Reactions of ZnR₂ Compounds with Dibenzoyl: Characterisation of the Alkyl‐Transfer Products and a Striking Product‐Inhibition Effect

Abstract

The first systematic theoretical and experimental studies of reaction systems involving ZnR(2) (R=Me, Et or tBu) with dibenzoyl (dbz) as a non-innocent ligand revealed that the character of the metal-bonded R group as well as the ratio of the reagents and the reaction temperature significantly modulate the reaction outcome. DFT calculations showed four stable minima for initial complexes formed between ZnR(2) and dbz and the most stable structure proved to be the 2:1 adduct; among the 1:1 adducts three structural isomers were found of which the most stable complex had the monodentate coordination mode and the chelate complex with the s-cis conformation of the dbz unit appeared to be the least stable form. Interestingly, the reaction involving ZnMe(2) did not lead to any alkylation product, whereas the employment of ZntBu(2) resulted in full conversion of dbz to the O-alkylated product [tBuZn{PhC(O)C(OtBu)Ph}] already at -20 °C. A more complicated system was revealed for the reaction of dbz with ZnEt(2). Treatment of a solution of dbz in toluene with one equivalent of ZnEt(2) at room temperature afforded a mixture of the O- and C-alkylated products [EtZn{PhC(O)C(OEt)Ph}] and [EtZn{OC(Ph)C(O)(Et)Ph}], respectively. The formation of the C-alkylated product was suppressed by decreasing the initial reaction temperature to -20 °C. Moreover, in the case of the dbz/ZnEt(2) system monitoring of the dbz conversion over the entire reaction course revealed a product inhibition effect, which highlights possible participation of multiple equilibria of different zinc alkoxide/ZnEt(2) aggregates. Diffusion NMR studies indicated that dbz forms an adduct with the O-alkylated product, which is a competent species for executing the inhibition of the alkylation event.

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 .…”

Probing the Role of π Interactions in the Reactivity of Oxygen Species: A Case of Ethylzinc Aryloxides with Different Dispositions of Aromatic Rings toward the Metal Center

“…Diffusion-ordered spectroscopy (DOSY), a routine NMR technique, gives an approximate size of the species that exist in solution and has been successfully applied to several organozinc systems. [24][25][26][27] As already discussed previously, [28,29] molecular weight can be easily obtained by combining the Stokes-Einstein equation (D = (kT)/(6phr H )) and the relationship between molecular weight M and molar radius, which is related by M = (4pr M 3 1N A )/3, where r H and r M are hydrodynamic and molar radii, respectively; h is the viscosity; and 1 is the density of the liquid. It has also been shown that these values are similar for small molecules, which is the case in our system; although, it should be noted that such an estimation can lead to errors in the range of 10-20 %.…”

Development of zinc alkyl/air systems as radical initiators for organic reactions

“…Building on our continuous interest and expertise in radicals' chemistry and dioxygen activation, 5,7,10,12 we herein report comparative experiments on the activity of carbon- and oxygen-centred radical species (derived from a series of zinc alkyl/air reaction systems with a clear mechanistic picture, see Scheme 1), in a model reaction of radical addition of THF to imines as a selected model reaction. The study of these model systems not only provides better understanding of how the oxygenated products participate in radical organic reactions but also assists in the development of new types of radical initiators.…”