Oxidation and dehydrogenation of a phenol ether in a pentasil zeolite (Na ZSM-5): an EPR study

Abstract

Incorporation of p-propylanisole 1 onto a pentasil zeolite (Na ZSM-5) gives rise to the EPR spectrum of anethole radical cation 2*+; this conversion requires three separate oxidation steps alternating with two deprotonations, a reaction sequence of remarkable complexity for a room temperature reaction, albeit on the internal surfaces of a zeolite.

Oxidation of Aryl- and Diarylcyclopropanes in a Pentasil Zeolite: Ring Opening with Deprotonation or Net Hydrogen Migration

“…[15] An iso-topomer of 9b (9b-2,2-D 2 ) gave rise to a particularly interesting spectrum (Figure 2, bottom). …”

“…[14] In others, it promoted more complex conversions (p-propylanisole to p-propenylanisole radical cation). [15] Model considerations indicate that trans-1 might fit into the zeolite channels (ID ഠ 5.5 Å ), whereas the cis isomer appears too bulky. The oxidation potential of trans-1 (E ox ϭ 1.17 V vs. Ag/Ag ϩ , ca.…”

Dual Behavior of ZSM-5 as Brønsted Acid and Electron Acceptor in the Adsorption ofN,N′-Diphenylhydrazine

“…Another recent example of an ET-induced conversion on a zeolite is the dehydrogenation of 4-propylanisole upon adsorption on pentasil zeolite which generates propenylanisole (anethole) radical cation, a process that formally involves the loss of three electrons and two protons. [19] The electron acceptor ability of zeolites has been attributed to the presence of acid sites, although a controversy seems to persist regarding the (Lewis or Brønsted) nature of the active site. Brønsted acid sites are associated with bridged ϵSi(OH)Alϵ hydroxy groups, whereas Lewis sites are due to octahedral oligomeric and polymeric nonframework positive aluminate clusters of the general composition Al n (OH) x O y…”

Oxidation of Arylcyclopropanes in Solution and in a Zeolite: Structure and Rearrangement of the Phenylcyclopropane Radical Cation

“…In some cases, zeolites serve the dual function of one-electron oxidant and proton acceptor, converting oximes to iminoxyl radicals or phenols to phenoxyl radicals. 16 In others, more complex conversions occur, such as formation of p-propenylanisole radical cation from p-propylanisole (net loss of 3 e -plus 2 H + ), 17 or the conversion of 2-phenyl-1,3-dithiane to 1,2-dithiolane radical cation, which requires ring contraction with extrusion of a benzyl function. 18 The available precedent left little doubt that 1 could be oxidized in the ZSM-5 channels, but the ultimate fate of 1 •+ could not be predicted.…”

“…These spectra were identified by analogy with the known spectra of anethol radical cation, 2 •+ (R ) OCH 3 ), as those of 1-arylpropene radical cations, 2 •+ (R ) H, OCH 3 ); the allylic methyl protons and the olefinic -proton are the strongly coupled nuclei. 17 The conversion of 1 to 2 •+ within the zeolite is yet another example of the interesting, complex, and occasionally specific reactions of organic substrates in redox-and/or acid-base-active zeolites. The restrictive environment of zeolites often increases the stability of radical cations by protecting them from external reagents and/or restricting their mobility.…”