Trans alkenes alkyne conversion

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

The observation was a significant finding since at the time, when the only synthetic method to reduce alkynes selectively was their conversion by heterogeneous catalytic hydrogenation (Raney nickel) to cis alkenes. The dissolving-metal reduction provided easy access to high-purity trans alkenes since the latter do not readily react further under the conditions used. The efficient reduction of 1-alkynes in this system requires the presence of ammonium ion.196... 

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... 

The conversion of an alkyne to a trans-alkene can be accomplished by heating with lithium aluminum hydride (LAH), by reaction with lithium in liquid ammonia (Li, NH3). Thus all of these reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing agents for alkynes and give alkenes as the reduced products. 

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. 

The steps in the mechanism for the conversion of an internal alkyne to a trans alkene are ... 

Another method for the conversion of an alkyne to an alkene uses sodium or lithium metal as the reducing agent in liquid ammonia as solvent. This method is complementary to the Lindlar reduction because it produces trans rather than cis alkenes. For example, 5-decyne gives trans-5 decene on treatment with lithium in liquid ammonia. 

Control of cis-trans-geometry is well illustrated by catalytic hydrogenation or alkali metal reduction of alkynes, as discussed in Sections 4.8.B and 4.9.C, respectively. The Lindlar catalyst (sec. 4.8.B) allows selective reduction of alkynes to the cis-alkene, as in the conversion of 23 to 24 in Kaiser s synthesis of niphatoxin B. This contrasts sharply with treatment of an alkyne with alkali metals (sec. 4.9.C) to give the... 

The first reaction is stereospecific cis addition of hydrogen to an alkyne to give the c/s-alkene. The intermediate is therefore a cfs,cfs-diene and it may seem remarkable that it should become a trans,trans-diene on elimination. However, when we draw the mechanism for the elimination, we see that there need be no relationship between the stereochemistry of the intermediate and the product as this is an El reaction and the cationic intermediate can rotate into the most stable shape before conversion to the aldehyde. 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

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