Steric variations

Although the evidence for the direct attack on the substrate by the nucleophile is overwhelming it was necessary to examine the ki pathway in depth to decide whether it represented a parallel dissociative process or associative solvolysis. Trapping,417 AF+ measurements446 and the general observation that kj and k2 respond in a similar fashion to electronic and steric variations in the other ligands in the substrate suggest that both pathways possess the same mode of activation. The possibility of dissociative activation in cfl planar complexes is discussed separately below (Section 7.1.7.6) and further consideration of this question is best reserved until then. 

Particularly in molecular graphics, a commonly used steric variation of this concept is the practice of superimposing molecules by minimizing the root mean square (rms) deviation between corresponding atoms, either all or selected atoms, in two molecules. Alternatively, one may superimpose or compare molecules on the basis of positions of proposed macromolecular binding... 

The chromium(II) reductions of a number of trans-dihalide ruthenium (III) complexes are inner sphere in nature with halide bridging groups. Reaction rates are sensitive to steric effects, whereas the corresponding reductions by vanadium(II) show little steric variation, exceed the V " ligand substitution rate, and must be considered outer sphere. 

Most conspicuous in Fig. 6.4 are the steric variations (x s, dotted lines) of the three methyl protons as they successively twist into coplanarity with the adjacent kn lone pair (with Hg achieving such coplanarity at 30° for the dihedral range shown in Fig. 6.4). However, the sum of the three Mn o ch interactions is constant (as the rigid Csv symmetry of the methyl group demands), so these repulsions make no net contribution to the overall methyl torsional dependence. Instead, the most important CH methyl repulsion is expected to be that with the lone pair (circles, solid line),... 

R.L. Cohen [19] examined the effect of substitution on the rate of disappearance of 2 in reaction (4). His selection of compounds was designed to emphasize steric variations in 2. A plot of Cohen s rate constants vs. —Ep/2 values for various imidazole-imidazolyl radical... 

Application of Snyder s theory of linear chromatographic adsorption (171) gives the variation in adsorption energy of the thiazole nitrogen atom as a function of this steric hindrance for silica and alumina (see Table III-22). These results show that alumina is more sensitive toward steric effects while silica shows a higher selectivity in the case of polar effects. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The relation of rates of reduction with NaBH4 to variations in structure in a wide variety of monocyclic and bridged bicyclic compounds has also been discussed for example, a methyl a to a ketone slows the rate of reduction. Brown ° stated that reactions should not be discussed in terms of axial and equatorial attack, since the rates simply reflect differences in the energies of the possible transition states and not enough is known about the transition state to analyze it. He accepted th concepts of SAC and PDC, but preferred to call them steric strain contrpl and product stability control. ... 

Olefins are normally hydrogenated more readily than any other functional group except acetylenes. However, because of steric differences, a considerable variation exists in the ease of saturation of steroidal double bonds. 

Correlations with o in carboxylic acid derivative reactions have been most successful for variations in the acyl portion, R in RCOX. Variation in the alkyl portion of esters, R in RCOOR, has not led to many good correlations, although use of relative rates of alkaline and acidic reactions, as in the defining relation, can generate linear correlations. The failure to achieve satisfactory correlations with cr for such substrates may be a consequence of the different steric effects of substituents in the acyl and alkyl locations. It has been shown that solvolysis rates of some acetates are related to the pA", of the leaving group, that is, of the parent alcohol. The pK of alcohols has been correlated with but this relationship... 

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

An immense number of phosphine complexes of rhodium have been synthesized and detailed compilations of information are available [3d]. In this section, some case studies are presented to illustrate the variations that arise as a result of changes in the steric requirements of the phosphine. 

Variation of c/s-bond lengths has been noted in some cases and is believed to reflect steric interaction. Therefore, in the series PtX(PEt3)3 (X = H, Cl, F) the trans-bond to X shows a dependence upon the electronegativity of X, while the cz s-Pt—P bond shows no such dependence (Figure 3.91) [148], instead increasing as X becomes larger. 

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