Ring-chain tautomerism in 1,3-diaza and 1,3-oxaza heterocycles

Lambert, Joseph B.; Majchrzak, Michał

doi:10.1021/ja00530a045

Cited by 53 publications

(19 citation statements)

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“…The results for accurate mass measurements for compound 5d are given in Table 2. The compounds studied have a methyl, ethyl, n-propyl, isopropyl or phenyl group on the 1-nitrogen, and an electron-donating or withdrawing substituent (NO 2 , Br, H, CH 3 , OCH 3 or N(CH 3 ) 2 ) on the 2-phenyl group. The weak molecular ion peak indicated that all the compounds studied were noticeably unstable to electron ionization.…”

Section: Resultsmentioning

confidence: 99%

“…Elimination of the phenyl group directly from the molecular ion seems to be a common feature for these kind of 5-membered heterocycles (unpublished results). 15 Another noteworthy fragment was the [C 7 H 6 X] ion, which was present when the phenyl ring was unsubstituted or had an electron-donating substituent X (X = CH 3 , OCH 3 , N(CH 3 ) 2 ), whereas in the case of strongly electron-withdrawing substituents (X = NO 2 , Br) it was totally absent. The formation of the [C 7 H 6 X] ion can be explained so that for electron-donating substituents X, the ionization takes place partly on the 2-phenyl ring and ions are formed via radical site initiated cleavage followed by a hydrogen transfer.…”

Section: Resultsmentioning

confidence: 99%

“…1 Imidazolidines, which are 5-membered 1,3-N,N-heterocycles, are most often prepared by condensation of a diamine with an aldehyde. 2,3 They have been studied in the liquid phase, and equilibrium has been demonstrated between the open-chain and ring forms. 4,5 Mass spectrometry can be used to study gas-phase ring-chain tautomerism by observing the ratio of the relative abundances of peaks relating to one or another tautomeric form.…”

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Unexpected results in gas-phase tautomerism of differently 1-nitrogen- and 2-aryl-substituted imidazolidines on electron ionization

Joutsiniemi

Vainiotalo

Fülöp

et al. 1998

Rapid Commun. Mass Spectrom.

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The 70 eV electron ionization mass spectra of 35 differently 2-phenyl-and 1-nitrogen-substituted imidazolidines were studied. Tautomerism was observed between the open-chain and ring forms in the gas phase. The fragment ions detected in the low-energy mass spectra showed that in the gas phase the compounds mostly existed in the ring form; except those having a nitro substituent on the 2-phenyl group, for which only open-chain fragment ions were found. The most important fragment ion relating to the ring form for Nmethyl-substituted compounds was the Received 8 March 1998; Revised 6 May 1998; Accepted 11 May 1998Ring-chain tautomerism involving the reversible addition of a heteroatom to a heteropolar double bond is a well known process for heterocycles. 1 Imidazolidines, which are 5-membered 1,3-N,N-heterocycles, are most often prepared by condensation of a diamine with an aldehyde.2,3 They have been studied in the liquid phase, and equilibrium has been demonstrated between the open-chain and ring forms. 4,5 Mass spectrometry can be used to study gas-phase ring-chain tautomerism by observing the ratio of the relative abundances of peaks relating to one or another tautomeric form.6-9 The validity of the method greatly depends on the reliability with which a fragment ion can be attributed to only one tautomeric form. Differences in the ionization efficiencies of the ring and open-chain forms, the absence of intense primary fragment ions, difficulties in the measurement of their decomposition efficiencies and the lack of thermal equilibrium before ionization are further factors which may cause problems. At best, the method is qualitative only. For example, the ring-chain equilibria of fiveand six-membered 1,3-O,N-heterocycles have been studied extensively by mass spectrometry, with varying results. [6][7][8][9][10][11][12] In the present work, 35 2-aryl-substituted imidazolidines, with either methyl, ethyl, n-propyl, isopropyl or phenyl on one nitrogen, were studied mass spectrometrically under electron ionization.The main question of interest was to examine whether it was possible to determine the position of the ring-chain equilibrium for the imidazolidines in the gas phase by mass spectrometry. The effect of the electron-donating or withdrawing substituent X on the 2-phenyl group on the ringchain tautomerism was carefully studied. The possible *Correspondence to: P. Vainiotalo,

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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Unexpected results in gas-phase tautomerism of differently 1-nitrogen- and 2-aryl-substituted imidazolidines on electron ionization

Joutsiniemi

Vainiotalo

Fülöp

et al. 1998

Rapid Commun. Mass Spectrom.

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“…The realization of a possible synthesis of novel 1,3-imidazolidines based on the indicated reaction is connected with the marked interest in the chemistry and the biochemical reactions of this class of compounds at this time [10][11][12][13][14][15][16][17].…”

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“…Since the ring-chain tautomerism of 5-6-membered 1,3-heterocycles affects their reactivity (and hence determines their synthetic use) and is also important for understanding and predicting biochemical processes [10,16,17] we have tried to show that compounds 3a and 4a are not isomeric (one of which is a labile intermediate) but are actually tautomers. With this in mind we have studied the change in their ratio upon heating the reaction mixture in CDCl 3 solution in the NMR spectrometer ampule.…”

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confidence: 99%