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Contrasting reactivity of mono- versus Bis-2,2,6,6-tetramethylpiperidide lithium aluminates towards polydentate lewis bases : co-complexation versus deprotonation

Campbell, R. and Crosbie, E. and Kennedy, A.R. and Mulvey, R.E. and Naismith, R.A. and Robertson, Stuart (2013) Contrasting reactivity of mono- versus Bis-2,2,6,6-tetramethylpiperidide lithium aluminates towards polydentate lewis bases : co-complexation versus deprotonation. Australian Journal of Chemistry, 66 (10). pp. 1189-1201. ISSN 0004-9425

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Abstract

Two closely related lithium alkylaluminium amides LiAl(TMP)2iBu2 and LiAl(TMP)iBu3 (TMP: 2,2,6,6-tetramethylpiperidide) have been compared in their reactivity towards six polydentate Lewis bases containing either N or O donor atoms or a mixed N,O donor set. Seven of the twelve potential organometallic products of these reactions, which were carried out in hexane solution, have been crystallographically characterised. Three of these structures, [Li(-Me2NCH2CHCH2CH2CHO)(-TMP)Al(iBu)2], [Li(-Me2NCH2CH2OCH2)(-TMP)Al(iBu)2], and [Li(-Me2NCH2CH2OCHCH2NMe2)(-TMP)Al(iBu)2] reveal that the bis-amide LiAl(TMP)2iBu2 deprotonates (aluminates) the multifunctional Lewis base selectively at the carbon atom adjacent to oxygen with the anion generated captured by the residue of the base. In contrast, the mono-amide LiAl(TMP)iBu3 in general fails to deprotonate the Lewis bases but instead forms co-complexes with them as evidenced by the molecular structures of [Me2NCH2CHCH2CH2CH2O·Li(-iBu)(-TMP)Al(iBu)2], [Me2NCH2CH2OMe·Li(- iBu)(-TMP)Al(iBu)2], and [MeOCH2CH2OMe·Li(-iBu)(-TMP)Al(iBu)2]. Providing an exception to this pattern, the mono-amide reagent deprotonates chiral R,R,-N,N,N′,N′-tetramethylcyclohexanediamine to afford [Li(-CH2NMeC6H10NMe2)2Al(iBu)2], the final complex to be crystallographically characterised. All new products have been spectroscopically characterised through 1H, 7 Li, and 13C NMR studies. Reaction mixtures have also been quenched with D2O and analysed by 2D NMR spectroscopy to ascertain the full metallation versus co-complexation picture taking place in solution.