Hydrogenation reactions using formates

As the transfer hydrogenation reactions using Ru-TsDPEN catalyst could take place in aqueous solution [108-111], the next stage was to develop a polymeric catalyst suitable for the aqueous conditions. One such example was the use of PEG as a polymer support, as reported by Xiao [112]. The PEG-supported TsDPEN 176 (Scheme 3.54) was highly effective in the Ru(II)-catalyzed transfer hydrogenation of simple ketones by sodium formate in water. The same polymeric catalyst was also effective for the same reaction by using a formic acid-triethylamine azeotrope [113]. 

Bose and coworkers have described hydrogenation reactions using ammonium formate as hydrogen donor and Pd/C as catalyst for selective transformations of fS-lactams 1, as shown in Scheme 10.3 and Table 10.1 [4]. The authors report reaction times similar to those achieved with a preheated oil bath at 130 °C on a small scale on a larger scale, however, microwave-assisted reactions seem to proceed more rapidly. 

The great reactivity of the sulfurane prepared by this procedure toward active hydrogen compounds, coupled with an indefinite shelf life in the absence of moisture, makes this compound a useful reagent for dehydrations,amide cleavage reactions, epoxide formation, sulfilimine syntheses, and certain oxidations and coupling reactions. 

When the catalyst coordinates to the pyrazoline nitrogen and carbonyl oxygen at the step of 1-pyrazoline formation, desilylation or deprotonation takes place at the same position to give either Na or Nb, respectively. On the other hand, when the catalyst coordinates to the two carbonyl oxygens, the methine hydrogen derived from the acceptor molecule is deprotonated to give Nc. In the reaction using a Le-... 

One of the most used systems involves use of horseradish peroxidase, a 3-diketone (mosl commonly 2,4-pentandione), and hydrogen peroxide." " " Since these enzymes contain iron(II), initiation may involve decomposition of hydrogen peroxide by a redox reaction with formation of hydroxy radicals. However, the proposed initiation mechanism- involves a catalytic cycle with enzyme activation by hydrogen peroxide and oxidation of the [3-diketone to give a species which initiates polymerization. Some influence of the enzyme on tacticity and molecular... 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

Note Hydrogen is used in a supplementary role in reactions where reduction is not the primary function, for instance where it is necessary to prevent the formation of oxides or carbides and generally improve the characteristics and properties of the deposited material. 

Based upon the above-mentioned assumptions, the reaction scheme in Figure 3.1 is reduced to the scheme shown in Figure 3.2A. It should be noted that active catalyst is used in the reaction scheme in Figure 3.1 while most asymmetric hydrogenation processes use a pre-catalyst (11). Hence, the relationship between the precatalyst and active catalyst needs to be established for the kinetic model. The precatalyst used in this study is [Et-Rh(DuPhos)(COD)]BF4 where COD is cyclooctadiene. The active catalyst (Xq) in Figure 3.2A is formed by removal of COD via hydrogenation, which is irreversible. We assume that the precatalyst is completely converted to the active catalyst Xq before the start of catalytic reaction. Hence, the kinetic model derived here does not include the formation of the active catalyst from precatalyst. 

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