Abstract
Bulky amido ligands are precious in s‐block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n‐butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe3)(Dipp)]− (Dipp=2,6‐iPr2‐C6H3). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s‐block metal amides. Solvation by N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) or N,N,N′,N′‐tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi‐solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe3)(Dipp)}2(μ‐nBu)]∞ (M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.
Highlights
Alkali metal amides of bulky secondary amines, [(MNR2)n], are popular tools in many synthetic campaigns due to a complementary combination of strong deprotonating power and weak nucleophilicity
Most common are lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP), referred to collectively as the “utility amides”.[1]. Though all these amides are useful in their monometallic form, the performance of the TMP anion in CÀH deprotonation applications can be significantly enhanced by incorporating it within multicomponent systems such as the turbo-Hauser base [TMPMgCl·LiCl][2] or magnesiate [(TMEDA)Na(m-TMP)(m-nBu)Mg(TMP)][3] (TMEDA = N,N,N’,N’-tetramethylethylenediamine) or alternatively by administering LiTMP in combination with an organometallic [e.g., Al(iBu)3,[4] (TMP)Al(iBu)2,[5] Zn(TMP)2[6]] or salt (MgCl2, ZnCl2 or CuCN)[7] trapping agent, which can drive equilibria to products by stabilising sensitive intermediates through lower polarity MÀC bonds (M = Al, Mg, Zn)
Casting the net wider for other bulky amides that might possess interesting cooperativity in magnesiate modifications our attention was drawn to the arylsilyl amido ligand [N(SiMe3)(Dipp)]À
Summary
Alkali metal amides of bulky secondary amines, [(MNR2)n], are popular tools in many synthetic campaigns due to a complementary combination of strong deprotonating power and weak nucleophilicity. Most common are lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP), referred to collectively as the “utility amides”.[1] Though all these amides are useful in their monometallic form, the performance of the TMP anion in CÀH deprotonation applications can be significantly enhanced by incorporating it within multicomponent systems such as the turbo-Hauser base [TMPMgCl·LiCl][2] or magnesiate [(TMEDA)Na(m-TMP)(m-nBu)Mg(TMP)][3] (TMEDA = N,N,N’,N’-tetramethylethylenediamine) or alternatively by administering LiTMP in combination with an organometallic [e.g., Al(iBu)3,[4] (TMP)Al(iBu)2,[5] Zn(TMP)2[6]] or salt (MgCl2, ZnCl2 or CuCN)[7] trapping agent, which can drive equilibria to products by stabilising sensitive intermediates through lower polarity MÀC bonds (M = Al, Mg, Zn). Both the starting metal reagent and, where relevant, the donor solvent employed in these reactions were added in slight excess compared to the parent amine
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