Hydrogen, donor

Avecia have scaled-up this process to the 200 L scale with acetophenone and 1-tetralone. A number of batches were run with consistently high enantiomeric excess and yield, with an acceptable concentration (see Example 1 later). Other alcohols such as methanol and ethanol will also work but are typically less effective as the aldehyde byproducts can interfere in the reaction. Isobutanol is an effective hydrogen donor and others such as glucose will also work but cannot be used in such high concentrations. Isopropanol can be mixed with an inert solvent, including water, but the rates of reaction fall linearly as expected. 

A hydrogen donor for ketones and imines is formic acid and its salts. This reaction is always under kinetic control and no erosion of ee is observed. Carbon dioxide is generated as a byproduct and this gases out of the system preventing the re- 

Ratio of HCOOH/Et3N (mole) Time (h) Conversion (%) 

The TEAF reagent is acidic, and the triethylamine may act to buffer the pH which changes as formic acid is consumed during the reaction. An excess of formic acid is often used, and as will be discussed later on, it is preferable to charge TEAF during the reaction to ensure a high yield of product. Other salts of formic acid have been used with good results, for example sodium and potassium formate in water with the water-soluble catalysts discussed above. 

Rather surprisingly alcohols are poor at reducing imines, yet TEAF works well. During our studies we rationalized that the TEAF system was sufficiently acidic (pH approximately 4) to protonate the imine (pK l approximately 6) and that it was an iminium that was reduced to an ammonium salt [14]. When an iminium was used in the I PA system, it was reduced albeit with a low rate and moderate enan-tioselectivity. Quaternary iminium salts were also reduced to tertiary amines. Hydrogen will not reduce ketones or imines using the CATHy catalysts, but hydrides such as sodium borohydride have been shown to work. 

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Oxidoreduciases. Enzymes catalysing redox reactions. The substrate which is oxidized is regarded as the hydrogen donor. This group includes the trivially named enzymes, dehydrogenases, oxidases, reductases, peroxidases, hydrogenases and hydroxylases. 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

Direct Hydrogenation. Direct hydrogenation of lignitic and other coals has been studied by many investigators. Lignite can be slurried with an anthracene-oil solvent, heated to a temperature of 460—500°C with 1 1 CO H2 synthesis gas at pressures to 28 MPa (280 atm) in a 2 kg/h reactor. The product hquids are separated, and in a commercial process, a suitable hydrogen-donor solvent would be recycled (54). 

Frcc-Radical Reactions. Eree-radical reactions of maleic anhydride are important in polymeri2ations and monomer synthesis. Nucleophilic radicals such as the one from cyclohexane [110-82-7] serve as hydrogen donors that add to maleic anhydride at the double bond to form cyclohexylsuccinic anhydride [5962-96-9] (20) (63). 

Biosynthesis of coen2yme A (CoA) ia mammalian cells incorporates pantothenic acid. Coen2yme A, an acyl group carrier, is a cofactor for various en2ymatic reactions and serves as either a hydrogen donor or an acceptor. Pantothenic acid is also a stmctural component of acyl carrier protein (AGP). AGP is an essential component of the fatty acid synthetase complex, and is therefore requited for fatty acid synthesis. Free pantothenic acid is isolated from hver, and is a pale yeUow, viscous, and hygroscopic oil. 

In Example 1 the solute, acetone, contains a ketone carbonyl group which is a hydrogen acceptor, i.e., solute class 5 according to Table 15-4. This solute is to be extracted from water with chloroform solvent which contains a hydrogen donor group, i.e., solvent class 4. The solute class 5 and solvent class 4 interaction in Table 15-4 is shown to give a negative deviation from Raonlt s law. 

Magnitudes of /cg, /cp, /c, and indicate the importance of direct reactions with coal, where and are for hydrocracking reactions in the conversion process. Data for and from the experiments with HPO indicate that oil production from coal is increased by the use of a good hydrogen donor solvent. 

H2/Pd-C. If hydrogenation is carried out in the presence of (BOC)20, the released amine is directly converted to the BOC derivative. H2/Pd-C, NH3, —33°, 3-8 h, quant.When ammonia is used as the solvent, cysteine or methionine units in a peptide do not poison the catalyst. Pd-C or Pd black, hydrogen donor, solvent, 25° or reflux in EtOH, 15 min-2 h, 80-100% yield. Several hydrogen donors, including cyclohex-... 

The relative basicity of carbonyl oxygen atoms can be measured by studying strength of hydrogen bonding between the carbonyl compound and a hydrogen donor such as phenol. In carbon tetrachloride, values of for 1 1 complex formation for the compounds shown have been measured. Rationalize the observed order of basicity. 

One of the most common reactions of photoexcited carbonyl groups is hydrogen-atom abstraction from solvent or some other hydrogen donor. A second common reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group ... 

For a given hydrogen donor S—H, replacement by S—D leads to a decreased rate of reduction, relative to nonproductive decay to the ground state." This decreased rate is consistent with a primary isotope effect in the hydrogen abstraction step,... 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

Upon heating the enediyne la rearranges reversibly to the 1,4-benzenediyl diradical 2a, which in its turn can rearrange to the enediyne lb or—in the presence of a hydrogen donor (e.g. cyclohexa-1,4-diene)—react to the aromatic compound 3a. 

In hydrogen-transfer hydrogenations, various olefinic hydrogen donors are not necessarily equivalent, neither in selectivity nor in rate. The point is illustrated by selected data of Tabor et al. 97) on the transfer hydrogenation of dimethyl bicyclo[2.2,l]heptane-2.5-diene-2,3-dicarboxylate. 

Two hydrogen-transfer systems have been developed that also give good yields of hydroxylamines. One uses 5% palladium-on-carbon in aqueous tetrahydrofuran with phosphinic acid or its sodium salt as hydrogen donor the other uses 5% rhodium-on-carbon in aqueous tetrahydrofuran and hydrazine as donor. These systems are complementary and which is the better may depend on the substrate (36). The reductions cannot be followed by pressure drop, and both require analysis of the product to determine when the reduction should be terminated. 

In the reaction of naphthenes with olefins, naphthenic compounds are hydrogen donors. They can react with olefins to produce paraffins and aromatics (Equation 4-12). 

Usually metal-free phthalocyanine (PcH2) can be prepared from phthalonitrile with or without a solvent. Hydrogen-donor solvents such as pentan-l-ol and 2-(dimethylamino)ethanol are most often used for the preparation.113,127 128 To increase the yield of the product, some basic catalyst can be added (e.g., DBU, anhyd NH3). When lithium or sodium alkoxides are used as a base the reaction leads to the respective alkali-metal phthalocyanine, which can easily be converted into the free base by treatment with acid and water.129 The solvent-free preparation is carried out in a melt of the phthalonitrile and the reductive agent hydroquinone at ca. 200 C.130 Besides these and various other conventional chemical synthetic methods, PcH2 can also be prepared electrochemically.79... 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

A fast, unimolecular reaction can be used to excellent advantage. The rm-butoxyl radical offers the advantage that /3-scission occurs with a known rate constant. For Eq. (5-31), ki = 1.4 X 106 s-1 in water.8 In the presence of a hydrogen donor, AH, the competition is... 

The product distribution derived from the disproportionation of sulfonyl radicals is expected to be dependent on the conditions under which the reaction is being carried out thus, in hydrogen donor solvents, the formation of ArS020H should be important while at higher temperatures the formation of an aryl radical, namely... 

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