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.
The mass spectrometric behaviour of condensation products of nine substituted benzaldehydes with 3-amino-1,2-propanediol and 3-amino-1-phenoxy-2-propanol was studied under electron impact and chemical ionization. The 70eV electron impact mass spectra showed that, as in solution, the amino diol derivatives existed as three different structural isomers in the gas phase: viz. open-chain Schiff base, oxazolidine and tetrahydro-1,3-oxazine. The fragmentations revealed a large amount of the decomposed molecular ions to have the open-chain structure, but also that the amount of both ring forms was considerable. In the same way, with the amino alcohol derivatives both the open-chain and the oxazolidine ring forms were present in the gas phase. In all cases, the ring formation was much more favourable in the gas phase than in solution. Substituents at the phenyl ring caused changes in the relative amounts of the different forms: electron-withdrawing substituents shifted the equilibria in favour of the ring forms while electron-donating substituents favoured the open-chain form. Under chemical ionization, methane, isobutane and acetone were used as reagent gas. Methane was the only reagent gas that led to some fragmentation of the protonated molecules.In general, compounds possessing a primary amino group easily react with aldehydes and ketones to produce imines by elimination of water.' This reaction has been the subject of interest in several investigations because imines are important intermediates in many biochemical reactions, as well as in organic ~yntheses.~ The imines formed in the reaction of 1,2-and 1,3-amino alcohols with a carbonyl compound usually react further giving rise to cyclic oxazolidines and tetrahydro-1 ,3-oxazines7 re~pectively.~ The formation of oxazolidines takes place fairly well, even though, according to Baldwin's rules,' this involves an unfavourable 5-endotrig process, whereas the formation of oxazines involves a favoured 6-endo-trig process. However, both oxazolidines and tetrahydro-173-oxazines with an unsubstituted ring nitrogen exist in rapid equilibrium between the open-chain Schiff base and the ring form. This ringchain tautomerism has been extensively studied in solution,6 and a few investigations have been done in the gas phase.7Author for correspondence.Recently, the reaction of substituted benzaldehydes with 3-amino-172-propanediol and 3-amino-1-phenoxy-2-propanol was studied in solution.8 In addition to the open-chain Schiff base, the reaction yields four different epimeric ring structures for 3-amino-l,2-propanediol derivatives: two stereoisomeric oxazolidine forms and two stereoisomeric oxazine forms (Scheme 1). With 3-amino-1-phenoxy-2-propanol derivatives, by contrast, the only possible ring forms are those of stereoisomeric oxazolidines (Scheme 2). In solution, the amino diol derivatives exist as 12.9% in the unfavoured oxazolidine and 46.4% in the favoured oxazine form, whereas the amino alcohol derivatives exist up to 74.9% in the open-chain form.In this work, we studied the ...
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