Steric effects directed nature

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

The isomerization shown in Scheme 6.32 involves two intermediary dianions and not the direct conversion of the initial anion-radical into the final one. However, what is the driving force for this isomerization The molecule of l,3,6,8-tetra(tcrt-butyl)naphthalene is nonplanar the tert-butyl group and carbon atom at position 1 of the naphthalene skeleton lie off the plane of the molecule. The corresponding anion-radical has the same stereochemical peculiarity (Goldberg 1973). Such bending removes the steric strain but, naturally, decreases the degree of the n-electron delocalization over the neutral molecule. As for the anion-radical, its unpaired electron delocalizes less effectively than the anion-radical of the unsubstituted naphthalene. Bending of the naphthalene skeleton... 

The directed nature of primary steric effects results in a conclusion of vital importance, that in general ... 

In the second step the bas is recognized by the receptor site and the bas-rep complex forms. As was noted above, the complex is generally bonded by inter-molecular forces. The bas is transferred from an aqueous phase to the receptor site. The receptor site is very much more hydrophobic than is the aqueous phase. It follows, then, that complex formation depends on the difference in intermolecular forces between the bas-aqueous phase and the bas-receptor site. The importance of a good fit between bas and receptor site has been known for many years. The configuration and conformation of the bas can be of enormous importance. Also important is the nature of the receptor. If the receptor is. a cleft, as is the case in some enzymes, steric effects may be maximal as it may not be possible for a substituent to relieve steric strain by rotating into a more favorable conformation. In such a system, more than one steric parameter will very likely be required in order to account for steric effects in different directions. Alternatively, the receptor may resemble a bowl, or a shallow, fairly flat-bottomed dish. Conceivably it may also be a mound. In a bowl or dish, steric effects are likely to be very different from those in a cleft. Possible examples are shown in Fig. 1, 2, and 3. 

The natural conclusion to be drawn from Table 12 is that steric effects play a very big part indeed in determining the rate and orientation of radical addition. We believe this conclusion to be correct, but, just as it is impossible to separate bond strength effects from polar effects, so it is impossible to separate steric effects in radical addition from polar effects. Any steric hindrance in the addition of a radical to an olefin will depend to a major extent on the shape of the radical. However, the shape of the radical is directly connected to the electron density at the trivalent carbon atom. 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

The effect of bulky ortho-substituted aza-aromatics on the stability of metal complexes depends mainly on the nature of the metal. In the case of com-plexation of Ag+ with methylpyridines, a Bronsted-type equation correlates with complexation AH with the pKa, indicating no direct steric effect to the approach of the Ag+ ion (74BSF2793). 

Nature of Cooperative Effects in Mixed-Ligand Thiocyanate Complexes Various explanations have been put forward to account for the effects of neutral ligands on the nature of the metal-thiocyanate bond TT-bonding, symbiosis, and antisymbiosis of hard and soft acids and bases as well as steric effects have all been discussed. In addition, directional... 

It is well documented that direct complexation of 1-alkylindane or 1-alkyltetrahn with Cr(CO)e produces a mixture of two possible diastereomerie ehromium complexes, endo and exo isomers, and the ratio depends on the steric effect and the nature of the substituents [46]. It might also be possible to synthesize either endo or exo isomers stereoselectively, from a common a-tetralone or a-indanone. 

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