbable. A striking difference between the spectra of diamines and monoamines was the considerable importance of the product ion of the nucleophilic addition–elimination reaction in the case of diamines. This difference might be due to the possibility for ring–chain tautomerism, although the product ions seem to decompose through the open-chain form, after the manner of protonated 1,3-dimethyl-1,3-diazolidine.