Catalysis of transimination by rate-limiting proton transfer to buffer bases

General base catalysis of the hydroxylaminolysis of benzhydrylidenedimethylammonium ion gives a Bronsted plot that follows the Eigen curve expected for rate-determining trapping of the initially formed addition intermediate by proton transfer to a buffer base or water. The solvent deuterium isotope effects for catalysis by oxy anions exhibit a maximum at the break point of the Eigen curve, close to the estimated pK/sub a/ for the addition intermediate. This maximum can be accounted for by a partial change in rate-limiting step of the proton-transfer process near ..delta..pK = 0. Water shows a positive deviation from the Bronsted plot and a solvent isotope effect of k/sub H/sub 2/O//k/sub D/sub 2/O/ = 4.7 that provide additional evidene for the trapping mechanism. The addition of glycerol increases the rate of the base-catalyzed reaction; much larger increases are observed with ethylene glycol and methanol. In contrast, the base-catalyzed hydrolysis of the cationic imine follows a linear Bronsted plot (..beta.. = 0.24), with a negative deviation for catalysis by water, and gives a constant value of k/sub H/sub 2/O//k/sub D/sub 2/O/ = 1.9 +- 0.2. A concerted mechanism of base-catalyzed attack by water is suggested for this reaction.