Hydrogenation regioselectivity

Regioselective Hydrogenation- allylic and hornoallylie alcohols are hydrogenated faster than isolated double bonds... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

Scheme 52 explains the [(Cp )Rh(MeCN)3]2+-assisted regioselective hydrogenation of pyridines, benzoquinolines, acridines as well as indoles and benzothiophene.258 The relative hydrogenation rates were attributed to both electronic and steric effects, the rate generally decreasing with increasing basicity and steric hindrance at the nitrogen atom. 

Scheme 15.16 Regioselective hydrogenation of an unsymmetrical succinic acid derivative.

A special example for a regioselective hydrogenation in ionic liquids was reported by our group and by DrieRen-Holscher [96, 97]. Based on investigations in the biphasic system water/n-heptane, the ruthenium-catalyzed hydrogenation of sorbic acid to ds-3-hexenoic acid according to Scheme 41.3 in the system [BMIM][PF6]/MTBE was studied [98],... 

Scheme 41.3 Regioselective hydrogenation of sorbic acid in the biphasic system [BMIM][PF6]/MTBE.

A characteristic example for the application of homogeneous catalysts in enantiose-lective and regioselective hydrogenation of dienic compounds is the hydrogenation of geraniol and nerol to citronellol with Ru-BINAP catalyst (equation 8)26,27. The high enantiomeric excesses (96-98%), the nearly quantitative yields (>95%) and the very low catalyst/substrate ratio (1 50000) are attractive attributes of this process. 

Keywords Aqueous multiphase catalysis Regioselective hydrogenation Hydrodynamics Mass transport Kinetic modelling... 

The difference in reactivity of different double bonds allows selective hydrogenation of dienes. Conjugated dienes with at least one trisubstituted double bond may be completely reduced by using excess reagents222 [Eq. (11.71)]. Regioselective hydrogenation of the more substituted double bond occurs in 1,5-dienes. The hydrogenation of 1,4-dienes, however, is not selective [Eq. (11.72)] ... 

Regioselective hydrogenation of anhydrides to lactones. The last step in the synthesis of arylnaphthalene lignans is usually selective hydrogenation of an an-... 

NATURE OF THE ACTIVE SITES IN THE REGIOSELECTIVE HYDROGENATION OF OXIRANES... 

The same Pt/SiC catalysts were poisoned by CS2 the hydrogenation of (+)-apopinene was used as an indicator reaction (ref. 14). The amount of CS2 necessary to eliminate the hydrogenation activity permits calculation of the fraction of metal sites active in olefin hydrogenation. This fraction correlates with the number of step sites. Similarly, a good correlation is found between this fraction and the rate of methyloxirane transformation (Table 1). These results reveal that the structure-sensitivity is caused by the variation in the number of active sites, and the steps appear to be the active sites for the regioselective hydrogenation of methyloxirane. 

The regioselective hydrogenation of 96 with Raney nickel as the catalyst was investigated by Wakisaka.529 However, 48 was the major, and 28 was the minor isomer obtained (no yields being reported). 

Rothenberg, G., Yatziv, Y. and Sasson, Y. (1998) Comparative autoxidation of 3-carene and a-pinene factors governing regioselective hydrogen abstraction reactions. Tetrahedron, 54, 593. 

They are also selective for C=C over C=0. Highly regioselective hydrogenations are possible, for example,... 

A considerable degree of control of the regioselective hydrogenation of polycyclic aromatic hydrocarbons has been achieved through the choice of catalyst and optimized conditions. The review by Nishimu-ra and Takagi and Rylander s monographs furnish many examples and references. - ... 

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