Isomerization during hydrogenation

The increase of the quantity of catalyst enhances the rate, but it does not influence the stereochemistry in the hydrogenation of phenol derivatives (6). The cis product formation is favored in acidic medium, and the trans product formation in neutral or alkaline medium (7). On Ru and Rh, about twice as much cis isomer is formed as trans isomer, whereas on Pt and Pd, the isomers are obtained in approximately equivalent amounts. Isomerization during the hydrogenation can be excluded (8). 

Unfortunately it is not known whether hydride and alkene coordinate to the same or different rhodium atoms in the dimer, but the former appears more probable in terms of our general mechanistic knowledge of hydrogenation. The observation of alkene isomerization during the reaction indicates that equation (34) is reversible. 

D. W. Harris and M. S. Feather, Evidence for a C-2- C-l intramolecular hydrogen-transfer during the acid-catalyzed isomerization of D-glucose to D-fructose, Carbohydr. Res., 30 (1973) 359-365. 

Catalytic activity in zeolitic materials is strongly influenced by the type of alkali metal cations, and maximum catalytic activity, e.g, in isomerization reactions, is explained by the formation of an imide species EuNH [305]. Synergetic effects were observed in bimetallic supported Si02 which showed considerable hydrogen uptake during hydrogenation reactions [307]. The formation of Ln-NH2, -NH, -N species seemed to be suppressed in the presence of transition metal powders and precipitation of elemental lanthanides is favored [309]. Lanthanide imides were favored as active species in the Ln/AC-mediated cyclization of ethyne and propyne [310]. 

Isomerization during hydrogenation may alter the functional groups present in the molecule. For instance, reduction of cyclohexen-2-ol over 5% platinum-on-carbon occurred with rapid absorption of 1 mole of hydrogen at substantially constant rate and quantitative formation of cyclohexanol. On the other hand, reduction over palladium ceased abruptly at about two-thirds of a mole, and the product was a mixture of cyclohexanol and cyclohexanone, the latter arising through double bond migration to the enol of cyclohexanone (29). 

Albright, L. and Wisniak, J., J. Amer. Oil Chem. Soc. 39, 14, 1962. Selectivity and Isomerization during Partial hydrogenation of cottonsead oil and methyl oleate Effect of operating variables. ... 

During the hydrogenation process, hydrogen is chemically attached at the double bond sites on the carbon chain of the unsaturated fatty acids. This reaction eliminates a double bond and converts an unsaturated fatty acid to a more saturated fatty acid. Isomerization during the hydrogenation process can also create frawi-isomers. Both of these chemical changes increase the melting point of the reacted oil. 

While various borohydride nickel ratios have been used in these nickel boride preparations maximum P-2 catalytic activity was observed with a 2 1 BH4 Ni ratio.However, when the reduction was run under a hydrogen atmosphere using a 4 1 BH4 Ni ratio, a hydrogenated nickel boride catalyst, the P-3 nickel boride, was obtained.27 This catalyst was somewhat more active than the P-2 catalyst for alkene hydrogenation but it induced signifieantly more double bond isomerization during the reaction. 

Organozinc reagents are useful for the transfer of alkyl groups, more so than their organotin analogs. Isomerization during the alkylation reaction, or elimination reactions due to the presence of a hydrogen on a /3-carbon to the metal, may lead to side reactions. Palladium catalysts are... 

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