Solution behaviour, kinetics and mechanism of the acid-catalysed cyclopalladation of imines

The cyclometallation reactions of N-benzylidene-benzylamines, -anilines and -propylamine with palladium acetate have been studied in acetic acid solution. Carbon-hydrogen electrophilic bond activation occurs to produce different types of metallacycles, given the polyfunctional nature of the ligands selected. The cyclo-metallated compounds formed indicate that the stability of the final species is, with respect to the activated C-H bond, in the order: five-membered aromatic endo > six-membered aliphatic endo > five-membered aromatic exo. >>> five-membered exo, four-membered. The nature of the final cyclometallated compounds in acetic acid solution has been ascertained via H-1 NMR spectroscopy: as a whole the spectra are complex, indicating that the nature of the cyclometallated species in solution is not simple, and that a wide variety of compounds is present depending on the imine used. The metallation reactions have been monitored kinetically vin UV/VIS spectroscopy at different temperatures and pressures in order to establish the mechanism through which these acid-assisted reactions occur. Although the thermal activation parameters cover a wide range of values (Delta H double dagger = 49 to 73 kJ mol(-1), Delta S double dagger = -52 to -138 J K-1 mol(-1)), the activation volume is in a very narrow range, -15 +/- 3 cm(3) mol(-1). The results are interpreted as the formation of a highly ordered four-centred transition state, involving the C-H and Pd-O (acetato) bonds, which is found to be very sensitive to the presence of any protons that could enhance the leaving-group characteristics of the MeCO2H ligand, converting it into its protonated MeCO2H2+ form.