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.