Selective Hydrogenation of Acetylene Alcohols

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

Continuous flow capillary microreactors with embedded monometallic (Pd) or bimetallic (Pd25Zn75) catalysts have been tested in the selective hydrogenation of alkyne reagents, among which was 2-methyl-3-butyne-2-ol [155]. Under conventional reaction conditions a number of side products can be formed. 

Under nitrogen atmosphere, oxidative dimerization dominates, and 2-methyl-3-butyn-2-ol (MBY) is converted over a Pd catalyst into 2,7-dimethyl-3,5-octadiyne-2,7-diol (1) which can further be hydrogenated to 2,7-dimethyl-5-octen-3-yne-2,7-diol (2). 

The highest selectivity toward the alkene product of 90% was obtained at 99.9% conversion on the Pd25Zn75/Ti02 catalyst. The selectivity was further increased to 

Further, selective Pd-catalyzed hydrogenation of 3-methyl-l- pentyn-3-ol to PI without over-hydrogenation to P2 was used to prove the advantageous characteristics of segmented flow in eliminating diffusional effects and axial dispersion. 

---

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

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. 

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

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]. 

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-... 

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). 

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... 

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 [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

---