Octyne, hydrogenation

The first task is to assemble a carbon chain containing eight carbons. Acetylene has two carbon atoms and can be alkylated via its sodium salt to 1-octyne. Hydrogenation over platinum converts 1-octyne to octane. 

Finally, the only example of a polynuclear homogeneous catalyst is the dinuc-lear complex [Pt P sH ]4- [66], which catalyzed the hydrogenation of styrene, phenylacetylene, 1-octyne, and 1-hexyne (i-PrOH, 60°C, 20.7 atm H2 pressure, Pd substrate ratio 1 1800) to the corresponding alkanes within 10 h of reaction. 

An example of these pressure studies is provided by the studies of Elsevier et al. [31], who investigated the dependence of the hydrogenation rate of 4-octyne by a Pd-catalyst on the dihydrogen pressure, which was varied between 0 and 40 bar. The hydrogenation rate was shown to depend linearly on the dihydrogen pressure. In order to elucidate the reaction mechanism, the dependence of the reaction rate on substrate and catalyst concentration, and on the temperature, was also measured. NMR experiments with deuterium gas as well as PHIP-ex-periments were also carried out. 

At high substrate, or low hydrogen concentration, the semihydrogenation of 4-octyne is inhibited by the formation of catalytically inactive palladacycle species. These species are formed by oxidative coupling of two substrate molecules. 

Davis 69) found no considerable variation in the o-xylene versus ethylbenzene ratio as a function of hydrogen pressure. He also observed that the relative amount of o-xylene from n-octane increased (a) with decreasing Pt loading of the catalyst (70) b) with increasing tin addition 69, 7J) (c) with the poisoning of the catalyst with thiophene (77) and d) if octenes or octynes... 

The H-P bond in hydrogen phosphonates readily adds across a C=C bond [22]. Upon treatment of 1-octyne with dimethyl phosphonate in the presence of a palladium complex in refluxing tetrahydrofuran, the addition reaction proceeds smoothly to afford dimethyl l-octen-2-ylphosphonate (2) regio-and stereo-selectively (Scheme 18). 

The complex RhCl(ttp), where ttp = PhP(CH2CH2CH2-PPh2)2, in the presence of either triethylaluminum or diethylaluminum chloride, is an effective homogeneous catalyst for hydrogenation of 1-olefins and 1-octyne. The rates of hydrogenation of substituted olefins are considerably slower than for terminal olefins. H-l and P-31 NMR spectra were used to identify several different chemical species [including RhH(ttp)] in these catalyti-cally active solutions. The observed rate of hydrogenation of 1-octene to n-octane at 20 0.3°C and under a constant H2 pressure of 750 torr is 6.4 x min 9... 

Materials. The complex RhCl(ttp) was prepared as described previously (6). Cyclohexanone was obtained from Mallinkrodt cyclohexene and 1-pentene were obtained from MCB. All other olefins and 1-octyne were purchased from ChemSampCo. The aluminum alkyls were obtained from Aldrich as 25% w/w solutions in toluene. Neat liquids used in the H-l NMR work were obtained from these solutions by removing the toluene under vacuum. Lithium dimethylamide, LiN(CH3)2, was obtained from Alpha/Ventron as a 10% w/w slurry in hexane. Hydrogen was passed through an Englehard Deoxo purifier and activated alumina before use. 

Alternatively, two successive alkylations of acetylene with CH3CH2CH2Br could be carried out to give 4-octyne (CH3CH2CH2C=CCH2CH2CH3), which could then be hydrogenated to octane. 

C. Excess 1 is destroyed by adding 22 mL (0.3 mol) of freshly distilled propionaldehyde and stirring for 1 hr at room temperature. Liberated a-pinene is then removed by vacuum (Note 7). Tetrahydrofuran, 200 mL, is added, followed by 150 mL of 3 M aqueous NaOH. Hydrogen peroxide (150 mL, 30%) is added dropwise (CAUTION Note 8). Oxidation is complete in 3 hr at 40°C. The reaction mixture is transferred to a separatory funnel and extracted with three 50-mL portions of ethyl ether. The ether layers are combined and dried with copious amounts of anhydrous magnesium sulfate, filtered, and concentrated by rotary evaporation to give an oil. Distillation at 60-65°C (3.0 mm) yields 31 g (0.245 mol) of l-octyn-3-ol, 86% yield (Note 9). The distillation pot residue is a thick oil consisting for the most part of cis-l,5-cyclo-octanediol. An NMR lanthanide shift study showed the alcohol to be 93% (R) and 7% (S), 86% ee, (Note 10 and 11). 

Is a waxy solid which does not lend itself to recrystallization. Attempts to form crystalline salts of the phthalate derivative with achiral alkyl amines only lead to waxy solids or thick oils. The phthalic amine salt made with racemic l-octyn-3-ol requires 3-4 recrystal 1 Izations from methylene chloride to resolve enantiomers. The first recrystallization may take several days, with successive recrystal 1Izations becoming easier. If the 86% ee l-octyn-3-ol is used to make the phthalic amine salt only one facile recrystalHzation is needed to provide optically-pure alcohol. The pure amine salt melts at 132-134°C. The enantiomeric purity of the salt may be determined by NMR by observing the ethynyl hydrogen doublets at 6 2.48 (minor) and 2.52 (major) (CDClj solvent). 

Hydrogenation of 1-octyne and phenylacetylene in C2 to n-C4 alcohols and n-C6 to n-Cg alkenes by palladium oxide catalysts 15... 

This catalyst (1) is a selective catalyst for hydrogenation of 4-octyne to cw-4-octene (90% yield). If the product is left in contact with 1, it isomerizes to the trans-isomer. Since monoalkenes are hydrogenated slowly with this catalyst, cyclo-octene can be obtained from either 1,3- or 1,5-cyclooctadiene. Of more interest, benzene can be hydrogenated to cyclohexane (83% yield). One drawback is that the catalyst loses about 90% of its activity after one cycle. 

In the initial studies by Campbell and Eby it was noted that 3- and 4-octyne, 3-hexyne and 5-decyne could be efficiently reduced to the corresponding rram-alkenes in good yields and with remarkably high stereoselectivity. Shortly thereafter, Henne and Greenlee reported the quantitative reduction of 1-alkynes to 1-alkenes using sodium in ammonia in the presence of ammonium ion. In the absence of ammonium ion, however, extensive metallation of the 1-alkyne occurs. In the presence of ammonium ion dialkylacetylenes are inefficiently reduced (extensive hydrogen evolution occurs, in which sodium is consumed). 

The nature of the solvent in liquid-phase alkyne hydrogenations and the extent to which it can influence the competitive adsorption factors needed to attain selectivity should also be considered. The semihydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts of varying metal load gave 1-octene with a selectivity of 100% over a wide range of metal loads when the reaction was run in heptane.38 n-propanol, however, reaction selectivity increased with decreasing metal load. Apparently the alcohol interacted with the catalyst to modify the active sites and influenced the relative adsorption characteristics of the acetylenic and olefinic species. This can affect reaction selectivity particularly if reactant diffusion assumes some importance in the reaction. 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

Ahyd/7 of all the linear monoalkynes in this series were measured in n-hexane solution. The slight trend toward less exothermic enthalpies of hydrogenation for 2-octyne to 4-octyne is due to the larger number of conformers in the alkane product. Thus, A H of the alkyne is not decreasing along this series, Af H of the conformational mix of products is increasing. Compare the linear nonynes and decynes. 

The use of secondary modifiers, e. g. quinoline, and the choice of solvent also play important roles in directing semi-hydrogenation selectivity. For example, in the hydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts metal loading had no effect on selectivity when the reaction was performed in n-heptane as solvent. When the same experiment was conducted in n-propanol, however, an inverse relationship between selectivity and catalyst metal loading was observed [56], This effect has been interpreted as a polar solvent-induced modification of the Pd active sites, which alters the relative adsorption behavior of the alkyne and alkene species [57], Modification by addition of quinoline is reported to benefit the selective production of a cij-vitamin D precursor from the related disubstituted alkyne [58] ... 

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