Kinetic Evidence for π-Complex Formation Prior to Oxidative Addition of Propargyl Halides to Triphenylphosphine−Platinum(0) Complexes

Abstract

The mechanism of oxidative addition of phenylpropargyl halides to Pt(PPh3)4 to give Pt(η1-CH2C⋮CPh)(X)(PPh3)2 has been investigated on the basis of the kinetics of the reaction and stereomeric analysis of the products. The kinetic results of the reaction showed the contributions of two pathways involving Pt(PPh3)3 and Pt(PPh3)2 complexes as active species. The second-order rate constants for both pathways were on the order of 103 larger than the corresponding rate constants of the oxidative addition reaction of CH3I. Moreover, both active species and the substrate halide gave, as the kinetic product, the isomer having two PPh3 groups located cis, which is also in sharp contrast to the case of the reaction of CH3I. It is proposed that the rate-determining step in the reaction involving Pt(PPh3)2 is the coordination of the C⋮C bond to Pt(PPh3)2 to form Pt(η2-PhC⋮CCH2X)(PPh3)2, which subsequently undergoes rapid collapse to the η3-propargyl complex [Pt(η3-CH2CCPh)(PPh3)2]X and eventually to cis-Pt(η1-CH2C⋮CPh)(X)(PPh3)2. The reaction of Pt(PPh3)3 may also involve a rate-determining C⋮C bond coordination step.