5.10 Phase-Transfer Catalysts

All four hydrogens of ammonium salt (NH₄⁺) are replaced by alkyl or aryl groups. Such compounds are called quaternary ammonium salts, which are soluble in both water and non-polar organic solvents and can be used as Phase-Transfer catalyst. It is used to move ionic reagents into organic solvents. For example, dichlorocarbene adds to cyclohexene to form a cyclopropane ring (Scheme 1).

Scheme 1

In this case, sodium hydroxide dissolves in water but not in chloroform whereas cyclohexene dissolves in chloroform but not in water. Very little reaction occurs even at vigorous stirring. But reaction occurs easily after adding tetrabutylammonium chloride. Tetrabutylammonium chloride reacts with sodium hydroxide, in aqueous phase, to give tetrabutylammonium hydroxide which has large alkyl groups that enhance its solubility in organic solvents. Thus, tetrabutylammonium hydroxide migrates to organic phase where it reacts with chloroform to give dichlorocarbene, which reacts with cyclohexene to give the addition product (Scheme 2).

Scheme 2

5.11 Reactions

5.11.1 Alkylation

Alkylation of amines with alkyl halides proceeds by S_N² mechanism to give alkyl ammonium halides. If this alkyl ammonium halide has N-H bonds, further alkylation takes place which means that the reaction do not, usually, stop at one alkylation (Scheme 3).

Scheme 3

Amines can be converted into quaternary ammonium salts with excess alkyl halide under vigorous condition. When excess methyl iodide is used for this conversion, this process is called exhaustive methylation (Scheme 4).

Scheme 4

5.11.2 Condensation

Primary and secondary amines form imines and enamines when they react with either aldehydes or ketones. In the presence of a reducing agent such as sodium borohydride (NaBH₄) and sodium cyanoborohydride (NaBH₃CN), imines and enamines can be reduced to amines (Scheme 5). Reduction of the C=N double bond is similar to reduction of C=O double bond. Nucleophilic substitution of aniline with benzaldehyde gives an imine which then reduced to N -benzyl aniline

Scheme 5