Oxygenation of a Me2Zn/α-Diimine System: A Unique Zinc Methylperoxide Cluster and Evidence for Its Sequential Decomposition Pathways

Autor: Zbigniew Ochal, Iwona Justyniak, Karolina Suwała, Janusz Lewiński, Janusz Lipkowski, Marcin Kubisiak
Rok vydání: 2008
Předmět:
Zdroj: Angewandte Chemie International Edition. 47:7888-7891
ISSN: 1521-3773
1433-7851
DOI: 10.1002/anie.200803254
Popis: Interest in the reaction of alkylzinc complexes with O2 has persisted for over 150 years since the pioneering studies by Frankland. The nature of the products, however, has been the subject of intense controversy. The widely accepted freeradical chain-reaction mechanism for these reactions, as found in the vast majority of text books, assumes the initiation by adventitious alkyl radicals (RC) followed by a cascade of fast reactions with little opportunity for the detection of intermediates. Recently, however, we provided unambiguous proof that R2Zn compounds, or their adducts with Lewis bases, have a marked tendency to undergo oxidation of only one alkyl group under controlled conditions with subsequent formation of RZnOOR or RZnOR species, and simultaneously structurally characterized the first examples of zinc alkylperoxides. We also proposed a plausible hypothesis for the mechanism of the reaction of alkylzinc complexes with O2. [5,6] In recent years there has been an increased interest in various radical additions initiated by the R2Zn/O2 system, especially regarding organic substrates which contain donor sites capable of forming the Lewis acid/base adducts with R2Zn that are actually involved in the reaction with O2. [7] This latter fact is usually ignored, and another assumption made in this field, which seems irrefutable, involves an alkyl radical RC (generated through the oxygenation reaction) acting as the chain carrier. Moreover, the most effective initiation systems involve Me2Zn, [7f,g] which, according to recent findings, can be selectively transformed into MeZnOMe without the generation of free MeC radicals. In light of this fact, as well as the lack of structurally characterized ZnOOMe species, it seemed reasonable to wonder how the oxygenated products participate in radical reactions. To gain a more in-depth view of both the role of the supporting ligands and the character of the radical species formed in the reactions of Me2Zn with O2, we have turned our attention to a-diimines, which have been widely used in fundamental coordination chemistry as noninnocent ligands. In the field of zinc chemistry, van Koten et al. have extensively studied the reactions of R2Zn compounds with 1,4-diazabutadiene (R-DAB) ligands and have demonstrated convincingly that this reaction system smoothly forms both paramagnetic and diamagnetic species. Herein we report the synthesis and structural characterization of a novel zinc oxo(methylperoxide) cubane along with the MeOC radical entrapped product, the formation of which involves ZnO OMe bond homolysis. Previous studies have demonstrated that the treatment of Me2Zn with tBu-DAB in diethyl ether at ambient temperature results in the formation of [Me2Zn(tBu-DAB)] (1). [11] According to the authors, the four-coordinate adduct 1 is rather stable under these reaction conditions and only upon heating above 35 8C does it undergo an inner-sphere single electron transfer to give the radical pair [MeZn(tBu-DABC)(MeC)], which subsequently dimerizes to the C C coupled dinuclear compound [{MeZn(tBu-DABC)}2] (2 ; Scheme 1). In light of these findings, we decided to modify the reaction system slightly and, in the first instance, stirred equimolar amounts of tBu-DAB andMe2Zn in toluene at 22 8C for 7 h. [12]
Databáze: OpenAIRE
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