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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
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An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
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Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...
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Shielding Effects of Aromatic (Indole) Ring for Structural Analysis.

Jian-Zi Liu1, Hao-Di Sun2,3, Lin Chen4

  • 1Key Laboratory of Bioactive Substances and Resources Utilization of Chinese Herbal Medicine, Ministry of Education, Institute of Medicinal Plant Development, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100193, People's Republic of China.

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|August 30, 2023
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This summary is machine-generated.

This study analyzes aromatic indole ring shielding effects in natural compounds like asterric acid analogs and diketopiperazines. These findings refine NMR analysis and structural configuration determination for these small molecules.

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Area of Science:

  • Organic Chemistry
  • Structural Elucidation
  • Natural Products

Background:

  • Aromatic rings, particularly indole, significantly influence NMR chemical shifts in small molecules.
  • Understanding these shielding effects is crucial for accurate structural analysis of complex natural products.

Purpose of the Study:

  • To critically analyze shielding effects of aromatic (indole) rings in specific natural compounds.
  • To develop empirical rules for predicting and interpreting these shielding effects.
  • To revise and assign structures and configurations of asterric acid analogs and diketopiperazines.

Main Methods:

  • Review and critical analysis of existing literature on shielding effects.
  • Classification of empirical rules governing aromatic (indole) ring shielding.
  • Application of established rules to revise 1H NMR chemical shifts and molecular structures.

Main Results:

  • Established empirical rules for aromatic (indole) ring shielding effects.
  • Revised 1H NMR chemical shift values and structures for asterric acid analogs.
  • Assigned or revised relative configurations for indole-containing diketopiperazines.

Conclusions:

  • The developed empirical rules offer an efficient method for NMR data analysis.
  • These rules facilitate accurate configuration determination for asterric acid analogs, diketopiperazines, and rubrolides.
  • This work enhances the structural elucidation of important classes of natural products.