Steric effects, secondary

Examples of effects of reactant stmcture on the rate of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. The general trends of reactivity of primaiy, secondary, and tertiaiy systems and the special reactivity of allylic and benzylic systems have been discussed in other contexts. This section will emphasize the role that steric effects can pl in nucleophilic substitution reactions. 

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates. ... 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

From the standpoint of geometrical considerations, the major difference is in the far greater steric requirements of the nitro group. This could result in either primary or secondary steric effects. Nevertheless, primary steric effects do not seem to be necessarily distinguishable by direct kinetic comparison. A classic example is the puzzling similarity of the activation parameters of 2-chloropyrimidine and 2,6-dinitrochlorobenzene (reaction with piperidine in ethanol), which has been described by Chapman and Rees as fortuitous. However, that nitro groups do cause (retarding) primary steric effects has been neatly shown at peri positions in the reaction with alkoxides (see Section IV,C, l,c). 

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

Secondary steric effects of the same kind have been found in the reaction of methyl derivatives of 22 with aniline. A methyl group at position 6 has a 4-fold rate-diminishing effect (mainly inductive), but when positions 4 and 6 are both methylated the effect is 81-fold and is mainly of steric origin. 

The reaction is quite susceptible to steric effects since hindered secondary hydroxyl groups were found to be unreactive. The method can therefore be used to selectively replace a primary hydroxyl group by halogen in the presence of more hindered secondary hydroxyl groups in the same molecule. An example (70) is the reaction of 52 with triphenylphosphite methiodide which affords the 6-deoxy-6-iodo derivative 53 (60%) in which the C-2 hydroxyl group remains intact. 

As proximity electrical effects are a function of the a and Or constants, and secondary bonding interactions when present may be a function of the Oi constants the effect of an ortho- or a ds-vinylene substituent may be represented by an equation including electrical and steric terms. The presence or absence of a steric effect may be ascertained in the following manner (72). There are four major cases of interest, (a) The steric effect obeys a linear free energy relationship. Then we may write the equation... 

The sodium of the organosodium compound is less effective as a Lewis acid than is the magnesium of the Grignard reagent, which may account for some of the differences. With sodium as the metal, the direction of reaction seems to be decided by steric effects, the bulkier reagents avoiding the secondary position. 

Secondary steric effects on chemical reactivity can result from the shielding of an active site from the attack of a reagent, from solvation, or both. They may also be due to a steric effect on the reacting conformation of a chemical species that determines its concentration. 

The authors (162) attempted to explain the stereochemical outcome of the reactions (Schemes 3.169 and 3.170) in the terms used earlier (337), that is, by steric factors, which destabilize the endo approach of a dipolarophile, and the electronic effect (secondary orbital interactions), which is most typical for electron-rich dipolarophiles and can slightly stabilize the endo approach of these olefins. 

SENAs derived from secondary AN are not involved in catalytic C,C-coupling reactions with silyl ketene acetals. This is possibly due to a decrease in both the effective concentration of the cationic intermediate (the steric effect) and its lower level of electrophilicity (see the lower entry in Table 3.23). 

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