Alkynes to cis alkenes

Lindlar catalyst (Section 8.5) A hydrogenation catalyst used to convert 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 ... 

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... 

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. 

Ni-Gr I.1 This nickel catalyst reduces disubstituted alkynes to cis-alkenes the stereoselectivity is comparable to that observed with Lindlar catalyst or nickel boride (3, 208 -210 5,471-472). 

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). 

Table 11.5 summarizes the catalytic hydrogenation reactions that can be used to add hydrogens to double or triple bonds, converting alkenes and alkynes to alkanes or alkynes to cis-alkenes. 

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). 

Poisoned catalyst (Section 12.5B) A hydrogenation catalyst with reduced activity that allows selective reactions to occur. The Lindlar catalyst is a poisoned Pd catalyst that converts alkynes to cis alkenes. 

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 ... 

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). 

---