FascinATES: Mixed-Metal Ate Compounds That Function Synergistically

Abstract

Ate complexes are generally thought of as having a cationic and an anionic moiety, each containing a metal atom, though these moieties can be contacted or separated depending on the system. They have been known for over 150 years but yet only recently has it dawned on synthetic chemists that ates can bring about lots of improvements to the metalation reaction. Alkali-metal magnesiates, zincates and aluminates in particular have been successfully utilized in metal-hydrogen exchange reactions of challenging weakly acidic aromatic substrates. The fascination of these mixed-metal reagents lies in their ability to effect unique deprotonations at ambient temperatures without attacking functional groups on the aromatic scaffold where conventional organolithium reagents would require subambient temperatures to reduce such attacks, to direct metalation to the meta position of certain substrates where ortho or lateral deprotonation is more common and to strip more than one hydrogen atom from multi-C-H bonded compounds where lithiation methods would generally fail. Furthermore some of these superficially simple ate-metal-induced deprotonation reactions produce complex but beautiful host-guest macrocyclic architectures referred to as inverse crowns. Since magnesium, zinc and aluminium organic compounds cannot replicate these metalation reactions, these alkali-metal-mediated reactions are synergistic in origin. This article is not intended to be comprehensive but provides a representative selection of these fascinates in action, with emphasis on their synergistic nature. Reactions of the heteroleptic alkyl-amido magnesiate TMEDA·Na(TMP)(nBu)Mg(TMP) and zincate TMEDA·Na(TMP)(tBu)Zn(tBu) are featured prominently (TMP is 2,2,6,6-tetramethylpiperidide).