Alcohols as hydrogen donors

Widdel F (1986) Growth of methanogenic bacteria in pure culture with 2-propanol and other alcohols as hydrogen donors. Appl Environ Microbiol 51 1056-1062. 

We report here the results obtained by using secondary alcohols as hydrogen donors towards steroidic conjugated enones and saturated ketones in the presence of 7.5% Cu on alumina. 

The possibility of using of aliphatic alcohols as hydrogen donors for the catalytic transfer reduction of nitro group over MgO was examined. Catalytic hydrogen transfer was found to be effective and selective method for reduction of nitrobenzene, A-nitrotoluene, A-chloronitrobenzene, 4-nitro-m-xylene, 3-nitro-styrene, 3-nitrobenzaldehyde, 1-nitropropane, and 1-nitrobutane. Conversion of starting nitro compound into desired product depended on the alcohol used as a donor. Adsorption of reactant and catalyst deactivation were studied by esr. New aspects of a role of one-electron donor sites in hydrogen transfer over MgD were demonstrated. 

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. " ... 

Transfer Hydrogenation using Alcohols as Hydrogen Donors... 

With the exception of reactions involving alcohols as hydrogen donors, the major process observed upon irradiation of diones in the presence of H-donors is formation of 1 1 adducts derived from coupling of the pair of radicals generated in the H-abstraction step. The ambident character of the semidione radical allows such coupling to occur at either of two... 

Selective reduction of the carbonyl group of a,/S-unsaturated aldehydes and ketones has been achieved by a vapor-phase hydrogen transfer reaction using saturated primary and secondary alcohols as hydrogen donors. The preferred catalyst for the reaction, which is reversible, is magnesium oxide. Application to the reduction of acrolein to allyl alcohol, methacrolein to methallyl alcohol, crotonaldehyde to crotyl alcohol, and methyl isopropenyl ketone to 3-methyl-3-buten-2-ol is described. 

Fig. 1. Process variables in the reduction of acrolein to allyl alcohol with ethyl alcohol as hydrogen donor, (a) Effect of temperature on yield of allyl alcohol and conversion of acrolein to allyl alcohol, (b) Effect of feed ratio on conversion of acrolein to allyl alcohol, (c) Effect of flow rate on yield of allyl alcohol and conversion of acrolein to allyl alcohol, (d) Effect of catalyst age on conversion of acrolein to allyl alcohol.

In an extension of the acrolein-allyl alcohol reaction, other alcohols were compared with ethyl alcohol as hydrogen donors. All primary and secondary alcohols which were tried were found to react however, with the secondary alcohols the extent of reaction appears to be governed less by equilibrium considerations than is the case with ethyl alcohol. Thus, the equilibrium constant for the reaction between acrolein and isopropyl alcohol at 396° was estimated from thermodynamic data to be about 350, whereas the experimental product ratio at 400° was 0.03. 

Isopropyl methyl ketone was successfully reduced on bulk magnesia to 3-methyl-2-butanoi using isopropyl alcohol as hydrogen donor. Reaction proceeded according to the equation ... 

The centres responsible for the activity of various oxides in CTR using aliphatic alcohol as hydrogen donor are usually of basic character. Acidic centres are mainly responsible for the undesired consecutive dehydration. 

Hj concentration as a function of time for various alcohols as electron donor. In the presence of primary and secondary alcohols the rates of Hj generation are equal although the induction periods at the start of illumination are different. In the presence of a tertiary alcohol the rate of formation is substantially lower. As mentioned in section 4.2 alcohols are not strongly adsorbed at the TiO particles and therefore do not react with the positive holes before they are trapped. The main scavenger in this system is the platinum deposit which scavenges electrons and catalyses the reduction of hydrogen ions 2 e" -I- 2 —> H2- A surprising observation was that the H2... 

Benzophenone has also been found to be photoreduced in the presence of amines as hydrogen donors, although less efficiently than in the presence of benzhydrol or isopropyl alcohol. The photoreduction of ketones in aromatic amines is thought not to go by the same mechanism as the photoreduction in alcohols, for the following reasons ... 

Different results were obtained using allylic alcohols as terminating agents. In spite of the expected reactivity of the alcoholic function as hydrogen donor only products deriving from the reaction of the double bond were obtained (23) according to the general pattern previously described for Heck-type reactions with allylic alcohols (24). 

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... 

The alcohol derivatives that have been successfully deoxygenated include thiocarbonates and xanthates.134 Peroxides can also be used as initiators.135 Dialkyl phosphites can also be used as hydrogen donors.136 (see Entry 4, Scheme 5.7)... 

While chemo- and regioselectivity are essentially unaffected by the hydrogen source, the stereoselectivity of the hydrogen addition to the conjugated olefinic moiety depends upon the alcohol used as hydrogen donor, and 5p 5a isomer ratios ranging between 48 and 85% were observed. 

Transfer hydrogenation of ketones catalyzed by a transition-metal complex or a main group-metal alkoxide is a useful method to produce secondary alcohols. Pure organic compounds such as 2-propanol [2,281,282] and formic acid [283] are preferably used as hydrogen donors in place of hydrogen gas. This method is convenient for a small- or medium-scale reduction... 

Ab initio calculations at the B3LYP/6-31+G level show the existence of only one conformation for the adduct. The blue shifted band assigned to the adduct indicates a-OH--7i interaction between water and the chromophore [110-112]. The calculations made by these authors show two alternative structures. The more stable is the one with the alcohol as HB donor, and one of the water hydrogens placed at a distance of 1.98 A with respect to the mass center of the aromatic ring in the alcohol molecule (Fig. 3.10). This -OH--7I interaction is responsible for the two blue shifted chromophore bands. The less stable structure has the alcohol as HB acceptor, and no -OH-"71 interaction. 

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