Oxygenation Chemistry of Magnesium Alkyls Incorporating β‐Diketiminate Ligands Revisited

Abstract

Despite the fact that extensive research has been carried out, the oxygenation of alkyl magnesium species still remains a highly unexplored research area and significant uncertainties concerning the mechanism of these reactions and the composition of the resulting products persist. This case study compares the viability of the controlled oxygenation of alkylmagnesium complexes supported by β-diketiminates. The structural tracking of the reactivity of (N,N)MgR-type complexes towards O at low temperature showed that their oxygenation led exclusively to the formation of magnesium alkylperoxides (N,N)MgOOR. The results also highlight significant differences in the stability of the resulting alkylperoxides in solution and demonstrate that [(BDI)Mg(μ-η :η -OOBn)] (in which BDI=[(ArNCMe) CH] and Ar=C H iPr -2,6) can be easily transformed to the corresponding magnesium alkoxide [(BDI)MgOBn] at ambient temperature, whilst [( BDI)Mg(μ-OOtBu)] (in which BDI=[(ArNCMe) CH] and Ar=C H F -2,4,6) is stable under similar conditions. The observed selective oxygenation of (N,N)MgR-type complexes to the corresponding (N,N)MgOOR alkylperoxides strongly contradicts the widely accepted radical-chain mechanism for the oxygenation of the main-group-metal alkyls. Furthermore, either the observed transformation of the alkylperoxide [(BDI)MgOOBn] to the alkoxide [(BDI)MgOBn] as well as the formation of an intractable mixture of products in the control reaction between the alkylperoxide [( BDI)MgOOtBu] and the parent alkylmagnesium [( BDI)MgtBu] complex are not in line with the common wisdom that magnesium alkoxide complexes' formation results from the metathesis reaction between MgOOR and Mg-R species. In addition, a high catalytic activity of well-defined magnesium alkylperoxides, in combination with tert-butyl hydroperoxide (TBHP) as an oxygen source, in the epoxidation of trans-chalcone is presented.

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

“…As already noted, the widely accepted mechanism implies that the formation of an alkoxide species is a consequence of a metathesis reaction between the resulting zinc alkylperoxide and the parent alkylzinc compound (Scheme a; metathesis) . Conversely, our results on the oxygenation of 1 and 2 at low temperature, along with earlier reported precedents concerning the oxygenation chemistry of zinc,, aluminum, and particularly magnesium alkyls, stand in clear contradiction to this widely accepted metathesis process. The formation of zinc alkoxides can be rationalized in terms of an intramolecular transformation of zinc alkylperoxides (Scheme ; the proposed η 2 ‐binding mode of alkylperoxide ligand has previously been observed for aluminum and magnesium alkylperoxide complexes).…”

“…During the last decade, our group's studies have provided a fresh perspective on such oxygenation processes, and have indicated that, despite the involvement of radical species, a radical‐chain process alone cannot fully explain all of the experimental observations . For example, we have convincingly demonstrated the essential role of both the coordination state of the organometallic species (M=Al, Zn,‐ or Mg) and the geometric requirements of an O 2 molecule approaching the metal center in the case of rationally designed aluminum alkyls . In the latter case, the combined structural and chemical patterns provided compelling experimental evidence that attack of O 2 on the metal center from a desired direction is the key feature in O 2 activation by tetrahedral organoaluminum complexes, prior to insertion of the dioxygen molecule into the Al−C bond .…”

“…In the latter case, the combined structural and chemical patterns provided compelling experimental evidence that attack of O 2 on the metal center from a desired direction is the key feature in O 2 activation by tetrahedral organoaluminum complexes, prior to insertion of the dioxygen molecule into the Al−C bond . The same issue has not yet been convincingly resolved for alkylzinc and alkylmagnesium complexes. So far, the data only indicate that four‐coordinate zinc alkyl species do not display reactivity towards dioxygen, in contrast to the three‐coordinate species, which react swiftly –…”

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

“…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 . Apart from the fundamental aspects, these systematic studies have advanced rational design of metal alkyl/air systems as radical initiators for organic reactions, stoichiometric and catalytic asymmetric epoxidation of enones and pave the way to well‐defined metal alkoxides (not accessible through the classical alcoholysis path) as potential initiators of cyclic esters polymerization as well as to a new approach for the preparation of semiconductor zinc oxide quantum dots based on organozinc/air systems …”

“…In later studies, the reactions of O 2 with dialkylaluminium and alkylzinc complexes incorporating monoanionic, bidentate ligands allowed for stabilization and subsequent isolation of the first aluminium and zinc alkylperoxides. Since that time, over a dozen examples of metal alkylperoxides with non‐redox‐active metal centres, that is, Li, Na, K, Mg, Zn, Cd, Al, Ga, In incorporating a variety of supporting ligands have been successfully synthesized. These compounds form typically dimeric structures or higher aggregates with bridging alkylperoxide moieties, however, to the best of our knowledge, a handful of examples of terminal metal alkylperoxide have also been described .…”