title: Mechanism of the palladium-catalyzed metal-carbon bond formation. A dual pathway for the transmetalation step.
abstract: The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [structure: see text], resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylides Bu(3)Sn-C [triple bond] C-(p-XC(6)H(4)) (11a, X = H; 11b, X = Cl) yields the products of transmetalation [structure: see text] (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR(3) (Ph > Bu > Me) and less so on the nature of the p-substituent X group, proceeds through two competing pathways, depending on the initial concentration of substrate. At high [3] (approximately equal to 10(-2) M), the transmetalation proceeds through an intermediate species (12) formed by the interaction of complex 3 with 11a. This associative complex accumulates in the presence of added PPh(3) and has been characterized spectroscopically. At low [3] (approximately equal to 10(-4) M), the reaction rate shows an inverse dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate species (13), in which PPh(3) has been substituted by a dimethylformamide (DMF) molecule, as shown by UV-vis and (31)P NMR spectroscopy. Values of k(obs) depend on the concentration and nature of the aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric iodide bridged complex [structure: see text](14) has been obtained during attempts at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give the transmetalation product.
authors.affiliation: Centro C.N.R. di Studio sui Meccanismi di Reazione, Dipartimento di Chimica, Università La Sapienza, Box 34-Roma 62, Piazzale Aldo Moro, 5 00185 Roma, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Ricci
authors.forename: Francesco
authors.initials: F
authors.lastname: Angelucci
authors.forename: Mauro
authors.initials: M
authors.lastname: Bassetti
authors.forename: Claudio
authors.initials: C
authors.lastname: Lo Sterzo