Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands

P Hanson, S C Rowell, P H Walton, A W Timms

Research output: Contribution to journalArticlepeer-review

Abstract

Relative to the rate observed for the hexa-aqua ion, Cu(OH2)(6)(2+) chelation of the copper catalyst by certain bidentate ligands enhances the rate of hydroxydediazoniation reaction (Sandmeyer hydroxylation); the ligands also provide a source of hydrogen in competitive hydrodediazoniation (H-transfer) reactions. By using the cyclisation of 2-benzoyl-phenyl radical as a radical clock, it has been possible to evaluate absolute rate constants for both processes effected by a variety of complexes involving one or two bidentate ligands (2-aminocarboxylate, 2-hydroxycarboxylate, 1,3-dicarboxylate, 1,2-diamine). The radical exhibits electrophilic character in both processes. The pattern of behaviour observed suggests the rate determining step in hydroxylation is reaction of the aryl radical at the metal centre to form an organocopper adduct which is rapidly converted into products. The relative reactivities of different complexes are explained qualitatively in terms of variations in the ligand field and Jahn-Teller distortion splittings of the copper d orbitals. Hydrodediazoniation is an S(H)2 H-abstraction process. Generally, coordination by Cu2+ deactivates the first added ligand relative to its reactivity as a free species in the same state of protonation. For the majority of complexes studied, the relative reactivity as H-donors of 1 : 1 and 1 : 2 complexes is statistically determined but an additional electronic effect is discerned for doubly charged ions.

Original languageEnglish
Pages (from-to)1838-1855
Number of pages18
JournalOrganic and Biomolecular Chemistry
Volume2
Issue number13
DOIs
Publication statusPublished - 2004

Keywords

  • ELECTRON-SPIN-RESONANCE
  • ABSOLUTE RATE CONSTANTS
  • ALIPHATIC FREE-RADICALS
  • AQUEOUS-SOLUTION
  • PULSE-RADIOLYSIS
  • CRYSTAL-STRUCTURE
  • METAL-IONS
  • MOLECULAR-STRUCTURES
  • METHYL RADICALS
  • WATER LIGANDS

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