The susceptibility of an amino substituted triarylmethane dye towards the nucleophilic and electrophilic attack of peracids: the disparate kinetic effects of β-cyclodextrin

摘要

Cyclodextrins are cyclic carbohydrate oligomers enzymaticallynproduced from starch. β-Cyclodextrin consists of seven α-(1–4)nlinked -glucopyranose units in a chair conformation, formingna hollow truncated cone with a hydrophilic exterior and anhydrophobic cavity, suitable for accommodating most simplenaromatic guests.1 Cyclodextrins are renowned for their propensitynto form inclusion complexes with wide range of guestnmolecules.2 This complex formation often becomes an importantnfactor that influences the kinetics of the guest’s reactionsnwith other substances. Although many reactions affected by thenpresence of cyclodextrin in solution show simple saturationnkinetics, some of them can be more complicated and in suchncases a detailed kinetic analysis is needed to reveal the mechanismnof catalysis or inhibition by the cyclodextrin host. Green Snis a triarylmethane dye from the Victoria Blue family, whosendistinctive feature is a hydroxyl positioned ortho to the centralncarbon of the dye. The bleaching of Green S by aqueous hydrogennperoxide or by water in alkali solution involves nucleophilicnattack on the sp2 hybridized central carbon of the dye vianintramolecular base catalysis by the ionized ortho-hydroxyl.3nA recent paper demonstrated that with increasing β-cyclodextrinnconcentration the reaction of Green S with hydrogennperoxide is first accelerated and then slowed down.4 This ratenmaximum is clear evidence for reaction pathways involvingnboth one and two molecules of cyclodextrin and is interpretednas follows. The first cyclodextrin binds to an aryl group of thendye with the central carbon in a position where the field effectnof the cyclodextrin attracts the nucleophilic oxygen atomnof hydrogen peroxide and accelerates the reaction. The secondncyclodextrin binds to another aryl group so as to stericallynhinder the approach of hydrogen peroxide to the central carbonnand slows down the reaction. Alkali bleaching of the dye is alsonaccelerated by cyclodextrin, and the reverse reaction is slowedndown, but no rate maxima or minima are observed. Thesenresults are also consistent with pathways involving both onenand two molecules of cyclodextrin provided the second cyclodextrinnstabilizes Green S and its alkali bleaching product to thensame extent. The present paper shows that peracetic acid, likenhydrogen peroxide, attacks the central carbon of Green S.nCaro’s acid, on the other hand, behaves as an electrophile andnattacks a tertiary amine nitrogen atom of the dye. The effect ofnβ-cyclodextrin on the reaction of peracetic acid is very similarnto that of hydrogen peroxide whereas its