New insights in the mechanism of amine catalyzed epoxidation: Dual role of protonated ammonium salts as both phase transfer catalysts and activators of Oxone

VK Aggarwal*, C Lopin, F Sandrinelli

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

98 Citations (Scopus)

Abstract

Amines have previously been reported to catalyze the epoxidation of alkenes using Oxone (2KHSO(5)+KHSO4+K2SO4), and significant levels of asymmetric induction were observed. From screening a series of amines based on 2-substituted pyrrolidines, it has now been found that more consistent and reproducible results are achieved with the HCl salt of the amine compared to the amine itself. Up to 66% ee was achieved in epoxidation of 1-phenylcyclohexene. The chiral amine could be reisolated in >90% yield when reactions were conducted at -10 degreesC, indicating that the integrity of the amine was maintained during the oxidation process. At -10 degreesC, (S)-2-(diphenylmethyl)pyrrolidine 1 reacted with Oxone to give a mixture of ammonium salts containing the peroxymonosulfate salt 6b. The enantioselectivity obtained with this salt was compared to the amine(.)HCl salt catalyzed process and identical results were observed, indicating that the true oxidant was the peroxymonosulfate salt 6b. The relative rates of oxidation of cis- and trans-beta-methylstyrenes together with the rho value of a series of 1-arylcyclohexenes were determined. These studies indicated that the amine catalyzed process involved electrophilic oxidation. On the basis of these findings, a new mechanism is advanced in which the protonated amine not only acts as a PTC but also activates Oxone, through hydrogen bonding, toward electrophilic attack.

Original languageEnglish
Pages (from-to)7596-7601
Number of pages6
JournalJournal of the American Chemical Society
Volume125
Issue number25
DOIs
Publication statusPublished - 25 Jun 2003

Keywords

  • POTASSIUM PEROXYMONOSULFATE
  • NUCLEOPHILIC-ADDITION
  • CERIUM(III) CHLORIDE
  • EFFICIENT METHOD
  • OXIDATION
  • ALKENES
  • ACID
  • REAGENTS
  • DIMETHYLDIOXIRANE
  • TERTIARY

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