TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation-alkylaluminium-trapping reagents?

摘要

The lithium TMP-aluminate bases "LiTMP-Al(~iBu)3" 1 and "LiTMP·Ai(TMP)(~iBu)2" 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as "aluminating" reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)·Li(μ-TMP)(μ-~iBu)Al(~iBu)2 1-THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1-THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole.