Steric effects solvolysis rate

A pronounced rate-retardation of 1.65 x 1 O -fold by an a-MesSi group relative to Me in the solvolysis of benzylic p-toluenesulfonates is due to a steric effect . The rate increment in ethanol decreases with increasing steric size of the a-sUyl group attached to the benzylic position and foUow the order listed in entry 54 of Table 1. 

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... 

The allyl cation thus formed may stabilize itself either by readdition of the leaving group — leading to a 2,3-dihalopropene — or by the addition of a nucleophile. The influences of steric and electronic effects on the stereochemistry and on the solvolysis rates of various alkyl-substituted chlorocyclopropanes have been investigated by Parham and co-workers [165, 166], who could show for example that os-2,3-dipropyl-l,l-dichlorocyclopropane solvolyzes 24 times faster than its trans-isomer, in accordance with predictions based on orbital symmetry arguments. When one propyl substituent of the trans-isomer is replaced by an ethoxy group the rate of solvolysis increases 200 fold. 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

Enhanced rates of solvolysis of c-PrP( = O) (Ph) NHPh relative to acyclic analogs were interpreted as possibly reflecting positive charge development in the rate-limiting transition states, but the importance of steric effects could not be discounted . Thus there is no firm evidence for electron donation by cyclopropyl to positively charged phosphorous . 

The effect of a 1-cyclopropyl substituent on the rate of solvolysis of 2-adamantyl tosylates (129) has been compared to a variety of other 1-substituents. These rate effects were attributed to electronic substituent effects on formation of a bridged ion, but steric effects are probably also involved and a detailed interpretation of the conjugative effect of cyclopropyl is not straightforward. 

Thus, in the concept of classical ions the steric factors at C fail to account for the deceleration of solvolysis at C. At the same time the models show that for 6,6-dimethyl-2-exo-norbomyl brosylate 45 in the transition state leading to a non-classical ion the non-bonded interactions between the CHj groups on C and the hydrogens on C and are markedly increased. It is clearly seen from the scheme on the page 18. In the transition state of the nondassical ion formation unfavourable steric effects occur irrespective of whether the CH3 group at C is in the endo- or exo-position. Insteed, the substitution of the hydrogen by the CH3 group in C decreases the solvolysis rate of endo-tosylate (for steric reasons) but since the solvolysis rate of the exo epimer decreases too this confirms the participation of the C —C bond in the exo-tosylate ionization. 

Table 6. Exo Endo Rate Ratio for the Solvolysis of Secondary and Tertiary Norbomyl Derivatives (after Corrections for Steric Effects, Internal Return )...

In his recent survey H. Brown points out that the high exo endo rate ratio (885) in the solvolysis of epimers 55 and 56 cannot be due to the relief of steric strain in ionization of exo epimer since the corresponding alcohols are comparably stable Neglecting whether it is correct to substitute the OPNB group by OH when considering steric effects the similarity of the ground state energies of epimers does not imply the equality of the steric strain relief in transition states from different non-bonded interactions. 

Thus steric effects are really the main factor determining the exo endo solvolysis rate ratio for tertiary substrates but this cause is not the steric hindrance to ionization of endo isomers, but the steric strain relief in the transition state of ionization of exo and endo epimers. 

Introduction to of electron-releasing substituents sharply decreases the exo endo rate ratio there is no such effect if two geminal methyl groups are introduced to C. The value 3 for the exo endo rate ratio resembles greatly the usual epimeric rate ratio for secondary substrates without anchimeric assistance (see above). Introduction of an electron-releasing substituent to levels the o-participation in the solvolysis of the secondary 2-exo-epimer, but it does not affect the rate ratio of the tertiary analogues for which the solvolysis rate is predominantly determined by steric factors, for example ... 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

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