D. A. Evans Enolate Structures from X-ray diffraction Chem 206 Ab initio calculations(Spartan)indicate M Metal Tautomerism that the partial negative charge on the alpha carbon is-0. 22 for the Li enolate Me S/H M=Li For alkali metal enolates(M= Li, Na, Ketc ) the o-metal tautomer is strongly favored. This generalization holds for most alkaline earth enolates(Mg) as well. These are the generally useful enolate nucleophiles For certain metal enolates from heavy metals such as M=Hg"the C-metal tautomer is sometimes favored Resonance Structures 0" resonance C resonance structure structure ince enolates some interest to ed contributing ray crystal ructures of a r One would predict that as the relative importance of the C structure J.Am.Chem.Soc.1985,107,5403 does hold, but the net change in 130- would lengthen increases the c-o bond would shorten and the c-c bond The prediction stated above 135A the C-C bond length is <2 %! 132A~134A CMea In solution and in the solid state metal enolates have a strong tendency to aggregate into dimers and tetramers to satisfy metal solvation requirements o-MaMg N O Li Li O M H Me H O R M R R R O O Li H R H Br Mg OEt2 O Me CMe3 H H R H O Me M OR RO M O O R M M Me O O CMe3 H Li N Me2 Me2 N Mg Br M M M O M R O R O R R O M R R O R M R R O M R O M ● ● 1.35 Å 1.34 Å + In solution and in the solid state metal enolates have a strong tendency to aggregate into dimers and tetramers to satisfy metal solvation requirements. Crystallized as the dimer Crystallized as the dimer Seebach & co-workers, J. Am. Chem. Soc. 1985,107, 5403. For certain metal enolates from heavy metals such as M = Hg+2 the C-metal tautomer is sometimes favored. The prediction stated above does hold, but the net change in the C–C bond length is < 2 % ! 1.36 Å 1.32 Å One would predict that as the relative importance of the C– structure increases, the C–O bond would shorten and the C–C bond would lengthen. Since enolates usually function as carbon nucleophiles, it is therefore of some interest to assess the relative importance of the illustrated contributing polar resonance structures. Within the last decade good X-ray crystal structures of a number of metal enolates have been obtained. C – resonance structure O – resonance structure For alkali metal enolates (M = Li, Na, K etc.) the O-metal tautomer is strongly favored. This generalization holds for most alkaline earth enolates (Mg+2) as well. These are the generally useful enolate nucleophiles Metal Tautomerism Resonance Structures D. A. Evans Enolate Structures from X-ray Diffraction Chem 206 d– Ab initio calculations (Spartan) indicate that the partial negatilve charge on the alpha carbon is ~ – 0.22 for the Li enolate M = H M = Li – 0.19 – 0.22