Influence of Carbonyl Groups

Difference spectrum of oak (Quercus obtusata) obtained by subtracting the spectrum of MWL from the spectrum of the same MWL after NaBH4 reduction. (Instrument Nicolet 20SX D, 4cm 1 resolution, 32 scans, KBr pellet technique) 

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Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

The nature of the cation is unimportant in aqueous or other highly polar solutions of borohydrides, but influences the rate of reaction in isopropanol or pyridine, where the reagent exists mainly as associated ion-pairs [44]. Lithium borohydride is more reactive than the sodium compound in these solvents Li+ can probably associate more closely than Na+ with the carbonyl oxygen, promoting polarisation of the C=0 group, and so aiding hydride transfer from the anion. Other cations [e.g. Ca +j and solvents e.g. dimethylformamide) provide variations in reactivity which can have valuable uses for selective reduction of carbonyl groups [42]. 

Further refinement of the model by calculated trajectory analysis enabled the steric influence of the group (R) attached directly to the carbonyl to be assessed. Increasing the size of R was known to enhance the reduction diastereoselectivity. This may be understood by the perturbation of the trajectory of the... 

It is therefore not surprising that the chemical shifts of carbonyl groups are influenced by other features of the structure more or at least to the same extent as they are by the main chain dihedral angles. Performing ab initio calculations on small amino acid-like models, de Dios and Oldfield detected a strong dependence on hydrogen bonds. The authors found a variation of... 

As electronic effects of additives have been claimed for the selective hydrogenation of a,p unsaturated aldehydes [9-14], the influence of metal salts was examined in the hydrogenation of acrolein with Co/silica-alumina catalysts. As shown in Table 2, a small amount of Fe (II or III) chloride added into the hydrogenation mixture exerted a favourable effect on the selectivity to allyl alcohol. TTie other metal chlorides examined were less effective on the catalyst performance. Therefore, not chloride anion but Fe cation seems to have positive effects on the hydrogenation of carbonyl group as illustrated by Richard et al.[12j. 

The influence of the amino gronp on the carbonyl, the carboxyl and the benzene chromophores was reviewed earho-. Since then significant work only with regard to the influence of varions groups, including the amino, substituted amino and ammonium groups, on the benzene chromophore has been published, including far-ultraviolet ECD observations on chiral a-phenylalkylamines and a -phenylalkylamine hydrochlorides , and reviewed , and has resnlted in the formulation of the benzene sector mle and the benzene chirality mle. ... 

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