SOLUTION BEHAVIOR, KINETICS AND MECHANISM OF THE ACID-CATALYZED CYCLOPALLADATION OF IMINES

The cyclometallation reactions of N-benzylidene-benzylamines, -aniline s 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 na ture of the ligands selected. The cyclo-metallated compounds formed in dicate that the stability of the final species is, with respect to the activated C-H bond, in the order: five-membered aromatic endo > six-m embered 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 th e cyclometallated species in solution is not simple, and that a wide v ariety of compounds is present depending on the imine used. The metall ation reactions have been monitored kinetically vin UV/VIS spectroscop y at different temperatures and pressures in order to establish the me chanism through which these acid-assisted reactions occur. Although th e 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 -1 38 J K-1 mol(-1)), the activation volume is in a very narrow range, -1 5 +/- 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 pres ence of any protons that could enhance the leaving-group characteristi cs of the MeCO2H ligand, converting it into its protonated MeCO2H2+ fo rm.