Reduction of Dienes and Polyenes

The behavior of dienes and polyenes - both open chain and cyclic - toward reduction depends especially on the respective positions of the double bonds. Carbon-carbon double bonds separated by at least one carbon atom behave as independent units and can be partly or completely reduced by catalytic hydrogenation or by diimide. cis, trans, /rani-l,5,9-Cyclododecatriene treated with hydrazine and air in the presence of copper sulfate (diimide in situ) gave cij-cyclododecene in 51-76% yield [269]. It appears as if the trans double bonds were reduced preferentially. 

If the double bonds are structurally different to such an extent that their rates of hydrogenation differ considerably they can be reduced selectively. Monosubstituted double bonds are saturated in preference to di- and trisub-stituted ones [13, 348, 349]. When 2-methyl-1,5-hexadiene was hydrogenated over nickel boride with one mole of hydrogen only the unbranched double bond was reduced, giving 95% yield of 2-methyl-1-hexene [349]. 

The monosubstituted double bond in 4-vinylcyclohexene is hydrogenated over P-1 nickel in preference to the disubstituted double bond in the ring, giving 98% of 4-ethylcyclohexane [73]. Similarly in limonene the double bond in the side chain is reduced while the double bond in the ring is left intact if the compound is treated with hydrogen over 5% platinum on carbon at 60° and 3.7 atm (yield 97.6%) [348]. 

Under the conditions which cause migration of the double bonds, e.g. treatment with sodium and especially potassium, the isolated double bonds can become conjugated, and thus they undergo reduction by metals. Some macrocyclic cycloalkadienes were reduced (predominently to trans cycloalkenes) by treatment with potassium on alumina in a hydrogen atmosphere [350]. 

In systems of conjugated double bonds catalytic hydrogenation usually gives a mixture of all possible products. Conjugated dienes and polyenes can be reduced by metals sodium, potassium, or lithium. The reduction is accomplished by 1,4-addition which results in the formation of a product with only one double bond and products of coupling and polymerization. Isoprene was reduced in 60% yield to 2-methyl-2-butene by sodium in liquid ammonia [357]. Reduction of cyclooctatetraene with sodium in liquid ammonia gave a 

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The electrochemical oxidation or reduction of dienes and polyenes is generally more useful than the corresponding reaction of monoolefins which is not substituted with activating groups, since the electrode potentials required in the reaction of dienes and polyenes are generally much lower than the potentials necessary in the reaction of monoolefins. 

Reduction of dienes and polyenes has attracted much attention since it is important from both practical and theoretical aspects. In these reactions the major interest is the selective reduction of a double bond in the presence of another. In general, saturation of all the multiple double bonds of nonaromatic compounds can be carried out with any of the catalysts which are suitable for low-pressure reductions or with some reducing chemicals. 

Reduction of dienes and polyenes 3. Reduction by metal hydrides and dissolving metals... 

In the reduction of dienes and polyenes, combinations of a metal hydride and transition metal halides can also be used. Sodium borohydride and cobalt(II) halides were applied in the selective reduction of unsaturated carbon-carbon double bonds93. LiAlH4, in the presence of Zrlv-, TiIV- or Vlv-halides, is a selective reducing agent of dienes94,95. The following reactions were carried out with sodium borohydride and iodine (equation 28)96. 

A special mild reducing agent called BER is prepared by treating an anion exchange resin with aqueous NaBILt. Addition of CUSO4 allows selective reductions of dienes and polyenes (equation 31)98. 

Besides the most widely used catalytic reductions of dienes and polyenes there are some other ways to saturate the C=C double bonds in these molecules. One of these rarely used methods is the enzymatic or microbial reduction. In the presence of bacteria and fungi the reactions progress just as over any classical catalysts. 

III. REDUCTION OF DIENES AND POLYENES WITH DIFFERENT STRUCTURES A. Allenes... 

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