Hexyne, selective

Typical concentration-time profiles during the 1-hexyne hydrogenation over 0.4wt.% Pd/ACF catalyst are presented in Figure 7 showing the experimental and simulated curves (Langmuir-Hinshelwood mechanism). Pd/ ACF materials with the same particle size but different Pd loading (0.4, 0.6, 1.2wt.%) show identical initial activity of 0.140 0.004 kmolHj/kgp(j/s. This indicates the absence of diffusion limitations. Selectivity to 1-hexene is 97.1 +0.4% up to 80% conversion, and 95.9 + 0.4% at 90% conversion. 

Hydrogenations involving consecutive reactions are common in the organic process industry and even in the hydrogenation of fats. In the fine chemicals industry we have examples of acetylenic (triple) bonds to be selectively converted to olefinic (double) bonds. Lange et al. (1998) have shown, for the comversion of the model substance 2-hexyne into cis-2-hexene, how catalytically active microporous thin-film membranes can accomplish 100% selectivity. This unusual selectivity is attributed to avoidance of backmixing. 

Alkynes are hydrogenated to cis olefins with the same catalytic systems, and subsequently undergo hydrogenation to yield the corresponding alkanes [7, 42, 45, 47, 49, 59, 93]. For example, Jordan et al. reported the selective hydrogenation of 3-hexyne into cis 3-hexene with a TOF of 25 IT1 [25], and cis 3-hexene is... 

Non-Sonogashira alkynylation procedures have also been employed with pyrimidines, and selective replacement of the 4- and 6-chlorine atoms of 2,4,6-trichloropyrimidine 258 was able to be achieved using potassium (1-hexyn-l-yl)-trifluoroborate 257 <2002JOC8416>. 

Although silylformylation of 3-butyn-I-ol 84 gives normal product 85 preferentially in the absence of EtsN, an appreciable amount (38%) of 7-lactone 86 is formed concomitantly." Protection of the hydroxy group in 84 leads to selective silylformylation of the acetylenic moiety as shown in Scheme 3. Hydrolysis of the silyl ether in 88 gives 85 as a single product. 4-Pentyn-I-ol 89 reacts with Mc2PhSiH under CO pressure to give a mixture of silylformylation product 90 (20%) and (5-lactone 91 (38%) after a short reaction time (0.5 h) (Equation (16)). The unusual lactone formation is not observed in the reaction of 5-hexyn-l-ol 92 in the presence of EtsN (Equation (17)). ... 

A stereospecific synthesis for cw-3-hexen-l-ol starts with the ethylation of sodium acetylide to 1 -butyne, which is reacted with ethylene oxide to give 3-hexyn-l-ol. Selective hydrogenation of the triple bond in the presence of palladium catalysts yields cw-3-hexen-l-ol. Biotechnological processes have been developed for its synthesis as a natural flavor compound, e.g., [12]. 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

The diphosphine of choice for obtaining good regioselectivity is the bis(diphe-nylphosphino)butane (Scheme 4). It enables addition of a variety of carboxylic acids to phenylacetylene and hexyne. The reaction temperature, which enables complete conversion can be reduced from 80 to 0 °C when the acidity of the carboxylic acid increases in the pK, range from 5 to 1.5. The milder temperature conditions always lead to higher regioselectivity of the addition. The preparation of functionalized (Z)-cnol esters can be conducted in apolar solvents such as toluene or pentane dienyl esters are selectively produced from conjugated enynes [9]. 

Mauret and Alphonse reported that a finely divided nickel prepared by reduction of nickel halide with magnesium in ethanol was more selective for semihydrogenation of both mono- and disubstituted acetylenes than Raney Ni or those obtained by reduction in THF.86 With the nickel produced in ethanol, the rate of hydrogen uptake after 1 equiv was almost nil in the case of 3-hexyne, and 98-99% of cA-3-hexene along with only 1-2% of hexane was produced. 

The hydroborylation of terminal alkynes with pinacolborane (PinBH) and RhCl(PPh3)3 or NiCl2(dppe) as catalyst has been studied in a variety of ionic liquids including chloroaluminates and chlorozincates, see Scheme 9.39.111451 In the absence of a catalyst, the reaction between 1-hexyne and PinBH is 100% selective whereas in the presence of a catalyst, faster rates are achieved at the cost of selectivity. 

The above mechanism agrees with many experimental observations regarding the steric effect e.g., i) 1-hexyne produces a mixture of cyclotrimer and polymer in the presence of Mo and W catalysts, while terr-butylacetylene, a bulkier monomer, selectively yields a polymer with these catalysts, and ii) monosubstituted acetylenes usually form cyclotrimers exclusively with Nb and Ta catalysts, whereas disubstituted acetylenes, which are sterically more hindered, give mainly polymers with them. 

Chemoselective cocycloaddition of two molecules of a terminal alkyne together with one molecule of an internal alkyne into a benzene product is possible due to the relative unreactivity of phosphine nickel carbonyls towards simple trimerization of the internal alkyne itself (equation 28). ° " Careful control of reaction conditions has also permitted selective intermolecular cocycloaddition in the presence of cobalt catalysts as well for example, diphenylacetylene and 3-hexyne give rise to a 57% yield of 1,2,3,4-tetra-pheny 1-5,6-diethylbenzene. ... 

The cocycloaddition of a,d)-cyanoalkynes with alkynes is another efficient entry to bicyclic pyridines. In this process an intermediate metallacyclopentadiene is formed intermolecularly from the cyanoalkyne and the alkyne. The larger alkyne substitutent ends up nearer the metal with very high regioselectivity except in the case of 2-hexyne, where selectivity is reduced to about 3 2 (equation 43). ... 

Supercritical water has the potential to be an interesting solvent system for organotransition metal systems. An example of this potential is found with the complex (7r- cp)Co(CO)2, which in organic solvents catalyzes the cyclotrimerization of hexyne-1, phenylacetylene, and butyne-2. At I40°C in aqueous media these reactions show poor selectivity to benzenes, but at 374°C in supercritical water the selectivity is again high (139). 

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