Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C−C Bond Forming Reactions

Abstract

Dedicated to Prof. Doug Stephan on the occasion of his 70 th birthday.

[{SiNDipp}MgNa]2: A Potent Molecular Reducing Agent

“…An early demonstration of the reducing nature of the β-diketiminato compounds 1 and 2 was provided by Jones and co-workers’ study of their behavior toward anthracene and benzophenone, which provided the respective deep red bimetallic and purple monometallic and radical products of reduction (Scheme a). , Compound 4 was, thus, reacted with anthracene ( E 0 = −1.98 V vs SCE) in C 6 D 6 at 40 °C for a period of 3 days. This procedure resulted in the gradual decolorization of the solution and the formation of colorless single crystals of compound 8 .…”

Pinacol Cross‐Coupling Promoted by an Aluminyl Anion

“…To probe the possible involvement of radical species in the aluminyl promoted pinacol coupling, one equivalent of 4‐dimethylaminopyridine (DMAP) was added to a solution of 4 to either trap (or promote formation of) the ketyl radical (Scheme 3). [28a] Colourless ( 5 ) and deep pink ( 6 ) crystals were isolated by fractional crystallisation from the reaction mixture, and analysed by X‐ray diffraction.…”

A Doubly Reduced Dipalladium Dibenzylideneacetone Complex

“…However, the isolation of such ketyl radicals has to cope with inherent challenges, such as potential hydrogen abstraction or coupling reactions. Hence, examples of isolated metal-bound carbonyl anions are scarce and mostly restricted to diaryl ketones, like benzophenone or fluorenone, for the radical anions of which alkali metal salts could be isolated (Figure ); − the reaction of dibenzylideneacetone (dba), which is of relevance in the context of the present work, with a Mg I precursor led to C–C coupling of dba, proposed via dba radical species . Early d-block metal-stabilized ketyl radical complexes were accessed by Hou and co-workers, and recent works exemplified that benzophenone ketyl radicals can also be stabilized by first-row late transition metals (Co, Fe) (Figure ).…”

“…Hence, examples of isolated metal-bound carbonyl anions are scarce and mostly restricted to diaryl ketones, like benzophenone or fluorenone, for the radical anions of which alkali metal salts could be isolated (Figure 1); 8−11 the reaction of dibenzylideneacetone (dba), which is of relevance in the context of the present work, with a Mg I precursor led to C−C coupling of dba, proposed via dba radical species. 6 Early d-block metal-stabilized ketyl radical complexes were accessed by Hou and co-workers, 12 and recent works exemplified that benzophenone ketyl radicals can also be stabilized by first-row late transition metals (Co, Fe) (Figure 1). 13,14 A few additional examples of other complexes bearing simple diaryl ketyl or fluorenyl radical anions were reported for rare-earth metals 15−17 and uranium.…”

“…The existence of metal ketyl radicals became first evident from the work of Gomberg and Bachman, who reported a reduction of benzophenone in contact with a Mg(s)/MgI 2 mixture yielding a pinacol coupling product via a transient ketyl intermediate, [IMg­(OCPh 2 ) • ] . Today it is clear that the formation of metal-bound ketyl radical anions and the resulting umpolung of the carbonyl function are crucial steps in many organic transformations, including C–C coupling reactions with carbonyl compounds and photoredox catalysis. − The role of reactive ketyl radicals in various transformations fueled substantial efforts to structurally characterize metal-stabilized ketyl intermediates to study their coordination chemistry and reactivity. However, the isolation of such ketyl radicals has to cope with inherent challenges, such as potential hydrogen abstraction or coupling reactions.…”