Ester hydrolysis catalysed by ortho-palladated aryl oximes

Abstract

Hydrolysis of 4-nitro- and 2,4-dinitro-phenyl carboxylates proceeds catalytically in the presence of ortho-palladated aryl oximes 4. The catalytic reaction involves two active species, identified as aquated complexes 4 with protonated (k_AH path) and deprotonated (k_A path) oximate groups, respectively. Both species contain a weakly basic coordinated hydroxide anion manifested in the k_AH path. Other cyclopalladated hydroxo complexes 8, lacking an oxime group, are shown to react with 2,4-dinitrophenyl acetate with rate constants ca. ten times greater than those expected from their basicity. The rate constants k_A obey a Brønsted correlation (β ca. 1) with the pK_a of the coordinated oxime group. In neutral solution, palladated oximes react with esters ca. 10³ times faster than the respective free oximes. The O-acyl oxime intermediate undergoes very fast hydrolysis through intramolecular attack by the coordinated hydroxide. Factors ensuring the combination of fast acylation and deacylation steps are discussed with reference to some related systems.