D. A. Evans The Aldol reaction Boron Imide enolates Chem 206 The Alpha substituent, X, plays pivotal role in aldol diastereoselection Imide Enolates: The problem of enolate face selectivity OR Me2CHCHO Br-CH2R Bu2B-OTf Et3N How can we rationalize these data X=SMe Model for Asymmetric Induction(unpublished) CHO favored Chelate organization precluded, therefore face selectivity uncertain RCHO The aldol reaction selects for the opposite enolate diastereoface △AG(273K)~26kca H 6 J.Am.Chem.Soc1981,103,212-2129 disfavored product diastereomer: The destabilizing interaction?O B O R N O R Me O L L B L L O Me O N R O R O O O N Me O R R O N Me O O Li Br-CH2R R Me R O N O O O B O O R N O X R R R H Me2CHCHO O N Me R O O BL2 O BL2 O R Me O N O O N R O R Me OH H O R B R R N O Me O R O O B O O O N Me R R R R H R O N Me O O B R R O N O O R Me R OH O B O O R N O Me R R R H OH Me N R O R O O O B O R O N R Me O L L RCHO Bu2B-OTf Et3N R N O H O Me H R O B L L O H O B O L L H R H Me N O O H R O O N R O R X OH OH X N R O O O R disfavored product diastereomer: The destabilizing interaction? disfavored favored Model for Asymmetric Induction (unpublished) DDG ‡ (273 K) ~ 2.6 kcal mol -1 How can we rationalize these data ? The Alpha substituent, X, plays pivotal role in aldol diastereoselection + Substituent Ratio > 300 : 1 60 : 1 1 : 1 X = Me X = SMe X = H Result discovered but not predicted diastereoselection > 98% The aldol reaction selects for the opposite enolate diastereoface LDA Face selectivity predicated on chelate organization RCHO Chelate organization precluded, therefore face selectivity uncertain Imide Enolates: The problem of enolate face selectivity J. Am. Chem. Soc 1981, 103, 212-2129 D. A. Evans The Aldol Reaction: Boron Imide Enolates Chem 206