Steric effects aminoalkylation

The ratio of the two isomers was also, in part, controlled by the structure of the thiols, but it was difficult to rationalize this effect. Due to steric hindrance, aminoalkyl thiols, which are disubstituted at the a-position to the sulfur atom, did not add to pristinamycin... 

Because of the utility of jS-aminoalkyl sulfates in the synthesis of aziri-dines, Dewey and Bafford" have evaluated the effects of substituents on the jS-aminoalkyl sulfate cyclization reaction (Table 5 and 6). From the data in Table 5 it is seen that j8 substituents are more rate enhancing than a substituents. In fact, as the a substituents increase in bulk, the rate enhancement vanishes, and with the a-t-butyl a rate retardation exists. This is steric retardation since an a-phenyl group greatly accelerates the reaction, presumably by stabilizing the partial positive charge that forms at carbon upon C—O bond breaking. The rate data in Table 6 closely parallel in order those of other S 2-type reactions of cycloalkyl substrates. ... 

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