Dimethylcyclohexenes

In Problem 5 17 (Section 5 13) we saw that acid catalyzed dehydration of 2 2 dimethyl cyclohexanol afforded 1 2 dimethylcyclohexene To explain this product we must wnte a mecha nism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one Another product of the dehydration of 2 2 dimethylcyclohexanol is isopropyhdenecyclopentane Wnte a mechanism to rationalize its formation... 

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... 

Consider the ozonolysis of tram 4 5 dimethylcyclohexene having the configuration shown... 

The alkene mixture obtained on dehydration of 2,2-dimethyl-cyclohexanol contains appreciable amounts of 1,2-dimethylcyclohexene. Give a mechanistic explanation for the formation of this product. 

What products would you expect from reaction of 4,4-dimethylcyclohexene with NBS ... 

A similar reaction the with rran.v-isomer 3b gave c -3,5-dimethylcyclohexene (4) with very high diastereoselectivity. Accordingly, the stereochemistry of this substitution is anti. Deuterium labeling experiments using the 1-deuterio or 3-deuterio derivative of 3 a showed that the ratio of SN2 /SN2 with lithium dimethylcuprate was about 50 50, while the ratio with lithium cyano(methyl)cupratc was >96 4. 

The stereochemistry of HX addition is varied. Examples are known of predominant syn, anti, and nonstereoselective addition. It was found that treatment of 1,2-dimethylcyclohexene (4) with HBr gave predominant anti addition, ° while addition of water to 4 gave equal amounts of the cis and trans alcohols ... 

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. 

Reports of the photoaddition of nitriles to alkenes have also been published. An intermediate azetine 282 has been identified in the photoaddition of benzonitrile to 1,2-dimethylcyclohexene (283) to give adducts 284 and 285,235 and stable azetines have been prepared by direct photoaddition of... 

The triethylsilane/trifluoroacetic acid reagent system reduces alkenes to alkanes in poor to excellent yields depending largely on the ability of the alkene to form carbocations upon protonation. Under these conditions the more substituted olefins are reduced in better yields and styrene double bonds are reduced in high yields.127,202,207,221-228 The reduction of 1,2-dimethylcyclohexene with this reagent gives a mixture of cis- and trans- 1,2-dimethylcyclohexane.229 The formation of the trifluoroacetate esters is a side reaction.205,230... 

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... 

Moving closer to a cationic transition state mimic, Hasserodt et al. (1996) used the amidinium ion [75] as a TSA for cyclization of the arenesulfonate ester [761- One antibody raised to this hapten, 17G8, catalysed the conversion of substrate [76] into a mixture of the 1,6-dimethylcyclohexene [77] and 2 methylene-1-methylcyclohexane [78] (Fig. 28) (Appendix entry 15.3). By contrast the uncatalysed cyclization of [76] formed a mixture of 1,2-... 

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. 

In a detailed study of the reduction of the xylenes in the liquid phase on a 5% rhodium on carbon catalyst, Siegel and Ku (105) showed that both 1,2- and 2,3-dimethylcyclohexenes are formed from orlAo-xylene (Fig. 19). The initial (extrapolated) ratio, l,2-/2,3-, lies between 0.5 and 1 but rises as the reaction proceeds. If the initial distribution of cyclo-alkenes was random as previously postulated (97) the ratio should be 0.5. 

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