Hydrogenation of alcohols

Alcohols. The direct synthesis of esters by dehydrogenation or oxidative hydrogenation of alcohols offers a simple method for the preparation of certain types of esters, such as ethyl acetate (96—98) ... 

In 2004, Whittlesey and Williams demonstrated that the reversible C-H activation of Ru-NHC complexes (e.g. 32a, Scheme 13.14) provides an effective manifold for tandem dehydrogenation/Wittig reaction/hydrogenation of alcohols, thus generating alkanes from alcohols and phosphorus ylides [56]. 

Table 21.5 Diastereoselective hydrogenation of alcohols with exocyclic double bond. 

Looking into the future, we expect that hydrogenation reactions will also be tremendously important for the conversion of renewable resources. Going from carbohydrates to valuable chemicals will require deoxygenating reactions. Thus, hydrogenation of alcohols, aldehydes and carboxylic acids will become very important topics. 

When an acyl group replaces the hydrogen of alcohols, carboxylic acids, hydrogen halides, ammonia or amines, we have the acyl compounds known as esters, anhydrides, halides, and amides, respectively. 

Substitution of the hydroxyl hydrogens of alcohols by hydrocarbon groups gives compounds known as ethers. These compounds may be classified further as open-chain, cyclic, saturated, unsaturated, aromatic, and so on. For the naming of ethers, see Sections 7-3 and 15-11 A. 

In the presence of this compound the enantiotopic methylene hydrogens of alcohol 10-12 are no longer equivalent, exhibiting a chemical shift difference up to 0.8 ppm at 100 MHz. Since these hydrogens are no longer equivalent, the coupling constant between them can be measured directly 2J = 13 Hz. Compare this approach with the method discussed in Section 9.4. 

Hydrogenation is also used for improving and accelerating separations. Drawert and coworkers [88, 89] employed hydrogenation for the determination of small amounts of ethanol in aqueous solutions and in blood. For the hydrogenation of alcohols to hydrocarbons, Raney nickel on Kieselguhr (1 10) was used as the catalyst at 160-200°C in a... 

Few applications of the theory of absolute reaction rates to particular catalytic reactions have been made so far. Among them hydrogenation of alcohols on copper [59] can be pointed out. As has been shown by the author for reactions of the type mentioned, the application of the theory of absolute reaction rates to the model given by the multiplet theory leads to such results that are in accordance with experiment. 

Catalysts containing chiral 1,2-diamine been developed for the hydrogenation of alcohols. ... 

The most distinctive feature of the H-NMR spectra of ethers is the chemical shift of hydrogens on the carbons bonded to the ether oxygen. Signals for this type of hydrogen fall in the range 8 3.3 to 4.0, which corresponds to a downfield shift of approximately 2.4 units compared with their normal position in alkanes. The chemical shifts of H—C—O— hydrogens in ethers are similar to those seen for comparable H—C—OH hydrogens of alcohols. 

Some of the most commonly reported C-alkylation methods involving the transition-metal-catalyzed transfer hydrogenation of alcohols can be described as follows (1) the a-alkylation of ketones and carbonyl compounds (2) alcohol-alcohol coupling reactions to afford ketones and (3) P-alkylation by alcohol dimerization, which is also referred to as a Guerbet reaction. 

C-Alkylation methods involving the transfer hydrogenation of alcohols represent atom-economical and green chemical transformations. With this in mind, we hope that this review will act as an informative reference and provide support to researchers interested in the utilization of hydrogen transfer as a C-alkylation method. We also hope that this review will inspire researchers to develop new innovative transformations in the field of C-alkylation involving transfer hydrogenation. 

Iridium-catalyzed transfer hydrogenation of alcohol substrates also provides a useful method of C-C bond formation. 

---