Steric effects, definition

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

In this chapter, the definitions used by Perrin in his book on pA a prediction (which also includes a very convenient compilation of o values) will be used. One must be alert to the importance of the number of hydrogens directly attached to the carbonyl carbon several groups have pointed out that aldehydes and ketones give separate but parallel lines, with formaldehyde displaced by the same amount again. What this means is that given one equilibrium constant for an aldehyde (or ketone) one may estimate the equilibrium constant for other aldehydes (or ketones) from this value and p for the addition using a value from experiment, if available, or estimated if necessary. This assumes that there is no large difference in steric effects between the reference compound and the unknown of interest. 

The observed trend - a decrease in the number of branches with increased steric bulk - is quite surprising. One could expect the opposite trend, since the steric effects increase the ratio between 1,2- and 2,1-insertions, and intuitively, this should lead to an increase in the number of branches (less 2,1-insertions - less removed branches). However, for the systems 3, 4, 5, and 7 the insertions at the secondary carbons happen with relatively large frequencies for the systems 3, 4, and 5 the probabilities of the insertion starting from the secondary carbon are c.a. 0.4-0.5, and for the system 7 - c.a. 0.25-0.33. Since every insertion into the secondary carbon by definition adds a branch, the global number of branches for the systems 3-5 and 7 is larger than for the more bulky catalyst 6, for which there are practically no insertions from the secondary carbons. An increase in the steric bulk leads to a decrease in the secondary-insertion probability, and... 

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

These Es parameters estimated by Eq. 4 for hetero atom substituents can be combined with those originally developed for various alkyl groups as a set of steric constants for QSAR studies of aromatic systems 6). Thus, apart from the original definition for the intramolecular steric effect, the combined set of Es parameters is able to represent intermolecular steric effects as well. The original Taft E, values for unsymmetrical alkyl groups seem to represent effective steric dimension of the groups which is scaled on the same standard as those for symmetric monoatomic substituents where the effective dimension coincides with the van der Waals radius. 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

Clearly, there is a distinct difference between the kinetic results for the benzologs of azines and azoles. The former definitely show a sizable steric effect in N-methylation reactions when the site of quaternization is a peri position, while any effect for the latter is insignificant. Perhaps this simply reflects the smaller internal bond angles for five- over six-membered rings. 

B Uoster 7 has pointed out the parallelism of the aldol and Darkens reactions -with respect to electronic effects. It is conceivable that steric effects can also play an important role in the reactivity of the carbonyl component, but there is insufficient evidence in the literature at present to warrant any definitive conclusion on this subject.183 ... 

Triorganosilyl groups can control stereochemistry in organic reactions through a steric, an electronic or a stereoelectronic effect. More than one of these effects may exist simultaneously in some chemical processes. The trend listed in Section II could provide chemists with a clear guideline, yet definitive order, to choose a silyl group with appropriate size in control of reactions based on the steric effect. 

It is important to note, though, that for these reactions the situation is quite different from that in Equations (33a), (34a), and (36). In pericyclic reactions aromaticity is mainly a special characteristic of the transition state whereas the reactants and products are not aromatic or less so than the transition state. This is quite different from the proton-transfer reactions discussed in this chapter where the aromaticity of the transition state is directly related to that of the reactants/products. An analogy with steric effects on reaction barriers may illustrate the point. In a reaction of the type of Equation (38), steric effects at the transition state will definitely increase the intrinsic... 

---