Reduction group 13 molecules

The azo-compounds are usually very stable, and can be directly chlorinated, nitrated and sulphonated. On vigorous reduction the molecule splits at the azo group to give two molecules of primary amines, e.g. bcnzene-azophenol gives PhNH2 and p-HOC H NHa. 

Radicals can be obtained from reduction of molecules followed either by protonation or departure of a nucleophile as illustrated in Schemes 6 [10] and 7 [11], respectively. In the first example, a generally accepted mechanism involves a reduction of a double bond activated by an electron-withdrawing group to a radical anion followed by protonation and cyclization of the resulting radical. The addition of a second electron and proton completes the process. 

Radical anions resulting from cathodic reductions of molecules react with electrophilic centers. As an example (Scheme 8), the reduction of compounds in which a double bond is not conjugated with a carbonyl group, involves an intramolecular coupling reaction of radical anion with alkene [12]. 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

If it is necessary to reduce one group in a given molecule without affecting any other unprotected reducible group, the following reactivity orders for ease of reduction toward catalytic hydrogenation, LiAlH, and diborane may serve as a guideline. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Since (A) does not contain any other functional group in addition to the formyl group, one may predict that suitable reaction conditions could be found for all conversions into (A). Many other alternative target molecules can, of course, be formulated. The reduction of (H), for example, may require introduction of a protecting group, e.g. acetal formation. The industrial synthesis of (A) is based upon the oxidation of (E) since 3-methylbutanol (isoamyl alcohol) is a cheap distillation product from alcoholic fermentation ( fusel oils ). The second step of our simple antithetic analysis — systematic disconnection — will now be exemplified with all target molecules of the scheme above. For the sake of brevity we shall omit the syn-thons and indicate only the reagents and reaction conditions. 

Neither sodium borohydride nor lithium aluminum hydride reduces isolated carbon-carbon double bonds This makes possible the selective reduction of a carbonyl group m a molecule that contains both carbon-carbon and carbon-oxygen double bonds... 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

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