Enolates de β-cétoesters: conformations et stabilités des formes dissociées

Abstract

634 (1982). The problem of the conformation of P-ketoester enolates has been investigated through IH nmr studies and through conductimetric studies. We have compared the dissociative properties of the K+ enolates of acyclic P-ketoesters which can a priori adopt any possible ZE, Z Z , EE, or EZ conformations and of hemicyclic P-ketoesters (2-carbethoxycyclopentanone, 2-acetyl y-butyrolactone) in which geometrical constraints excluded some of them. The results lead to the conclusion that the EE conformation is more stable than either the ZZ or EZ ones. Confirmation of this point, which implies that the acyclic dissociated enolates exist in the EE conformation, is afforded by 'H nmr spectroscopy. Those experimental results have been complemented with ab initio STO-3G calculations performed on the methyl formylacetate anion and on the formylacetone anion (the diketonic analog). The method of conductimetric titration by cryptand (2,2,2) used for the determination of the dissociation coefficient of K+ enolates in DMSO is discussed in some detail.

(1,1,1,5,5,5-Hexafluoro-2,4-pentanedionato-O,O')(1,4,7,10,13-pentaoxapentadecane-κ5O)sodium

“…Thus, the Na + ion is on the same side of the mean plane as two of the crown-ether O atoms (O12 and O14). The literature compounds most closely related to (I) are (15-crown-5)(4-ethoxybutan-2-one-4-olato-O,O')sodium (Cambillau et al, 1982) and (15-crown-5)[diphenylethyl(diphenylmethyl)cyanoacetonato]sodium (Coleman et al, 1991), both of which contain a six-membered Na--O==C(R)C(R)C(R)==O /3-diketonate ring system. For these compounds, the mean Na--4)(crown ether) and mean Na--Q(ketonate) distances are 2.491/2.309 and 2.478/2.320A, respectively.…”

The structure of the 15-crown-5–NaClO4 (1/1) complex

“…One of the most interesting studies turned out to be the series of complexes of 15-crown-5 [ 15-C-5, (1)] and benzo-15-crown-5 [B-15-C-5, (2)] with various sodium and potassium salts. Although the structures of alkali-metal complexes of B-15-C-5 have been extensively studied (Dally, 1978;Poonia & Bajav, 1979;Weber & V6gtle, 1981;Hilgenfeld & Saenger, 1982), only very few structural studies have been done on the related complexes of 15-C-5, its more flexible analog (Groth, 1981;Cambillau, Bram, Corset & Riche, 1982). Here we present the crystal and molecular structure of the complex of 15-C-5 with NaC10 4 (la), a complex which is a good structural representative of the 15-C-5/Na complexes.…”

Tetrabutylammonium α-acetyl-γ-butyrolactonate containing a three-dimensional hydrogen-bonded network

“…deviation 0.035 A Ê ). This geometry corresponds to the most stable conformation found for -ketoesterenolates in solution, as demonstrated by 1 H NMR spectroscopy (Cambillau & Guibe, 1982). This contrasts with most malonates, which adopt the s-cis conformation (Ushape) in the solid state and in solution.…”