Reduction of conjugated enones

IV. REDUCTION OF CONJUGATED ENONES AND DIENONES, SATURATED KETONES AND KETOL ACETATES. 

Reaction times of from 5 to 60 minutes have been employed for the reduction of conjugated enones. Although the longer times apparently do not seriously diminish the yields of products, they usually are not necessary. If a conjugated enone is sufficiently soluble in the reaction medium, it is reduced almost instantly when added to lithium-ammonia solutions. 

A widely used procedure for the reduction of conjugated enones to saturated ketones is that of Bowers, Ringold and Denot, Procedure 5 (section V). Ether-dioxane (1 1) is used as the organic co-solvent and solid ammonium... 

For the reduction of conjugated enones to saturated alcohols, Procedure 5 (section V) may be modified by adding methanol in place of ammonium chloride a sufficient excess of lithium is present to effect reduction of the intermediate saturated ketone to the alcohol. Procedure 2 (section V) for effecting Birch reductions is also useful for reduction of conjugated enones to saturated alcohols. Thus, 17-ethyl-19-nortestosterone affords crude 17a-ethyl-5a-estrane-3) ,17) -diol of mp 174-181°, reported mp 181-183°, in quantitative yield. 

In section V-A it has been pointed out that catalytic reduction of conjugated enones is usually a good method for the preparation of p- or y-labeled ketones. To overcome certain stereochemical problems, however, it is occasionally necessary to use the lithium-ammonia reduction. In this case deuteration takes place at the / -carbon and generally leads to the thermodynamically more stable product (see chapter 1). 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

If the equilibrium were established rapidly, reduction of the free ketone as it formed would result in a substantial loss of product. Lithium enolates are more covalent in character than are those of sodium and potassium and consequently are the least basic of the group. This lower thermodynamic basicity appears to be paralleled by a lower kinetic basicity several workers have shown that lithium enolates are weaker bases in the kinetic sense than are those of sodium and potassium.42,78,94 As noted earlier, conjugated enones have been reduced to saturated ketones almost exclusively with lithium the above considerations may explain why this metal has been favored by most chemists. The following observations, however, suggest that sodium may function nearly as well as lithium in the reduction of conjugated enones. 

Chemoselective reduction of conjugated enones to allylic alcohols via hydrogen transfer from propan-2-ol over metal oxides is investigated in vapour phase conditions. The unique ability of Mgo to reduce exclusively carbonyl group is observed. However, because of the high basicity of MgO side reactions are present. It is shown that by doping the Mgo catalyst with HC1 a significant decrease of its basicity occurs and consequently side reactions are minimized. 

Although asymmetric 1,4-reduction of conjugated enones is recorded, the enantiomeric excess is not high (equation 58)." ... 

Lithium aluminium hydride in ether selectively reduces tosylates to alkanes in the presence of alkyl iodides and bromides in contrast, with diglyme as solvent only the halides are reduced. 1-5 Mol% of Cul in LiAlH4 facilitates the selective reduction of conjugated enones to saturated ketones. 

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