Double bonds conjugated bond reduction

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

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. " ... 

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]. 

In an interesting reaction, -santonin was reduced with lithium in liquid ammonia so that the lactone was hydrogenolyzed to an add and one of the double bonds conjugated with the carbonyl was reduced. The other double bond as well as the keto group did not undergo reduction [1091]. 

Various examples of the use of dissolving metal reduction on exocyclic double bonds, conjugated either with another C —C double bond53 or a carbonyl group,54 have been reported. For example, reaction of 2,6,6-trimethyltricyclo[5-4.0.0I,5]undec-7-en-9-one with lithium metal in liquid ammonia gave, after trapping of the intermediate enolate, 2,6,6-trimethyl-9-tri-fyloxy[5.4.0.015]undec-8-enc (3).53... 

Reduction of ol, -unsaturated esters.2 These esters are reduced to the saturated esters by Mg in CH3OH in almost quantitative yield. Even double bonds conjugated with an aromatic ring are reduced. 

Based on the fact that other unsaturated compounds containing a double bond conjugated to an aromatic nucleus were reduced rather than hydroformylated, the products of the reaction were carefully analyzed and, in accordance with prior work, it was found that a mixture of two aldehydes resulted. But in addition, a 25 % yield of the reduction product,... 

Disubstituted peroxides are cleaved reductively by the same reagents as are used for hydroperoxides. The usual products are alcohols derived from the two substituents, but considerable selectivity is possible with some substrates. This is illustrated by the reduction of cyclic endoperoxides of the general formula (50) these compounds are available from the cycloaddition of singlet oxygen to cyclic conjugated dienes. The reduction of such compounds provides a good method of synthesis of cis-1,4-diols, which can be formed with retention of the double bond or with reduction of it (Scheme 29). It is also possible to prepare unsaturated epoxides by reduction with triphenylphosphine or other phosphorus(III) reagents. 

Carbon-carbon double bonds conjugated to multiple bonds other than aromatic systems may also be reduced with metals. Again the nature of the reduction product is dependent on the availability of a proton donor in the reaction medium. In the absence of an excess of proton donors, dimerization of the initially formed anion radical is observed. Both the reduction of 1,3-dienes and trapping experi-... 

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. 

The empirical formula is C15H21ON and the IR- and NMR-spectra supplied evidence of a furan ring, a imTis-quinolizidine system, and a double bond conjugated with furan. On reduction with sodium borohydride dehydrodeoxynupharidine gave the dihydro derivative, C15H23ON, which was identical with ( —)-deoxynupharidine hence the problem of its structure was reduced to that of the position of the double bond. 

The reduction of the double bonds conjugated to electron-withdrawing groups is examined later (Section 3.2.9). 

Reduction of conjugated double bonds. This selective reduction method is valuable in situations in which other double bonds must be retained. Such a reaction, further facilitated with ultrasound, has been employed during a synthesis of penitrem-D. 

This type of reaction may be divided into two categories (1) reduction of isolated double bonds and (2) reduction of conjugated double bonds. There are very few examples of the former in the steroid field, all involving the use of unidentified fecal anaerobes to reduce A -compounds to the 5j3-series (C-133, S-914), and there is no adequate basis for the presentation of arguments about mechanism. 

The reduction of double bonds conjugated with keto groups may be viewed as part of the illustrated equilibrium process, the reverse of which is the dehydrogenation of ketosteroids. The nature of the reagent X, be it protein-bound flavin, quinone. 

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