¹H and ¹³C NMR studies of E/Z 2‐hydroxyindolenines

Abstract

The 'H NMR spectra of E/Z 2-hydroxyindolenines are reported in both CDCI, and DMSO-d, solvents. The data suggest a preferred conformation of the heterocyclic ring, which is slightly distorted from planarity, in agreement with the x-ray crystallographic data of one of the isomers. The 13C NMR spectra for the Z-hydroxyindolenines are also described. Substituent effects on E/Z isomerism about the exocyclic C=C bond are discussed.

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“…8.0 ppm for the Z-isomer) because of the deshielding effect of the nearby carbonyl group. [15,[26][27][28][29] In CDCl 3 solution, the proton at C8 in the E-isomer experiences a deshielding effect caused by the influence of the ring oxygen. On changing the solvent from CDCl 3 to (CD 3 ) 2 SO, this signal was shifted downfield by 0.63-0.74 ppm for Zisomers and 0.13-0.29 ppm for E-isomers.…”

“…Isomerization was observed upon dissolving the pure Z-or E-isomer, leading to the formation of both isomers in the equilibrium mixture (Tables 7 and 8). It is known [15,27,28,[31][32][33] that polar substituents linked to a double bond can lower the rotational energy barriers of this bond. However, the degree of fluorination in the R F substituent in (6)-(8) did not influence the equilibration rate (Tables 7 and 8).…”

“…The solvent dependence in (6)-(8) is indicative of a dipolar transition state associated with the C3=C8 bond rotation. [28] In the case of adding traces of water to the deuterated solvent, the participation of the polyfluoroacyl oxygen in intermolecular hydrogen bonding obviously additionally lowers the doublebond character of the C3=C8 bond (Tables 7 and 8), cf. ref.…”

Electron ionization mass spectra of indolenines obtained using sector and ion trap mass spectrometers

“…The preparation of undeuterated compounds 1 – 4 , 6 – 9 and 12 – 18 has already been described3b, 3e, 4a, 5a–c (see Table 1).…”

“…The characterization of indolenines 1 – 18 by NMR in CDCl 3 solution has shown that all of them exist as the Z ‐isomer. This follows5a from the chemical shift differences of H‐2, situated within the sphere of influence of the carbonyl group, when compared to the E ‐isomer also observed in very small amounts in polar deuterated dimethyl sulfoxide (DMSO‐ d 6 ). The Z ‐configuration was further substantiated by single crystal X‐ray diffraction analysis 5b…”