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

摘要

The complexes [Pd(eta~2-dmfu)(P-N)]{dmfu=dimethyl fumarate;P-N=2-(PPh_2)C_6H_4-1-CH=NR,R=C_6H_4OMe-4 (1a),CHMe_2 (2a),C_6H_3Me_2-2,6 (3a),C_6H_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(eta~2-fn)(P-N)] (fn=fu-maronitrile1b) has a greater thermodynamic stability than its dmfu analogue 1a.The kientics of the oxidative addition of ArI (Ar=C_6H_4CF_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(eta~2-dmfu)(solvent)(k~1-P-N)] formed in a pre-equilibrium step from Pd-N bond breaking.The complexes 1c and 2c react with PhC ident to CSnBu_3 in the presence of an activated olefin (ol=dmfu,fn) to yield the palladium(0) derivatives [Pd(eta~2-ol(P-N)] along with ISnBu_3 and PhC ident to CAr.The kinetics of the transmetallation step,which is rate-determining for the overall reaction,obey the rate law;k_(obs)=k_(2t)[PhC ident to CSnBu_3].The k_(2T) values are markedly enhanced in more polar solvents such as CH_3CN and DMF.The solvent effect and the activation parameters suggest an associative S_E2 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 ident to CSnBu_3 with ArI catalysed by 1a or 2a,the rate-determining step is represented by the oxidative addition and that CH_3CN is the solvent in which the highest rates are observed.