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

David R. Armstrong, Elaine Crosbie, Eva Hevia, Robert E. Mulvey, Donna L. Ramsay, Stuart D. Robertson

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48 Citations (Scopus)
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Abstract

The lithium TMP-aluminate bases "LiTMP·Al(iBu) 3" 1 and "LiTMP·Al(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. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1·THF to complex equilibria involving five major species in LiTMP·THF, Al( iBu)3·THF, [{Li(THF)4} +{Al(TMP)(iBu)3}-] 1·(THF)4, [(THF)·Li(μ-TMP)(μ-OC4H 7)Al(iBu)2], 4, and (TMP)Al( iBu)2·THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP·THF and (TMP)Al( iBu)2·THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.

Original languageEnglish
Pages (from-to)3031-3045
Number of pages15
JournalChemical Science
Volume5
Issue number8
Early online date7 May 2014
DOIs
Publication statusPublished - 1 Aug 2014

Keywords

  • aluminium–hydrogen exchange
  • x-ray crystallography
  • lithium-mediated alumination

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