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Titolo:
Mechanism of catalysis of the hydrolysis of a formamidinium compound
Autore:
Dalby, KN;
Indirizzi:
Univ Texas, Coll Pharm, Div Med Chem, Austin, TX 78712 USA Univ Texas Austin TX USA 78712 Pharm, Div Med Chem, Austin, TX 78712 USA
Titolo Testata:
JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2
fascicolo: 10, , anno: 2001,
pagine: 1961 - 1967
SICI:
1472-779X(200110):10<1961:MOCOTH>2.0.ZU;2-N
Fonte:
ISI
Lingua:
ENG
Soggetto:
GENERAL-BASE CATALYSIS; AZETIDINYL AMIDINIUM SALTS; PROTON-TRANSFER; TRANSITION-STATES; BETA-VALUES; ACID; SUBSTITUTION; SOLVATION; MOTION; IONS;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
41
Recensione:
Indirizzi per estratti:
Indirizzo: Dalby, KN Univ Texas, Coll Pharm, Div Med Chem, Austin, TX 78712 USA Univ Texas Austin TX USA 78712 v Med Chem, Austin, TX 78712 USA
Citazione:
K.N. Dalby, "Mechanism of catalysis of the hydrolysis of a formamidinium compound", J CHEM S P2, (10), 2001, pp. 1961-1967

Abstract

The hydrolysis of a fluoroformamidinium ion to give a urea is subject to general base catalysis, where the addition of water is catalyzed by carboxylate monoanions (pK(a) 2.22-5.52) with a Bronsted coefficient of beta = 0.80through a class n mechanism. The Bronsted coefficient and small solvent isotope effect of k(H)/k(D) = 1.2 +/- 0.2 for catalysis by acetate anion are consistent with significant movement of a hydron towards the catalyst in a late asymmetric transition state. A concerted mechanism is proposed that isenforced by proton transfer within a hydrogen-bonded intermediate species,in which the difference in pK(a) between the catalyst and the intermediateis similar to 16 units. Hydroxide and carbonate deviate below an extrapolation through a series of carboxylate anions, suggesting that over a wider range of pK(a) the Bronsted plot displays downward curvature; possibly due to a 'Hammond effect' on the proton transfer component of the reaction, thatresults in a change in transition-state structure with increasing pK(a) ofthe base catalyst. A 30-fold positive deviation of the uncatalyzed, reaction from the limiting Bronsted line through the carboxylates suggests that the uncatalyzed reaction occurs via a different mechanism. The solvent isotope effect of k(H)/k(D) = 1.7 +/- 0.1 and Bronsted type coefficients (for donating nitrogen substituents) of beta (ArN)(dg) = -0.3 and beta (RN)(dg) -0.4 for the uncatalyzed reaction are consistent with either a stepwise mechanism of addition and proton transfer, or a cyclic transition state containing two hydrogen bonded water molecules.

ASDD Area Sistemi Dipartimentali e Documentali, Università di Bologna, Catalogo delle riviste ed altri periodici
Documento generato il 22/01/20 alle ore 18:40:29