Variable-Temperature and -Pressure Kinetics and Mechanism of the Cyclopalladation Reaction of Imines in Aprotic Solvent

Abstract

The cyclometalation reactions of benzylidenebenzylamines, -anilines, and -propylamine with palladium acetate have been studied in toluene solution. The cyclometalated compounds are formed via C−H electrophilic bond activation to produce different types of metallacycles, depending upon the polyfunctional nature of the ligand selected. If a five-membered endo metallacycle is possible, it is formed via aromatic C−H bond activation. Formation of a five-membered exo metallacycle only takes place when endo cyclometalation requires the formation of a six-membered compound via aliphatic C−H bond activation or when the steric hindrance affects the planarity of the imine in an important way. The activation of aliphatic C−H bonds has been achieved for endo six-membered metallacycles only when the alternative aromatic C−H bond activation would produce a four-membered exo compound; no activation of aliphatic C−H bonds to form exo five-membered complexes has been observed. The reactions have been monitored kinetically at different temperatures and pressures in order to establish the mechanism through which these spontaneous reactions occur. Three different sets of activation parameters are apparent from the results obtained: those related to the benzylamines and propylamine (ΔH⧧ = +45/+67 kJ mol-1, ΔS⧧ = −100/−180 J K-1 mol-1, ΔV⧧ = −11/−17 cm3 mol-1), those related to the aniline derivatives (ΔH⧧ = +66/+75 kJ mol-1, ΔS⧧ = −96/−123 J K-1 mol-1, ΔV⧧ = −21/−25 cm3 mol-1), and finally those related to the imines producing aliphatic bond activation (ΔH⧧ = +48/+49 kJ mol-1, ΔS⧧ = −167/−177 J K-1 mol-1, ΔV⧧ = −20/−24 cm3 mol-1). The results are interpreted as the formation of a highly ordered four-center transition state, involving the C−H and Pd−O(acetato) bonds, which is found to be very sensitive to the flexibility and steric hindrance of the imine ligands and very insensitive to electronic changes of the C−H bonds.