Tertiary alkyl systems, solvolysis

Nucleophilic substitution reactions that occur under conditions of amine deamination often differ significantly in stereochemistry, compared with that seen in halide or arenesulfonate solvolysis. The results of four key substrates are summarized in Table 5.13. It can be seen (entry 1) that displacement of nitrogen on the 1-butyldiazonium ion is much less stereospecific than the 100% inversion observed on acetolysis of the corresponding brosylate. Similarly, the secondary system (entry 2) affords 2-butyl acetate with only 28% inversion of configuration. Furthermore, a crossover to net retention of configuration is observed as the alkyl group becomes better able to stabilize a carbonium ion. The small net retention (10%) observed in deamination of 1-phenylethylamine increases to 28% retention in the tertiary benzylic system 2-phenyl-2-butylamine. 

Table 5 shows that for tertiary systems the exo endo rate ratio predicted only with steric factors agrees with experiments in the case of secondary systems there is no such accordance. Thus the difference in activation ener for the solvolysis of secondary exo- and endo-derivatives which is due to the a-participation upon solvolysis of exo isomer coincides by chance with the difference for tertiary systems due to a steric strain decrease in the transition state for exo isomers owing to the removal of 2-endo-alkyl from 6-endo-hydrogen The relief of the steric strain in the transition state of solvolysis of 2-exo-esters makes it necessary to correct for the steric accelerating effect in tertiary systems in comparing tertiary and secondary substrates. 

Steric effects in solvolysis has been the subject of a further report by Brown and Peters this year. The change in the alkyl group R in (116), (117), and (118) from Me to Bu increases the rates by factors of 39600, 140(KX), and 1120000. The differences are much greater than those found in the more flexible aliphatic or alicyclic systems, and the use of such models to estimate steric interactions in norbornyl derivatives can therefore lead to serious errors. Intramolecular steric interactions have also been monitored in a study of the kinetics of methanolysis of various tertiary chlorides and p-nitrobenzoates based on the norborn-2-yl system. 

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