Acetylene alcohols, selective hydrogenation

Partial hydrogenation of acetylenic compounds bearing a functional group such as a double bond has also been studied in relation to the preparation of important vitamins and fragrances. For example, selective hydrogenation of the triple bond of acetylenic alcohols and the double bond of olefin alcohols (linalol, isophytol) was performed with Pd colloids, as well as with bimetallic nanoparticles Pd/Au, Pd/Pt or Pd/Zn stabilized by a block copolymer (polystyrene-poly-4-vinylpyridine) (Scheme 9.8). The best activity (TOF 49.2 s 1) and selectivity (>99.5%) were obtained in toluene with Pd/Pt bimetallic catalyst due to the influence of the modifying metal [87, 88]. 

Acetylenic alcohols, usually of propargylic type, are frequently intermediates in the synthesis, and selective reduction of the triple bond to a double bond is desirable. This can be accomplished by carefully controlled catalytic hydrogenation over deactivated palladium [56, 364, 365, 366, 368, 370], by reduction with lithium aluminum hydride [555, 384], zinc [384] and chromous sulfate [795], Such partial reductions were carried out frequently in alcohols in which the triple bonds were conjugated with one or more double bonds [56, 368, 384] and even aromatic rings [795]. 

The nitrile group can be selectively hydrogenated in the presence of aldehydes, ketones but not in the presence of olefins, acetylenes, and nitro groups. Cyanohydrins are reduced to amino alcohol mostly over platinum oxide in acetic acid . 

We studied the behavior of catalytic nanoparticles formed in a nanostructured polymeric environment in the hydrogenation of long chain acetylene alcohols and the direct oxidation of a monosaccharide (L-sorbose). These reactions were chosen because of their industrial relevance and also because of the special importance of high selectivity, which can be achieved using a polymeric environment in mild... 

It is well established that low molecular weight modifiers such as quinoline, pyridine, etc. [51] increase the selectivity of the hydrogenation of acetylene alcohols, but often the modifiers leach and selectivity deteriorates. In the case of pyridine units of the P4VP block, the modification is fairly permanent. The stability of modification, which governs the stability of catalytic properties and high selectivity, is one of the important advantages of catalytic nanoparticles stabilized in the polymeric media [47]. 

Table 3.1 Catalytic properties and kinetic parameters of selective hydrogenation of acetylene alcohols with micellar catalysts based on PS-b-P4VPl l... 

In Table 3.1 C is the initial acetylene alcohol concentration, Q is the catalyst concentration, S is the selectivity (%), A is the acetylene alcohol conversion (%), turnover frequency (TOP) is the mole of substrate converted over a mole (Pd) of the catalyst per second, Xt is the relative concentration Xi= Q /Q (where Q is the current concentration of the substrate at i= 1 and product at i=2). Strictly speaking TOP should be calculated per Pd atoms participating in the catalytic reaction (available surface atoms), but for the sake of comparison with hterature data, in this chapter we will use the TOP definition given above. To find the kinetic relationships, we have studied the reaction kinetics at different substrate-to-catalyst ratio SCR=Co /Q. Kinetic curves for DHL hydrogenation with Pd and bimetallic catalysts are presented in Pig. 3.4. 

The catalytic properties of Pd-containing aluminas were studied in the selective hydrogenation of aU three long chain acetylene alcohols DMEC, DHL and DHIP (Scheme 3.1, Table 3.3). Because the structure of aluminum sites in both A-Pd-... 

Table 3.3 Catalytic activity and selectivity of microgel-templated mesoporous alumina in the hydrogenation of acetylene alcohols. ...

The hydrogenation of dimethylethynylcarbinol (DMEC), an alcohol containing an acetylene bond was conducted with the most active catalyst, P4VP-Pd(NaBH4) [70]. The rate of reduction of the triple bond of DMEC is lower than that of the double bond. This is clearly seen from the kinetic curve in Fig. 26a i . the reaction rate sharply increases after the absorption of a half of the calculated quantity of hydrogen. The chromatographic analysis of reaction products shows relatively selective hydrogenation of the acetylenic bond (Fig. 26b). 

Moreover, n-hexanal may serve for the manufacture of l-octen-3-ol (mushroom alcohol) by the addition of acetylene followed by selective hydrogenation (Scheme 6.4) [19]. The unsaturated alcohol gives flavors and fragrances an earthy... 

Methylation of the terminal acetylenic C atom requires deprotonation by strong bases since the pKi, of acetylene is ca. 25. Z-selective hydrogenation of the triple bond is followed by reduction to alcohol and bromination to TM 12a. In the last steps of synthesis, alkylation of ethyl acetoacetate TM 2.12b then decarboxylation... 

The selective hydrogenation of acetylene alcohols is an important step in the synthesis of fine chemicals, especially vitamins A and E [153,155]. 

Practical Exercise PdZn/Ti02-Catalyzed Selective Hydrogenation of Acetylene Alcohols in a Capillary Microreactor [111]... 

Yanov, I., Leszczynski, J., Sulman, E., Matveeva, V., Semagina, N. (2004). Modeling of the molecular structure and catalytic activity of the new fullerene-based catalyst (i/ -C6o)Pd(PPh3)2 An application in the reaction of selective hydrogenation of acetylenic alcohols. International Journal of Quantum Chemistry, 100, 810-817. 

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