Field effects, carbonyl compounds

The effect of conjugation also is reflected in infrared carbonyl frequencies (Section 16-3A) and nmr spectra. With respect to the latter, it is found that the protons on the (3 carbon of a,/3-unsaturated carbonyl compounds usually come at 0.7 to 1.7 ppm lower fields than ordinary alkenic protons. The effect is smaller for the a protons. 

Protons on the/3 carbon of an a,/3-unsaturated carbonyl compound absorb at very low fields j (about 57) because of the electron- withdrawing resonance effect of the j carbonyl group. 

The most interesting series of observations in this field have been made on the catalytic action of Lewis acids, which are also effective catalysts of Diels-Alder reactions (Section 4.1.6). In the presence of BF3 or its derivatives it has been possible to obtain 1,3-cycloadditions that do not proceed under uncatalysed conditions, e.g. those of benzonitrile oxide to aliphatic nitriles and carbonylic compounds and to oximes . The method has been applied also to the cycloaddition of diazomethane to CN multiple bonds, by using alkyl aluminium halides . Lewis acid catalysts probably act by making the dipolarophiles more polar, through complexing. 

An integral equation formalism (lEF) has been developed as particularly suitable for the description of solvent effects on spectral transition energies within the PCM model. The respective theoretical equations have been applied for the calculation of solvatochromic shifts of several carbonyl-group containing molecules at the self-consistent field (SCF), configuration interaction (Cl) and multiconfiguration self-consistent (MC SCF) field level of theory. The calculated spectral shifts accompanying the transfer of a solvatochromic compound from the gas phase to water were comparable with the experimental data. In Table 11.1.4, the results of calculations are presented for three carbonyl compounds, formaldehyde, acetaldehyde and acetone. 

Kabachnik-Fields reaction is a three-component reaction between a carbonyl compound (ketone and aldehydes), an amine, and a dialkyl phosphite [144] to form a-aminoalkylphosphonates 79 (Scheme 12.26). It was first described in 1952 independently by Kabachnik [145] and Fields [ 146]. This reaction constitutes a general and effective method for the synthesis of a-amino phosphonates, which, in the last five decades, have danonstrated to be a kind of compounds of relevance. 

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