Kinetic evidence for pi-complex formation prior to oxidative addition of propargyl halides to triphenylphosphine - Platinum(0) complexes

The mechanism of oxidative addition of phenylpropargyl halides to Pt(PPh3)( 4) to give Pt(eta (1)-CH2C=CPh)(X)(PPh3)(2) has been investigated on the ba sis of the kinetics of the reaction and stereomeric analysis of the product s. The kinetic results of the reaction showed the contributions of two path ways involving Pt(PPh3)(3) and Pt(PPh3)(3) complexes as active species. The second-order rate constants for both pathways were on the order of 10(3) l arger than the corresponding rate constants of the oxidative addition react ion of CH3I, Moreover, both active species and the substrate halide gave, a s 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 propo sed 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(eta (2)-PhC=CCH 2X)(PPh3)(2), which subsequently undergoes rapid collapse to the eta (3)-pr opargyl complex [Pt(eta (3)-CH2CCPh)(PPh3)(2)]X and eventually to cis-Pt(et a (1)-CH2C=CPh)(X)(PPh3)(2). The reaction of Pt(PPh3)(3) may also involve a rate-determining C=C bond coordination step.