Mechanism of the Palladium-Catalyzed Metal−Carbon Bond Formation. A Dual Pathway for the Transmetalation Step

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 (3, R = Ph; 15, R = Bu; 16, R = Me), resulting from oxidative addition of a Mo−I moiety to a palladium center, with aryltributyltinacetylides Bu3Sn−C⋮C−(p-XC6H4) (11a, X = H; 11b, X = Cl) yields the products of transmetalation (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR3 (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] (≅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 PPh3 and has been characterized spectroscopically. At low [3] (≅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 PPh3 has been substituted by a dimethylformamide (DMF) molecule, as shown by UV−vis and 31P 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 (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.