Enolates continued

The curves for log(kK/s ) and log(kK/s ) of acetophenone are parallel in the range pAT << PH << pAT and the vertical distance between them then equals pAtE = log(kK/s 1) — log /s-1). Most ketones are very weak bases, pAt < 0, so that the parameter does not affect the shape of the pH-rate profiles in the range pH > 1. Base catalysis of ketonization saturates at pH = pAr , while the rate of enolization continues to rise, so that the curves for kE and kK eventually cross at higher pH. At still higher pH, the rate constant kE exceeds that of kK = k o and kobs follows kK. The crossing point, for which kE = kK, lies at pH = pAT = 18.3 for acetophenone (Fig. 3), which is outside the accessible pH range when ionic strength I is limited to 0.1 m, but pA is readily calculated from Equation (2). 

High-coordination-number complexes of 0-keto-enolates continue to be obtained with the metals such as zirconium(IV),8 hafnium(IV),8 cerium(IV),9 and the lanthanons(III),10 the last being tetrakis anionic species. At least one example of a volatile tetrakis 0-keto-enolate salt has been reported,11 Cs[Y(CF3-COCHCOCF3)4]. The ionic charge on the 0-keto-enolate complex has been shown to produce12 a high field nuclear magnetic resonance for anions and low field shifts for cations, relative to the positions observed for the neutral species. 

The use of preformed enolates continues to provide a valuable route for performing mixed Claisen condensations. Reaction of a 2-indolylacetic ester with 2 mol of LDA and treatment of the product with an a-trisubstituted acid chloride gave the corresponding keto ester as a diastereomeric mixture in 74% yield (equation 1) ... 

The rate of publication of investigations into systems for carrying out diastereo- and enantio-selective aldol condensations has continued unabated during the year (cf. 9, 241). Chiral boron enolates continue to invoke much interest the dimethylborolane enolates of thioesters form adducts in which the anti-aldols... 

The Claisen rearrangement of ester enolates continues to provide interesting syntheses of unsaturated acids. Rearrangement of the ester (32) leads to the allyl silane (33) in 61 and 53% yield for two examples protiodesilylation then provides the Se-unsaturated acids (34) in quantitative and 85 % yield, with a ratio of >8 1 of the diastereoisomers when = H and R = Me (Scheme 17). ... 

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