Interaction between alkali metal aromatic ketone radical anions and the chlorides of lithium and magnesium in solution. A case of a carbon–carbon bond strengthening through complex formation

Abstract

The interaction between lithium, sodium and potassium salts with the pre-formed radical anions of benzophenone, 2-methylbenzophenone, 2,4,6-trimethylbenzophenone or fluorenone, and lithium or magnesium chlorides has been studied by nuclear magnetic resonance spectroscopy. The interaction between the radical anions and the metal salt depends on both the anion and the countercation and is stronger with magnesium chloride than with lithium chloride. Interaction leads either to the establishment of equilibria (lithium chloride) or to the formation of diamagnetic complexes in solution with rather well defined stoichiometry (magnesium chloride). Sodium 2-methylbenzophenone radical anion, unlike sodium benzophenone radical anion, is soluble and paramagnetic in tetrahydrofuran (THF). The highly hindered 2,4,6-trimethylbenzophenone radical anion exhibits a very weak interaction with magnesium chloride. It appears to exist in solution either as a very loose aggregate or as a monomeric entity. It is concluded that complexation between unhindered aromatic ketone radical anions with magnesium chloride causes a C–C bond strengthening in the incipient pinacolate dianion moiety within the radical anion cluster. On the basis of the above information, the success of the Gomberg–Bachmann pinacol synthesis by reducing aromatic ketones with Mg–MgI₂, can now be understood.