4.5- Dimethylcyclohexene

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... 

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. 

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. 

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... 

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. 

Acid-catalyzed dehydration of 2,2-dimethylcyclohexene in ether yields a mixture of 1,2-dimethylcyclohexene and isopropylidenecyclopentane. 

MULTIPLE BONDS 1,2-Dimethylcyclohexene HC1 solvent- and temperature-dependent a... 

Dimethylcyclohexene (209) is converted to two tertiary hydroperoxides, 210 (12%) and 211 (88%).85,135 1-Methylcyclohexene (30) gives rise to 45% of the tertiary hydroperoxide 31 and to 55% of a non-resolved mixture of secondary hydroperoxides, 52.135 Since the ratio tertiary/secondary hydroperoxides is the same as with limonene (16) and carvomenthene (19), a similar product distribution among the secondary hydroperoxides 32 as was found for 16 and 19 has been assumed. No product distribution has been reported for the photosensitized oxygenation of 1-methylcyclopentene, 213.85,123... 

Very few data are available concerning the stereochemistry of these additions. Hydration of 1,2-dimethylcyclohexene was found to be nonstereoselective.27 In contrast, predominant or exclusive exo-syn addition was shown to take place in the reaction of 2,3-dideuteronorbomene with water and alcohols 28... 

A more detailed picture could be obtained from the results of stereochemical studies.3111 Since 1,2-dimethylcyclohexene, 1,6-dimethylcyclohexene, and 2-methylmethylenecyclohexane give different proportions of cis- and tram-1,2-dimethylbromocyclohexanes, simultaneous formation of the C-H and the C-Br bonds was suggested.112 A similar conclusion was arrived at on the basis of the stereoselective, predominantly anti addition of HC1 and HBr to 1,2-dimethylcyclo-pentene in pentane.113... 

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

The relative reactivities toward diimide cover a range of -10, from 1,2-dimethylcyclohexene to norbornene (ref. 6). Electron attractive substituents increase the reactivity of the double bond towards diimide although the data to place compounds such as maleic acid or acrylonitrile on the scale for Garbisch s hydrocarbons is lacking (ref. 21b). Garbisch et al. found that the main factors that contribute to the observed reactivities in diimide reductions of unsaturated hydrocarbons, eqn. (3), are torsional strain, bond angle... 

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