Allyl alcohols homogeneous hydrogenation

RESOLUTION OF RACEMIC ALCOHOLS AND RELATED SUBSTRATES. Under certain conditions, enantiomers react at sufficiently different rates and they provide a chemical means to resolve racemates (Scheme 31) (64). The Sharpless epoxidation efficiently resolves allylic alcohols that have flex-le structures (64, 65). Homogeneous hydrogenation with (/ )- or (S)-... 

As Table 1 shows, several bimetallic DENs have been employed as homogeneous catalysts, predominately for hydrogenations. The most detailed studies have been with allyl alcohol hydrogenation and the partial hydrogenation of 1,3-cyclooctadiene (1,3-COD) test reactions. In these studies, turnover fre-... 

Homogeneous hydrogenation of chiral allylic and homoallylic alcohols is accomplished with a high degree of diastereoselectivity, as shown in the hydrogenation of 3-phenylbut-3-en-2-ol (14) and 4-phe-nylpent-4-en-2-ol (16) in the presence of (15). ... 

Compound A is known as Wilkinson s catalyst and was oneof the first of the homogeneous catalysts to be developed. The cationic rhodium complex B is a useful catalyst for hydrogenations that involve a chelated structure between the reactant and the catalyst, such as allylic alcohols. The catalyst is activated by the hydrogenation of the norborna-... 

The most important characteristic of heterogenized homogeneous catalysts is their stability. The stability of polyvinylpyridine complexes of palladium was studied during the hydrogenation of sequential portions of allyl alcohol. On all the examined samples, the hydrogenation of 10-30 substrate portions occurs without a significant rate decrease (Fig. 25). However with Pd black each subsequent sample is hydrogenated at a lower rate than a previous one. 

Together with hydrogenation and isomerization, epoxidation completes the trio of commercially significant applications of enantioselective homogeneously catalyzed reactions. Stereospecific olefin epoxidation is distinctive in that it creates two chiral centers simultaneously. The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with tert.-butyl hydroperoxide and titanimn isopropoxide in the presence of optically active pure tartrate esters (Eq. 3-14). 

Figure 4.6. Internal asymmetric induction in homogeneous hydrogenation of allylic and homoallylic alcohols.

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