Solvents ethanol, olefin hydrogenation

Sodium hydrogen telluride, (NaTeH), prepared in situ from the reaction of tellurium powder with an aqueous ethanol solution of sodium borohydride, is an effective reducing reagent for many functionalities, such as azide, sulfoxide, disulfide, activated C=C bonds, nitroxide, and so forth. Water is a convenient solvent for these transformations.28 A variety of functional groups including aldehydes, ketones, olefins, nitroxides, and azides are also reduced by sodium hypophosphite buffer solution.29... 

The selective hydrogenation of the olefinic double bond in the acetylated carbohydrate C-nitroolefins, using palladium black as catalyst, depends to a marked degree on the solvent employed. In absolute ethanol a sharp decrease in the rate of hydrogen absorption usually occurs when the double bond has been saturated. This decrease in rate is not sharply defined in 95 percent ethanol and is not detectable when absolute ethyl acetate is employed as the hydrogenation solvent. 

When primary alkyl phenyl tellurium or secondary alkyl phenyl tellurium compounds in methanol were treated with an excess of 3-chloroperoxybenzoic acid at 20, the phenyltelluro group was eliminated and replaced by a methoxy group. This reaction, which converts alkyl halides used in the synthesis of alkyl phenyl telluriums to alkyl methyl ethers, produced the ethers in yields as high as 90%3-4 Olefins are by-products in these reactions4 With ethanol as the solvent, ethyl ethers were formed. Other oxidizing agents (hydrogen peroxide, ozone, (ert.-butyl hydroperoxide, sodium periodate) did not produce alkyl methyl ethers. 

In the hydrogenation of methylenecyclohexane, 16, over a palladium catalyst, 1-methylcyclohexene, 17, is also produced by double bond isomerization (Eqn. 5.4). >5 In an ethanol solution both of these isomers are hydrogenated over palladium at essentially the same rate but, as depicted in Fig. 5.1, when a 1 1 ethanohbenzene solvent is used the rate of hydrogenation of 17 is decreased but the hydrogenation of 16 is not affected initially, but drops off significantly after only a short reaction time. At this point analysis of the reaction mixture showed that the only olefin present was 17 which, as shown by the bottom curve in Fig. 5.1, was hydrogenated very slowly under these conditions. These results indicate that benzene evidently competes with 17 for adsorption sites on the catalyst but that 16 is more readily adsorbed than is benzene. > 5... 

The P-2 Ni(B) is an effective catalyst for the saturation of double bonds.68> With this catalyst, terminal alkenes are hydrogenated with greater than 95% selectivity in the presence of di- and tri-substituted olefins in ethanol solution at room temperature and atmospheric pressure.69 Solvent Effect... 

Only limited data are available for the kinetics of oxo synthesis with the water-soluble catalyst HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [115]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures. The lack of data is obvious and surprising at this time, when the Ruhrchemie/ Rhone-Pou-lenc process has been in operation for more than ten years [116]. Other kinetic studies on rhodium-catalyzed hydroformylation have been published, too. They involve rhodium catalysts such as [Rh(nbd)Cl]2 (nbd = norbomadiene) [117] or [Rh(SBu )(CO)P(OMe)3]2 [118], or phosphites as ligands [119, 120]. 

PMHS can also be used with a catalytic quantity of 5 % palladium on charcoal to effect hydrogenation of terminal and cur-olefins, aromatic nitro compounds, and aromatic aldehydes. Ethanol (95 %) containing one drop of concentrated HC1 is used as solvent the reaction is conducted at 40-60° (caution the hydrogenations are sometimes exothermic). This method provides a convenient form of low-pressure hydrogenation. 

The use of catalysts based on polymers with inverse temperature solubility, often copolymers of TV-isopropy-lacrylamide, to allow recovery by raising the temperature to precipitate the polymer for filtration,9 was mentioned in Chap. 5. The opposite, if the catalyst is soluble hot, but not cold, has also been used in ruthenium-catalyzed additions to the triple bonds of acetylenes (7.1).10 The long aliphatic tail of the phosphine ligand caused the catalyst to be insoluble at room temperature so that it could be recovered by filtration. There was no loss in yield or selectivity after seven cycles of use. A phosphine-modified poly(A-iso-propylacrylamide) in 90% aqueous ethanol/heptane has been used in the hydrogenation of 1-olefins.11 The mixture is biphasic at 22°C, but one phase at 70°C, at which the reaction takes place. This is still not ideal, because it takes energy to heat and cool, and it still uses flammable solvents. 

It has been reported that use of a suitable co-solvent increases the concentration of the olefin in water (catalyst) while retaining the biphasic nature of the system. It has been shown that using co-solvents like ethanol, acetonitrile, methanol, ethylene glycol, and acetone, the rate can be enhanced by several times [27, 28], However, in some cases, a lower selectivity is obtained due to interaction of the co-solvent with products (e.g., formation of acetals by the reaction of ethanol and aldehyde). The hydroformylation of 1-octene with dinuclear [Rh2(/t-SR)2(CO)2(TPPTS)2] and HRh(CO)(TPPTS)3 complex catalysts has been investigated by Monteil etal. [27], which showed that ethanol was the best co-solvent. Purwanto and Delmas [28] have reported the kinetics of hydroformylation of 1-octene using [Rh(cod)Cl]2-TPPTS catalyst in the presence of ethanol as a co-solvent in the temperature range 333-353 K. First-order dependence was observed for the effect of the concentration of catalyst and of 1-octene. The effect of partial pressure of hydrogen indicates a fractional order (0.6-0.7) and substrate inhibition was observed with partial pressure of carbon monoxide. A rate eqution was proposed (Eq. 2). 

Limited data are available for the kinetics of the oxo synthesis with HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in the presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [4]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures (Eq. 1). 

It has been claimed3 that no selectivity is observed in ethyl acetate, but, in recent work (see Section IV,2c, p. 128) selective hydrogenations have been achieved by using this solvent. Sodium borohydride in ethanol has also been found9111 to reduce the double bond of nitro-olefinic sugars selectively. 

The kinetics of hydroformylation of 1-octene using [Rh(COD)Cl]2 as a catalyst precursor with TPPTS as a water-soluble ligand and ethanol as a co-solvent was further studied by Deshpande et al. [15] at different pH values. The rate increased by two- to fivefold when the pH increased from 7 to 10, while the dependence of the rate was found to be linear with olefin and hydrogen concentrations at both pH values. The rate of hydroformylation was foimd to be inhibited at higher catalyst concentrations at pH 7, in contrast to linear dependence at pH 10 (Figure 5). The effect of the concentration of carbon monoxide was linear at pH 7, in contrast to the usual negative-order dependence. At pH 10, substrate-inhibited kinetics was observed with respect to CO (Figure 6). 

In a subsequent report, iron(iii) triflate could be used in combination with sodium borohydride for the hydrogenation of olefins (Scheme 12.22). The reaction operated well in a range of alcoholic solvents at ambient temperature, with the highest yields obtained using ethanol. This procedure provides an operationally simple method for the hydrogenation of alkenes, which can be conducted in solvents with excellent green credentials. 

Unconjugated olefins and acetylenes are rapidly hydrogenated in a ds manner in the presence of V fiUdnson s catalyst. These reactions typically occur at ambient hydrogen pressure and temperature in benzene when the reaction is conducted with a polar co-solvent, such as ethanol. These polar co-solvents might facilitate migratory insertion, which is the turnover-limiting step of the catalytic cycle presented later in this chapter. 

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