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

Abstract

The new mixed lithium−zinc enolate compounds [(TMEDA) 2 Li 2 Zn{OC(═CH 2 )Mes} 4 ] ( 2 ) and [{TMP(H)} 2 Li 2 Zn{OC(═CH 2 )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···Zn···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(═CH 2 )Mes} 2 ] ( 4 ) as a crystalline solid, which has been characterized in the solid state by X-ray crystallography, and Me 2 Zn·TMEDA and TMP(H) as coproducts, showing that the dimethylamido zincate behaves as an amide base. The homoleptic zinc enolate [(TMEDA)Zn{OC(═CH 2 )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 1 H, 13 C, and 7 Li NMR spectroscopy in C 6 D 6 solution. DFT studies of the metalation of 1 by Zn(TMP) 2 and Et 2 Zn revealed that the former amide has a much greater kinetic basicity than the latter alkyl reagent.