Enhancing the Metalating Power of ZnEt2 via Formation of an Alkyl/Alkoxide Potassium Zincate

Abstract

AbstractAdvancing the use of alkali‐metal alkoxides as additives to activate organometallic reagents, here we report the synthesis and characterization of a novel potassium zincate [{(PMDETA)KZn(OtBu)Et2}2] obtained by co‐complexation of equimolar amounts of potassium tert‐butoxide, diethyl zinc and the tridentate donor PMDETA (N,N,N’,N’’,N’’‐pentamethyldiethylenetriamine). Demonstrating its ability to activate both of the Et groups towards alkyne C−H metalation, this zincate reacts at room temperature with 2 equivalents of phenylacetylene to furnish [(THF)2KZn(CCPh)2(OtBu)}2], whereas, for the homometallic [ZnEt2(TMEDA)] (TMEDA=N,N,N’,N’‐tetramethylethylendiamine) only one Et group is reactive towards the same substrate under the same conditions. Investigations on the reactivity of these complexes to undergo N−H amine metalation using 2, 6‐diisopropylphenylamine (NH2Dipp) and 1,2,3,4‐tetrahydroquinoline (THQ(H)) as model substrates revealed that while homometallic [ZnEt2(TMEDA)] is completely inert towards these amines, heterobimetallic [{(PMDETA)KZn(OtBu)Et2}2] reacts through one of its Et groups to form [(THF)2KZn(OtBu)(Et)(NHDipp)] as well as [(THF)4K2Zn(THQ)4]. The latter is obtained as a side product of ligand redistribution of a putative [(THF)nKZn(OtBu)(Et)(THQ)] intermediate. An alternative method to access mixed amide/alkoxide potassium zincates is also described by assessing the co‐complexation of KOtBu with Zn(HMDS)2 in the presence of PMDETA which gave [(PMDETA)KZn(HMDS)2(OtBu)].