Reduction of conjugated alkenes

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

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

The ability to change the course of electrolytic reduction of conjugated alkenes by use of low Hj overvoltage cathodes to effect reduction of the C=C bond has been reported to have been successful in the commercialization of the reduction of maleic acid to succinic acid in India. 

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

Cyanide-containing cobalt catalysts, particularly potassium pentacyanocobalta-te(II) K3[Co(CN)5], are used in the reduction of activated alkenes (conjugated dienes).26,31 [Co(CO)4]2 is best known as a hydroformylation catalyst, but hydrogenation is also possible under specific conditions. Phosphine-substituted analogs are more successful. 

Various diphenylethylenes are reduced in high yield by magnesium in methanol. Expectedly, reductions of conjugated double bonds via one-electron transfer processes can also be carried out electro-chemically. Scheme 87 illustrates both the reduction of a conjugated alkene and partial trapping of the intermediate anion radical to yield a mixed hydrodimerization product. 

A more difficult problem is encountered when it is necessary to reduce selectively multiple bonds in the side chain, especially when they are conjugated with the nucleus. Up to the present time little has been published on this subject. The best method for the side chain reduction of /3-2-furylacrylic acid seems to be with Raney alloy in dilute NaOH. If the reaction is carried out more drastically, the nucleus undergoes easy reduction to give 4-hydroxy-valerolactone. The method of Railings and Smith also appears to be successful this involves the use of palladium strontium carbonate for the reduction of the alkene side chain of 2 -furylalkenes, without affecting the ring. Such compounds as j8-2 -furylacrylic esters are exceptions, but the potassium salt of the acid can be selectively reduced in aqueous solution. 

Reduction of alkenes. Siegel et al have discussed the stereochemistry of diimide reduction of alkenes as compared with catalytic hydrogenation. The same laboratory investigated the reduction of conjugated dienes to enes no evidence for 1,4-reduction was obtained. 

It was previously mentioned that zinc reduces alkynes to alkenes. Niif showed that reduction of the conjugated alkyne (541) to the conjugated cis alkene (in 542) occurred with zinc in the presence of potassium cyanide (KCN). This reduction was not possible by hydrogenation with a Lindlar catalyst, due to the poor selectivity for reaction with only the alkyne, without reduction of any alkenes.5 4 The yields obtained by zinc reduction were not always reproducible, however. ... 

Silane hydrides can be used for the reduction of carbonyls and alkenes. Reaction of methylcyclohexene with a mixture of triethylsilane (EtsSiH) and trifluoroacetic acid (CF3CO2H) reduced the alkene moiety to give methylcyclohexane in 72% yield. 2 Under the same conditions, however, 1-pentene was not reduced. More commonly, this reagent is used for reduction of conjugated carbonyls, probably via formation of a silyl enolate (secs. 9.2, 9.3.B) as in the reduction of cyclohexenone to cyclohexanone in 85% yield with Ph2SiH2. Addition of transition metals such as ZnCl2, or copper salts to the silane facilitates the reduc-tion,594 as in the conversion of 576 to 577 in 96% yield. ... 

Reduction of conjugated unsaturated aldehydes or ketones is sometimes accompanied by a shift in the position of the double bond (7.110). In other cases pyrazoline derivatives may be formed these decompose yielding cyclopropanes isomeric with the expected alkene (7.111). 

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