ION-MOLECULE REACTIONS OF SIMPLE AMINES WITH CARBONYL-COMPOUNDS IN THE GAS-PHASE - AN EXPERIMENTAL AND THEORETICAL-STUDY OF ENAMINE FORMATION

Ion-molecule reactions of mono- and diamines with acetone and pentan-3 -one were investigated under chemical ionization, using the carbonyl c ompound as reagent gas. To check the reactivity of different plasma io ns, the reactions of selected ions with neutral butylamine were carrie d out under low pressure in the cell of a Fourier transform ion cyclot ron resonance mass spectrometer. All the primary monoamines gave rise to the nucleophilic addition-elimination reaction product, formed by t he reaction of the protonated ketone dimer with a neutral amine. Proto nated 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 tbe structure of the ketone. Co mparison of the collision-induced dissociation mass spectra of the pro duct ions with those of authentic protonated imines showed that, with acetone as reagent gas, only protonated imines were formed. However, w hen 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 theoret ical calculations show that a high energy barrier is separating them f rom each other, making isomerization improbable. A striking difference between the spectra of diamines and monoamines was the considerable i mportance 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 protona ted 1,3-dimethyl-1,3-diazolidine.