Formates as hydrogen donor

Bose and coworkers [10] have described hydrogenation using ammonium formate as hydrogen donor and Pd/C as catalyst for selective transformations (Tab. 8.1) of /7-lactams 10, as shown in Scheme 8.6. 

Cinnamic acid and its derivatives were reduced in high yields under mild conditions using ammonium formate as hydrogen donor, in the presence of palladium(II) as catalyst in ionic liquids.294... 

Sheep rumen contents are known to detoxify pyrrolizidine alkaloids. A new, obligate anaerobe, Peptococcus heliotrinreductans, has been identified from the rumen.74 It is a Gram-positive coccus, and reduces 1,2-dehydropyrrolizidines, using hydrogen gas or formate as hydrogen donors. 

Homogeneous catalytic hydrogenation over tetrakis(triphenylphosphine)palladium using hydrogen or sodium formate as hydrogen donors converts p-bromostyrene to styrene in 37% and 35% yields respectively (the other products being polymers) ... 

Hydrodehalogenation of aliphatic or benzylic halides were catalyzed by water soluble ruthenium phosphine complexes in the presence of sodium formate as hydrogen donor [194], Hydroxycarbonylations could also be performed with high palladium catalyst activities in biphasic systems [195-197]. 

Under the conditions of Table 3.10, HCOOH is decomposed and the yield of CHC13 reaches only 3.5% (in contrast to HCOONa, where the 478 TON corresponds to 60 % conversion). The use of 5 bar H2 instead of formate as hydrogen donor leads to 1.8 % conversion. The reason for this low conversion can be the low concentration of dissolved hydrogen (<0.004 M under the reaction conditions) as opposed to that of formate (1.67 M) such a limited reactivity has also been observed in related non-aqueous systems [270]. It is worth mentioning that Na2HPC>2 also decomposes during the reaction as indicated by the pressure rise in the reactor, however, the conversion oi CCl4 still reaches 31 %. 

Preparation from l-(4-benzyloxy-3-methoxyphenyl)-2-phenylethanone (m.p. 136-138°) by catalytic hydrogen transfer using ammonium formate as hydrogen donor and 10% Pd/C catalysis in refluxing methanol for 30 min (94%) [5193]. 

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... 

Other salts of formic acid have been used with good results. For example, sodium and preferably potassium formate salts have been used in a water/organic biphasic system [36, 52], or with the water-soluble catalysts discussed above. The aqueous system makes the pH much easier to control minimal COz is generated during the reaction as it is trapped as bicarbonate, and often better reaction rates are observed. The use of hydrazinium monoformate salts as hydrogen donors with heterogeneous catalysts has also been reported [53]. 

Methyl 3,4,5,6-tetra-0-acetyl-2-deoxy-2-diazo-D-arabino-hexonate (84) has been irradiated in methanol and in 2-propanol.177 In methanol, the only photoproduct is the enol acetate 85 however, irradiation in 2-propanol results in formation of minor proportions (6%) of 85 and the alkene 86 (7%), but the major product is the deoxy sugar 87 (61%). The difference in reactivity of 84 in these two solvents is probably a reflection of the difference in the ability of methanol and 2-propanol to function as hydrogen donors when reacting with a carbene (see Scheme 31). In methanol, a 1,2-hydrogen atom shift to the divalent... 

One of the methods used in the production of liquid fuels from coal is to heat coal in the presence of solvents in order to dissolve and stabilize low molecular weight fragments. Many studies (1-6) have been devoted to elucidating the chemical mechanism of product formation in hydrogen donor and non-donor solvents. In most of these studies, the time dependence of the product yield was used as a measure of the rates of reaction, or the product yield was correlated with the solvent, the rank of the coal or other properties of the coal. In order to gain a better understanding of the nature of the coal-solvent interactions, we... 

Promise is held in MPV reactions carried out under catalytic conditions. Instead of, for example, stoichiometric amounts of aluminum as the metal ion activator, catalytic quantities of complexes of rhodium and iridium can sometimes be used to bring about the same reactions. Although the catalytic mechanisms have not been established, postulation of the usual six-membered transition state in the critical step of hydride transfer appears reasonable. The strongly basic conditions of the MPV reaction are avoided. Reductions of aryl ketones (69 equation 30) using (excess) isopropyl alcohol as hydrogen donor and at partial conversions have led to the formation of alcohol (70) in modest enantiomeric excesses with various chiral ligands. " ... 

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