Ether, dimethyl coordination energy

Fig. 2.5. Comparison of energy profile (AG) for pathways to E- and Z-product from the reaction of lithio methyl dimethylphosphonoacetate and acetaldehyde. One molecule of dimethyl ether is coordinated to the lithium ion. Reproduced from J. Org. Chem., 64, 6815 (1999), by permission of the American Chemical Society.

When dissolved in nonpolar solvents such as benzene or diethyl ether, the colourless (2a) forms an equally colourless solution. However, in more polar solvents [e.g. acetone, acetonitrile), the deep-red colour of the resonance-stabilized carbanion of (3a) appears (1 = 475... 490 nm), and its intensity increases with increasing solvent polarity. The carbon-carbon bond in (2a) can be broken merely by changing from a less polar to a more polar solvent. Cation and anion solvation provides the driving force for this heterolysis reaction, whereas solvent displacement is required for the reverse coordination reaction. The Gibbs energy for the heterolysis of (2a) correlates well with the reciprocal solvent relative permittivity in accordance with the Born electrostatic equation [285], except for EPD solvents such as dimethyl sulfoxide, which give larger values than would be expected for a purely electrostatic solvation [284]. 

In the model system for (/ ,5)-2 in a polar solvent with a coordinating dimethyl ether molecule at the lithium center (Epimerization 2 in Scheme 1), the energy difference between MIN-7 and TS-4 is smaller by 25 kJ/mol. Furthermore, in contrast to the nonpolar case described, there is an energy difference of 9 kJ/mol between the two diastereomeric minimum structures MIN-7 and MIN-8. The conclusion from these studies is that from ether solution, a highly diastereomerically enriched (,R,S)-2 should be obtained by epimerization at room temperature, while a diastereotopos-differentiating deprotonation without noticeable epimerization should be possible in nonpolar solution (under the reaction conditions described). 

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