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

Abstract

Advancing 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)].