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 benzylami nes [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, respective ly, and these carry out the hydrolysis according to rate expression k(obs) = k(o) + k(cat)[Pd(II)](t) at pH 8 and 25 degrees 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 c omplexes 2a-c, 4a-c, respectively, demonstrating that the benzylamine (4a) and alpha-methylbenzylamine (4b,c) complexes are approximately ten and hund red times, respectively, catalytically more active than N,N-dimethylbenzyla mine complex 2a. Such a drastic discrimination is observed when the activat ed 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 ) approximate to 0.8 M-1 s(-1)) in hydrolysis of 2,4-dinitrophenyl acetate. The formation of the catalytically active intermediate between aquated pal ladacycles derived from either 2 or 4 and 5 is postulated. Different cataly tic properties of the primary and tertiary palladacycles with respect to th e methionine ester 5 are rationalized in terms of the steric effects in com bination with the hydrophobic interactions playing a role in aqueous soluti on. (C) 2000 Elsevier Science S.A. All rights reserved.