Sodium Dialkyl-amidozincates:AlkyI or Amido Bases? An Experimental and Theoretical Case Study

摘要

Alkali metal zincates are fast establishing themselves as a new vanguard class of organozinc reagent,to add to the old guard of organozinc halides and diorganozincs,which have long been actively engaged in organic synthesis.Among the best developed alkali metal zincates are homoleptic triorganozincates [M~+ZnR_3~-] and heteroleptic diorgano-amidozincates [M~+ ZnR_2(R_2N)~-].In the latter category,lithium di-tert-butyltetramethylpiperidinozincate (TMP-zincate) is receiving special attention in view of its high chemo-/regioselectivity in deprotonative reactions of,for example,functionalized aromatics (including benzyne precursors) and functionalized heteroaromatics (including halopyridines).The heteroleptic character of TMP-zincate makes ligand-transfer selectivity an issue,as in theory either the amido ligand or the alkyl ligand could function as the base (zincating agent) in these metal-hydrogen exchange applications.Though hitherto no definitive structural evidence has been forthcoming to decide between the two possibilities,it has been presumed that TMP-zincate is an amide base,that is,following deprotonation the new (putative) zincate retains both tert-butyl ligands and TMPH is formed (eq 1).