Alkynes to alkanes

The use of stronger acid conditions provides somewhat better synthetic yields of alkanes from alkynes. A useful method consists of treatment of the substrate with a combination of triethylsilane, aluminum chloride, and excess hydrogen chloride in dichloromethane.146 Thus, treatment of phenylacetylene with 5 equivalents of triethylsilane and 0.2 equivalents of aluminum chloride in this way at room temperature yields 50% of ethylbenzene after 1.5 hours. Diphenylacetylene gives a 50% yield of bibenzyl when treated with 97 equivalents of triethylsilane and 2.7 equivalents of aluminum chloride after 2.8 hours. Even 1-hexyne gives a mixture of 44% n -hexane and 7% methylpentane of undisclosed structure when treated with 10 equivalents of triethylsilane and 0.5 equivalent of aluminum chloride for 0.5 hour.146 

The reductive cyclization of enynes has been used to prepare exo-methylene-cycloalkanes. Two systems have proven successful in this transformation, namely PMHS/Pd2(dba)3 CHCl3245 (Eq. 102) and Et3SiH/Pd(dppe)Cl2/HC02H (Eq. 103).246 

The triethylsilane/Pd2(dba)3 combination is also used for these reductive cycli-zations, although lower yields are reported.247 1,6-Diynes are reductively cyclized to 1,2-dialkylidenecyclopentanes in good yields with Et3SiH/Pd2(dba)3 CHCl3 (Eq. 104).248 

2- ethylcyclopropane gives an alkane reaction mixture consisting primarily of 

After ten days at room temperature in the presence of one equivalent 

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

Hydrogenation of alkynes to alkanes (Section 9.9) Alkynes are completely hydrogenated, yielding alkanes, in the presence of the customary metal hydrogenation catalysts. 

Complete reduction of alkynes to alkanes is easily accomplished by catalytic hydrogenation, especially using palladium [386, 387], platinum oxide and active nickel catalysts [559]. 

The complete hydrogenation of alkynes to alkanes occurs quite readily under the usual alkene hydrogenation conditions. This may or may not involve an intermediate alkene 81... 

Another important catalyst of the dihydride type is [Rh(nbd)(PR3)2]BF4.150 This reacts with H2 to liberate norbomane and the active Rh(PR3)2- moiety. In polar solvents [RhH2(solv)2(PR3)2]+ is obtained. Such catalysts can selectively reduce alkynes to alkanes and dienes to monoenes. They are also active for the reduction of ketones, substrates which are much more difficult to reduce.150,151... 

Water is essential for this reduction. This system reduces alkynes to rij-alkenes with high stereoselectivity. Addition of methyl propynoate (10 mole %) increases the reactivity and can effect reduction of alkynes to alkanes. 

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. 

Ti(Cp)2(CO)2l is a catalyst for the hydrogenation of phenylacetylene to ethylbenzene, while alkyl-substituted terminal alkynes are reduced to alkenes. Electron rich titanium(II) complexes, [Cp2Ti(PhC OPh)(PMe3)], [(MeCp)2Ti(PhC=CPh)(PMe3)] and [CpCp Ti(PhCsCPh)] are also catalyst precursors for the hydrogenation of alkynes to alkanes at 20 C under atmospheric pressure of hydrogen. "... 

All of the common hydrogenation catalysts can effect the complete saturation of the alkyne to alkane, but all catalysts are not equally effective in the selective hydrogenation to produce alkenes. Selectivities for cis 2-pentene formation from 2-pentyne decreased in the order Pd > Rh > Pt > Ru > Ir. Palladium is the most selective of the noble metal catalysts for alkyne semihydrogenation with respect... 

Another important catalyst of the dihydride type is [Rh(nbd)(PR3)2]BF4. This reacts with H2 to liberate norbomane and the active Rh(PR3) moiety. In polar solvents [RhH2(solv)2(PR3)2] is obtained. Such catalysts can selectively reduce alkynes to alkanes and dienes to monoenes. They are also active for the reduction of ketones, substrates which are much more difficult to reduce. The cationic rhodium catalysts are useful for asymmetric hydrogenation. In this variant, the presence of a chiral phosphine leads to differences in the rates of H2 addition to the two faces of a prochiral alkene. Where the alkene has groups such as C02Me suitably placed to bind to the metal, the selectivity can become very great enantiomeric excesses of the product over its enantiomer can reach 95-98% (equation 67). The mechanism has recently been elucidated by Halpern. ... 

In aqueous solution mer-[Ir(H)(H)Cl(PMe3)3] catalyzed the hydrogenation of various alkynes to alkanes [19]. 

The answer is D. There is nothing there to dehydrate or dehydrohalogenate in an alkyne. This eliminates Choices A and B. Hydrolysis will not help the direct conversion of an alkyne to alkane. A simple hydrogenation is the best way to accomplish this conversion. 

Aikynes to alkenes Alkenes/alkynes to alkanes dichloride PCI5... 

For the reduction of alkenes or alkynes to alkanes in laboratory we use metal catalysts such as Pt or Pd and often high pressures. The heating of alkane precursors with these metal catalysts reoxidizes alkanes to alkenes. In biosynthesis these reactions proceed with special reagents like flavine adenine dinucleotide FAD or its reduced form FADH2. 

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