Influence of substitutents

The most extensive mechanistic studies of quinone Michael addition chemistry involve the arylsufinic acids, which yield reduced product (50,51). The sulfones produced in such reactions have been examined electrochemicaHy (48) and kineticaHy (52). The influence of substitutents in the quinone has... 

It shoiild be noted that the influence of substituting solvents of widely differing viscosities upon the interfacial area a can be very large. One therefore should be cautious about extrapolating data to account for viscosity effects between different solvent systems. 

The acid hydrolysis of diaziridines has been investigated kinetic-ally. The reaction is first order and shows a relatively high temperature coefficient. Thus one finds a relatively high activation enthalpy (23-28 kcal) and a positive activation entropy (2-6 eu). The influence of substitution on nitrogen is small. The velocity of the diaziridine hydrolysis depends only in the weakly acid region on the acid concentration. Between pH 7 and 3 the fc-values rise by nearly 10 . For the... 

On the other hand, the rate constant k does not depend on the changing steric influence of substitutents in the 8-position, but correlates surprisingly well with the Hammett-Brown constant cr. This result indicates that the formation of an sp3-hybridized carbon atom (at the 1-position of the o-complex) leads to a compound without significant steric interaction of the electrophile with substituents in the 8-position. The o-complex cannot be planar and is asymmetric. The preferred conformation of a o-complex of this type is illustrated in Figure 12-6. The pseudoax-ial position of the electrophile E reduces the steric interaction between this group and the peri substituent R. 

Worthington, S. E., Cramer, C. J., 1997, Density Functional Calculations of the Influence of Substitution on Singlet-Triplet Gaps in Carbenes and Vinylidenes , J. Phys. Org. Chem., 10, 755. 

Figure 20. Influence of substitution with different halogens and halogen with methyl in the phenyl ring on blood pressure activity...

Many observations document that modifications of peptides by substitution with artificial amino acids or D-amino acids can often protect against enzymatic degradation. This is especially true when more than one modification is made. We begin here with two examples in which replacement of a single natural amino acid with an artificial one had a favorable impact on biological stability. Then, the influence of substitution with D-amino acids is described. Examples involving two or more modifications with artificial amino acids and D-amino acids combined are presented in the following section. 

Some of these mutants (Asn-38, Glu-38 and Ala-38) were examined by NMR spectroscopy to assess the influence of substitutions at position-38 on the pK of the adjacent His-33 and His-39 residues. The results of these experiments indicated that the behavior of these two residues was minimally perturbed by these substitutions. Interestingly, theoretical electrostatic calculations, based on... 

The X-ray crystal structures of a number of differently substituted 1,3-dioxanes have been published the structures of chair conformers are given in Table IX. The influence of substitution on the geometry of the 1,3-dioxane ring in the solid state has been discussed in detail in two reviews (88ZSK110 90MI2). 

FIGURE 2.11 Influence of substitution on solute retention. Functional groups with phenol as base. Column Lichrosorb RP CIS 250 x4 mm mobile phase water. 

For examples of Ru-catalyzed hydrogenation of N-(3.4-dihydro-2-naphthalenyl)-acetamide see (a) Renaud. J.L., Dupau, P., Hay. A.-E.. Guingouain. M., Dixneuf P.H. and Brtmeau. C. (2003) Ruthenium-catalysed enantioselective hydrogenation of trisubstituted enamides derived from 2-tetralone and 3-chromanone Influence of substitution on the amide ai m and the aromatic ring. Adv. Synth. Catal.. 345. 230-238. 

From the experimental data for the Sommerfeld constant yN in fig. 56, conclusions on the influence of substitutional disorder on the rare-earth site of YxLui Ni2B2C... 

Commercial products with molecular substitution (MS) usually ranging from 1.8 to 2.5 are water soluble (9). Several studies have been done on the influence of substitution on the enzymolysis rate. 

Table 15.8 compiles SARs for 02 reactions with anilines, sulfides, eneamines, furans, phenols and styrenes, all compounds which have high reactivity toward 102. Most reactions are in organic solvents. All p values are large, negative values indicating significant influence of substitution on the rate at the reaction center. 

I, 2-disubstituted 3-hydroxy-4(1H)-quinolines and the influence of substitution on the course of cyclization. J. Heterocyclic Chem. 1999, 36, 141-144. 

In CHEC-II(1996), some semi-empirical calculations at the AMI level of approximation for the typically conjugated l,4-(oxa/thia)-2-azoles 5, 7, and the parent cross-conjugated azolynones 8 and the azolynethione 9 were reported. Comments about their stability and on the influence of substitution on their frontier molecular orbitals (FMOs) are given <1996GHEC-II(4)493>. 

Every zirconium atom forms an active complex, as shown by Tait (103) and Chien and Wang (104), producing about 46,000 polymer chains per hour. (This efficient use of the zirconium atoms is contrasted to Ziegler-Natta and Phillips polymerization, in which only a small fraction of the metal centers are active.) The insertion time of one ethylene unit is only 3 x 1(T5 s. The rates are comparable to those observed for some enzymes catalyzing synthesis reactions. The analogy to enzymes is manifested in others ways as well—for example, the influence of substitution, regioselectivity, and stereospecificity. 

Figure 24. Structures and long axis polarizations of the intense short wavelength UV transitions of benzoate (a) and p-substituted benzoates (b) and (c) showing the influence of substitution on the intensity (e) and wavelength (Xmax). (d) p-Dimethylaminocinnamate.

The 1-ADPM rearrangement of 1-aza- 1,4-dienes affords the corresponding cyclopropylimines regioselectively. However, in order to predict the outcome of the reaction in pyrazinobarrelenes, quinoxalinobarrelenes, and other polycyclic systems, two different factors should be taken into account. One is the possible competition between the DPM and the 1-ADPM rearrangements, and the other is the influence of substitution on the stability of the reaction intermediates. However, in most of the cases studied, the 1-ADPM process takes place preferentially over the DPM reaction [3e]. An example of competition between these two reactions is shown in Sch. 4 for compound 5a [10a]. 

The activity of the 4-alkylpyrocatechols (la), 3-alkylpyro-catechols (lb), 3,5-dialkylpyrocatechols (Ic), 3,6-dialkylpyro-catechols (II), 4,5-dialkylpyrocatechols (III), 3,4,6-trialkyl-pyrocatechol (TV), and 4,5- and 4,6-dialkyl-2-alkoxyphenols (V and VI) was studied in isotactic polypropylene at 180 =t 0.1° C. The relative activities Ar were correlated with substituent constants and redox potentials. In the la series the activity of 4-n-alkyl derivatives exceeded that of 4-tert-alkyl derivatives. The length of the main alkyl chain in la and lb and the steric effects in the latter exerted a specific favorable influence. In agreement with these effects, 3-tert-alkyl-5-methyl derivatives are the most active compounds in Series Ic compounds with other combinations of alkyls are weaker antioxidants as are the isomeric substances II and III or compound IV. Etherification of one hydroxyl group exerts an unfavorable effect influences of substitution in 2-alkoxyphenols (V and VI) are generally the same as in the pyrocatechols Z-ZZZ. 

Contrary to our results, other workers (4, 9, 20, 36) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyrocatechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation. The participation of phenolic antioxidants in the inhibition of autoxidation can be demonstrated (1, 2, 3) simply as a reaction between the molecule of antioxidant AH and the alkylperoxy radical ROO formed duririg the autoxidation of the substrate RH. During this process, an aryloxy radical (A ) is first generated. 

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