Ketones polar effects

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

Kinetically controlled deprotonation of a,p-unsaturated ketones usually occurs preferentially at the a -carbon adjacent to the carbonyl group. The polar effect of the carbonyl group is probably responsible for the faster deprotonation at this position. 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term.7,76 Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical... 

Ketone and the formed a-ketoperoxyl radical are polar molecules. Hence the polar effect influences the reactivity of the ketones and the peroxyl radicals. Polar solvents also influence the reactions of peroxyl radicals with ketones as well as other free radical reactions. 

Rate constants and Arrhenius parameters for the reaction of Et3Si radicals with various carbonyl compounds are available. Some data are collected in Table 5.2 [49]. The ease of addition of EtsSi radicals was found to decrease in the order 1,4-benzoquinone > cyclic diaryl ketones, benzaldehyde, benzil, perfluoro propionic anhydride > benzophenone alkyl aryl ketone, alkyl aldehyde > oxalate > benzoate, trifluoroacetate, anhydride > cyclic dialkyl ketone > acyclic dialkyl ketone > formate > acetate [49,50]. This order of reactivity was rationalized in terms of bond energy differences, stabilization of the radical formed, polar effects, and steric factors. Thus, a phenyl or acyl group adjacent to the carbonyl will stabilize the radical adduct whereas a perfluoroalkyl or acyloxy group next to the carbonyl moiety will enhance the contribution given by the canonical structure with a charge separation to the transition state (Equation 5.24). 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

Steroidal 5a hydroxy-6-ketones are readily converted into 5a-halo-6-ketones by the action of hydrogen halide in chloroform [gi]. The mechanism is not clear, for it seems necessary to invoke a C(s) carbonium ion intermediate, yet the 6-ketone must exert a strong polar effect opposing development of electron deficiency at C(5). It is also curious, if a carbonium ion is indeed involved, that it does not suffer either skeletal rearrangement or loss of a proton from C(4> (see p. 258). These difficulties may be circumvented by a mechanism (Fig. 16) in which ionisation of a C(5)-hydroxyl/hydrogen halide complex... 

After some comments on what was named the medium polarity effect and the medium dielectric constant the authors came to the following conclusions such nucleophilic addition reactions of diphenylmagnesium compounds with ketones and metallation reactions of 1-alkynes have some common features. Apparently, they can both be considered as a special case of aromatic electrophilic substitution, differing from (what the authors named) the classical examples of this reaction, in that the latter one has a more pronounced reagent-like transition state. 

Aldehyde and ketone functions, and thione and thials, if suitably placed, can strongly interact with the ring sulfur by hypervalent or possibly polar effects <94SR3i7>. The exocyclic thione function is readily oxidized by, for example, peracetic acid. Thione-S -oxides (110) have been isolated. The reaction with more oxidizing agent under acidic conditions is a valuable synthesis of... 

Twisted non-charge transfer excited triplet states of aromatic aldehydes and ketones are revealed by resonance Raman scattering and transient dielectric loss interconversion between two populated triplet states in the case of 2-naphthaldehyde and 2-acetophenone in cyclohexane. Solvent polarity effects on the nlT and nn states of 1,2-naphthoquinone and 9,10-phenanthrenequinone examined... 

In the hydrogenation of optically active azomethines (prepared from chiral a-phenylcthyl-amine and ketones), the effect of reaction parameters, i.e., catalyst dispersion, mass and solvent polarity, on the diastereoselectivity has been studied20. The diastereoselective hydrogenation of several related chiral imines using nonchiral or chiral diphosphane ligands/rhodium has been reported to yield the corresponding amines with diastereomeric ratios up to 99.7 0.3 141. 

Fries rearrangement, review. Martin2 has reviewed the rearrangement of aryl esters to o- and p-hydroxyaryl ketones, usually effected with AICI3. The rearrangement can occur at room temperatures in polar solvents (nitroalkanes). Photo-induced rearrangement usually provides the same products as obtained by Lewis acid induced rearrangement. The review lists 281 references to literature mainly from 1964 to 1990. 

The effect of 17-substituents on the base-catalysed equilibration of steroidal 2j5,3y -disubstituted 6-ketones has been investigated The presence of a range of functional groups at position 17 results in marked changes in the equilibrium between 5a- and 5) -isomers and this cannot be explained in terms of conformational transmission or by simple transmission of polar effects. ... 

The transfer and the adsorption are strongly influenced by the size of the compound (surface and volume) and by the functional groups present in the molecule (alcohols, aldehydes, ketones, carboxylic acids, amines, mercaptans, halogenated molecules) inducing some polarization effects. Some studies have reported a quantitative structure activity relationship (QSAR) between molecular structure and adsorption parameters (adsorption capacities, energies) [32,37,78]. The reactivity of some compounds leads to oxidation at the adsorbent surface, which plays a catalytic role. Moreover, a mixture of molecules in air... 

This indicates that log P can be used to reproduce predictive equations. The aldehyde-ketone results indicate that the bulkiness of the substituents on the carbonyl group does not determine the level of odor intensity. Suprathreshold correlations indicate that the polar effects of the groups bonded to the carbonyl group does not determine the level of odor intensity. Similar conclusions regarding the importance of hydrophobic and steric effects can be made from the alkane odor intensity -log P and E equations in Table III. 

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