Selectivity of hydrogenation

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

The catalytic alcohol racemization with diruthenium catalyst 1 is based on the reversible transfer hydrogenation mechanism. Meanwhile, the problem of ketone formation in the DKR of secondary alcohols with 1 was identified due to the liberation of molecular hydrogen. Then, we envisioned a novel asymmetric reductive acetylation of ketones to circumvent the problem of ketone formation (Scheme 6). A key factor of this process was the selection of hydrogen donors compatible with the DKR conditions. 2,6-Dimethyl-4-heptanol, which cannot be acylated by lipases, was chosen as a proper hydrogen donor. Asymmetric reductive acetylation of ketones was also possible under 1 atm hydrogen in ethyl acetate, which acted as acyl donor and solvent. Ethanol formation from ethyl acetate did not cause critical problem, and various ketones were successfully transformed into the corresponding chiral acetates (Table 17). However, reaction time (96 h) was unsatisfactory. 

Selection of hydrogen feedstock and production centres outside the EU... 

The selectivity of hydrogenation of dimethyl tetramethylbicyclo[2.2.0]hexa-2,5-diene-5,6-dicarboxylate depends on the catalytically active metal and on the bulk of the ester substituent (equations 59 and 60)147. 

Fig. 7, Percent selectivity of hydrogenative C, ring closure as a function of the hydrogen content of the carrier gas. Pulse system catalyst, 0.4 g platinum black T = 360°C. Starting hydrocarbons ( ) 3-methylpentane ( ) 3-methyl-1-pentene (T) tra . -3-methyl-2-pentene (A) di-2-methyl-2-pentene. Selectivity is expressed as methylcyclopentane (MCP) % in the total C5 cyclic product (MCP + MCPe) (55).

When alkadienoic acids were hydrogenated with [RhCl(PPh3)3] or [RhCl P(p-tolyl)3 3] catalysts an unusual effect of water was observed [65]. In dry benzene, hydrogenation of 3,8-nonadienoic acid afforded mostly 3-nonenoic acid. In sharp contrast, when a benzene-water 1 1 mixture was used for the same reaction the major product was 8-nonenoic acid with only a few % of 3-nonenoic acid formed. Similar sharp changes in the selectivity of hydrogenations upon addition of an aqueous phase were observed with other alkadienoic acids (e.g.3,6-octadienoic acid) as well. 

Detailed studies have been conducted on the selectivity of hydrogenation of naphthalene.67 The reaction proceeds via tetrahydronaphthalene as an intermediate and eventually yields a mixture of the isomeric decalins ... 

Figure 1 compares the selectivities of hydrogenation products as a function of UAL conversion for liquid-phase reaction over the two Ru catalysts. Both catalysts produced significant amounts of SAL. However, selectivity towards UOL, the desired product, increased threefold for the K-exchanged catalyst compared to Ru/NaY. 

From a thermodynamic point of view, the hydrogenation of the C=C bond is more favorable than that of the C=0 group. Since there are numerous other important processes to synthesize saturated aldehydes and ketones, the main objective of recent research efforts is to increase the selectivity of hydrogenation of unsaturated oxo compounds into unsaturated alcohols. The results summarized below clearly indicate the significant success achieved in the selective hydrogenation of unsaturated aldehydes. The hydrogenation of unsaturated ketones, in turn, cannot be accomplished yet with similar selectivities. 

Bortolini, O., Conte, V., Di Furia, F. and Modena, G. (1986) Metal catalysis in oxidation by peroxides. Part 25. Molybdenum- and tungsten-catalyzed oxidations of alcohols by diluted hydrogen peroxide under phase-transfer conditions. /. Org. Chem., 51, 2661. Barak, G. and Sasson, Y. (1989) Effect of phase-transfer catalysis on the selectivity of hydrogen peroxide oxidation of aniline. /. Org. Chem., 54, 3484. 

The regioselectivity observed in Eq. (6) with diprotonated quinoxaline (reaction performed in 96% H2S04) is of particular interest. Whereas in monoprotonated quinoxaline (Eq. 1) the C-2 carbon atom has the lowest electron density and substitution occurs at C-2 position only, in diprotonated quinoxaline (Eq. 6) the electron density of the equivalent C-2 and C-3 is as low as that of the equivalent C-6 and C-7 carbon atoms (NMR and IN DO calculations) [9] and substitution occurs at both C-2 and C-6 positions. To minimize polysubstitution, the conversions in Eq. (6) were limited to about 50 % (selectivity, based on the reacted bases, is >90 % in any case). Another feature of Eq. (6) is the exceptional selectivity of hydrogen abstraction from the C-5 position of w-hexyl derivatives by the aminium radical i-Bu2NH+, generated from i-Bu2NHCl+ and Fe(II) [2]. 

Economic and market analyses presented in this session appropriately complement previous technical sessions and address cmcial aspects that will determine which processes of nuclear hydrogen production will be developed beyond lab-scale experiments and may be ultimately commercialised. They contribute in particular to orient decisions to be taken in 2009 about the selection of hydrogen production process to be tested with the NGNP and about the creation of a consortium to support the next phases of the project. 

DOE-NE also proceeds with such analyses (H2A) to document the selection of hydrogen production processes retained for future work in the United States and possible demonstration with the NGNP. 

Fig. 15. Effect of degree of sulfuration of palladium on the selectivity of hydrogenation and isomerization of l-butene (T = 22°C, P = 10 bars thiophene A, H2S O, DMDS).

In general, the yields of the products follow the same trend as that of the methane conversion. The selectivities of the respective products did not vary much with either reactor size or residence time. Figure 3 shows that the selectivity of hydrogen has little variance between the different reactor sizes and residence times. The largest variance is 10% with experimental uncertainty accounting for at least 2-3% of this. 

A variety of levels of reporting hydrogen atom positions can be found in the literature. These are (1) no hydrogen atomic coordinates reported (in some journals, deposition is required) (2) a selection of hydrogen atomic coordinates are reported. These could be only those which could be calculated from the nonhydrogen positions, or only those observed on the difference maps, or a combination of both (3) all hydrogen coordinates reported (especially in more recent publications). Here again, practices differ. Some authors prefer to report calculated positions when they can be calculated, and observed positions only when they cannot be calculated. 

Fig. 6.25 Selectivity of hydrogen abstraction in the reaction of OH radicals with 2-propanol Asmus et al., 1973).

An example from micromolecular chemistry involves the selectivity of hydrogen and chlorine atoms in abstraction of hydrogen from propionic acid (7-1). Hydrogen atoms attack at carbon atom 2 more rapidly than at carbon 3, in accordance with the relative strengths of the... 

Site Selectivity of Hydrogen in Metals and Alloys.—Entropy Data for PdjH. The partial molar entropy (5h) of H in Pd and its alloys is of great interest because of the information which can be obtained concerning the nature and distribution of H atoms in the metal lattice. At low temperatures hydrogen atoms are randomly distributed over the octahedral sites in the lattice, of which there is 1 per Pd atom. However, at high temperatures it is possible that both tetrahedral and octahedral sites are partially occupied. 

Carbonaceous depositions on a nickel catalyst change the activity and selectivity of hydrogenation reactions. The source of carbonaceous residue is of minor importance. Similar changes in activity and selectivity are found after pretieating the catalyst with 2-ethylhexenal. cydopentadiene and carbon monoxide. 

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