Kinetic Studies of the Oxidative Addition and Transmetallation Steps Involved in the Cross‐Coupling of Alkynyl Stannanes with Aryl Iodides Catalysed by η2‐(Dimethyl fumarate)(iminophosphane)palladium(0) Complexes

Abstract

AbstractThe complexes [Pd(η2‐dmfu)(P−N)] {dmfu = dimethyl fumarate; P−N = 2‐(PPh2)C6H4−1‐CH=NR, R = C6H4OMe‐4 (1a), CHMe2 (2a), C6H3Me2‐2,6 (3a), C6H3(CHMe2)2‐2,6 (4a)} undergo dynamic processes in solution which consist of a P−N ligand site exchange through initial rupture of the Pd−N bond at lower energy and an olefin dissociation‐association at higher energy. According to equilibrium constant values for olefin replacement, the complex [Pd(η2‐fn)(P−N)] (fn = fumaronitrile, 1b) has a greater thermodynamic stability than its dmfu analogue 1a. The kinetics of the oxidative addition of ArI (Ar = C6H4CF3‐4) to 1a and 2a lead to the products [PdI(Ar)(P−N)] (1c, 2c) and obey the rate law, kobs = k1A + k2A[ArI]. The k1A step involves oxidative addition to a reactive species [Pd(solvent)(P−N)] formed from dmfu dissociation. The k2A step is better interpreted in terms of oxidative addition to a species [Pd(η2‐dmfu)(solvent)(κ1‐P−N)] formed in a pre‐equilibrium step from Pd−N bond breaking. The complexes 1c and 2c react with PhC≡CSnBu3 in the presence of an activated olefin (ol = dmfu, fn) to yield the palladium(0) derivatives [Pd(η2‐ol)(P−N)] along with ISnBu3 and PhC≡CAr. The kinetics of the transmetallation step, which is rate‐determining for the overall reaction, obey the rate law: kobs = k2T[PhC≡CSnBu3]. The k2T values are markedly enhanced in more polar solvents such as CH3CN and DMF. The solvent effect and the activation parameters suggest an associative SE2 mechanism with substantial charge separation in the transition state. The kinetic data of the above reactions in various solvents indicate that, for the cross‐coupling of PhC≡CSnBu3 with ArI catalysed by 1a or 2a, the rate‐determining step is represented by the oxidative addition and that CH3CN is the solvent in which the highest rates are observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)