Conformational equilibria of C-methyl groups in piperidines and N-methylpiperidines

Eliel, Ernest L.; Kandasamy, Duraisamy; Kenan, William R.

doi:10.1016/s0040-4039(00)93104-6

Cited by 10 publications

(1 citation statement)

References 11 publications

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“…The 8.9 ppm difference between the resonance signals of the epimeric 3-methylquinolizidines 21 and 22 shows the same influences as in l3C NMR. The upfield shift per gauche interaction is also not very constant:1413 the effect of the gauche -methyl group on the nitrogen shift of 8(e)-methyl-fra/Ls--decahydiOquinoline (17a) is only +5.5 ppm and the upfield shift per interacting methylene group is 5.3 ppm for m-decahydroquinoline (16a) and 4.4 ppm for m-4-?m-butylcyclohexylarnine (23).…”

Section: Roberts Et Al /mentioning

confidence: 98%

Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Steric and electronic effects on nitrogen-15 chemical shifts of piperidines and decahydroquinolines

Duthaler¹,

Williamson²,

Giannini³

et al. 1977

J. Am. Chem. Soc.

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Section: Roberts Et Al /mentioning

confidence: 98%

Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Steric and electronic effects on nitrogen-15 chemical shifts of piperidines and decahydroquinolines

Duthaler¹,

Williamson²,

Giannini³

et al. 1977

J. Am. Chem. Soc.

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On the Concept of Isomerism

Eliel

1976

Israel Journal of Chemistry

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The question as to whether two species of the same molecularcomposition are identical or isomeric in the conventional sense cannot be answered in the absence of stated experimental criteria of conditions and methods of observation. A new definition is proposed which minimizes such dependenceon experimentalcriteria. "Residual isomerism" is defined as an auxiliary term to make the new definition practically useful.

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Carbon‐13 NMR studies of twenty‐four methyl‐substituted morpholines

Nilsson¹,

Hernestam²

1978

Org. Magn. Reson.

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Carbon-13 N M R spectra of all the isomers of monomethyl-, 2,3-, 2'5, 2'6-, 3,5-dimethyl-, 2'3,s-, 2,3,6-6ethyl-and 2,3,5,6-tetramethyl&orpholine have been obtained at both ambient (25 "C) and low temperature (--100 to -120 "C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r=0.99tt9). The values were used in an attempt to assign, conformationally, the 'all cis' isomer 2,3,5,6-tetramethylmorpholine, which from 'H N M R spin-spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted 'steric shat' values for such carbons were compared to their corresponding proton shift data.It is well established that the heterocyclic ring in morpholine, like other six-membered ring compounds, exists predominantly in the chair conformation with rapid interconversion at room temperature between the two conformational states.' In a recent variable temperature 'HNMR study2 we found that some of the isomers of 2,3,5,6-tetramethylmorpholine exist to a high degree in one conformation, due to too large differences in energy between a molecule with, for example, an eeea conformation compared to a conformer with the aaae arrangement of the methyl groups. The isomer with an 'all cis' configuration (XXIV) of which only one conformer was detectable at low temperatures was of special interest. In our investigation of the magnitude of methine proton coupling constants it was not possible to elucidate the conformation of the methyl groups in this isomer i.e. if the 2-and 6-methyl groups are axial (3-and 5-methyl equatorial) or if instead these groups are equatorial (3-and 5-methyl axial).A new approach to stereochemical problems has emerged in recent years by the development of 13C NMR spectro~copy."~ In the cyclohexane system, for example, axial and equatorial methyl groups have clearly been disting~ished.~ Furthermore, reports on polymethyl-substituted cyclohexanes, 1,3-dioxanes, piperidines and piperazines clearly show that methylinduced 13C chemical shifts in cyclic systems are additive. It was thus of interest to see if this additivity holds also for the morpholine system, and if a correct assignment of the energetically most favourable conformer of the 'all-cis' isomer could be made. In this paper we report results from a systematic 13C NMR study of 24 methyl-substituted morpholines. The molecular configuration of the mono-: di-? trL5 and tetramethyl-substituted2 compounds have been previously established by proton NMR studies at 60, 100 and/or 270 MHz.* Author to whom correspondence should be addressed. RESULTS AND DISCUSSIONThe observed 13C chemical shifts at low temperature ( --100 to -120 "C) are.presented in Table 1. In this table (footnote) we have also included shifts at the probe temperature (25 "C) for those isomers in which different conformers were detected. For the sake of clarity the labelling of the morpholine carbons is g...

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Conformational equilibria of C-methyl groups in piperidines and N-methylpiperidines

Cited by 10 publications

References 11 publications

Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Steric and electronic effects on nitrogen-15 chemical shifts of piperidines and decahydroquinolines

Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy. Steric and electronic effects on nitrogen-15 chemical shifts of piperidines and decahydroquinolines

On the Concept of Isomerism

Carbon‐13 NMR studies of twenty‐four methyl‐substituted morpholines

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