Dimethylcyclohexenes, hydrogenation

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 ... 

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

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. 

Nevertheless, during hydrogenation of the dimethylcyclohexenes, the fact that the quantities of trans isomer increase with the double bond migration ability of the various catalysts90 suggests that double bond migration sites (or the sites nearby) are involved in apparent trans addition. 

Tetrasubstituted Alkenes. Tetrasubstituted alkenes lacking electron-withdrawing substituents undergo facile ionic hydrogenation to alkanes in very good yields. Simple examples include 2,3-dimethyl-2-butene,208,214 1,2-dimethyl-cyclopentene, 1,2-dimethylcyclohexene,229 and A9(10)-octalin.126,204,212... 

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°. 

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. 

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

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 ... 

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 ... 

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. 

When 1,4-dimethylcyclohexene is hydrogenated on Pt and Rh, ds-l,4-dimethylcyclo-hexane is formed in slight excess (Table 7). These data, again, are consistent with the involvement of 7r-adsorbed species with an almost equal degree of hindrance in the alternate adsorption modes. The homoallylic substituent, consequently, does not exert any strong influence on stereochemistry. The trans compound, however, is the main product on Pd (Table 7) indicating a favored trans adsorption mode (18). 

The two products formed by addition of hydrogen bromide to 1,2-dimethylcyclohexene cannot be regioisomers. Stereoisomers are possible, however. 

The same two products are formed from 1,6-dimethylcyclohexene because addition of hydrogen bromide follows Markovnikov s rule in the absence of peroxides. 

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. 

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