Phosphine complexes, reaction kinetics

Quite different types of rhodium compound can give very similar reaction rates in a system which shows a kinetic dependence on the rhodium catalyst concentration. In particular, rhodium(III) halides and rho-dium(I) phosphine complexes give almost identical reaction rates after an initial induction period. Thus, in the case of these two systems, it appears that a common species is being formed. 

The third and last example in this chapter illustrates a case where kinetic data were used to derive relative enthalpies for a series of similar reactions. Consider the ligand (phosphine) exchange reaction 15.15 (see figure 14.5 forthe structure of the complex),... 

No detailed investigation of the reaction mechanism (kinetic measurements, etc.) was performed. The actual catalyst is probably the coordinatively unsaturated CoH(CO)2(PR3) species which may be formed by the loss of one ligand from both hydrido carbonyl phosphine complexes present in the reaction mixture ... 

In the case of trani -[PdEt2(PR3)2], where the phosphines include PEt3, PMc2Ph, and PEtPh2, warming to between 25 and 32 °C caused decomposition to equimolar amounts of ethane and ethene. Essentially, no butane is formed. Added phosphines had little effect on the rate of the reaction. Kinetic studies are consistent with the mechanism shown in Scheme 8, in which /3-elimination takes place predominately from the four-coordinate complex, with only minor reaction from the partially dissociated species [PdEt2(PR3)]. 

Amatore and coworkers have studied the addition of aryl halides to Pd(0) complexes formed by the addition of chelating phosphines to Pd2(dba)3171. Addition of these ligands to Pd2(dba)3 generates mixed phosphine/dba complexes. The kinetic behavior of the complex containing BINAP as phosphine ligand indicated that two competing pathways for addition occur, one by direct reaction of aryl halide with (BINAP)Pd(dba) and one from the (BINAP)Pd intermediate formed by dissociation of dba. 

Another convenient method to synthesize palladium(O) phosphine complexes involves the reduction of CpPd( 7r-all i) [70], The starting 7c-allyl complex can be readily prepared and stored as a relatively stable crystalline solid. In the presence of tertiary phosphine ligands in solution, this complex undergoes reductive elimination to give palladium(O) phosphine complexes. The reaction proceeds cleanly under mild conditions without formation of inorganic by-products, leading to a facile synthesis of kinetically unstable species. This method was used to prepare coordinatively unsaturated bis(tertiary phosphine)pa)ladium(O) complexes (eq (51)) [71]. 

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