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

Abstract

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 cyclometallated 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 via¹H 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 via 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 (ΔH^‡ = 49 to 73 kJ mol^–1, ΔS ^‡ = –52 to –138 J K^–1 mol^–1), the activation volume is in a very narrow range, –15 ± 3 cm³ 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.