Compounds to Alcohols

Many biological processes involve oxidation of alcohols to carbonyl compounds or the reverse process reduction of carbonyl compounds to alcohols Ethanol for example is metabolized m the liver to acetaldehyde Such processes are catalyzed by enzymes the enzyme that catalyzes the oxidation of ethanol is called alcohol dehydrogenase... 

The transfer hydrogenations of carbonyl compounds to alcohols catalysed by a variety of NHC complexes have been intensively studied. The strong bond... 

Reduction of Carbonyl Compounds to Alcohols - Sodium Borohydride-... 

Although there are now several catalysts useful for hydrogenation of saturated carbonyl compounds to alcohols (see Section XII), an alternative approach has involved initial hydrosilylation (Chapter 9 in this volume) followed by acid hydrolysis [Eq. (41)]. The area first developed using principally the RhCl(PPh3)3 catalyst (207-210), and has since proved particularly useful in asymmetric syntheses (see Section III,A,4). Besides simple aliphatic and aromatic aldehydes and ketones, the ter-pene-ketones camphor and menthone were stereoselectively reduced to mainly the less stable alcohols e.g., camphor gave 9 (209). 

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Reduction of aldehydes and ketones.4 In the presence of hydrogen chloride (or a Lewis acid), the reagent reduces carbonyl compounds to alcohols in high yield and with high stereoselectivity. However, the reduction of hindered ketones requires a strong Lewis acid (A1C13). Aldehydes are reduced so much more readily than ketones that selective reductions are possible. The reagent is also useful for reduction of a,/5-cnals to allylic alcohols. 

It is interesting that electrocatalyzed reduction of carbonyl compounds to alcohols takes place by using trialkoxysilanes (equation 101)128. 

Hydroboration, reduction of G=0. This complex is as efficient as BH3 THF lor hydroboration it forms diborane when dissolved in benzene. It reduces carbonyl compounds to alcohols at 0° in hexane or CH2C12. The stereoselectivity resembles that of BH3-THF. It reduces carboxylic acids to primary alcohols in 60-75% yield. 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Chapter 4 centers on two key transformations in organic synthesis (1) oxidation of alcohols and of unsaturated hydrocarbons (i.e., alkenes and alkynes) to carbonyl compounds (2) reduction of various carbonyl compounds to alcohols. 

The reader is already familiar with the reductions of carbonyl compounds to alcohols, since they were described in Sections 10.2-10.4. (To complete the overall picture in a,fi-unsaturated carbonyl compounds the C=C- instead of the C=0 double bond can also be reduced according to Figure 13.20, through the Birch and L-Selectride reduction of such carbonyl compounds.)... 

The reader already is familiar with the reductions of carbonyl compounds to alcohols, since they were described in Sections 8.2-8.4. 

A variety of other electron transfer reagents have been employed in reactions which appear to be mechanistically similar to the more common metal-NH3 or metal-alcohol systems. These include K-graphite, Zn-KOH-DMSO and both Li and AP amalgams. The amalgams from Zn, Mg, Ni, Cu, Sn and Pb have been found not to be effective in the reduction of cyclohexanone in aqueous THF. Also, several low-valent metal cations have been employed in the reduction of carbonyl compounds to alcohols. Among these reagents are low-valence salts of Ti, " Ce and Sm. ... 

Oxiranes are converted by organometaliic compounds to alcohols. This reaction, which is one of the most important methods for the formation of a C-C 6-bond, proceeds by nucleophilic attack of an organic carbanionic species on the least-substituted carbon. (Eq. 228). 

Reduction The stable Ni(0) complex is a useful catalyst for reduction of carbonyl compounds (to alcohols), sulfonyl azides (to sulfonamides), imines and nitro compounds (to amines) with ammonium formate. 

Reduction of carbonyl con unds. The reduction system made up of Zn-FeClj in aqueous DMF is capable of reducing carbonyl compounds to alcohols at room temperature. 

This reaction, known as the Bouveauft-BJanc reduction, used to be used to reduce carbonyl compounds to alcohols, but now aluminium hydrides and borohydrides are usually more convenient. You met an example of ttie Bouveault-Blanc reduction in Chapter 33 (conformational analysis-reduction of cyclohexanones). oy raii ... 

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