Alkene dissociation

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

Neutral lanthanide complexes are convenient models for the cationic zirconocene systems and avoid complications due to the presence of counteranions and the limited solubility of ionic compounds. Dynamic NMR studies on yttrium complexes 44-46 has allowed the determination of the alkene binding enthalpy, the activation enthalpy of alkene dissociation, and the relative rates of dissociation and alkyl site exchange (site epimerisation) (Scheme 8.20). Compared to the Zr... 

Generally the detailed mechanism will include steps such as precoordination of the alkene, dissociation of an auxiliary ligand, etc. Bercaw has noted that most commonly the actual hydrometallation step is not rate determining. Since the most important hydrometallation reagent is an insoluble polymer, it is easy to see why mechanistic understanding has not kept pace with applications in this field. 

The catalytic cycle starts with oxidative addition of 73b to palladium(O) (77 78), followed by enantiofacial alkene-coordination to form the diastereomeric complexes (R)-79 and (S)-79 (78 (R)-79 and 78->(S)-79). In both cases, sy -selective migratory insertion R)-79— R)-S0 and (S)-79 (S)-80) and subsequent stereospecific P-hydride elimination furnishes a pair of diastereomeric alkene-hydridopaUadium(II) complexes (R)-81 and (S)-81 ((R)-80 (R)-81 and (S)-80—> (S)-81). The Ji-bound Pd—H fragment in complex (S)-81 dissociates rapidly ((S)-81—>77), whereas (R)-81 undergoes hydropaUadation but with reversed regjoselec-tivity ((R)-81—>(R)-82) P-hydride elimination and alkene dissociation then affords... 

In other cases, it has been shown that isomeric cobalt-alkyl complexes interconvert without dissociation of alkene from the hydridocobalt-alkene complex. For example, isomerization through a tertiary alkyl intermediate without alkene dissociation is required by retention of optical activity and deuterium labeling in the products of hydroformylation of the non-racemic alkene in Equation 17.5. - These results are consistent with extensive, rapid, intramolecular isomerization of the intermediate cobalt alkyl to place the metal at terminal and internal positions along the alkyl chain. 

At -20°C 15a and 15b demonstrated rapid exchange with each other and the free alkene 14-[Ds], thus indicating fast equilibration between isomers via alkene dissociation/association. This evidently means that other less stable isomers may be kinetically accessible. 

The isomerization is intramolecular and a mechanism involving alkene dissociation with a four-co-ordinate intermediate is suggested. Stereochemical non-rigidity of the caged phosphite complexes [MH P(OCH2)3CPr 4] has been studied. ... 

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