INSERTION OF ALKYNES INTO THE PD-C BOND OF PALLADACYCLES - MECHANISTIC INFORMATION FROM HIGH-PRESSURE KINETIC AND X-RAY STRUCTURAL DATA

The reaction between halo-bridged ring-substituted orthopalladated NN-dimethylbenzylamines, [Pd(C6H3RCH2NMe2)X]2 1 (R = 4-MeO, 5-Me, H, 5-F, X = Cl, I) and alkynes R'C=CR'' (R'/R'' = Ph/Ph, Ph/C6H4CF3-3, Ph/C6H4NO2-4, and Et/Et) to afford double insertion products 3 is first-order in both the reagent concentrations in chloroform at 25.0-44.1-degrees-C. The insertion of the first alkyne molecule is the rate-determining step. Electron-donating groups R on the phenyl ring of 1 increase the rate constants and the slope of the Hammett plot equals -2.2. Variation of the nature of the alkyne results in smaller and less systematic changes in the rate constants. The volume of activation ranges from -18.6 to -14.4 cm3 mol-1 in the case of PhC=CPh but is practically zero in the case of EtC=CEt. The monomerization of dimers 1 by either pyridine or [NEt4]Cl.H2O to afford the species [Pd(C6H3RCH2NMe2)CI(L)] (L = py or Cl, charges are omitted) retards the insertion markedly, suggesting that only the dimers are the reactive species and that a halo bridge is essential for the entrance of alkyne into the coordination plane of Pd(II). The insertion of the second alkyne was kinetically studied by reacting the seven-membered complex 4 with PhC=CPh. Although this step is associated with a cis --> trans rearrangement about the C=C bond of 4, as shown by X-ray crystallography, its formal kinetic characteristics are very similar to those of the first insertion step. Both the steps are driven by the nucleophilic attack of the phenyl or alkenyl carbon on the coordinated alkyne. A more complicated mechanism of the second insertion, which is essential to account for the cis --> trans isomerization, is discussed in some detail.