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

D.R. Armstrong, A.M. Drummond, L. Balloch, D.V. Graham, E. Hevia, A.R. Kennedy

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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
Original languageEnglish
Pages (from-to)5860-5866
Number of pages6
JournalOrganometallics
Volume27
Issue number22
DOIs
Publication statusPublished - 24 Nov 2008

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Keywords

  • metalation
  • 2
  • 4
  • 6-trimethylacetophenone
  • organozinc reagents
  • role
  • base
  • composition
  • structure
  • developing enolate

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