Reactions migration

It is appropriate at this juncture to address some of the more useful transformations of 2,3-epoxy alcohols.913 A 2,3-epoxy alcohol such as compound 14 possesses two obvious electrophilic sites one at C-2, and the other at C-3. But in addition, C-l of a 2,3-epoxy alcohol also has latent electrophilic reactivity. For example, exposure of 14 to aqueous sodium hydroxide solution results in the formation of triol 19 in 79% yield (see Scheme 5). In this interesting transformation, hydroxide ion induces the establishment of an equilibrium between 2,3-epoxy-l-ol 14 and the isomeric 1,2-epoxy-3-ol 18. This reversible, base-induced epoxide migration reaction is a process known as the Payne rearrangement.14... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Since the migration reaction is always toward the end of a chain, terminal alkenes can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the alkene by heating with an alkene of molecular weight higher than that of the product (17-14). Photochemical isomerization can also lead to the thermodynamically less stable isomer. ... 

In the presence of transition-metal complexes, organic compounds that are unsaturated or strained often rearrange themselves. One synthetically useful transition-metal catalyzed isomerization is the olefin migration reaction. Two general mechanisms have been proposed for olefin migrations, depending on the type of catalyst employed (A and B) (Scheme 3.8).137... 

Fan, L., and T. Ziegler. 1990. The application of density functional theory to the optimization of transition state structures. I. Organic migration reactions. J. Chem. Phys. 92, 3645. 

In contrast, nucleophilic tantalum alkylalkylidene complexes show no tendency to undergo this migration reaction (15). 

Explain why the methyl migration reaction in CH3Mn(CO)5 has a rate when DMF is the solvent that is about 50 times that when mesitylene is the solvent. Give a reasonable estimate of the relative rate when the solvent is CHC13. Give a reasonable estimate of the relative rate when the solvent is n-C6H14. You may wish to consult... 

STEELE, C.L., GLJZEN, M., QUTOB, D., DIXON, R.A., Molecular characterization of the enzyme catalyzing the aryl migration reaction of isoflavonoid biosynthesis in soybean, Arch. of Biochem. and Biophys., 1999, 367, 146-150. 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

Y Sohma, M Sasaki, Y Hayashi, T Kimura, Y Kiso. Novel and efficient synthesis of difficult sequence-containing peptides through O-N intramolecular acyl migration reaction of O-acyl isopeptides. Chem Commun 124, 2004. 

There were several attempts to use homogeneous catalysts in the hydrogenation of fatty acid esters. Pt, Pd and Mo complexes were investigated. The monoene selectivity and the activity of Pt catalysts were good, but their separation from the product remained an unsolved problem. They were active in a double-bond migration reaction which, however, was accompanied by the disadvantageous cis-trans isomerization9,31. 

A second important migration reaction involves alkenes instead of carbon monoxide. Figure 2.6 gives a schematic representation of a hydride that migrates to a co-ordinated ethene molecule cis to the hydride. The figure shows the hydride migration, which may result in an empty space in the co-ordination sphere of the metal. This co-ordinative unsaturation can be lifted in two ways ... 

The migration reaction of hydrides to alkenes can be described as a 2+2 addition reaction. The reaction takes place in a syn fashion with respect to the alkene the two atoms M and H add to the same face of the alkene (Figure 2.8). This has been unequivocally established by experiments. Later we will see reactions where this is not the case although the overall stoichiometry is the same for both types. 

The detailed kinetics determine how this happens precisely. We don t know whether the complexation reaction or the insertion reaction is rate-determining. Theoretical work on insertion reactions of early-transition metal catalysts indicates that the complexation is rate determining and that the migration reaction has a very low barrier of activation. If the complexation is irreversible, it also determines the enantioselectivity. 

Crossover experiments have been used to establish that the novel N to C acyl migration reaction of acyclic imides (69), to give o -amino ketones (70), proceeds by intramolecular reaction of the base-generated carbanion. ... 

---