Substituent effect Steric effects

One-bond carbon-carbon spin-spin coupling constants are included in a review by Krivdin and KalabinR". They are analyzed in terms of hybridization, substitution effects, lone pair effects and steric effects as well as respective applications to structural determination. The carbon-carbon spin-spin coupling constants between carbons that are separated by more than one bond were reviewed by Krivdin and DeUa and are discussed in terms of experimental techniques, the effects of hybridization, substituent effects, steric effects and respective additivity patterns. 

There are further subtle infiuences of structure on the strength of allylic C—H bonds. Oxidation reactions in which there is initial removal of allylic hydrogen proceed with probabilities governed in part by the relative strength of this bond. Detailed experimental data are beginning to appear, as noted later, on the more subtle effects of olefin structure, including substituent effects, steric effects, and charge effects. Theoretical treatments of such effects are not far advanced indeed, these will... 

Alkyl substituents. The steric effect of 1-alkyl substituents in the pyrrole series has been demonstrated in, for example, Vilsmeier formylation reactions. Thus as the bulk of the alkyl substituent on nitrogen is increased e.g. from Me to Bu ) so does the proportion of /3 substitution (70JCS(C)2573). A similar trend has been observed in a series of experiments on the trifiuoroacetylation of A-alkylpyrroles with trifluoroacetic anhydride (80JCR(S)42). 

It is general practice to divide the influence of substituents into steric effects (for a review, see Gallo, 1983) and polar effects (Reynolds, 1983 ... 

The redox system i (R = alkyl Iqx Viologenes was the first to be interpreted correctly (E. Weitz ). It is especially well suited for studying the effects of N-substituents because steric effects are virtually absent. In spite of the great importance of some of these quaternary salts as universal herbicides (R = CH3, paraquat only potentials Ej were known for a long period of time, since the reductions SEM/RED in aqueous medium are mostly irreversible In aprotic media, however, as in acetonitrile and DMF, E2 and Ei are ideally reversibleThis holds true for most of the investi ted substituents as can be seen from Table 1. 

A single o- or p-methyl substituent has no influence on the rate of cyclization of the singlet tolylnitrene to the favored azirine. ° The methyl group has no bystander effect on benzazirine formation. Cyclization of 2,6-dimethylphenyl or 2,4,6-tri-methylphenylnitrenes necessarily proceeds toward a carbon bearing a substituent. A steric effect raises the barrier to cyclization by 1.5-2.0 kcal/mol, in excellent agreement with the predictions of Karney and Borden. The steric effect extends the lifetime of 2,6-dimethylphenylnitrene at ambient temperature to 13 ns in Freon-113 and of 2,4,6-trimethylphenylnitrene to 8 ns, in the same solvent (Table 11.4). ° ... 

Bulky substituents show steric effects which change the usual selectivity, e.g., large 1-alkyl groups increase -substitution in trifluoroacetylation (80JCR(S)42) and in the Vilsmeier formylation (70JCS(C)2572 of iV-alkyl-pyrroles. With larger substituents like triphenylmethyl in position-1 of pyrrole, bromination, formylation, and trifluoroacetylation occur selectively... 

The method does not appear to apply to aliphatic compounds, the acid-base properties of substituents of which are not easily transmitted through the chain, nor does it apply to ortho substituents, because steric effects... 

Besides electronic effects, steric effects have also been investigated. Contrary to expectations, the presence of reasonably hindered substituents at the ortho position, favours the reaction, preorganising the formation of the spirocyclic intermediate. In the presence of a single ortho substitution, the rate of reaction follows the trend t-butyl > z-propyl > methyl [21]. The... 

The influence of fluorine on the physical, chemical and biological properties of molecules is described. A discussion of the effect of fluorination on bond lengths and bond strengths is presented in the course of an analysis of the steric demand of fluorinated substituents. These steric effects as well as electronic interactions are illustrated in a discussion of fluorinated vitamin D3 and prostaglandin and thromboxane analogs. 

The regiochemistry of the reaction of trifluoropyrazine derivatives with nucleophiles is influenced by the nature of the substituent as well as the presence of the remaining fluorine atoms. If the substituent is either an alkoxy or amino group, the site of attack is generally ortho to the substituent, although steric effects can also influence the outcome of this reaction. In contrast, when the substituent is an alkyl group or chlorine, the site of attack is para to the substituent. 

The diastereoselectivity of the Type n C—Pd process significantly depends on the nature of the chiral group in the substrates, as exemplified by the results shown in Scheme 17. The results can be explained by assuming (i) a coplanar arrangement of the C—Pd and the participating C=C bond and (ii) a boat-hke transition state for the transition state of the exo-mode cyclic carbopalladation. The overall outcome may be determined by both steric and electronic (especially chelation) effects. Steric effects favor placement of substituents in pseudoequatorial positions, whereas chelation effects can favor pseudoaxial arrangements. 

The structure, substituents, and steric effect can influence reactivity. For example in 2,4-toluene diisocyanate (TDf), the isocyanate group para to the methyl group is 25 time more reactive than the other NCO group at the ortho position [28]. Moreover, the reactivity of the second NCO group can change as a result of the initial reaction. 

It has been shown that when both the carbon and the nitrogen contain an alkyl substituent, a steric effect exists between the coordinated amides, causing Dq to decrease, thereby dividing the above substituted amides into two groups. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

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