Hydrogenations cyclohexene hydrogenation over

Turnover Numbers in Cyclohexene Hydrogenation over Pt/S102 Samples... 

Figure 7 Relative rates of benzene and cyclohexene hydrogenation over Ru/Si02 catalyst [adapted from ref. 16].

Carba-y5-DL-idopyranose (77) was prepared from (readily accessible) (l,2,4/3)-5-(hydroxymethyl)-5-cyclohexene-l,2,3,4-tetrol (75) by hydrogenation over a platinum catalyst, acetylation of the product, and 0-deacetylation. 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane (> 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts... 

AUylic ethers were reduced by treatment with lithium in ethylamine to alkenes [636]. Benzyl ethers are hydrogenolyzed easily, even more readily than benzyl alcohols [637], 3,5-Bis(benzyloxy)benzyl alcohol gave 3,5-dihydroxy-benzyl alcohol on hydrogenation over palladium on carbon at room temperature and atmospheric pressure in quantitative yield [638. Hydrogenolysis of benzylic ethers can also be achieved by refluxing the ether with cyclohexene (as a source of hydrogen) in the presence of 10% palladium on carbon in the presence of aluminum chloride [639]. 

Occasionally with catalysts having high activity for aromatic ring reduction and relatively low activity for olefin saturation, partial reduction to olefins may be useful in synthesis. For example, hydrogenation of benzene over ruthenium afforded cyclohexene (12) in 20% yield and cis-A2-tetrahydroterephthalic acid was obtained in 73% yield from tereph-thalicacid (31). 

Petrov and Shchekin (297) showed that below the cracking temperature (250-316°C.) cyclohexene undergoes over silica-alumina hydrogen disproportionation and dimerization. Identical results were obtained from 1-methyl-l-cyclopentene. Ring expansion of lower alkylated cyclopentanes occurs simultaneously with polymerization. However, no bicyclic compounds with similar rings were formed. 

In contrast to 1,2-dimethylcyclohexene, methyl cyclohexene-1,2-dicarboxylate was reported to yield only the cis saturated product in the hydrogenation over platinum oxide in acetic acid at 26-27°C, independently of the pressure of hydrogen (0.1-20 MPa) and the concentration of the substrate (0.05-1,0M).140 Hydrogenation of methyl cyclohexene-1,6-dicarboxylate also gave the same result at about 1 atm H2, but some of the trans isomer (6+2%) was formed at a pressure of 13 MPa H2. 

---