Metalation of 2,4,6-Trimethylacetophenone Using Organozinc Reagents: The Role of the Base in Determining Composition and Structure of the Developing Enolate

The new mixed lithium-zinc enolate compounds [(TMEDA)(2)Li2Zn{OC(=CH2)Mes}(4)] (2) and [{TMP(H)}(2)Li2Zn{OC(=CH2)Mes}(4)] (3) were prepared by reaction of the sterically demanding ketone 2,4,6-trimethylacetophenone (1) with the all-amido homoleptic zincate [LiZn(TMP)(3)] (TMP = 2,2,6,6-tetramethylpiperidide). X-ray crystallographic studies revealed that these compounds adopt a trinuclear Li center dot center dot center dot Zn center dot center dot center dot Li chain arrangement with enolate O bridges. In contrast, the metalation of 1 with heteroleptic [(TMEDA)LiZn(TMP)Me-2] afforded the dimeric lithium enolate [(TMEDA)(2)Li-2{OC(=CH2)Mes}(2)] (4) as a crystalline solid, which has been characterized in the solid state by X-ray crystallography, and Me2Zn center dot TMEDA and TMP(H) as coproducts, showing that the dimethylamido zincate behaves as an amide base. The homoleptic zinc enolate [(TMEDA)Zn{OC(=CH2)Mes}(2)] (5) was obtained by reaction of 1 with the zinc amide Zn(TMP)(2), and its structure was determined by X-ray crystallography. 5 adopts a rarely observed monomeric arrangement where the two enolate groups bind terminally to the zinc. New enolates 2-5 have also been characterized by H-1, C-13, and Li-7 NMR spectroscopy in C6D6 solution. DFT studies of the metalation of 1 by Zn(TMP)(2) and Et2Zn revealed that the former amide has a much greater kinetic basicity than the latter alkyl reagent.