1.2- Dimethylcyclohexene, hydrogenation stereochemistry

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. 

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 ... 

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). 

Similarly, from 1,2-dimethylcyclohexene the fraa.v-dinitro derivative 4 was prepared (together with several minor byproducts) with more than 30 1 stereoselectivity118 the trans stereochemistry was demonstrated by the separation of the enantiomers on a chiral capillary GC column. Slow addition of the alkene to an excess of dinitrogen tetroxide is crucial to the success of the dinitration. The trans-diamine 5 was obtained by catalytic hydrogenation. 

The stereochemistry of addition of hydrogen halides to unconjugated alkenes is usually anti. This is true for addition of HBr to l,2-dimethylcyclohexene, cyclohexene, 1,2-dimethylcyclopentene, cyclopentene, Z- and -2-butene, and 3-hexene, among others. Anti stereochemistry is also dominant for addition of hydrogen chloride to 1,2-dimethylcyclohexene and l-methylcyclopentene. Temperature and solvent can modify the stereochemistry, however. For example, although the addition of HCl to 1,2-dimethylcyclohexene is anti near room temperature, syn addition dominates at —78°C. °... 

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