Alkynes to trans alkenes

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

Group I metals—sodium is the one usually employed—in liquid ammonia as the solvent convert alkynes to trans alkenes The reaction proceeds by a four step sequence in which electron transfer and proton transfer steps alternate... 

We saw m Section 9 10 that the combination of a Group I metal and liquid ammonia is a powerful reducing system capable of reducing alkynes to trans alkenes In the pres ence of an alcohol this same combination reduces arenes to nonconjugated dienes Thus treatment of benzene with sodium and methanol or ethanol m liquid ammonia converts It to 1 4 cyclohexadiene... 

Scheme 20.29 Reduction of alkynes to trans-alkenes by chromous sulfate.

Quite remarkably partial hydrogenation of alkynes to trans alkenes is also possible with homogeneous rhodium complexes 168169... 

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... 

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) ... 

Solutions of ytterbium in liquid ammonia are capable of reducing aromatic systems to 1,4-dihydroaromatics or alkynes to trans alkenes [293]. The a,fi-unsaturated cholest-4-en-3-one was reduced to eholestanone in 80% yield by threefold excess of ytterbium in ammonia followed by oxidation with Jones reagent (Scheme 16). In the absence of a proton source, (HOEt) and THF as co-solvent, the pinacol dimer was obtained as the major product. 

To form a trans alkene, two hydrogens must be added to the alkyne with anti stereochemistry. Sodium metal in liquid ammonia reduces alkynes with anti stereochemistry, so this reduction is used to convert alkynes to trans alkenes. 

Write a stepwise mechanism analogous to that of the dissolving metal reaction that reduces alkynes to trans alkenes for the following reaction ... 

Lithium or sodium in liquid ammonia reduces disubstituted alkynes to tran -alkenes. The reaction is carried out by addition of the alkyne in ether to a mixture of Na in NH3 (1), and the alkene produced is the (ii)-isomer. Overreduction and isomerization of the alkene are suppressed by addition of t-BuOH. ... 

In the reduction of alkynes to trans alkenes, the second electron transfer occurs after the first protonation. The stereochemistry-determining step is protonation of the carbanion obtained after the second electron transfer. The thermodynamically more stable trans product is obtained. 

Reductions for which group 1 metals are used in organic chemistry can be divided into two broad classes those that involve addition of H2 across a tt bond, and those in which a bonds between non-H atoms are formed or broken. The conjugate reduction of a,j3-unsaturated ketones, the reduction of alkynes to trans-alkenes, and the Birch reduction of arenes belong to the first class. The reduction of C—X bonds, C—O bonds a to a carbonyl, and acyloin and pinacol condensations belong to the second class. 

Keeping in mind the mechanism for the dissolving metal reduction of alkynes to trans alkenes in Chapter 12, write a stepwise mechanism for the following reaction, which involves the conversion of an a,p-unsaturated carbonyl compound to a carbonyl compound with a new alkyl group on the a carbon. 

Reduction of a variety of organic functional groups has long been carried out using ammonia solutions of alkali metals (Birch and SubbaRao, 1972). Given the strongly electropositive character of lanthanides such as ytterbium (which features a 4f 6s electron configuration), it follows that ytterbium/ammonia solutions should convert a,i8-unsatiu ated ketones to saturated ketones, alkynes to trans-alkenes and aromatics to 1,4-dihydroaromatics (White and Larson, 1978). 

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