Stabilization of transition states of organic reactions by cyclodextrins, micelles, and dendrimers

• Main Author: Fedortchenko, Alexei A
• Format: Others
• Published: 1997
• Online Access: http://spectrum.library.concordia.ca/414/1/MQ39955.pdf; Fedortchenko, Alexei A <http://spectrum.library.concordia.ca/view/creators/Fedortchenko=3AAlexei_A=3A=3A.html> (1997) Stabilization of transition states of organic reactions by cyclodextrins, micelles, and dendrimers. Masters thesis, Concordia University.

Micelles of cetyltrimethylammonium bromide (CTAB) catalyze the basic cleavage of p-nitrophenyl alkanoates, whereas micelles of sodium dodecyl sulphate (SDS) inhibit this reaction. The strength of binding of the esters to both CTAB and SDS shows the same linear dependence and sensitivity to the acyl chain length, but the catalysis by CTAB and the inhibition by SDS do not depend on this parameter. Micelles of SDS catalyze the hydrolysis of aromatic acetals and orthoesters in the acidic solution. There is no sharp linear correlation between the substrate and the transition state binding for different acetals and orthoesters, which reflects the importance of both nature of the substituent, and its location in the molecule of the substrate. Amino- and carboxylate-terminated dendrimers modestly catalyze the basic cleavage of p-nitrophenyl alkanoates. Both substrate and transition state binding are independent on the ester acyl chain length, but were found to be a function of the dendrimer generation number. Carboxylate-terminated dendrimers catalyze the hydrolysis of aromatic acetals and orthoesters in the acidic solution. The observed dependence of the substrate and the transition state binding on the nature of the substituent supported the previously made suggestion that the interior of PAMAM dendrimers possesses hydrophilic character. The hydrolysis of aromatic acetals and orthoesters is retarded by cyclodextrins. The magnitude of the effect depends on the size of cyclodextrin cavity. The observed rate retardation is attributed to the unfavourable transition state binding to cyclodextrins, and possible retardation of diffusional separation of carboxonium ion and the newly formed alcohol molecule.