Neighboring-group-effect 846 Subject

In general, one speaks of a neighboring group effect only when the cyclic and usually positively charged intermediate cannot be isolated but is subject to an SN2-like ring opening by the... 

Clearly a variety of other chemical reactions may be carried out on existing block polymers. Complete derivatization is, however, often subject to the usual problems of polymer reactions such as steric hindrance, neighboring group effects, solvent problems, and gelation. However, judicious choice of reagent, conditions, and moderate conversions can produce an attractive array of block polymers that can be synthesized in no other way. 

The last subject to be discussed in this section is one found with Group IV elements, and probably also with many Group II elements (cf. mercury, Section IV, E)—neighboring group effects and participation. Hawthorne (192) has pointed out that y-halopropylboranes, produced by hydroboration of allyl halides, undergo basic hydrolysis to yield cyclopropanes. 

The term intramolecular catalysis introduced by Bender is also widely used to describe neighboring group effects, especially when analogous intermolecular catalysis is observed. Thus this term is commonly used when referring to reactions that are subject to acid, base, and/or nucleophilic catalysis such as hydrolysis of esters, amides, and acetals the mutarotation of aldoses or the enolization of ketones. It is rarely used when referring to nucleophilic substitution reactions at saturated carbon. 

Studies of vinyl polyelectrolyte solutions connected with problems of this type have dealt with specific effects of neighboring groups in intramolecular reactions and rates of reaction between polyelectrolytes and simple substrates. Comprehensive reviews of this subject have been published (27, 22, 23). 

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