Isomerization, configurational with hydrogenation catalyst

A double bond in the 7,8-(VI) or 8,9-(VII) position of a steroid which has the trans A/B ring configuration isomerizes to the 8,14 position (VIII) when treated with hydrogen and a palladium catalyst or platinum in the presence of acetic acid (69) (Fig. 12). Once the double bond reaches the 8,14 position it cannot be hydrogenated however it may be isomerized by treatment with HCl in chloroform to yield a isomer which can be reduced catalytically. 

When the hydrogenation function is embedded in the crystal voids of an MFI topology, the formation of trans-isomers is strongly reduced. After partial reduction of soy bean oil with such catalyst from an iodine value of 140 to 80, virtually no trans-isomers are obtained (56). This is the result of pore mouth catalysis combined with zeolite shape selectivity. Due to the bent character of the cts-isomer chains in triglycerides, trans-configured chains preferentially enter the pore mouths for hydrogenation. In this environment, metal-catalyzed cis-trans isomerization is restricted for steric reasons as multiple readsorption is minimal. 

Hydrogenation of epoxides lends itself well to both synthetic applications and mechanistic studies. The reaction is complex, for either carbon-oxygen bond may break with or without inversion of configuration, and the product may contain deoxygenated products (92,93) as well as ketones derived by isomerization (26). The reaction is especially sensitive to both catalyst and environment (74). 

In contrast to the high enantioselectivity achieved for the Z-isomeric substrates, hydrogenation of the S-isomeric substrates usually proceeds at a much lower rate and gives poor enantioselectivities [86]. With the Rh-BINAP system as the catalyst and tetrahydrofuran (THF) as solvent, hydrogenation of the Z-and S-isomeric substrates generates products with different configurations [2]. 

Another possibility is based on a model of active sites proposed by Hubaut et al. (138), which derives from a previous investigation by the same group (139) (Fig. 40). In this model, the reactions involved in hydro-treating on Mo/Al catalysts are associated with the CUS configuration. Hydrogenation of aromatic compounds (toluene, pyridine, etc.) and isomerization of dienes require a or site, whereas sites... 

Partial catalytic hydrogenation of polyynes to give polyenes with cw configuration has been accomplished with a variety of substrates. Palladium catalysts, particularly the Lindlar catalyst ", are generally the most satisfactory. Side-reactions include over-hydrogenation, isomerization of cis to Irons isomers and double-bond migration. 

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