Hydrogenation reactions during donor solvent

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

Data for the kinetics of coal liquefaction have been published in the literature (1-11). A review of the reported studies has recently been given by Oblad (12). The reported data were mostly obtained in bench-scale reactors. Guin et al. (7) studied the mechanism of coal particle dissolution, whereas Neavel (7), Kang et al. (8), and Gleim (10) examined the role of solvent on coal liquefaction. Tarrer et al. (9) examined the effects of coal minerals on reaction rates during coal liquefaction, whereas Whitehurst and Mitchell (11) studied the short contact time coal liquefaction process. It is believed that hydrogen donor solvent plays an important role in the coal liquefaction process. The reaction paths in a donor solvent coal liquefaction process have been reviewed by Squires (6). The reported studies examined both thermal and catalytic liquefaction processes. So far, however, very little effort has been made to present a detailed kinetic model for the intrinsic kinetics of coal liquefaction. 

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

Most researchers distil solvents for radical reactions in the same manner as they might for use with a reactive organometallic. This practice is recommended to ensure that solvents are sufficiently pure. That the solvents are simultaneously dried during purification is of little consequence. Water is a much poorer hydrogen atom donor than any common solvent (conversely, the hydroxyl radical is a powerful hydrogen atom abstractor). Thus, the presence of trace quantities of water will have no adverse effect on most radical reactions. As a corollary, water can be a useful solvent or cosolvent provided that the reagents or substrates are not susceptible to hydrolysis or protonolysis. 

Reactive solvents dissolve coal by active interaction. Such solvents are usually hydrogen donors (e.g., tetralin, 1,2,3,4-tetrahydronaphthalene) and their chemical composition is affected appreciably during the process. Again, using tetralin as the example, the solvent is converted to the aromatic counterpart (in this case, naphthalene) and the products from the coal can vary in composition, depending on the reaction severity and the ratio of the solvent to the coal. In addition, the extracts differ markedly in properties from those obtained with degrading solvents. 

The reaction is most probably initiated by radical attack of a reduced carbonyl function on the aromatic ring in the adjacent system. The product is formed as the radical anion but reoxidized by air during work-up. In the presence of proton donors, or in alcoholic solvents, reduction of 98 gives a mixture of acyclic and partly hydrogenated cyclic products [288]. Substituted 98, such as the 4,4, 5,5 -tetracarboxylic acid, gives coupling in basic alcoholic medium but not in DMF [289]. 

Rhodium l(R)-amino-2(S)-hydroxyindanyl pentamethylcyclopentadienyl chloride was selected from a screen of catalysts as giving the highest rate and enantiomeric excess for this reaction. A further screen showed isopropanol was the best hydrogen donor and solvent. In the laboratory a closed vessel was used and without actively removing the acetone, the maximum concentration that could be achieved with high conversion was 0.05 M, and during the reaction the enantiomeric excess fell markedly. 

Superoxide is believed to react as an effective hydrogen atom acceptor. Thermodynamically, it is one of the weakest oxidants in Table 9.10, but the electron transfer alternative is highly disfavored by the inaccessibility of 022 in aqueous solution Taube estimates that H02- has a pXa value of 21.110 Thus, a HAT mechanism is inferred for the oxidations of [Con(sep)]2 +, [Fen(tacn)2]2 +, [Run(tacn)2]2 +, and [Run(sar)]2 + nl in these reactions the ligands are the hydrogen atom donors, but it is the metal centers that are oxidized. The bimolecular disproportionation of H02 is another likely example of hydrogen atom transfer this mechanism may also apply to the disproportionation via the reaction of H02 with 02, 27 Curiously, the reaction of 02- with the hydrated electron is fast and seems not to be pH dependent. This latter reaction may yield genuine 022- as an intermediate, or it may be a form of PCET where the solvent donates a proton during the addition of the electron. 

The same conclusions were drawn from infrared and Raman work ". Data on the chloride ion donor strength of the oxyhalide solvents is, however, not available. One of the main difficulties is the removal of the last traces of water and its elimination during the reactions. Although experimental methods have been considerably improved, it must be born in mind that apparently nobody has ever been successful in working in the complete absence of moisture or of hydrolysis products. Traces of water appear to remain even in reactive liquids, such as the oxychlorides under consideration. It seems that in phosphorus oxychloride small amounts of water form HaO+Cl" which is dissociated in the solution and contrasts with the behaviour of anhydrous hydrogen chloride when dissolved in the same solvent. Thus purified phosphorus oxychloride contains approximately moles of water per liter and this must be taken into consideration, when the pure solvent or when dilute solutions are considered. The assumption of the seK-ion-ization equilibria in the liquid sol vents, 79... 

Dibromaniline in tetrahydrofuran added dropwise during 1-2 hrs. to a refluxing soln. of 1.5-2 moles amyl nitrite in the same solvent 1,3-dibromo-benzene. Y 87%. - Tetrahydrofuran serves as hydrogen donor in this one-pot reaction, which does not involve separate diazotization. F. e. s. J. I. G. Cadogan and G. A. Molina, Soc. Perkin I 1973, 541 hydroquinone as hydrogen donor cf. R.N. McDonald and J. M. Richmond, Chem. Commun. 1973, 605. 

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