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Metalation of 2,4,6-trimethylacetophenone using organozinc reagents: the role of the base in determining composition and structure of the developing enolate

Armstrong, D.R. and Drummond, A.M. and Balloch, L. and Graham, D.V. and Hevia, E. and Kennedy, A.R. (2008) Metalation of 2,4,6-trimethylacetophenone using organozinc reagents: the role of the base in determining composition and structure of the developing enolate. Organometallics, 27 (22). pp. 5860-5866. ISSN 0276-7333

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Abstract

The new mixed lithium−zinc enolate compounds [(TMEDA)2Li2Zn{OC(CH2)Mes}4] (2) and [{TMP(H)}2Li2Zn{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···Zn···Li chain arrangement with enolate O bridges. In contrast, the metalation of 1 with heteroleptic [(TMEDA)LiZn(TMP)Me2] afforded the dimeric lithium enolate [(TMEDA)2Li2{OC(CH2)Mes}2] (4) as a crystalline solid, which has been characterized in the solid state by X-ray crystallography, and Me2Zn·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 1H, 13C, and 7Li 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. Copyright © 2010 American Chemical Society