Micellar catalysis of organic reactions. 24. Comparison of SNAr reactions in hydroxy‐functionalized micelles and in the presence of cyclodextrins

The basic hydrolysis of 2,4‐dinitrochlorobenzene (DNCB) and 2,4‐dinitrofluorobenzene (DNFB) was studied in the presence of β‐cyclodextrin (CDOH) and in the presence of hydroxy‐functionalized micelles containing either a primary hydroxy group [hexadecyl‐2‐hydroxyethyldimethylammonium bromide (CHEDAB)] or a secondary hydroxy group [headecyl‐2‐hydroxypropyldimethylammonium bromide (CHPDAB) and 2‐hydroxyhexadecyltrimethylammonium bromide (2‐OHCTAB)]. In all systems a biphasic reaction was observed. The first phase consisted of a competition between the additive (either micelle or cyclodextrin) and hydroxide ion for the aromatic substrate, and the second phase consisted of the hydrolysis of the trapped aryl micellar or cyclodextryl ether. The percentage of trapping of the aromatic substrate by the cyclodextrin was similar to that found for reactions in the hydroxy‐functionalized micelles (CHPDAB and 2‐OHCTAB) which contained secondary hydroxy groups. The relative rates of reaction for DNFB and for DNCB, i.e. F/Cl rate ratios, in the presence of CDOH were similar to those obtained in the presence of 2‐OHCTAB but less than that obtained in the presence of CHEDAB. These results support the assumption that in CDOH the secondary hydroxy groups of C‐2 or C‐3 are involved in covalent bond formation with the aromatic substrate rather than the primary hydroxy group of C‐6. All the reactions studied proceed much more slowly in the presence of CDOH than in the presence of the hydroxy‐functionalized micelles. This may reflect a catalytic effect of the positively charged surface present in the micelles but not in the cyclodextrin. Copyright © 1989 John Wiley & Sons Ltd.