Catalysis by Cyclodextrins in Nucleophilic Aromatic Substitution Reactions. 2. Amines as Nucleophiles

Article abstract of DOI:10.1021/jo00231a030

The kinetics of the reactions of 1-chloro-2,4-dinitrobenzene and 1-fluoro-2,4-dinitrobenzene with piperidine, butylamine, and morpholine in the presence of β-cyclodextrin (CD) was studied.There is an increase in the observed rate constant for the reactions of the first two amines at pH < pK_AH when CD is added, but there is no change in the rate for these two reactions at pH > pK_AH and for the reaction of morpholine either at pH < pK_AH or at pH > pK_AH.The three amines (A) as well as their conjugated acids (AH) form inclusion complexes with the CD.The association equilibrium constants for the amines are 50.3, and 17 M^-1 for piperidine, butylamine, and morpholine, respectively.Part of the observed catalysis is attributed to the fact that the amines and the complexed amines (ACD) react with the substrate at similar rates and at constant pH, the ratio (A + ACD)/(AH + AHCD) increases with the concentration of CD.Besides that, the complexed substrates react with the complexed amines at faster rates than that of the free substrate with the free amine.The latter reaction is not detected in the reactions at high pH and in the reactions of morpholine.