Increased catalytic activity of primary amine palladacycles in biomimetic hydrolysis of N-t -BOC- S-methionine p-nitrophenyl ester

Abstract

The kinetics of hydrolysis of N-t -BOC- S-methionine p-nitrophenyl ester ( 5) promoted by the cyclopalladated complexes of tertiary and primary benzylamines [Pd(C 6H 4CHR′NR 2)Cl(py)], where R=Me ( 1) and H ( 3), R′=H ( a), S-Me ( b) and R-Me ( b), has been studied. In buffered solutions complexes 1 and 3 undergo aquation of the chloro ligand to afford species 2 and 4, respectively, and these carry out the hydrolysis according to rate expression k obs= k o+ k cat[Pd(II)] t at pH 8 and 25°C. The rate constants for the pathway k cat equal 2.5, 2.1, 2.2, 32, 220 and 260 M −1 s −1 for complexes 2a– c, 4a– c, respectively, demonstrating that the benzylamine ( 4a) and α-methylbenzylamine ( 4b, c) complexes are approximately ten and hundred times, respectively, catalytically more active than N, N-dimethylbenzylamine complex 2a. Such a drastic discrimination is observed when the activated ester has a donor center capable of binding with Pd(II). In accord with this is the fact that all complexes studied display similar activity ( k cat≈0.8 M −1 s −1) in hydrolysis of 2,4-dinitrophenyl acetate. The formation of the catalytically active intermediate between aquated palladacycles derived from either 2 or 4 and 5 is postulated. Different catalytic properties of the primary and tertiary palladacycles with respect to the methionine ester 5 are rationalized in terms of the steric effects in combination with the hydrophobic interactions playing a role in aqueous solution.