Induction substitutent effect

The enantiomers are obtained as a racemic mixture if no asymmetric induction becomes effective. The ratio of diastereomers depends on structural features of the reactants as well as the reaction conditions as outlined in the following. By using properly substituted preformed enolates, the diastereoselectivity of the aldol reaction can be controlled. Such enolates can show E-ot Z-configuration at the carbon-carbon double bond. With Z-enolates 9, the syn products are formed preferentially, while fi-enolates 12 lead mainly to anti products. This stereochemical outcome can be rationalized to arise from the more favored transition state 10 and 13 respectively ... 

The fluorination of chromium oxide caused an increase of surface site Lewis acidity. Kemnitz and al.[12] as well as Peri [13], showed that on fluorinated alumina the progressive substitution of F for O and OH led, thanks to inductive attracting effect of fluorine, to an increase of the Lewis acidity of a sites. Hence the dehydrofluorination reaction was ftivoured on strong acide sites. 

The stabilisation is particularly marked in that not only is an extra (fourth) canonical state involved in the stabilisation of the o- and p-positive charge is located on oxygen, are inherently more stable than their other three complementary forms, in which the positive charge is located on carbon (cf. 48a— 48c, and 49a— 49c). This effect is sufficiently pronounced to outweigh by far the electron-withdrawing inductive (polar) effect also operating in these two cr complexes, substitution is thus almost completely o-lp- ( 1% of the m-isomer is obtained in the nitration of PhOMe), and much more rapid than on benzene itself (kcsH50Me/fcc 6H = 9-7x 10 for chlorination). 

In diamines such as 4,4 -diaminodiphenylmethane, 4,4 -diaminodiphenyl sulphone or 1,6-hexamethylenediamine, the substitution effect is practically absent641 although this may not be the case for di- and polyamines where the distance between the amino groups is not large allowing inductive and steric effects to be operative 59,68 ... 

Among the first of them are the so-called substitution effects due to steric, induction, catalytic, or some other types of influence of the reacted active centers on the reactivity of neighboring unreacted centers. In order to take account of such short-range effects it has been suggested (Kuchanov, 1978) to use under mathematical modeling an extended Flory principle. In line with this principle, the reactivity of any active center of a molecule is supposed to be controlled exclusively by local chemical structure of the... 

Results of additional note are that 4-chlorobenzo[b]thiophene gives 2-substitution (84%) and the 6-isomer gives mainly 2- (64%) together with some 3-substitution (26%) [78JCR(S)10]. This suggests that the +M (mes-omeric donor) effect of chlorine is more effectively transmitted between the two rings than the -/ (inductive acceptor) effect, which is not unreasonable. This view is supported by the preferential 3-acetylation of 5-... 

Stabilization by the introduction of bulky /er/-butyl groups into the azulene rings should be attributable to not only their steric and inductive electronic effects but also C-C hyperconjugative effects induced by the o-bond of the tert-butyl groups. The p/ R value of 14.3 for methyl cation 2b with six tert-butyl groups is the highest value ever reported for a methyl cation substituted with only hydrocarbon groups. 

Increasing the reaction temperature and the amount of halogen used results in the introduction of further halogen atoms. It was noted in Chapter 2 that the halogen atom in chlorobenzene is ortholpara directing, but deactivating to electrophilic substitution as a result of opposing mesomeric (+M) and inductive (-1) effects. Consequently, disubstitution leads to a mixture of the 1,2- and, mainly, 1,4-isomers under conditions similar to those required to attack benzene. 

Halogen atoms such as chlorine and bromine deactivate the ring to electrophilic substitution by the inductive (-1) effect, but the mesomeric (+M) donation of electrons directs substitution to the ortho and para positions. 

An ab initio theoretical treatment of inductive-field effects has been carried out for -substituted ethylamines, where hyperconjugative and polarizability effects on proton affinities were minimized and intramolecular... 

A study employing a combined QSAR-CoMFA (comparative molecular field analysis) used observed [ H]diazepam pICso values, calculated HOMO and LUMO energies and total dipole moments hydrophobic, steric, and field/inductive substituent effects were also considered (116). Equation 5.13 was derived for a set of 30 compounds that varied only in substitution at the Cl, C2, and N1 positions (the latter varied only between H and CH3). 

While g parameters from the dissociation of 4-substituted bicyclo-[222]octane-l-carboxylic acids are excellent measures of the inductive polar effect they suffer from the problem that the carboxylic acids cannot be synthesised readily for a wide range of X-substituents. A more substantial (but not universal) range of Gj constants is known than for g parameters. ... 

The destabilization of sp -bound fluorine by p-jt repulsion activates fluorinated aromatic compounds totvard nucleophilic attack and subsequent substitution. The susceptibility of the carbon center toward nucleophiles is also enhanced by the negative inductive (—T) effect of fluorine. In particular, if the aromatic compound is also activated by —M electron-withdrawing substituents, for example a nitro or cyano group, in the ortho or para positions the fluorine is easily replaced by a variety of nucleophiles even under very mild conditions via a resonance stabilized Meisenheimer complex (Scheme 2.39). The ease of nucleophilic halogen replacement - F > Cl > Br > I - is in the opposite order to that for aliphatic nucleophilic substitution. 

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