The Chemistry of Metal Enolates [Part 1] by Jacob Zabicky

By Jacob Zabicky

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By Jacob Zabicky

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Castella-Ventura and coworkers70 applied the DFT approach at the B3LYP/6-31+G* level to determine the geometrical, energetic and vibrational characteristics of the different aggregates of the acetaldehyde lithium enolate involved in the cleavage of thf by n-BuLi. Calculations showed that two types of anions may be distinguished in these aggregates, the O and π(CC)-Li bound pyramidal structure (55) and the O-Li only bound linear structure (56). In parallel, IR and FT-Raman spectroscopy allowed one to determine the vibrational properties of the different enolate aggregates and to specify the mechanism of the thf cleavage.

Then, the abstraction from the thf molecule of an α-hydrogen followed by a cycloreversion mechanism73 leads to an ethylene molecule and a butane molecule. The acetaldehyde enolate anion in the CH2 CHOLi mixed aggregate with n-BuLi (58) (equation 15) probably interacts by its oxygen atom bound to the two lithium cations. Its structure corresponds to a linear enolate anion. Mixed aggregate 58 was characterized by the 1612 cm−1 Raman band, and their larger stability was confirmed by ab initio calculations74 .

These results were in agreement with experimental NMR data on the structure of mibLi in the presence of dme, 12-crown-4 and cryptand-2,1,1. 1, originally attributed to a ligand-separated monomer, mibLi(2,1,1)+ , was well reproduced by HF-calculated NMR shifts for the predicted ligand-separated triple ion pair. A noticeable increase in rate was also observed in the polymerization of mma in the presence of the Na+ (2,2,2) countercation55 . Wang and coworkers53 postulated that 1. General and theoretical aspects of the metal enolates 21 O O Li N N O O O O O O Li O O (48) the counteranion within the ion pair formed in the presence of cryptand-2,1,1 was the monomeric mib anion.

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