Double bonds, conjugated hydrogenation

Both enols have their carbon-carbon double bonds conjugated to a carbonyl group and can form an intramolecular hydrogen bond They are of comparable stability... 

By homogeneous reaction of the conjugated double bond system selectively the C=C double bond is hydrogenated [63-66] the ester function is not affected. Moreover, by action of the chiral catalyst, a chiral hydrogenated product is created with good enantioselectivity. 

In addition to the described reduction of double bonds conjugated to a carbonyl group, sodium hydrogen telluride and phenyltellurol reduce double (and triple) bonds conjugated to aromatic systems. " ... 

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... 

Double bonds conjugated with aromatic rings and with carbonyl, carboxyl, nitrile and other functions are readily reduced by catalytic hydrogenation and by metals. These reductions are discussed in the appropriate sections aromatics, unsaturated aldehydes and ketones, unsaturated acids, their derivatives, etc. 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

Localization of double bonds in unknown compounds has frequently been determined by ozonolysis. Unsaturated fatty acids of biological membranes are susceptible to ozone attack, but there are some important differences from autoxidation reactions. These include the fact that malonaldehyde is produced from linoleate by ozonolysis (53) but not autoxidation and also that ozonolysis does not cause double bond conjugation as judged by absorption at 233 nm (52). Reactions with the polyunsaturated fatty acids produce several possibilities for toxic reactions direct disruption of membrane integrity and toxic reactions caused by fatty acid hydroperoxides, hydrogen peroxide, and malonaldehyde. 

The C-C double bonds conjugated with carboxyl functions are usually much more readily hydrogenated than usual olefinic bonds, especially with nickel and palladium catalysts. Ethyl cinnamate is rapidly hydrogenated over Raney Ni under mild conditions (eq. 3.19).115 It is also hydrogenated over palladium oxide much faster (eq. 3.20) than over platinum oxide with which 2.9 h were required under the same conditions.5 Cinnamic acid was hydrogenated smoothly to dihydocinnamic acid as the sodium salt over Urushibara Ni in water under ordinary conditions (eq. 3.21).116... 

The precursor to an amino acid prepared by enantioselective hydrogenation has a Z double bond conjugated with a carboxylic acid carbonyl group. 

The simplest polymer with a conjugated backbone is polyacetylene. Its structure is similar to that of the saturated polymer polyethylene, but has one of the hydrogen atoms removed from each carbon of the polyethylene chain. Each carbon atom in the polyacetylene chain thus has one excess electron which is not involved in the basic chemical binding. And if the separation of the carbon were constant, polyacetylene would conduct along the chain in other words it would behave like a metal in one dimension. But unfortunately this is not true as the free electrons tend to get localized in shorter double bonds. Conjugated polymers can at best be expected to display semiconducting properties. 

In a-ionone, peroxybenzoic acid and peroxyphthalic acid epoxidize solely the endocyclic double bond to give 3,4-epoxy-a-ionone in 96.5 and 66% yields, respectively [299]. Alkaline hydrogen peroxide, on the other hand, epoxidizes only the double bond conjugated with carbonyl to give a 28-31% yield of a-ionone-ot, p -epoxide [332] (equation 440). 

The ease of reduction of an olefin decreases with increasing substitution of the double bond. Conjugation of a double bond with a carbonyl group can markedly increase the rate of hydrogenation of the double bond. 

In the steroid series tetrasubstituted double bonds are hydrogenated with difficulty or not at all, but they nevertheless consume perbenzoic acid. The product isolated, however, is not an epoxide but an allylic alcohol or a conjugated diene. The first case was encountered by Windaus and Luttringhaus, but a better documented example was reported by Windaus, Linsert, and Eckhardt. Reaction of A -cholesteryl acetate (1) with perbenzoic acid in chloroform afforded an unsaturated 98 17 CaH,. 

Y Kojima and T Kayama. Reactions of a lignin model compound containing a ring-conjugated double bond with hydrogen peroxide. Res. Bull. Coll. Expt. Forest Hokkaido University 40 1 3-194, 1983. 

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