Supramolecular catalysis by cucurbit[7]uril and cyclodextrins: similarity and differences.
To understand the analogies and differences between the cucurbituril and cyclodextrin cavities different solvolytic reactions have been studied in the presence of cucurbit[7]uril, CB7, and beta-CD or its methylated derivative, DM-beta-CD. Solvolysis of 1-bromoadamantane has been used as a test to evaluate the ability of the cavities to solvate the Br(-) leaving group. Obtained results show that in both cases the polarity inside the cavity is similar to that of a 70% ethanol:water mixture. Solvolysis of substituted benzoyl chlorides shows a great difference between the CB7 and DM-beta-CD cavity. Solvolysis of electron withdrawing substituted benzoyl chlorides (associative mechanism) is catalyzed by DM-beta-CD and inhibited by CB7. However, solvolysis of electron donating substituted benzoyl chlorides (dissociative mechanism) is catalyzed by CB7 and inhibited by DM-beta-CD. These experimental behaviors have been explained on the basis of different solvolytic mechanisms. Participation of the hydroxyl groups of the cyclodextrin as a nucleophile can explain the catalytic effect observed for solvolysis of benzoyl chlorides reacting by an associative mechanism. Solvolysis of benzoyl chlorides reacting by a dissociative mechanism is catalyzed by CB7 due to the ability of the CB7 cavity to stabilize the acylium ion developed in the transition state by electrostatic interactions.
Basilio
Nuno
N
Departamento de Química Física, Facultad de Química, Universidad de Santiago, 15782 Santiago, Spain.
García-Río
L
L
Moreira
J A
JA
Pessêgo
M
M
D001952:Bridged-Ring Compounds / Q000737:chemistry*; D002384:Catalysis; D003505:Cyclodextrins; D007093:Imidazoles / Q000737:chemistry*; D046911:Macromolecular Substances / Q000737:chemistry*; D008958:Models, Molecular; D012995:Solubility; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils
D001952:Bridged-Ring Compounds; D003505:Cyclodextrins; D007093:Imidazoles; D046911:Macromolecular Substances; C456276:cucurbit(7)uril; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils