Isomerization during oxidation, selectivity

Imidazolium dichromate is a selective oxidant for allylic and benzylic hydroxy groups. (Allylic alcohols are oxidized faster than benzylic alcohols.) The selectivity over saturate alcohols is similar to that of 4-(dimethylamino)pyridinium chlorochromate. DMF is recommended as the solvent for oxidations, since it appears that the choice of solvent is critical to obtaining high yields. This reagent has also been observed to cause some ( )/(Z)-isomerization during the oxidation of allylic alcohols. 

However, due to the artifacts resulting from oxidation, hydrolysis of esters or ethers, or isomerization of phenolics during pretreatment of wines, as well as due to the low recovery rates of some phenolics, analysis of wine phenolics via direct injection of the filtered wine into the chromatographic column is often selected (80,82-84). For the red wine and musts (80), which were injected directly into the HPLC without sample preparation, a ternary-gradient system was often employed for phenolic compounds. Twenty-two phenolic compounds, including 10 anthocyanins, were analyzed from red wine. The separation of cinnamic acid derivatives (313 nm),... 

One further complication concerning the origin of selectivity changes during structural isomerization reactions has been indicated by the work of Kramer and Zuegg.352 They observed that the percentage of n-hexane produced from methyl cyclopentane increases with the amount of interface between Pt and the support. They propose that isomerization occurs by two parallel routes, one on the metal the other at the metal/oxide interface. Such effects, if confirmed, should be more important for small metal particles, and may have influenced the selectivity observed in the other work quoted above. 

A complete restoration of the low level of isomerization selectivity was not possible by oxidation and low-temperature reduction (at 300°C). Hence, it was thought that some oxidized silicon species stay on the metal surface after regeneration (207). Suggested mechanism of interactions between Pd and Si02 during reduction at high temperatures is presented in Fig. 18. 

For reactions that occur with different selectivities or kinetics on the induced sites, the implications are far more complex. Teichner has found that the selectivities for the isomerization of methylcyclopropane and for cracking of benzene and cyclohexadienes may be substantially different on activated silica or alumina than on the normal oxides (see above). In these cases, a separate rate expression may need to be included. Depending on the rate constants and relative kinetics, this can substantially change the reaction-rate expression. Further, differences in the activation energies may affect the contribution at different temperatures. If the reaction temperature is sufficient, the activation of the support may be able to occur during the course of the experiment. 

In the first approach shown in Scheme 9, ketoester 77 was alkylated successively with 4-bromobutene and 1,3-dibromopropene. After decarboxylation, 78 was converted into iV-aziridinylimine 79 in good yield. The pivotal radical cyclization reaction proceeded smoothly to produce a mixture of isomeric propellane compounds 80, which was purified after the epoxidation step. For the synthesis of modhephene, the mixture of epoxides was rearranged into the corresponding allylic alcohols 81 and then the allylic alcohols were oxidized, giving a separable mixture of unsaturated ketones 82a and 82b. The major product 82a possessed the correct stereochemistry of the methyl group of modhephene. Since 82a had already been converted into modhephene, a formal total synthesis of dZ-modhephene has thus been completed. The isomeric ratio of 80 reflects the stereoselectivity during the radical cyclization reaction. The selectivity was very close to the ratio reported by Sha in his radical cyclization reaction. ... 

By using PDC as a suspension in dichloromethane it becomes a selective oxidant for the preparation of aldehydes, saturated or unsaturated. Allylic alcohols are oxidized faster than saturated alcohols, but some ( )/(Z)-isomerization has been observed during the preparation of a,3-unsaturated aldehydes with PDC in dichloromethane. 

Chromium(VI) oxide can be used as a catalytic oxidant for alcohols with r-butyl hydroperoxide as the cooxidant.This reagent iq>pears to be selective for allylic and benzylic over satura alcohols, though ( )/(Z)-isomerization has b n observed during the preparation of a, -unsaturated aldehydes. This reagent is also a good oxidant for allylic and benzylic C—H bonds these may be competing pathways in more sophisticated substrates. ... 

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