Kinetics migratory insertion

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

The resting state of this catalytic system was found to be the dimer shown. The migratory insertion is the rate-determining step and not the oxidative addition of aryl halide to a palladium zero species, see Figure 13.17. These kinetics were found for phenyl iodide phenyl bromide already showed less clear-cut kinetics indicating that the oxidative addition is somewhat slower now. The system shown in Figures 13.16-17 gives at least half a million turnovers. 

The effect of solvent polarity on the rate of the individual steps was also deduced from a comparison of the kinetics determined by IR. It was concluded that, comparing MeOH/Mel (80 20 v/v) with CH2Q2/MCI (80 20 v/v), the overall increase in rate of reaction of [Rh(CO)2l2] with Mel to give [Rh(C(0)Me)( CO)I] included contributions due to enhancement of the forward rates of both oxidative addition (ca. 50%) and migratory insertion (ca. 100%). 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

Bianchini has reported that the migratory insertion reactions of [Pd(R)(CO)(P-P)]+ complexes (R = Me, Et) are reversible and follow first-order kinetics irrespective of the chelating diphosphine (P-P = dppp, dppe, meso-dppb, rac-dppb, meso-bdpp, rac-bdpp) [5e, f]. The free energies of activation for these reactions were calculated from the half-life times (tj 2) obtained by P( H HP NMR spectroscopy as all the rates of conversion of the methyl carbonyl complexes were independent of the CO pressure. Therefore, the AG values associated with the migratory insertion of the methyl carbonyl complexes could be straightforwardly calculated from the values using the equation AG = RT(ln k -ln kT/h) with = In First-order... 

Warming an equimolar solution of 41 and NGAr at —41 °G for 2h led to / -migratory insertion and formation of the cyclopentylmethylpalladium complex 42 in 96 10% yield ( H NMR) as a single diastereomer. Disappearance of 41 at —41 °G obeyed first-order kinetics to >3 half-lives with the following activation parameters AG = 16.9 0.1 kcal mol ... 

To explain this degradation, two different reaction pathways have been proposed (Scheme 29). The reaction may proceed by concerted reductive elimination or alternatively by migratory insertion, giving the intermediate (175) and subsequent heterolytic cleavage. Kinetic studies and DFT calculations supported the second proposed pathway. An... 

It is believed that the initial migratory insertion step does not involve N coordination . Although CO insertion into a metal-carbon bond is kinetically preferred, formation of... 

The migratory insertion of CO into the Ti-Me bond in Cp2TiMe2 has been investigated by both static and dynamic density functional theory calculations. CO coordination prior to insertion has been analyzed considering both lateral and central approaches, and the two pathways were found to be kinetically equivalent. The 0- outside r -bound acyl complex is more stable than the 0- inside isomer by 4.0kcal mol-1, with an isomerization energy barrier of 9.6kcal mol-1.1334... 

Electron poor alkynes are readily trapped by migratory insertion of the highly fluorinated phenyl ligand but competitive formation of 4-electron donor alkyne complexes is observed for electron rich alkynes [48]. If a CO ligand is removed by photolysis, migratory insertion is rapid at room temperature and detailed kinetic studies of the thermal reaction [49] have been reported. The above... 

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