Structurally Stimulated Deprotonation/Alumination of the TMP Anion**

Derived from the cyclic secondary amine 2,2,6,6-tetramethylpiperidine [TMP(H)] by removal of its nitrogen-attached hydrogen atom, the TMP anion 1 has had a 40 year career in the business of selective proton abstraction. Possessing attributes of high steric bulk and low nucleophilicity, without any b-hydrogen atoms (the presence of which often disadvantages its diisopropylamido analogue through b-hydride decomposition), 1 is usually employed as its lithium salt, LiTMP, which exists as a cyclotetramer in the crystal. NaTMP is a cyclotrimer, whereas KTMP remains still to be crystallographically characterized. Besides being strong Brønsted bases, these alkali metal TMP compounds are strong Lewis acids. Adding the Lewis base TMEDA (N,N,N’,N’-tetramethylethylenediamine) to them causes steric inflation of the alkali metal coordination sphere, concomitantly reducing the TMP complex aggregation state. Tetrameric LiTMP is reduced to the hemisolvated open dimer [(TMEDA)Li(m-TMP)Li(TMP)]; whereas the larger alkali metal complexes become fully solvated cyclodimers [{(TMEDA)M(m-TMP)}2] [7] (M=Na or K). Since such structural changes can modify the reactivity of the “TMP anion”, accommodating it within a more altered mixed-metal (and/or mixed-anion) environment can exert a strong influence on reactivity. This idea has stimulated much current research into TMP-based bimetallic reagents including TMP-magnesiates, TMP-zincates, TMP-cadmates, TMP-chromates, TMP-manganates, and TMP-ferrates, which can show significant improvements in, for example, functional group tolerance or kinetic stability compared to that of LiTMP. Of particular interest in this study are TMP-aluminates. Uchiyama and co-workers introduced the first TMP-aluminate “iBu3Al(TMP)Li” in 2004 [13] showing its high chemoand regioselectivity in proton abstraction reactions of functionalized aromatic substrates. In these and subsequent reactions of this type, the TMP anion is invariably the sole active (or most active) base ligand executing deprotonation upon the substrate and disembarking the assembly as TMP(H). The prospect of deprotonating this most powerful of bases to generate a TMP-based dianion would seem a challenge too far. However, here we present a rational structurally controlled approach that has met this challenge. Recent inorganic structurally focused studies have elaborated the design characteristics of a TMP-bimetallic complex that can render it a powerful metalating (magnesiating, zincating, aluminating etc.) agent. Commonly, the two distinct metals connect through a TMP bridge, with the dior trivalent metal carrying two or three additional anions (alkyls and/or amides), while a neutral aggregation-blocking capping ligand (for example, multiple THF molecules or TMEDA) binds terminally to the alkali metal. With “iBu3Al(TMP)Li”, organic substrates such as benzamides can undergo ortho deprotonation/alumination with evolution of TMP(H) (Scheme 1). While such reactions are intermolecular, it is