Four-Electron Reduction of Benzene by a Simple Samarium(II)-Alkyl

Abstract

Abstract Benzene reduction by molecular complexes remain a significant synthetic challenge, requiring harsh reaction conditions involving group 1 metals. Even under such forcing conditions only the one- or two-electron reduction of benzene has been achieved. The high stability of benzene is attributed to its aromaticity, imparted by the delocalisation of electrons across its six-carbon framework. All reductions of benzene to date result in a loss of aromaticity, although the hypothetical benzene tetra-anion is calculated to be stable and aromatic. The four-electron reduction has yet to be realised due to the lack of a sufficiently potent reductant. Here we present examples of four-electron reductions of benzene and its derivatives by a simple β-diketiminate samarium(II) alkyl reagent under mild conditions and without the need for group 1 metals. Whereas the reactivity of organo-samarium(II) compounds are typically defined by one-electron processes, the compounds reported here feature an unprecedented two-electron process. Density functional theory calculations implicate a transient samarium(I) intermediate involved in the reduction of benzene, which ultimately dimerise to give the inverted sandwich complex where two samarium(III) ions are bridged by a benzene tetra-anion. The remarkably strong reducing power of this samarium(II) alkyl implies a rich reactivity, providing applications as specialised reducing agents. This finding enhances the opportunities for the strategic reduction of aromatic rings in synthetic design, with implications for several scientific fields.