Kinetics of oxidative addition

Besides these determinations of static structures, further studies of the kinetics of oxidative addition to Ir(I) complexes have been made. Beginning with Ir(CO)H(PPh3)3 (VIII), the first step is seen as dissociation to the four-coordinate planar intermediate (IX). This then deforms and undergoes concerted cis addition to yield the adduct (V) (179,224). In a comparison of group IV hydrides, PhsMH, the rates increased in the order Sibond energy but to the effects of differing polarizability and solvation of M-Ir products on the activation energy for addition. 

Oxidative addition of aryl haUdes to an electron-rich paUadium(O) species depends substantially on the nature of the C(sp )—X bond. Relatively weak bonds, as in aryl iodides or triflates, lead to a faster oxidative addition, but the activations of stronger bonds, as in aryl bromides-and especially chlorides-require very good donor ligands. This dependence of oxidative addition rates on the nature of the electrophile in multistep reactions may lead to different rate-deterriiiiittig steps. Apart from the nature of the aromatic substrate, the possible interaction of the terminal alkyne starting material and the internal alkyne product with the metal center of the catalyst can alter the kinetics of oxidative addition. The kinetics of addition of... 

We are on the right path in attempting to establish the kinetics of oxidation reactions in flow systems. This is the scientific basis of continuous processes in chemical industry and an invaluable source of additional information on reaction mechanisms. 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. 

Reaction Steps 3a and 3b also can be used to rationalize the observed para-substituent effects presented in Table III the more electron-releasing, para-substituted benzaldehydes retard the rate of oxidative addition (18) for RhCl(PPh3)3. Therefore, p-methyl- and p-methoxybenzaldehyde are expected to be decarbonylated slower than the unsubstituted benzaldehyde, as is observed in Table III. (This argument requires that Reaction 3a be saturated to the right, which is expected, in neat aldehyde solvent with electron-releasing, para-substituted benzaldehydes.) The unexpected slower rate for p-chloro-benzaldehyde could be accounted for ifK for this aldehyde is small and saturation of equilibrium in Equation 3a is not achieved. Note that fcobs is a function of K and k (see Equation 4b) under this condition. It is also possible that the rate-determining step is different for this aldehyde. Present research includes a careful kinetic analysis using several aldehydes so that K and k can be determined independently. 

Probing C—H addition/elimination in Pt(ll)/Pt(IV) systems The importance of oxidative addition of aromatic and aliphatic C—H bonds to Pt(II) centers and its microscopic reverse, reductive elimination of C—H from Pt(IV) species, is ubiquitous in the context of both catalysis and synthesis. It is thus inevitable that the chemical, mechanistic, and kinetic facets of such reactions have become a prominent focus of group 10 poly(pyrazolyl)borate research, although this remains a relatively nascent area. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

In ABS, because the BR units are more photosensitive than the PS units, they are photooxidized in the first steps of the reaction. The radicals which are formed can attack the neighboring PS units. Moreover, the grafted sites of the PS macromolecules are the starting sites of an additional route of photooxidation of the PS units. Therefore, the kinetics of oxidation of the copolymer ABS are twice as fast as expected on the basis of only addition of the photooxidation rates of the two polymers studied separately. 

The kinetics of oxidation of Am (III) by sodium persulfate in the presence of Ag+ ions were reinvestigated by studying the effect of additions of small amounts of reagents which do not drastically change the distribution coefficients of Am (VI) or Cm (III) ions. 

A kinetic study by Harrod and Smith of oxidative addition to a square planar cationic iridium complex also supports the three-center mechanism (258). The rate law is first order in the iridium complex and first order in hydrosilane. Determination of the activation parameters indicated a moderate activation enthalpy (AH — 5-6 kcal/mol) and a large negative activation entropy (AS — -47 e.u.) No variations were observed on changing the solvent. Harrod and Smith concluded that oxidative addition proceeds via a concerted three-center transition state in which little bond-making or bond-breaking had occurred. The activation enthalpy was attributed to a deformation of the square planar complex on its approach to the transition state. 

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