Strain and Steric Effects

We have to stress again that there might be special reasons that make a disrotatory process or a stepwise mechanism favored over the conrotatory process such as ring-strains and steric effects [88],... 

The use of the magnetic susceptibility exaltation as a measure of aromaticity has been advocated. The exaltation A is defined as the difference between the magnetic susceptibility and a sum of group increments. Although, like its resonance energy counterpart, this definition employs a fictitious reference system, the magnetic susceptibility is believed to be less sensitive to strain and steric effects than the total energy. Both and the index ... 

Because of the frequent mutual interference of electronic, inductive, and steric effects, and because of the influence of ring strain, the carbonyl stretching frequency is naturally not an absolute criterion for the methylation course. The heterocyclic systems in question are too diverse for this to hold. Careful inspection of Table I discloses certain deviations from the relationships mentioned. These deviations will now be discussed. 

According to the orbital phase theory, the Gloss s diradical 62 is predicted to have a triplet ground state due to the same orbital-phase topology as the TM (2). In derivatives of 62, electronic and steric effects of various substituents as well as ring strains in the cyclic diradicals have successfully been applied to modulate the... 

A QSAR model was developed to test the mutagenicity of 117 nitro-aromatic compounds in bacterial strain cells without metabolic activation [74]. It was shown that hydrophobicity plays a crucial role in determining the relative mutagenicity of most of the compounds studied. The mutagenicity of the nitro-aromatic compounds increases slowly at low logR and then decreases rapidly at high log P. It was suggested that a combination of adverse hydrophobic and steric effects could explain these observations. In a subsequent analysis, Debnath and Hansch [75]... 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

The Felkin-Anh model has also been used to explain the preference for axial attack by nucleophiles on cyclohexanones and the effect of proximate substituents on facial selection. The anti periplanar geometry that Anh regarded as important in nucleophilic attack of carbonyl compounds is compromised by torsional strain in the reactions of cyclohexanones from the equatorial face. Felkin slated Whereas both torsional strain and steric strain can be simultaneously minimised in a reactant-like transition state when the substrate is acyclic... this is not possible in the cyclohexanone case.. ..These reactions all proceed via reactant-like transition states. In the absence of polar effects, their steric outcome is determined by the relative magnitude of torsional strain and steric strain [in the axial and equatorial transition states] [16]. 

One approach, presented by Taft (10) and later confirmed by Becker (101), is based on the separation of the free energy of activation into contributions coming from polar and steric effects (10). The contribution from the steric effect was then postulated to contain both "steric strain" or repulsion and "steric hindrance of motions" obtained from the statistical thermodynamic partition function Q. This separation therefore represents the changes in potential and kinetic energies related to the steric effect. Since the total steric effect AAE is related to... 

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