Triple bond, hydrogenation, reduction

This chapter is devoted to the discussion of the reduction of carbon-carbon double and triple bonds by noncatalytic methods, These methods include reductions by diimide, by dissolving metals in the presence or absence of proton donors, by low-valent metal ions, by metal hydride-metal halide combinations and by so-called ionic hydrogenation procedures. Of these widely diverse methods of reduction of carbon-carbon double and triple bonds, the reduction by diimide appears to be the most versatile. The reduction of carbon-carbon double and triple bonds by diimide occurs with complete stereoselectivity and stereo-specificity, and can be effected in the presence of a variety of other, very chemically reactive functional... 

When the polarographic reduction of phenyl(4-methylphenyltelluro)acetylene was examined in MeCN in the presence of benzoic acid, the first reduction wave corresponding to the 2-electron electrochemical cleavage of the Csp—Te bond was progressively superseded by the 4-electron process of the hydrogenation of the triple bond. This reduction has been interpreted as involving an electron transfer to the adsorbed substrate yielding an anion radical, which after protonation and further reduction would form the saturated telluride (Scheme 1). 

Terminal alkynes are only reduced in the presence of proton donors, e.g. ammonium sulfate, because the acetylide anion does not take up further electrons. If, however, an internal C—C triple bond is to be hydrogenated without any reduction of terminal, it is advisable to add sodium amide to the alkyne solution Hrst. On catalytic hydrogenation the less hindered triple bonds are reduced first (N.A. Dobson, 1955, 1961). 

Reductions of Nitriles. In the reduction of nitriles, hydrogen is added progressively across the carbon—nitrogen triple bond, forming first the imine and then the amine. 

Acetylenes have hijh synthetic utility, and hydrogenation of the triple bond occurs in many reaction sequences (7). Often the goal of this reduction is formation of the cis olefin, which usually can be achieved in very high yields (for an exception, see Ref. 10). Continued reduction gives the paraffin. Experimentally, both the relative and absolute rates of acetylene and olefin hydrogenation have been found to depend on the catalyst, substrate, solvent, reaction conditions, and hydrogen availability at the catalyst surface. Despite these complexities, high yields of desired product usually can be obtained without difficulty. 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

Various other reducing methods are employed for the conversion of (3-nitro alcohols to amino alcohols, namely, electrochemical reduction.107 The selective electrohydrogenation of ni-troaliphatic and nitroaromatic groups in molecules containing other groups that are easy to hydrogenate (triple bond, nitrile, C-I) are carried out in methanol-water solutions at Devarda copper and Raney cobalt electrodes (Eq. 6.55).107... 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

The scope of hydrogen transfer reactions is not limited to ketones. Imines, carbon-carbon double and triple bonds have also been reduced in this way, although homogeneous and heterogeneous catalyzed reductions using molecular hydrogen are generally preferred for the latter compounds. 

Transfer Hydrogenation Catalysts for Reduction of C-C Double and Triple Bonds... 

Heterocyclic compounds are frequently used as hydrogen donors in the reduction of C-C double and triple bonds catalyzed by complexes of transition metals. Cyclic ethers such as [l,4]dioxane (39) and 2,3-dihydrofuran are known to donate a pair of hydrogen atoms to this type of compound. 2,3-Dihydro-[l,4]di-oxine (41), the product of dioxane (39), is not able to donate another pair of hydrogen atoms [46, 60, 73, 74]. These heterocyclic compounds are in general also very good solvents for both the catalyst and the substrates. 

Reduction of enynes to (Z)-atkenes. Lindlar s catalyst is not useful as a hydrogenation catalyst for reduction of trienynes or of dienediynes. The best results can be obtained in CH3OH with zinc activated by successive treatment with Cu(OAc)2 (10%) and AgN03 (10%). This reduction results in conversion of the triple bond to a (Z)-double bond. The system does not reduce simple, nonactivated alkynes, and a-branched enynes are reduced slowly. The reduction is effected at 25° with (Z)-enynes, but temperatures of 45° are necessary for the (E)-isomers. Yields of pure tetraenes are 25-65%. 

Hydrogenation of double and triple bonds has already been discussed. If the compound contains reducible functional groups, they may also be reduced but selective reduction of double bonds has been possible under suitable conditions. 

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. " ... 

Isolated double and triple bonds are reduced readily, whereas conjugated alkenes and aromatic systems are difficult to hydrogenate. Carbonyl double bonds react only very slowly, if at all, so it is possible to achieve selective reduction of C=C double bonds in the presence of aromatic and carbonyl functions. 

---