Kinetic Studies of the Oxidative Addition and Transmetallation Steps Involved in the Cross‐Coupling of Alkynyl Stannanes with Aryl Iodides Catalysed by η 2 ‐(Dimethyl fumarate)(iminophosphane)palladium(0) Complexes

The complexes [Pd(η 2 ‐dmfu)(P−N)] {dmfu = dimethyl fumarate; P−N = 2‐(PPh 2 )C 6 H 4 −1‐CH=NR, R = C 6 H 4 OMe‐4 ( 1a ), CHMe 2 ( 2a ), C 6 H 3 Me 2 ‐2,6 ( 3a ), C 6 H 3 (CHMe 2 ) 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(η 2 ‐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 = C 6 H 4 CF 3 ‐4) to 1a and 2a lead to the products [PdI(Ar)(P−N)] ( 1c , 2c ) and obey the rate law, k obs = k 1A + k 2A [ArI]. The k 1A step involves oxidative addition to a reactive species [Pd(solvent)(P−N)] formed from dmfu dissociation. The k 2A step is better interpreted in terms of oxidative addition to a species [Pd(η 2 ‐dmfu)(solvent)(κ 1 ‐P−N)] formed in a pre‐equilibrium step from Pd−N bond breaking. The complexes 1c and 2c react with PhC≡CSnBu 3 in the presence of an activated olefin (ol = dmfu, fn) to yield the palladium(0) derivatives [Pd(η 2 ‐ol)(P−N)] along with ISnBu 3 and PhC≡CAr. The kinetics of the transmetallation step, which is rate‐determining for the overall reaction, obey the rate law: k obs = k 2T [PhC≡CSnBu 3 ]. The k 2T values are markedly enhanced in more polar solvents such as CH 3 CN and DMF. The solvent effect and the activation parameters suggest an associative S E 2 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≡CSnBu 3 with ArI catalysed by 1a or 2a , the rate‐determining step is represented by the oxidative addition and that CH 3 CN is the solvent in which the highest rates are observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)