Kinetics and mechanism of the cyclization of ω-(p-nitrophenyl)-hydantoic acid amides: steric hindrance to proton transfer causes a 104-fold change in rateElectronic supplementary information (ESI) available: Observed first-order rate coefficients, constants for solvent and buffer catalysis for the cyclization reactions. See http://www.rsc.org/suppdata/ob/b2/b211040g
| Autor: | Ivan G. Pojarlieff, Anthony J. Kirby, Asen H. Koedjikov, Violina T. Angelova |
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| Rok vydání: | 2003 |
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| Zdroj: | Organic & Biomolecular Chemistry. 1:859-865 |
| ISSN: | 1477-0539 1477-0520 |
| DOI: | 10.1039/b211040g |
| Popis: | The pH-rate profiles for the cyclization of primary 2,3-dimethyl and 2,2,3-trimethyl-hydantoinamides (2-UAm and 3-UAm respectively) differ strikingly from those for the cyclizations of the corresponding N-methylated amides 2-MUAm and 3-MUAm; which are dominated by the water reaction, spanning some 6 pH units. For the cyclization of UAm the plateau extends over no more than two pH units. The difference is due to the slower base-catalyzed cyclization of the N-methylamides. The solvent kinetic isotope effect for this hydroxide-catalyzed reaction is close to 1.2, consistent with a slow protonation by water of the amino-group of the negatively charged tetrahedral intermediate. General base catalysis was observed with bases of pKBH+ up to 8. The Bronsted β are compatible with a hydrogen bonding mechanism for the GBC. In the gem-dimethyl compounds 3 the leaving group is flanked by substituents on both sides. The N-methyl group in 3-MUAm hinders frontal access of the proton, causing a 14000 fold decrease in rate. This is only 3800 fold in the compound with one methyl group at position 2. |
| Databáze: | OpenAIRE |
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