Selective Formation of Triplet Alkyl Nitrenes from Photolysis of β-Azido-Propiophenone and Their Reactivity

Pradeep N. D. Singh, Sarah M. Mandel, Jagadis Sankaranarayanan, Sivaramakrishnan Muthukrishnan, Mingxin Chang, Rachel M. Robinson, Paul M. Lahti, Bruce S. Ault, and Anna D. Gudmundsdóttir*
Contribution from the Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, and Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
J. Am. Chem. Soc., 2007, 129 (51), pp 16263–16272
DOI: 10.1021/ja077523s
Publication Date (Web): November 23, 2007
Copyright © 2007 American Chemical Society

Abstract

Abstract Image

Photolysis of β-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (λmax = 325 nm, τ = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,π* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. β-Azido-p-methoxy-propiophenone, with a π,π* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

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History

  • Published In Issue December 26, 2007
  • Received September 29, 2007

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