Iodide anion nucleophilic catalysis

The C-F bond is both strong and less readily polarised than a C-I bond, and the OH- ion, even though electron rich and capable of forming strong C-0 bonds, is not easily polarised. Hence, both species are reluctant to undergo the required movement of electrons within the bonds to form the transition state. Iodide ions, however, are more easily polarised and so may readily attack a saturated 8+ carbon, and form the transition state, which may then yield the iodo compound, R-I, as an intermediate. This, in turn, may now be more readily attacked by the OH- anion as the C-I bond is readily polarised. The use of an iodide anion in this manner is an example of nucleophilic catalysis. 

Substitutions. The cyanamide anion is strongly nucleophilic and reacts with most alkylating or acylating reagents (4) addition to a variety of unsaturated systems occurs readily (4). In some cases, a cyanamide salt is used in others, base catalysis suffices. Ethyl iodide reacts with sodium hydrogen cyanamide [17292-62-5] to form a trisubstituted isomelamine. 

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl bromides, iodides or tosylates. The carboxylate anion is not a particularly reactive nucleophile so reaction proceeds best in polar aprotic solvents or with crown ether catalysis. Acetone has also been found to be a good solvent for reactions with alkyl iodides. Carboxylate alkylation procedures have been particularly advantageous for preparation of hindered esters which can be relatively difficult to prepare by the acid-catalyzed esterification reactions to be discussed in Section 3.4. Sections F and G of Scheme 3.2 give some specific examples of ester alkylation by both diazoalkanes and other alkylating agents. 

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