Heterolytic dediazoniation of 2-, 3-, and 4-methylbenzenediazonium tetrafluoroborate in the presence of α-, β-, and γ-cyclodextrins

The rates and product distribution for the spontaneous dediazoniation of 2-, 3-, and 4-methylbenzene-diazonium ions, OMBD, MMBD, and PMBD, respectively, in the presence of alpha-, beta-, and gamma-cyclodextrins (CDs) in aqueous acid are reported. No evidence of reaction between arenediazonium ions and CDs was found. Quantitative conversion to the phenol derivative, ArOH, is achieved for all diazonium ions, and observed rate constants, k(0), and product distributions are independent of both acidity and cyclodextrin concentration. All evidence is consistent with a D-n + A(n) dediazoniation mechanism, that is, rate-determining formation of an aryl cation which reacts immediately, with low selectivity, with available nucleophiles. Data are discussed in terms of formation of an inclusion complex in which the methylbenzene portion of the arenediazonium ion is inserted, to some extent, in the CD cavity, the diazonium group being located in a water-rich region with similar properties to those of the hulk solvent. Results consistent with the D-n + A(n) mechanism contrast with those in the literature, which indicate that cyclodextrins promote free-radical dediazoniations in a solvent where the ionic process is especially favored.