Reduction of conjugated

Addition of dihydrosilane to a, /J-unsaturated carbonyl compounds such as citral (49), followed by hydrolysis, affords saturated citroneJlal (50) directly. The reaction is used for the selective reduction of conjugated double bonds[45,46]. In addition to Pd catalyst, the use of a catalytic amount of... 

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). 

IV. Reduction of Conjugated Alkenes with Chromium (II) Sulfate... 

Selective reduction of conjugated diolefins, such as 1,3-peniadiene, falls with metal in the sequence palladium > rhodium > ruthenium > platinum... 

D. Conjugate Reduction of Conjugated Aldehydes, Ketones, Acids, Esters and Nitriles... 

The concentrations of PUFA-derived conjugated hydroperoxydienes and oxodienes in biological samples can also be determined by a modification of a spec-trophotometric method originally developed by Fishwick and Swoboda (1977). This assay system involves (1) reduction of conjugated hydroperoxydienes and oxodienes to their corresponding hydroxydienes with... 

Table 9.2 Selective reduction of conjugated aromatic alkenes catalyzed by Pd-PMHS nanocomposites. (Reprinted with the permission of the American Chemical Society [52])...

The reduction of conjugated dienes by dissolving metals is not extensively reported. This method appears to be nonselective, giving rise to a mixture of the expected olefins and polyolefins as by-products". 

A difference in the reactivities and selectivities between tetra-n-butylammonium borohydride and sodium borohydride in the reduction of conjugated ketones is well illustrated with A1-9 2-octalone (Scheme 11.3) [17], Reduction with the sodium salt in tetrahydrofuran is relatively slow and produces the allylic alcohol (1) and the saturated alcohol (2) in a 1.2 1 ratio whereas, in contrast, tetra-n-butylammonium borohydride produces the non-conjugated alcohol (3) (50%) and the saturated alcohol (2) (47%), with minor amounts of the ketone (4), and the allylic alcohol (1) [16]. It has been proposed that (3) results from an initial unprecedented formation of a dienolate anion and its subsequent reduction. 

Reduction of conjugated carbonyl compounds using stoichiometric amounts of the ammonium salt shows little advantage over the sodium salt in acidic methanol [11] with both reagents producing allylic alcohols (58-88% for acyclic compounds and 15-64% for cyclic compounds) by selective 1,2-reduction of the conjugated systems. Aldehydes, ketones and conjugated enones are also reduced by tetra-n-butylammonium cyanoborohydride in HMPA [11, 12], whereas haloalkanes and alkanesulphonic esters are cleaved reductively under similar conditions [13]. 

Selected examples of the 1,4-reduction of conjugated a,p-unsaturated carbonyl compounds... 

Ruthenium chloride mediated reduction of conjugated ketones... 

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]. 

REDUCTION OF CONJUGATED ALKENES WITH CHROMIUM(II) SULFATE DIETHYL SUCCINATE... 

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