Of alkynes to cis-alkenes

Chromium complexes,163 cationic homogeneous complexes of rhodium164 and ruthenium,165 166 and supported palladium complexes141167 have been found to be extremely selective in reduction of alkynes to cis alkenes ... 

Selective reduction of alkynes to cis alkenes using hydrosilane functions immobilized on silica gel in acetic acid in the presence of a Pd(0) catalyst system has been reported.239... 

Palladium catalysts, 230 of alkyl silyl ethers to alkanes Nickel boride, 197 of alkyl sulfonates to alkanes Lithium triethylborohydride, 153 of alkynes to cis-alkenes... 

As catalytic semihydrogenation of alkynes to Cis-alkenes is not only a very important synthetic operation (ref. 1) but also of industrial interest, it is a challenging task for both synthetic and catalytic chemists. For instance, the importance of the problem is illustrated by numerous recent publications on different aspects of the selective hydrogenation of many compounds related to the propargyl alcohol structure (refs. 2-7). In this respect, 1,4-butenediol, obtained by the liquid-phase semihydrogenation of 1,4-butynediol, is a raw material for insecticides and Vitamin Bg (refs. 2,8,9). Furthermore, the total and selective liquid-phase hydrogenation of this compound is one of the procedure for making butanediol, the top 95 chemical produced in the United States (refs. 10,11), whose major use is in the manufacture of polyesters. 

Reduction of alkynes to cis-alkenes.1 This relatively stable copper hydride (1) reduces terminal alkynes at 25° internal alkynes are reduced at 80° to cis-alkenes exclusively. These reductions require 2 hydride equivalents of the complex, and 5-10 equiv. of H20 is necessary for satisfactory yields. Under these conditions, a tertiary propargyl acetate was converted to an allene (equation I). 

A heterogeneous catalyst for the hydrogenation of alkynes to cis alkenes. In its most common form, it consists of a thin coating of palladium on barium sulfate, with quinoline added to decrease the catalytic activity, (p. 406)... 

Hydrogenation of alkynes to cis-alkenes. This reduction can be carried out with sodium borohydride and a catalytic amount of PdCI, which is more effective than Pd/C, in polyethylene glycol and CHiCh. This system is superior to PdCI, and H, in polyethylene glycol. 

CHj, R = C2H5]. Indeed, [(C5H4R)2Y( t-H)( ]2 and (CsH4R)2Y-[C(R )=CHR ](thf) can be interconverted in this way and both initiate catalytic hydrogenation of alkynes to cis alkenes ( 5). Hence, one plausible rationale exists for the lanthanide-based catalytic hydrogenation reaction which originally stimulated the study of molecular lanthanide hydrides (50, 60). 

Diisobutylaluminium hydride has found considerable use for the selective 1,2-reduction of a,p-unsaturated carbonyl compounds to allylic alcohols. For example, the ester 95 was reduced to the allylic alcohol 96 (7.82). The reagent has also found use for the reduction of alkynes to cis -alkenes (see Section 2.6). 

The conversion of an unsaturated acid or ester to its perhydro derivative-sometimes a significant step in the identification of new fatty acids-is readily achieved by catalytic hydrogenation using a platinum, palladium, or nickel catalyst. The partial and stereospecific reduction of alkynes to cis alkenes is most commonly effected with Lindlar s catalyst (Section 7.1). 

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... 

Alkynes can be reduced electrolytically. Internal alkynes gave 65-80% yields of cis alkenes when electrolysed in 10% sulfuric acid in ethanol at spongy nickel cathode [127], or predominantly trans alkenes if the electrolysis was carried out in a methylamine solution of lithium chloride. The yields of the alkenes and the ratios of trans to cis alkenes varied depending on whether the electrolysis was carried out in divided or undivided cells (yields 24-80%, composition of product 89-99% of trans alkene) [379]. 

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... 

Besides heterogeneous and homogeneous catalytic hydrogenations, chemical reductions can also transform alkynes to cis alkenes. Interestingly, activated zinc in the presence of a proton donor (alcohol), although a dissolving-metal reagent, reduces disubstituted alkynes to cis alkenes 199... 

Although it does not act as selectively as the Pd catalysts, finely divided Ni is sometimes useful for reduction of isolated triple bonds, especially in the presence of nitrogenous bases such as pyridine, ammonia, or piperidine. The heterogeneous catalyst NaH/RONa/Ni(OAc)2 presents practical advantages it is cheap, easily and reproducibly prepared, and can be stored for long periods. By monitoring the uptake of Hj, selective reduction of alkyne to alkene and then to alkane is achieved, particularly in the presence of quinoline. Only mild conditions are required to reduce disubstituted or terminal alkynes to cis-alkenes in high yields ... 

Like the hydrogenation of alkenes hydrogenation of alkynes is a syn addition CIS alkenes are intermediates in the hydrogenation of alkynes to alkanes... 

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... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Hydrogenation of alkynes to alkenes (Section 9.9) Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts. Lindlar palladium is the metal catalyst employed most often. Hydrogenation occurs with syn stereochemistry and yields a cis alkene. 

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

Another hydride, magnesium hydride prepared in situ from lithium aluminum hydride and diethylmagnesium, reduced terminal alkynes to 1-alkenes in 78-98% yields in the presence of cuprous iodide or cuprous r rt-butoxide, and 2-hexyne to pure cij-2-hexene in 80-81% yields [///]. Reduction of alkynes by lithium aluminum hydride in the presence of transition metals gave alkenes with small amounts of alkanes. Internal acetylenes were reduced predominantly but not exclusively to cis alkenes [377,378]. 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

In the presence of the Wilkinson catalyst, 1-alkynes are reduced slowly and non-selectively to give saturated hydrocarbons.30 In contrast, internal alkynes give cis alkenes that are resistant to further reduction. Other catalysts can reduce 1-alkynes selectively161 162 [Eqs (11.31) and (11.32)] and be more active in the hydrogenation of internal alkynes ... 

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