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Rate constants for anilidyl radical cyclization reactions.

E Martinez1, M Newcomb

  • 1Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA.

The Journal of Organic Chemistry
|January 18, 2006
PubMed
Summary

N-Aryl-5,5-diphenyl-4-pentenamidyl radicals undergo rapid 5-exo cyclization reactions. These radicals act as electrophilic reactants, with one notable exception exhibiting unexpectedly fast cyclization kinetics.

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Area of Science:

  • Organic Chemistry
  • Photochemistry
  • Reaction Kinetics

Background:

  • N-Aryl-5,5-diphenyl-4-pentenamidyl radicals are key intermediates in organic synthesis.
  • Understanding their cyclization mechanisms is crucial for developing new synthetic methodologies.

Purpose of the Study:

  • To directly measure the rate constants for the cyclization of N-Aryl-5,5-diphenyl-4-pentenamidyl radicals.
  • To investigate the electronic and steric effects governing these cyclization reactions.

Main Methods:

  • Generation of radicals via 266 nm laser-flash photolysis of N-(phenylthio) derivatives.
  • Direct measurement of radical cyclization rate constants.
  • Variable temperature studies to determine activation parameters.
  • Hammett correlation analysis to probe substituent effects.

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Main Results:

  • 5-exo cyclization reactions were found to be very fast (k(c) > 2 x 10(5) s(-1)).
  • Radicals generally exhibited electrophilic behavior, as indicated by a positive Hammett rho value (1.9).
  • A p-methoxyphenyl-substituted radical showed significantly faster cyclization than predicted by the Hammett analysis.
  • Arrhenius parameters were determined for the parent radical, and a rate constant for hydrogen atom transfer was measured.

Conclusions:

  • The 5-exo cyclization of these radicals is a rapid process.
  • Substituent effects play a significant role, though deviations from expected Hammett correlations highlight complex reaction dynamics.
  • The study provides valuable kinetic data for understanding radical cyclization mechanisms in organic chemistry.