Alkynes, reduction with metals

Reduction with metals in weakly acidic solvents is not restricted to arenes. A useful related reaction reduces alkynes to trans-alkenes, and provides a useful alternative to catalytic hydrogenation, which favors formation of cw-alkenes (Section 11-2A) ... 

The liquid-phase hydrogenation of various terminal and internal alkynes under mild conditions was largely described with metal nanoparticles deposited/in-corporated in inorganic materials [83, 84], although several examples of selective reduction achieved by stabilized palladium, platinum or rhodium colloids have been reported in the literature. 

Reactions with Protic, ionic, Poiar Reagents. The reactions of radical anions with proton donors include the reduction of arenes, the well-known Birch reduction, as well as alkynes by alkali metals in liquid ammonia. Both reactions have synthetic utility and belong to the few radical ion reactions included in elementary textbooks. 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

Catalytic reduction of alkynes to ds-alkenes. This reduction is not possible with 10% Pd/C alone because this metal is too reactive and the alkane is formed readily. The selective reaction is possible if the Pd/C is deactivated by either Hg(0) or Pb(0), obtained by reduction of metal acetate with NaBH4. Sodium phosphinate, H2P02Na, is the preferred hydride donor. Since this donor is not soluble in the Organic solvents used, a phase-transfer catalyst, benzyltriethylammonium chloride, is added.3... 

Alkynes react with hydrogen gas in the presence of a metal catalyst and the process called hydrogenation. It is an example of a reduction reaction. With a fully active catalyst like platinum metal, two molecules of hydrogen are added to produce an alkane. 

Na, or Li in liquid ammonia, for example) to reduce aromatic rings and alkynes. The dissolving metal reduction of enones by lithium metal in liquid ammonia is similar to these reactions—the C=C bond of the enone is reduced, with the C=0 bond remaining untouched. An alcohol is required as a proton source and, in total, two electrons and two protons are added in a stepwise manner giving net addition of a molecule of hydrogen to the double bond. 

The preparation of pure isolated E olefins is readily accomplished by the reduction of an alkyne with metallic sodium or lithium in liquid ammonia (27,32). This reaction is preferably carried out by the addition of the alkyne in an ether to a mixture of sodium (or lithium) in liquid ammonia at -30°. 

Both disubstituted alkynes (Chapter 3.3, this volume) and isolated terminal double bonds may be reduced by alkali metals in NH3, but isolated double bonds are usually stable to these conditions. However, 16,17-secopregnanes (10 equation 8) afford mixtures of cyclization products (11) and (12) in 61% to 80% yield with Na naphthalenide-THF, Na-NHs-THF, Na-THF or Li-NHs-THF. With Na-NHa-THF-r-butyl alcohol, a 91% yield of a 72 28 mixture of (11) (12) (R = Me) is obtained. This type of radical cyclization of alkenes and alkynes under dissolving metal reduction conditions to form cyclopentanols in the absence of added proton donors is a general reaction, and in other cases it competes with reduction of the carbonyl group. Under the conditions of these reactions which involve brief reaction times, neither competitive reduction of a terminal double bond nor an alkyne was observed. However, al-lenic aldehydes and ketones (13) with Li-NHs-r-butyl alcohol afford no reduction products in which the diene system survives. ... 

The metal-NHs reductions of carbonyl groups are exceedingly fast reactions for the reaction of acetone with an ammoniated electron the rate is 9 x 10 M" s". Although many, particularly older, published experimental procedures for the metal-NHs reduction of ketones employ prolonged reaction times with excess metal, these conditions are unnecessarily harsh. The reactions of carbonyl compounds with metals in NH3 are effectively instantaneous and by using short reaction times it appears that reduction of terminal alkenes and disubstituted alkynes can be avoided.In addition to the functional groups mentioned above, alcohols, amines and ethers, other than epoxides, are usually stable to reductions of aldehydes and ketones by dissolving metals. " ... 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

Vinyl halides add to allylic amines in the presence of Ni(cod)2 where cod=l, 5-cyclooctodine, followed by reduction with sodium borohydride. Aryl iodides add to alkynes using a platinum complex in conjunction with a palladium catalyst. A palladium catalyst has been used alone for the same purpose, and the intramolecular addition of a arene to an aUcene was accomplished with a palladium or a GaCl3 catalyst, " AUcyl iodides add intramolecularly to aUcenes with a titanium catalyst, or to alkynes using indium metal and additives. The latter cyclization of aryl iodides to alkenes was accomplished with indium and iodine or with Sml2. " ... 

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