Variable-Temperature and -Pressure Kinetics and Mechanism of the Cyclopalladation Reaction of Imines in Aprotic Solvent Montserrat Go ´mez, Jaume Granell, and Manuel Martinez* Departament de Quı ´mica Inorga ` nica, Facultat de Quı ´mica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain Received December 30, 1996 X 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 q )+45/+67 kJ mol -1 , ΔS q )-100/-180 J K -1 mol -1 , ΔV q )-11/-17 cm 3 mol -1 ), those related to the aniline derivatives (ΔH q )+66/+75 kJ mol -1 , ΔS q )-96/-123 J K -1 mol -1 , ΔV q )-21/-25 cm 3 mol -1 ), and finally those related to the imines producing aliphatic bond activation (ΔH q )+48/+49 kJ mol -1 , ΔS q )-167/-177 J K -1 mol -1 , ΔV q )-20/-24 cm 3 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. Introduction Cyclometalation reactions on Pd(II) complexes have been throughly studied by a number of research groups. 1 The synthesis of cyclopalladated complexes is a field of great interest as a consequence of their applications, for example, in organic synthesis 2 or in the design of new metallomesogenes 3 or antitumor drugs. 4 Optically ac- tive cyclopalladated derivatives have also been used for the enantiomeric excess determination 5 and optical resolution of amines and phosphines. 6 The number of these studies dealing with kinetic- mechanistic information is limited. 7 Very little infor- mation is available about the nature of the species existing in these reaction solutions; even the nature of the starting material, palladium acetate, is rather unclear. 8 Although it is generally assumed that cyclo- palladated compounds maintain their dimeric structure in solution, it has been shown recently that acetone is able to break the bridge of the cyclopalladated complex [Pd(μ-Br)(C 6 H 4 CH(Me)NH 2 )] 2 , producing mononuclear species. 9 Our interests have been centered in studies of this type of reaction on iminic ligands 10 as well as in the mechanisms operating in organometallic reactions in- volving the activation of C-H bonds on Pt(II) and X Abstract published in Advance ACS Abstracts, May 1, 1997. 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