Electronic influence

The work by Hammett and Taft in the 1950s had been dedicated to the separation and quantification of steric and electronic influences on chemical reactivity. Building on this, from 1964 onwards Hansch started to quantify the steric, electrostatic, and hydrophobic effects and their influences on a variety of properties, not least on the biological activity of drugs. In 1964, the Free-Wilson analysis was introduced to relate biological activity to the presence or absence of certain substructures in a molecule. 

The electronic influence of the 4-substituent corresponds to a relative increase in the kinetic acidity of the C-5 proton when an electron-withdrawing group (R=Ph) is situated at the 4-position and to a relative increase in the kinetic acidity of the 2-methyl group when an electron-donating group (R = Me) is at the same position (Table 1-59). 

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

Chloro-/3-(trifluoromethyl)acroleins reacted with 2-aminothiophenol at room temperature to give a mixture of a benzothiazole and quinoline. This was in contrast to the /3-methyl analogue, which gave a benzothiazep-ine, the difference being due to the electronic influence of the CF3 group (91TL643). 

The electronic influence on the donor ability of the monomers as well as the acceptor behavior of the corresponding cation has been characterized by variation of the para-substituents of the styrenes. More detailed assertions can be expected when one varies the structure directly on the double bond by substitution at a- and P-position. A suprisingly good correlation appears between the energetic level of the HOMO and energy of theCT-absorption of the complexes with TCNE (ECT = 20.8 5.55x(HOMO) r = 0.80 n = 12) for styrenes, both with and without a methyl substituent at a- and p-position ... 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

The substituents bound to Al and the group 15 element E generally adopt a staggered conformation relative to one another. The Al—N bond distances, which range from 197.2(4) 63 to 198.8(3) pm 60, are almost equal, indicating no significant electronic influence of the specific group 15 element on the... 

The principles involved in the conformational analysis of six-membered rings containing one or two trigonal atoms, for example, cyclohexanone and cyclohexene are similar. The barrier to interconversion in cyclohexane has been calculated to be 8.4-12.1 kcal mol . Cyclohexanone derivatives also assume a chair conformation. Substituents at C2 can assume an axial or equatorial position depending on steric and electronic influences. The proportion of the conformation with an axial X group is shown in Table 4.4 for a variety of substituents (X) in 2-substituted cyclohexanones. 

The enthalpies of reaction for nucleophilic carbencs depend on the stereoelec-tronic properties of the ligands affecting the availability of the carbene lone pair. An example of electronic influence is the 3.5 kcal/mol enthalpy difference between the isosteric pair IMes and IMesCI that shows the electron-withdrawing nature of Cl compared to H. This trend again is in line with electron donor/withdrawing ability of arene substituents. The effect in this la.st case is a long range electronic... 

Chiral Catalysts Electronically Influenced by a Ferrocene Core.152... 

A Hammett plot for para-substituted benzaldehydes showed that electron-rich aldehydes gave higher ees (r = -0.4). As in Shibuya s related results (Section 5.3.3.1 above), this indicates that aldehyde coordination is important in enantiodifferentia-tion, but the lower rvalue (compared to Shibuya s r = -1.30) suggests a weaker electronic influence, probably due to the relative Lewis acidities of A1 and La. For ortho-substituted aldehydes, lower ees were observed, presumably due to steric effects. Although Al-Cl and Al-triflate complexes 29-30a-b did not catalyze the reaction, they... 

In traditional models of an eleetrified interfaee, metal electrons are artificially localized within the eleetrode. This leads to misinterpretation of the electronic influences on the compact layer, Ch in those models would always be smaller than its ideal conductor limit (with electrode eharge spread over an infinitesimally thin region at the electrode surface x = 0)... 

There was a substantial electronic influence with electron-withdrawing substituents decreasing enantioselectivity. Interestingly, steric bulkiness at this remote part of the molecule was found to be highly effective for asymmetric induction. The bulky ANM group provided 97% ee with a high isolated yield. 

On silylation/germylation of 17, compounds 18 were formed, which were subsequently lithiated on phosphorus to yield 16. It appears that the success of the next step from 16 to 15 depends on different factors, but the following three parameters may be regarded as critical (1) The reaction temperature for the elimination of LiF should not exceed 80°C (2) the concentration of 16 dissolved in hexane or toluene should be about <0.1 mol L (3) the steric bulk and electronic influence of the substituent attached to phosphorus should be optimized. In the first place, the nature of the substituent at phosphorus determines the structures of the precursors 16. This has been demonstrated by a study of a structure-reactivity... 

Anthryl, 1-naphthyl, mesityl, and 4-quinolyl groups in 37h-k promote C/C aryl shift (see Table 2) their proclivity to migrate is far greater than expected from the electronic influence of the x-systems. This is presumably due to the intermediacy of spiro-linked benzenium betaines 30 31 , occurring in place of the carbenes otherwise encountered. The formation of phosphinic esters 42h-k is partly or completely suppressed 30). 

Wagner and co-workers have reported the synthesis of ferrocene-bridged tris(l-pyrazolylborate) oligomeric systems (80) (Fig. 49).62 Such polymers are of interest, as they offer a multiplicity of options for tailoring the access space to a metal held by the tris(l-pyrazolylborate) ligand, as well as a means to electronically influence the environment of a transition metal.113,114... 

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