A New Look at the Reactivity of TEMPO toward Diethylzinc

Abstract

Reactions of diethylzinc with TEMPO were investigated. Dropwise addition of 1 equiv of TEMPO to Et 2 Zn at −10 °C leads to the nitroxide complex EtZn(TEMPO) in high yield, whereas upon addition of 2 equiv of TEMPO the corresponding homoleptic nitroxide compound Zn(TEMPO) 2 is formed. Diffusion ordered NMR spectroscopy experiments revealed that both zinc nitroxide compounds exist in monomeric forms in solution, while single-crystal X-ray diffraction confirmed their dimeric structure in the solid state.

Unprecedented Variety of Outcomes in the Oxygenation of Dinuclear Alkylzinc Derivatives of an N,N‐Coupled Bis(β‐diketimine)

“…Thus, the presence of a three‐coordinate zinc center is a crucial requirement for the initial step of the investigated reaction. Next, noncovalent activation of the O 2 molecule by the three‐coordinate CZnNN′ zinc center followed by electron transfer from the Zn−C bond to dioxygen (it is noteworthy that this step is in good agreement with our recent studies on the reactivity of zinc alkyls with TEMPO) results in the formation of an unstable monomeric dinuclear alkyl(alkylperoxide), [L][ZnR][ZnOOR], in which only one three‐coordinate Zn‐R unit is oxygenated (Scheme ). Despite the close proximity of the ZnOOMe and Zn‐Me subunits in the putative [L][ZnMe][ZnOOMe] species, the commonly assumed metathesis reaction, which would mediate oxygen transfer and afford the ZnOMe species, has not been encountered in this reaction system.…”

“…[10b] Thus, the presence of at hree-coordinate zinc center is ac rucial requirement for the initial step of the investigated reaction. Next,n oncovalent activation of the O 2 molecule by the three-coordinate CZnNN' zinc center followed by electron transfer from the ZnÀCb ond to dioxygen (it is noteworthy that this step is in good agreement with our recent studies on the reactivity of zinc alkylsw ith TEMPO [20] [ZnOOMe] species, the commonly assumed metathesis reaction, which would mediate oxygen transfer and afford the ZnOMe species, has not been encountered in this reaction system.Subsequently,there are an umber of possible scenarios that may be responsible fort he observed reaction outcomes. Firstly,a tl ow temperature, transformation of the putative [L] [ZnR][ZnOOR] complex to the corresponding alkoxide [L] [ZnR] [ZnOR], followed by association to form ad imeric {[L] [ZnR] [Zn(m-OR]} 2 moiety,p roceeds much more rapidly than further oxygenation of the initial mono alkylperoxide.…”

Reaching Milestones in the Oxygenation Chemistry of Magnesium Alkyls: towards Intimate States of O2 Activation and the First Monomeric Well‐Defined Magnesium Alkylperoxide

“…In their entirety, our results provided a basis for a new mechanism for the oxygenation of organometallics with non‐redox‐active metal centres, an inner‐sphere electron‐transfer mechanism (ISET) . The crucial step of the ISET mechanism involves a non‐specific activation of an O 2 molecule on the metal centre followed by single electron transfer (SET) from the M−C bond to the O 2 molecule (we note that the ISET mechanism nicely collaborates with our systematic studies on both the mentioned above controlled oxygenation of organometallics with non‐redox‐active metal centre as well as the reactivity of organozincs incorporating non‐innocent ligands, that is, stable nitroxide radicals, α ‐diimines and α ‐diketones). Moreover, we have also emphasized the essential role of the coordination state of the oxygenated organoaluminum and organozinc compounds, as well as the geometric requirements of an O 2 molecule approaching the metal centre .…”

Oxygenation Chemistry of Magnesium Alkyls Incorporating β‐Diketiminate Ligands Revisited

“…[14] Interestingly, the authors also showed that the metathesis reaction between the resulting magnesium alkylperoxide and the parenta lkylmagnesium compound did not take place at room temperature and suggested that it likely occurs at 80 8Co vernight. [16,17] Moreover,t he statement by Parkin and coworkers that "the isolation of discrete products by the reaction of dioxygenw ith metal alkyl derivatives and investigation of their subsequentr eactivity have provided major challenges" [14b] still remainsa ctual. [16,17] Moreover,t he statement by Parkin and coworkers that "the isolation of discrete products by the reaction of dioxygenw ith metal alkyl derivatives and investigation of their subsequentr eactivity have provided major challenges" [14b] still remainsa ctual.…”

“…[15] We also note that in this study the authors, like others previously, [13] used galvinoxyl as an inhibitor of the oxygenation process to substantiate the radical-chain mechanism.H owever,i ts hould be emphasized that recent studies convincingly demonstrate that stable nitroxyl radicals are not aq ualitative evaluation tool for this type of investigation, because they act as noninnocentl igandst owards metal alkyls and themselves initiate the formation of alkyl radicals. [16,17] Moreover,t he statement by Parkin and coworkers that "the isolation of discrete products by the reaction of dioxygenw ith metal alkyl derivatives and investigation of their subsequentr eactivity have provided major challenges" [14b] still remainsa ctual.…”