Hydrogenation of 2-hexyne

Figure 2. Hydrogenation of 2-hexyne in ethanol using 1 wt% of 1,10-phenanthroline-...

Figure 3-47. Hydrogenation of 2-hexyne with Pd7/8 on TiOj (1% weight) and H2 at normal pressure and room temperature. During the first 2 hours, the 2-hexyne is almost quantitatively transformed into cis-2-hexene. Only then does the formation of trans-1-hexene and other isomers begin.

Hydrogenation of 2,5-diacetoxy-2,5-dimethyl-3-hexyne 10 over 0% palladium-on-carbon is exceptionally complex. Seven different products are formed together with acetic acid. All are hydrogenolysis products arising from the initially formed 2,5-diacetoxy-2,5-dimethyl-3-hexene 11. One of these, 2,5-dimethyl-2-acetoxy-4-hexene 12 forms in as much as 4S yield. 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

From the difference between the enthalpies of formation of 2,5-dimethyl-2,5-dihydroperoxyhexane and 2,5-dimethyl-2,5-dihydroperoxyhex-3-yne, the enthalpies of hydrogenation of the alkyne are derived as —298.5 kJmol (g) and —309.6 kJmol (s). The gas phase value is quite a bit larger than that for its deoxygenated counterpart, 3-hexyne, of —272.4 kJmol (g) - However, the calculated solid phase enthalpy of hydrogenation of 2,5-dimethylhex-3-yn-2,5-dior to 2,5-dimethylhexane-2,5-diol is -295.7 kJmor. ... 

Hexyne is best suited to study activity and selectivity of a hydrogenation catalyst. A mixture of 1,10-phenanthroline-stabilized seven- and eight-shell Pd clusters (Pd 7/8) on Ti02, dispersed in ethanol, catalyzes the transformation of 2-hexyne to cA-2-hexene with ca. 95 % selectivity and a turnover frequency (TOF) of 35 min . As can be seen from Figure 2, tranx-2-hexene, n-hexane, and other hexene isomers are only formed in consecutive reactions. If the phenan-... 

The hydrogenation of dialkylalkynes is widely used in organic chemistry for the preparation of olefins in the cis-configuration (93), but only recently have studies been reported which have sought to elucidate the mechanisms that occur in this type or reaction. Studies have now been reported on the hydrogenation of 2-butyne (56, 84, 94, 95), 2-pentyne (58, 91), 2-hexyne (91, 92), and 3-hexyne (.91, 92, 94). The characteristics of these reactions have been shown to be fairly independent of the complexity of the molecule and of the phase in which the reaction was carried out. 

The nickel-catalyzed hydrogenation of 2- and 3-hexyne showed a selectivity of about 0.97 and the olefins were principally of the cis-configuration (92). Using alumina-supported palladium (96) and no solvent, the initial olefin distribution from 3-hexyene was cis-3-hexene, 92% [Pg.182]

The complex [(OC)4Fe(//-PPh2)Pd( -Cl)]2 is a selective catalyst for the isomerization of 1-octene to 2-octene and the hydrogenation of 1-hexyne in the presence of 1-hexene. At 448 K, under 100 atm H2, 93% of a sample of 1-hexyne in benzene was reduced to hexene and only 3% to hexane. This is unexpected because palladium is usually an excellent catalyst for the hydrogenation of olefins. It also catalyzes the carbonylation of 1-octene under mild conditions (348 K, 50 atm). The total yield of esters was ten times greater than with [PdCl2(PPh3)2] as a catalyst. This bimetallic complex was also an effective catalyst for the carbonylation of 1,5-cyclooctadiene. ... 

Another example of the application of supported nanometal colloids in line chemicals catalysis is the cw-selective partial hydrogenation of 3-hexyn-l-ol to leaf alcohol, a valuable fragrance in 1996 the amount produced was 400 tons, including esters (see Eq. 2). 

A surfactant was found to control the selectivity in the cis-selective partial hydrogenation of 3-hexyn-l-ol giving leaf alcohol, which is a valuable fragrance (Eq. (2.5)) [140]. 

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