Cyclisation and decarboxylation in zwitterionic micelles: effects of head group structure

Abstract

The spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion is strongly catalysed by micelles of zwitterionic surfactants, viz., sulfobetaines [C₁₄H₂₉N⁺R₂(CH₂)₃SO₃^–, R = Me, Pr and C₁₆H₃₃N⁺Me₂(CH₂)₃SO₃^–] and amine oxides (C₁₄H₂₉N⁺R₂O^–, R = Me, Pr), with rates enhanced by factors of up to 1800. These micelles and those of the corresponding carboxybetaines are more effective catalysts than those of the corresponding cationic surfactants. In all cases a change from Me to Pr at the head group speeds reaction by factors of ca. 5–8 for the sulfobetaines and amine oxides and ca. 14 for the cationic surfactants. Cyclizations of the o-3-halopropyloxyphenoxide ions (halogen = Br, I), which are intramolecular S_N2 reactions, are modestly micellar catalysed, but structural effects on the micellar catalysis by cationic and betaine surfactants are in the same sequence, as for decarboxylation.