Catalytic oxidations by peroxy, peroxo and oxo metal complexes: an interdisciplinary account with a personal view

The Role of Radicals in Metal‐Assisted Oxygenation Reactions

“…[155] The idea of at least two oxidizing species is shared by Adam et al, who suggested a bifurcation step in the catalytic cycle to account for the dependence of the diastereoselectivities on the oxygen source. [152] A concerted Lewis acid activated oxygen transfer (as has been suggested for the case of PhIO by Valentine et al [153] and recently by Que et al for iron porphyrin complexes [156] ) was proposed to compete with stepwise epoxidation via the Mn=O species [151] (Scheme 19, compare ref.…”

“…[148,149] The experimentally observed counterion effect as well as the effects of additives should thus be attributed to ligand-dependent reaction profiles and stereoselectivities on the individual PESs. [151] Recently, Linde et al studied the electronic effects of the substrate on the diastereoselectivity and enantioselectivity of the catalytic epoxidation by employing para-substituted cis stilbenes in reaction with PhIO and the manganese salen catalyst. [154] They concluded that cis and trans epoxide formation follow separate pathways, and that the possibility that the reaction mixture contains multiple, rapidly equilibrating oxidizing species, each of which preferentially follows one of the reaction pathways cannot be excluded.…”

Transition Metal Peroxides. Synthesis and Role in Oxidation Reactions*Dedicated, on the occasion of his 80th birthday, to our ‘vitae ac studiorum magister in secunda et in adversa fortuna’ Emeritus Professor Giorgio Modena, Padova University, who showed us the way toward Chemistry.

“…As far as the structure of the active complexes indicated in Scheme 1 is concerned, both in the solid state and/or in solution, several pieces of evidence indicate that most of them share a η 2 triangular arrangement of the peroxo moiety around the metal center (see below). In principle, all the µ-peroxo complexes structures, named following the definition made by Vaska 16 quite a long time ago, indicated in Scheme 2, are possible and have been found in numerous cases for different metals 14,17 .…”

Die Rolle von Radikalen bei metallvermittelten Oxygenierungen

“…[148,149] Der experimentell beobachtete Effekt von Gegenionen und Additiven sollte daher ligandenabhängigen Reaktionsprofilen und den damit verbundenen individuellen Stereoselektivitäten der Reaktionen auf den jeweiligen Hyperflächen zuzuschreiben sein. [151] Kürzlich haben Linde et al zudem anhand von parasubstituierten cis-Styrolderivaten untersucht, wie sich die elektronischen Verhältnisse im Substrat auf die Diastereound Enantioselektivität der (Salen)mangan-katalysierten Epoxidierung mit PhIO auswirken. [154] Aus den Ergebnissen folgerten sie, dass die Bildung von cis-und trans-Epoxiden auf unabhängigen Wegen erfolgt.…”

“…[151] Demnach können die Oxidantien in zwei Klassen eingeteilt werden: Mit PhIO, C 6 5 À , NaOCl und Dimethyldioxiran ist auch der konzertierte, Lewis-Säure-katalysierte Prozess (Weg C) wirksam, was sich in einer höheren cis-Selektivität ausdrückt. [152] Dieser Mechanismus würde auch erklären, warum Vinylcyclopropan-Radikaluhren bei Oxidationen mit NaOCl unversehrt bleiben, nicht aber bei Oxidationen mit PhIO.…”