Hydrogenation of 1,2-dimethylcyclohexene

Construct a molecular model of the product formed by catalytic hydrogenation of 1 2 dimethylcyclohexene Assume syn addition occurs... 

Fig. 39. Reaction scheme for the hydrogenation of 1,2-dimethylcyclohexene by a Horiuti—Polanyi mechanism as proposed by Siegel [221].

In the hydrogenation of 1,2-dimethylcyclohexene over a platinum catalyst, the suprafacial addition product is formed. Assuming that the mechanism of this hydrogenation is as shown in Figure 11-2, what conditions must be put on the stereochemistry of each of the postulated steps in order that the overall reaction be suprafacial ... 

The hydrogenation of 1,2-dimethylcyclohexene (17 Pt02, HOAc, 1 atm) yields trans-(19) as well as cis-1,2-dimethylcyclohexane (18), but Ir and Os in t-butyl alcohol are much more selective (equation lb). The addition of hydrogen to (17) competes with isomerization to 1,6-dimethylcyclohexene (20), which not only reacts more rapidly but yields trans (19) as well as the cis product (18) via syn addition of the two hydrogen atoms (equation 17). In the Pt-catalyzed reaction, the intermediate (20) can be observed, although its steady-state concentration remains low (0.21% of the 1,2-isomer 17) because of its... 

Sabadie and Germain [20] have investigated the stereoselectivity of polymer-supported metal catalysts in the hydrogenation of 1,2-dimethylcyclohexene. Depending on the pressure of hydrogen, different ratios of cis- and trans-isomers (0.44 and 0.57 at 1.25 and 10 MPa, respectively) of 1,2-dimethylcyclohexanes were obtained over Pd/APSDVB catalysts. 

However, such stereoselectivity is not always observed. For example, hydrogenation of 1,2-dimethylcyclohexene (3) yields trans-4 as well as cis-4, with the product ratio dependent on the metal and the reaction conditions employed 6 19. In contrast to osmium and iridium, which give nearly exclusively the expected co-product, hydrogenation in the presence of platinum results in a higher proportion of the tra .v-product. In the latter case hydrogenation competes with isomerization to 1,6-dimethylcyclohexene. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

In the transformation of 1,2-dimethylcyclohexene the fraction of the cis isomer was observed to increase with increasing hydrogen pressure. This is a clear indication that the hydrogen partial pressure affects step 3 (equation 5) in the Horiuti-Polanyi mechanism by shifting the equilibrium to the formation of the half-hydrogenated state, therefore suppressing isomerization and increasing selectivity. 

Therefore, for either antipode, the diastereomeric activated complex controlling optical yield could be either the one corresponding to the formation of the x-complex or the one corresponding to the olefin insertion into the metal-hydrogen bond. In the case of rhodium, it appears from the results of the hydroformylation of 1,2-dimethylcyclohexene and of 2-methylmethylidencyclohexane, that the second case is more probable 10). In the case of platinum, the fact that isomerization of the substrate, which is very likely to occur via metal alkyl-complex formation, proceeds at a rate similar to or even higher than the hydroformylation rate seems to indicate that the same situation can also be assumed. 

TABLE 3.14 The Stereoselectivity and Isomerization Ability of the Platinum Metals in the Hydrogenation of 1,6-Dimethylcyclohexene and 2-Methyl met hvlenecyclohexanet... 

Although not many examples are known, properly constrained organic compounds can transfer the elements of hydrogen in what appears to be a concerted process akin to reductions by diimide, HN=NH. The reduction of 1,2-dimethylcyclohexene (72) by c -9,10-dihydronaphthalene (71) is an example (equation 31). Yields are only moderate, but the stereospecificity is consistent with a concerted pro-... 

A comparison of the proportion of the saturated stereoisomers, formed under the same conditions (Pt02, AcOH) from the isomeric xylenes and the derived dimethylcyclohexenes, leads to the proposal that the reaction proceeds through the desorbed cyclohexenes. Low concentrations of the intermediate cyclohexenes were detected later. The effect of the metal on the stereoselectivity of hydrogenating o-xylene follows closely the effect of the metal on the hydrogenation of 1,2- and 1,6-dimethylcyclo-hexene. The highest selectivity for the conversion of o-xylene to the cis isomer is given by iridium... 

Under some experimental conditions, particularly with tetrasubstituted double bonds, some percentage of the product may appear to be formed by anti addition of hydrogen. Catalytic reduction of 1,2-dimethylcyclohexene, for example, yields predominantly ds-l,2-dimethylcyclohexane. Along with the cis isomer are formed lesser amounts of frflns-l,2-dimethylcyclohexane as a racemic mbcture. 

Two different compounds having the molecular formula CgHi5Br are formed when 1 6 dimethylcyclohexene reacts with hydrogen bromide in the dark and in the absence of peroxides The same two compounds are formed from 1 2 dimethylcyclohexene What are these two compounds ... 

Dimethylcyclohexene is an example of an alkene for which the stereochemistry of hydrogen chloride addition is dependent on the solvent and temperature. At —78° C in dichloromethane, 88% of the product is the result of syn addition, whereas at 0° C in ether, 95% of the product results from anti addition.8 Syn addition is particularly common with alkenes having an aryl substituent. Table 4.1 lists several alkenes for which the stereochemistry of addition of hydrogen chloride or hydrogen bromide has been studied. 

Fiq. 5. Variation with the pressure of hydrogen of the proportion of cis- and trans-diinethylcycloalkanes obtained from 1,2-dimethylcyclohexene (0), 2,3-dimethyIcycIo-hexene (A), 1,2-dimethylcyelopentene ( ), and 2,3-dimethylcyclopentene (V) reduced Pt02 in glacial acetic acid at 25°. 

The stereochemistry of addition depends largely on the structure of the alkene, but for simple alkenes and cycloalkenes, addition occurs predominantly in an antarafacial manner. For example, hydrogen bromide reacts with 1,2-dimethylcyclohexene to give the antarafacial addition product ... 

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