n‐π‐Interaction in Enamines and Enaminoketones. A <sup>15</sup>N‐NMR. Study

Schwotzer, Willi; Philipsborn, W. Von

doi:10.1002/hlca.19770600507

Cited by 52 publications

(6 citation statements)

References 14 publications

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“…Most notably, the rotation about the C1–C2 bond in 4c and 6c is largely frozen out at −10 °C, and the species are best depicted by the quinoid resonance extreme F . In addition to the inequivalent protons of the Me 2 NC 6 H 4 – ring, the characteristic 15 N NMR shift of the Me 2 N– group (δ N = −282.6 ppm) is in excellent agreement with this description . However, the rotation about the Rh1–C1 bond is unrestricted on the NMR time scale, which confirms the low bond order of the carbene unit deduced from the crystallographic data.…”

Section: Resultssupporting

confidence: 78%

Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis

et al. 2016

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Owing to its tremendous preparative importance, rhodium carbene chemistry has been studied extensively during past decades. The invoked intermediates have, however, so far proved too reactive for direct inspection, and reliable experimental information has been extremely limited. A series of X-ray structures of pertinent intermediates of this type, together with supporting spectroscopic data, now closes this gap and provides a detailed picture of the constitution and conformation of such species. All complexes were prepared by decomposition of a diazoalkane precursor with an appropriate rhodium source; they belong to either the dirhodium(II) tetracarboxylate carbene series that enjoys widespread preparative use, or to the class of mononuclear half-sandwich carbenes of Rh(III), which show considerable potential. The experimental data correct or refine previous computational studies but corroborate the currently favored model for the prediction of the stereochemical course of rhodium catalyzed cyclopropanations, which is likely also applicable to other reactions. Emphasis is put on stereoelectronic rather than steric arguments, with the dipole of the acceptor substituent flanking the carbene center being the major selectivity determining factor. Moreover, the very subtle influence exerted by the anionic ligands on a Rh(III) center on the chemical character of the resulting carbenes species is documented by the structures of a homologous series of halide complexes. Finally, the isolation of a N-bonded Rh(II) diazoalkane complex showcases that steric hindrance represents an inherent limitation of the chosen methodology.

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Section: Resultssupporting

confidence: 78%

Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis

et al. 2016

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“…Introduction of the acetyl group into the 4-position of the pyrazole causes a considerable deshielding of the triligant N-resonance, whereas the imine resonance is little affected, thus indicating mesomeric interaction of the N (1)-atom with the carbonyl group (i.e., a vinylogous amide structure). Further deshielding of the tertiary N-atom is observed in the N-ethoxycarbonyl derivatives 23 and 24, and in particular in l-nitropyrazole (20).…”

mentioning

confidence: 94%

“…The 15N chemical shifts (in (CD3),SO) of 1,3-dimethylimidazolidine (63) (-342.8 pprn), 3-methyloxazolidine (64) (-322.7 ppm) and thiazolidine (65) (-327.0 ppm) are typical for tertiary amines and may be compared with the value reported for 1-methylpyrrolidine (-340.2 ppm, in benzene, [20] [18]) have been used successfully to measure N, H-coupling constants in azaheterocycles, by direct observation of the '5N-resonances. In the following we discuss N, H-coupling constants obtained by the INEPT and DEPT techniques on selected representatives of the present series of azoles.…”

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confidence: 98%

¹⁵N‐NMR Spectra of Azoles with Two Heteroatoms

Chen

Philipsborn

Nagarajan³

1983

Helvetica Chimica Acta

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105

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SummaryThe 15N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro-and tetrahydroazoles. "N, 'H spin-coupling constants have been determined from spectra obtained by ['HI 4 I5N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.

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“…A stereoscopic view of the crystal packing is presented in Fig. 2 The 7r-electron delocalization is another interesting feature of enamines (Schwotzer & Philipsborn, 1977). The title compound exhibits delocalization in the azoenamine group.…”

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confidence: 99%