Insertion, migratory apparent

Several cases have been reported in which organometallic Lewis acids promote the migratory insertion reaction, apparently by initial attack on the oxygen of coordinated CO (220). An example of this type of reaction is given in Eq. (49). Two types of products can be obtained in these reactions those... 

The enantioselectivity determining step. Above we learnt that the oxidative addition of hydrogen is the rate-determining step. This step is irreversible and it also determines the enantioselectivity. This complex could still epimerise via substrate dissociation, but apparently it does not and migratory insertion is faster than epimerisation. We remember that two diastereomeric intermediates are involved, the major and the minor species and the minor species is the... 

While the main carbonylation cycles are now understood in considerable detail for these apparently simple catalytic systems, there will undoubtedly be considerably more work done on these and related sytems to understand the factors influencing the principal steps of oxidative addition, migratory insertion and reductive elimination and, in particular, further work to understand the unwanted reactions that lead to by-products. 

Still more problematic is the apparent migratory insertion of nitric oxide into transition metal-carbon bonds, an important reaction in metal nitrosyl complexes and one that may be relevant to biochemical reactions (7). On the evidence of isotopic labeling and kinetic experiments, the insertion of NO into the C0-CH3 bond of the (cyclopentadie-nyl)cobalt complex (i75-C5H5)Co(NO)(CH3), which occurs in Reaction (3),... 

Analogous reaction of Pt(Me)(OMe)(dppe) with CF2=CF2 in THF to give 6-methoxytetrafluoroethyl complex was proposed to involve migratory insertion step, without dissociation of OMe ion, though no direct stereochemical evidence has been provided [51], Apparent migratory insertion of alkenes to metal-hydroxo bond has also been observed in other metal complexes such as those of Ir [52]. 

Furthermore, numerous experimental data confirm that facile migratory insertion is possible only when a coplanar orientation of the double bond and P-Rh-H " is achieved in a dihydride intermediate (e.g.. Figure 1.4). Apparently, the migratory insertions with alternative coordination of the double bond are higher in energy, but computations of such steps were not tried so far. Nevertheless, only four possible pathways must be considered as competitive (e.g.. Scheme 1.34). 

It is obvious that the chirality will be eroded if the p-hydride elimination occurs at the chiral a-position of tosyUiydrazones. The selectivity of p-hydride elimination in this reaction can be interpreted as follows. Alkylpalladium complex, the intermediate generated after migratory insertion of palladium carbene, is favorable to afford 1,1-disubstituted chiral olefin (Fig. 18, path a) because of the less steric interactions in the transition state at p-hydride elimination. Apparently the alternative syn p-hydride elimination will lead to the erosion of chiral center. However, the latter pathway is not preferred because it leads to the eclipse of the bulky substituent with the methyl group of the substrate (Fig. 17, path b). 

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