Micellar catalysis of Diels-Alder reactions: substrate positioning in the micelle.
We have studied the kinetics of the Diels-Alder reactions of cyclopentadiene, sorbyl alcohol, and sorbyltrimethylammonium bromide with a series of N-substituted maleimides in micellar media. Micellar rate constants have been determined and were found to be 20-40 times lower than the respective aqueous rate constants. Nevertheless, it was found that upon addition of sodium dodecyl sulfate the observed rate constants could be enhanced up to a factor of about 4.5. The low micellar rate constants can be attributed to the relatively apolar (water-poor) region of the micelle, in which the reactions take place. NMR experiments indicate that the reactants usually reside near the alpha- or beta-CH2 groups of the surfactant molecules in the micelle. Comparison of the micellar rate constants with rate constants in water/1-propanol mixtures suggests a concentration of water of 10-15 M in the micellar region where the diene and dienophile react.
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Physical Organic Chemistry Unit, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
Engberts
Jan B F N
JB
D000438:Alcohols / Q000737:chemistry; D002384:Catalysis; D000077286:Cetrimonium; D002593:Cetrimonium Compounds; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008301:Maleimides / Q000737:chemistry*; D008823:Micelles; D015394:Molecular Structure
D000438:Alcohols; D002593:Cetrimonium Compounds; D008301:Maleimides; D008823:Micelles; D000077286:Cetrimonium