Hydrogenation preferential

The variation of the IR band intensities upon nitrogen incorporation for RF plasma-deposited a-C(N) H films is shown in Figure 26, as reported by Schwan et al. [53], As mentioned before, the intensity of the C—H stretching band decreases upon nitrogen incorporation, at the same time that an increase in the N —H stretching band intensity is observed. This suggests that hydrogen preferentially... 

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... 

In pyridylpyrrole derivatives the pyridine ring was hydrogenated preferentially, giving piperidylpyrroles, which were further hydrogenated to piper-idylpyrrolidines over platinum oxide in acetic acid at room temperature and 2 atm [453]. 

Pyrazole was hydrogenated over palladium on barium sulfate in acetic acid to 4,5-dihydropyrazole (A -pyrazoline), and 1-phenylpyrazole at 70-80° to 1-phenylpyrazolidine [478]. In benzopyrazole (indazole) and its homologs and derivatives the six-membered ring is hydrogenated preferentially to give... 

With a reliable access to 15 at my disposal (see Scheme 5), the construction of 25 was first attempted. Thus, 15 was carefully hydrogenated over 5% Pd-C to afford the dihydro-product 51, which expectedly dehydrated to give 25 in 32% overall yield (Scheme 11 It was apparent that the least hindered double bond had been hydrogenated preferentially. 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

In the reaction of peracetic acid with acetaldehyde (in the absence of oxygen) the majority of the methyl radicals abstract hydrogen, preferentially from acetaldehyde, to form methane ... 

XPS data suggest that the surface of the CVD catalysts is finely covered by a homogeneous silica layer except for Pt particles, where small holes are formed. Since hydrogenation exclusively occurs on Pt particles, selectivity is brought about by steric hindrance around the Pt site in the holes. Less hindered double bonds, consequently, are hydrogenated preferentially. 

Because the rate of alkene hydrogenation generally decreases as the number and size of the substituents on the double bond increase, the least hindered double bond should be hydrogenated preferentially in the competitive hydrogenation of olefin mixtures. This selectivity, however, is not always observed either because of diffusion constraints or the presence of concurrent double-bond isomerization. Isomerization modifies the olefin structure, which changes the alkene reactivity and makes reaction selectivity difficult to attain. 

The selectivity observed in the hydrogenation of pyrroles is strongly dependent on the catalyst and conditions used for the reactions. The pyrrole ring in nicotyrine, 42, was hydrogenated preferentially over palladium in acetic acid, 7 but over platinum the pyridine ring was saturated first (Eqn. 17.42), 48,149 K.eto and ester groups on an N-unsubstituted pyrrole were... 

The nitro group of 4-nitropyridine-N-oxide (1) was hydrogenated preferentially over palladium in neutral medium -2 but in acid the N-oxide was also reduced (Eqn. 19.1).2 Hydrogenation of 1 or 4-nitroquinoline-N-oxide over Raney nickel at elevated temperatures and pressures resulted in the reduction of both the nitro group and the N-oxide.3.4 Hydrogenation of 2-nitropyridine-N-oxide gave only 2-aminopyridine.5... 

The effects of the nature of the alcoholic solvent on the activity and diastereoselectivity, have been studied on catalysts Rh/C and Rh/Al O, pretreated at 100°C (Table 2). Without EDCA, the nature of the solvent had little effect on the activity of Rh/C and Rh/AljO, catalysts. In the presence of EDCA, the initial rates on both catalysts were in the following order MeOH > iPrOH > EtOH. In methanol, a gradual deactivation was observed, so that the reactions could not be run to completion. In the presence of EDCA, the d.e. were clearly lower in isopropanol since a value of ca. 25% was observed compared to ca. 42% and 68% over Rh/C and Rh/AljO, respectively. This could be partly due to the formation of larger amounts of intermediate cyclohexenic compound 6 in isopropanol, which was hydrogenated preferentially into 2, thus decreasing the final d.e. values. 

As noted in Section 2.5.1.1.1.1.3.2., enol ethers and esters are less reactive than compounds with isolated double bonds. Double bonds in conjugation with an enol ether are also hydrogenated preferentially. Thus, enol lactone 5 is hydrogenated over palladium in a regio- and diastereoselective manner in excellent yield to give the partially saturated enol lactone 67. 

Some further evidence that the smallest particles of nickel may adsorb hydrogen preferentially, if not by a different mechanism, is shown in Fig. 6, in which the fractional change of magnetization is plotted against volume of hydrogen adsorbed. (The more familiar type of pressure-volume isotherms for the identical samples is shown in Fig. 7). These data are for a... 

Group VIII metals supported on silica are known to hydrogenate preferentially the C=C double bond of a,B-unsaturated aldehydes (as e.g. crotonaldehyde). Only for a few metals, like Os and Co, a significant activity for the hydrogenation of the C=0 group was reported [1]. Improved selectivity for carbonyl group hydrogenation was described for platinum catalysts modified by Sn [2,3,4], Fe... 

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