Mechanism of catalysis of the hydrolysis of a formamidinium compound

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 Brønsted coefficient of β = 0.80 through a class n mechanism. The Brønsted 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 is enforced by proton transfer within a hydrogen-bonded intermediate species, in which the difference in pK_a between the catalyst and the intermediate is ∼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 Brønsted plot displays downward curvature, possibly due to a ‘Hammond effect’ on the proton transfer component of the reaction, that results in a change in transition-state structure with increasing pK_a of the base catalyst. A 30-fold positive deviation of the uncatalyzed reaction from the limiting Brønsted 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 Brønsted type coefficients (for donating nitrogen substituents) of β_dg^ArN = −0.3 and β_dg^RN = −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.