140678-00-8Relevant articles and documents
Intramolecular ureido and amide group participation in reactions of carbonate diesters
King, Stephen W.,Natarajan,Bembi, Ramesh,Fife, Thomas H.
, p. 10715 - 10721 (2007/10/02)
The cyclization of ethyl and phenyl 2-ureidophenylcarbonates in H2O at 30 °C involves two discrete steps with benzoxazolinone as the final product. The formation of benzoxazolinone is quantitative. Phenol is released in the initial step from both esters in an apparent OH--catalyzed reaction, which shows that intramolecular nucleophilic attack by the ureido group is via an anionic species. The pH-rate constant profile for the second step in the reaction of the ethyl ester is sigmoidal with pKapp = 8.9. The initial reaction involves a rearrangement, and the neighboring phenoxide ion of the intermediate participates in the second step. In view of the D2O solvent isotope effect (kH2O/kD2O = 1.2), this participation must be via a nucleophilic mechanism. The second step of the reaction of the phenyl ester, in which benzoxazolinone is formed, involves an apparent OH--catalyzed reaction of a cyclic intermediate. This intermediate was identified as N-carbamoylbenzoxazolinone, which thereby indicates that the initial nucleophilic attack is by nitrogen through a 5-membered-ring transition state. p-Nitrophenol release from p-nitrophenyl 2-ureidophenylcarbonate is only 18-fold faster than phenol release from the corresponding phenyl ester. Ratios of kOH(ortho) for phenol release from the 2-ureido-substituted esters to kOH(para) for OH--catalyzed hydrolysis of the corresponding 4-ureido-substituted esters are approximately 104 in all cases. In the intramolecular nucleophilic reactions of the ureido-substituted carbonate esters, a neutral species reaction is not observed, even at pH values as low as 3, in contrast with substituted benzoate esters having phenolic leaving groups. Also, in the apparent OH--catalyzed reactions of the carbonate diesters, only one cyclic product is obtained, whereas both oxygen and nitrogen attack occur in the nucleophilic reactions of carboxylate esters. The neighboring amide group of p-nitrophenyl o-(carboxamido)phenylcarbonate participates with nitrogen attack and apparent OH- catalysis. Intramolecular attack of nitrogen provides a rate enhancement of 103 in the release of p-nitrophenol over the OH--catalyzed hydrolysis of the para-substituted compound. Thus, in the intramolecular nucleophilic reactions of the carbonate diesters, nitrogen anion attack takes place preferentially when such attack is sterically favored (five-membered-ring transition state) and when there is an equal opportunity for oxygen or nitrogen attack.
