Dynamic aspects in host–guest interactions. Part 2. Directional inclusion reactions of some azo guest molecules with α-cyclodextrin

Abstract

The inclusion reactions of the azo guest molecules, 3′,5′-(Me)₂–4′-(OH)Ph–NN–Ph–Y 1–4 and 3′-Pr–4′-(OH)Ph–NN–Ph–Y 5–8(Y = CO₂^–, SO₂NH₂, SO₃^– and AsO₃H^–). with α-cyclodextrin were investigated. The formation constants of the inclusion complexes ranged from 5.0 × 10³ to 1.75 × 10⁴ mol^–1 dm³. The rate constants for the inclusion reactions in aqueous solution at 25 °C were determined by the stopped-flow method. The rate constants and reaction mechanism were quite sensitive to the substituent group Y on C-4 of one phenyl ring. The reactions of 1 and 5(Y = CO₂^–) were found to be very fast (millisecond or less) and to proceed as one step processes. On the other hand, the reactions of 4 and 8(Y = AsO₃H^–) were quite slow (several tens of minutes). In the case of the reaction of [graphic omitted] 2, 6(Y = SO₂NH₂) and 3, 7(Y = SO₃^–), the inclusion clearly proceeds as a two-step process. A specific orientation and/or direction of reactant within the inclusion complex is also discussed.