Mixed Lithium Amide-Lithium Halide Compounds: Unusual Halide-Deficient Amido Metal Anionic Crowns

摘要

Alkali metal halide salts can dramatically influence the reactivity/selectivity of organic transformations in either beneficial or detrimental ways. In many circumstances, the metal halide salt formed in situ in a metathesis reaction is dismissed as an innocent by-product. Recently, more cases have come to light where lithium halides affect organometal-lic reactions in a non-innocent, often dominant way. Knochel et al. has exploited this effect by adding stoichiometric amounts of LiCl to conventional Grignard or Hauser reagents to induce an enhanced reactivity with respect to that of monometallic magnesium reagents. Collum et al. presented the surprising and profound role that LiCl plays in a series of deprotonation and addition reactions, establishing that LiCl catalysis is detectable with miniscule quantities of LiCl, and that "striking accelerations" (70 fold) are elicited by less than 1.0 mol LiCl for 1,4-addition reactions of lithium diisopropylamide to unsaturated esters.Despite this, firm structural evidence of the crucial halide-incorporated species that may be involved in these reactions is rare. In one example, we recently synthesized and characterized the magnesiate (thf)2Li(μ-Cl)2Mg(TMP)(thf) and found that it functions identically to Knochel's in situ Grignard system (TMP = 2,2,6,6-tetramethylpiperidide).