Role of the distance to the S-donor center in the hydrolysis of amino acidesters, catalyzed by palladacycles

The hydrolysis of N-acetyl-S-methylcysteine and N-acetylmethionine p-nitrop henyl esters, catalyzed by the dimeric palladacycle [Pd(C6H4-2-CH2NMe2)Cl]( 2), is described by the kinetic equation k(ap) = k(0) + k(cat). The catalyt ic rate constants k(cat) are equal to 881 +/- 96 and 45 +/- 3 1 mol(-1) s(- 1), respectively, for the S-methylcysteine and methionine esters at pH 8.0 (25 degreesC). The equilibrium constant K-1 for dissociation of the dimeric catalyst by the action of amino acid esters in chloroform at 25 degreesC a re equal to (2.9 +/- 0.3) x 10(3) and (3.0 +/-0.6) x 10(4) l/mol, respectiv ely. Thus the catalysis in the hydrolysis of N-acetyl-S-methylcysteine p-ni trophenyl ester is by an order of magnitude more effective, and the formati on of catalytically active species is by an order of magnitude less effecti ve than in the case of the methionine ester. This finding was explained in terms of intramolecular mechanism of hydrolysis with formation of a six-mem bered cyclic tetrahedral intermediate.