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 (pKa 2.22–5.52) with a Bronsted coefficient of β = 0.80 through a class n mechanism. The Bronsted coefficient and small solvent isotope effect of kH/kD = 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 pKa 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 pKa the Bronsted 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 pKa of the 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 kH/kD = 1.7 ± 0.1 and Bronsted type coefficients (for donating nitrogen substituents) of βdgArN = −0.3 and βdgRN = −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.