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

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(C6H4CHR′NR2)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 kobs=ko+kcat[Pd(II)]t at pH 8 and 25°C. The rate constants for the pathway kcat 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 (kcat≈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.