Solvolysis steric effects

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 k term is independent of Y and would, therefore, appear to be dissociative, but it is in fact found to be solvent-dependent and so it is thought to be associative. (It is also found to be sensitive to steric effects in the same manner as the k2 pathway.) A plausible pathway for the k route is slow solvolysis followed by fast substitution... 

Solvolysis of y-allenic tosylates afforded cyclic compounds with the regioselectiv-ity determined by the steric effect of the two C=C bonds of the allene moiety [9]. 

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. 

The limiting nature of the solvolysis reactions of 2-adamantyl solvolyses apparently arises as a result of steric inhibition of rearside nucleophilic solvent participation by the axial hydrogens shown in 95. Such steric effects are absent... 

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. 

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