Selectivity in hydrogenation

The cylindrically chiral diphosphine neither changed, nor lost its reactivity and selectivity in hydrogenation reactions even after long exposure to air. In a 31P-NMR study, no detectable air-oxidation was observed even after a long exposure (3 weeks) to the atmosphere. The procedures for the synthesis of the chiral ligand and the asymmetric reaction described above are very simple, giving high enan-tioselectivity with many dehydroamino acids (Table 12.4). 

The selectivity for hydrogenation of dienes in the presence of monoolefins arises from the exceptional stability of jr-ally 1 complexes. In the case of Pt catalysts the reactions shown can compete with one another (equation 6)14. The second pathway is favored, especially when the olefin or diene must compete with excess ligands (phosphine, CO, SnCp ) for a coordination site. This is why the diene is almost completely hydrogenated before the concentration of olefin increases to the point that the olefin gains access to the catalyst. A similar phenomenon can be responsible for selectivity in hydrogenation of dienes with heterogeneous catalysts. 

The site-selective reduction of a,co-dienes, in which the two double bonds are differentiated only by the presence of an allylic substituent, is a challenging task. A pentamethylyttrocene complex that is extremely selective in hydrogenating terminal alkenes exhibits good selectivity in this reaction 142... 

An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene s pi orbital. 

Cooper, B. J. Platinum-Carbon Catalysts with Molecular Sieve Properties. Shape Selectivity in Hydrogenation Catalysis. Platinum Metals Rev, 14, 133 (1970). 

The ratio of 2,2-dimethyl butane to 2-methyl pentane produced by these reactions will be k5a/k5h. In general, chlorine atom is less selective in hydrogen abstraction reactions than are hydrocarbon free radicals and hence fc2a/ 2b > > k5a/k5h. Consequently, one would expect that the first increment of HCl would decrease the ratio of 2,2-dimethyl butane to 2-methyl pentane in the C6 alkylation product. 

Hydrogenations using heterogeneous catalysts usually require above ambient T and Hj pressures > 10 Pa. Homogeneous catalysts are often more selective and involve lower T (e.g., 25°C) and Hj (ca. 10 Pa). Highly useful selectivity in hydrogenation is shown in reactions of H2 with prochiral substrates in the presence of chiral catalysts, e.g., op-... 

Figure 11 Selectivity in hydrogenation of styrene/l-octene mixtures a test case octenes versus styrene conversion (aromatics hydrogenation always much less than 0.2%). (From Ref. 33.)...

THE EFFECT OF CARBONACEOUS DEPOSITIONS ON ACTIVITV AND SELECTIVITY IN HYDROGENATION OF AN UNSATLRATED ALDEHYDE... 

Thus, our results demonstrated that the trends of selectivity for hydrogenation of 1,4-butenediol and cyclopentadiene are in tight connection with preliminary chitosan chemical modification as well as mineral support nature. Pd/chitosan modified with 2-pyridinealdehyde deposited on SiOa demonstrated high selectivity in hydrogenation of cyclopentadiene into cyclopentene. 1,4-butynediol into cis-l,4-butenediol hydrogenation proceeded very selectively over Pd-Pb catalytic systems based on chitosan succinate form. 

Table 6.5 Relative selectivity in hydrogen abstraction from alkanes by halogen atoms at 300 Ka...

Table 1. Selectivity in Hydrogenation of Butynediol Using Pd/Ti02 Catalyst Cb = 20 g/L, T = 25 C, w = 5 g/L, solvent = water, 100 ml STR...

HIGH SELECTIVITIES IN HYDROGENATION OF HALOGENONITRO-BENZENES ON Pd, Pt OR RANEY NICKEL AS CATALYSTS... 

To directly reflect the enantiotopic selectivity in hydrogen atom abstraction in the ee of the resulting alcohol, the radical decay must be suppressed efficiently. To solve this problem, complex 11 in which the manganese ion is covered by the /-butyldiphenylsilyl group introduced into the phenyl substituent was synthesized [13b]. Good < es (84 and 83%) observed at the initial stage of the oxidation of 1,1-... 

Hydrogenation. The Pd-C(en) catalyst shows selectivity in hydrogenation such that an epoxide group is retained. The same system provides a means for the hydrogenation of alkenes that also contain an acid-labile TBS ether. ... 

Solvents and additives are known to influence activity and selectivity in hydrogenation reactions. In the present study we used four solvents having different dielectric constants (methanol, i-propanol, cyclohexane and 10% water in methanol) to check their influence on activity and selectivity over the most active Ni-Fe/Ti02 catalyst. The results are summarised in Table 2. 

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