Cis- and trans- 1,2-Dimethylcyclohexane

Wedge and dash drawings fail to show conformation and it s important to remember that the rings of cis and trans 1 2 dimethylcyclohexane exist m a chair conformation This... 

A reaction that introduces a second chirality center into a starting material that already has one need not produce equal quantities of two possible diastereomers Con sider catalytic hydrogenation of 2 methyl(methylene)cyclohexane As you might expect both CIS and trans 1 2 dimethylcyclohexane are formed... 

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... 

The H2-chemisorption data indicated fliat the average platinum particle size was not affected by the metal uptakeAoading and all three catalysts had a platinum particle size of ca. 1.5 nm in diameter. The catalyst exhibited high activities in o-xylene hydrogenation. Cis and trans 1, 2-dimethylcyclohexane were the only reaction products. The hydrogenation rate was found to increase by increased hydrogen partial pressure. Coirplete hydrogenation of o-xylene could be achieved at 460 K at H2 to o-xylene molar ratio of 12. 

Cis, trans isomers of disubstituted monocyclics generally exhibit similar MS profiles. However, subtle differences in fragmentation (relative abundance) profiles were found, e.g. in cis- and trans-1,2-dimethylcyclohexane. ... 

FIGURE 5.36 Three-dimensional representations of cis- and trans-1,2-dimethylcyclohexane and cis- and rra r-l,2-dimethylcyclopropane. 

The same kind of conformational analysis just carried out for cis- and trans-1,2-dimethylcyclohexane can be done for any substituted cyclohexane, such as ds-l-tert-butyl-4-chlorocyclohexane (see Worked Example 4.3). As you might imagine, though, the situation becomes more complex as the number of substituents increases. For instance, compare glucose with mannose, a carbohydrate present in seaweed. Which do you think is more strained In glucose. 

Problem 9.16 Use l, 3-interactions and gauche interactions, when needed, to find the difference in energy between (a) cis- and trans-1,3-dimethylcyclohexane (b) (ee) trans-1,2- and (aa) trans-1,2-dimethylcyclohexane. ... 

Ziegler catalysts prepared from nickel or cobalt 2-ethylhexanoate and triethylaluminum (3 or 4 1 ratio) are very efficient for the hydrogenation of aromatic compounds.o-Xylene (51) gives a mixture of cis- (52) and trans-1,2-dimethylcyclohexane (53) in 65 35 ratio. Phenol (54), dimethyl phthalate and dimethyl terephthalate are also reduced in high yield, while nitro-substituted benzene and phenols cannot be reduced. Catalytic activity toward benzene hydrogenation decreases in the order Ni > Co > Fe > Cr > Cu. ... 

The O18 exchanges accompanying the pinacol rearrangement in the cyclohexane-1,2-diol and cyclopentane-l,2-diol systems have also been investigated. The pinacolic rearrangement of both cis- (17) and trans-1,2-dimethylcyclohexane-l,2-diol (18) in aqueous acid gives 1-acetyl-methylcyclopentane and a small amount of 2,2-dimethylcyclohexanone... 

The stereochemistry of nitrene insertion into unactivated C—H bonds has been studied using substituted cyclohexanes as substrates. For arylnitrenes which usually exhibit triplet reactiviqr, the reaction is nonspecificbut most other nitrenes undergo stereospecific C—H insertion. For example, bcnzoylni-trene inserts selectively into the tertiary C—H bond of both cis and trans-1,4-dimethylcyclohexane with retention of crnifiguration. Similariy with cis- and rrun -1,2-dimediylcyclohexane as substrate, ethoxycarbonyl-, methanesulfonyl- and cyano-nitrenes all insert with retention of configuration at the tertiary C—H bond. 

Relative rate studies of cyclic cis- and trans-l 2-diols give no clear pattern of results. C -cyclohexane-l 2-diol is oxidised by V(V) faster than the trans isomer, but the reverse is found for the isomers of 1 2-dimethylcyclohexane-1 2-diol. 

Like the 1,4-dimethyl derivatives, trans-1,2-dimethylcyclohexane has a lower heat of combustion (see Table 3.2) and is more stable than cis-1,2-dimethylcyclohexane. The cis stereoisomer has two chair conformations of equal energy, each containing one axial and one equatorial methyl group. 

Construct cis- 1,2-dimethylcyclohexane by placing one —CH3 group axial and the other equatorial. Do ring flips and examine the two chair conformations. Which is the more stable conformation Explain your answer (11a). Given the two isomers, trans- 1,2-dimethylcyclohexane and cis- 1,2-dimethylcyclohexane, which is the more stable isomer Explain your answer (lib). 

This mechanism corresponds to one considered earlier by Dannenberg 12SK The consideration of a simple triplet energy transfer from the ferrocene T% (54 kcal/mole) [see section B] to piperylene [7T = 56.9 for the cis isomer and 58.8 kcal/mole for the trans isomer] has been rejected, since ferrocene was thought to behave like a high-energy sensitizer 128,129). However, in trans-1.2-dimethylcyclohexane it was shown to... 

Problem 9.14 (a) Draw the possible chair conformational structures for the following pairs of dimethylcyclohex-anes (i) cis- and tram-1,2- (ii) < is- and trims-1,3- (iii) cis- and trans-1,4-. (b) Compare the stabilities of the more stable conformers for each pair of geometric isomers, (r) Determine which of the isomers of dimethylcyclohexane are chiral. ... 

For the cis isomer of 1,2-dimethylcyclohexane, both conformations have one axial and one equatorial methyl, so the e,a and a,e conformations are equivalent in energy. Thus, we conclude that the cis isomer has two 13-diaxial methyl-hydrogen interactions for the axial mettiyl group, pitas one additional gauche interaction between the two methyls. The trans isomer has only the butane gauche interaction between the two methyls in the e,e conformation (which represents the great majority of molecules), so we predict that the trans isomer should be more stable than the cis isomer by two methyl 1,3-diaxial interactions or 1.8 kcal/mol. The literature value (Table 3.3) is 1.87kcal/mol. One can similarly analyze other dimethylcyclohexanes and compare the... 

Probably the most interesting observation in Table 3 2 concerns the 1 3 dimethylcyclohexanes Unlike the 1 2 and 1 4 dimethylcyclohexanes in which the trans stereoisomer is more stable than the cis we find that cis 1 3 dimethylcyclo hexane is 7 kJ/mol (17 kcal/mol) more stable than trans 1 3 dimethylcyclohexane Why" ... 

Obtain energies for diequatorial and diaxial conformers of cis-l,3-dimethylcyclohexane, trans-1,2-dimethyl-cyclohexane and trans-l,4-dimethylcyclohexane. 

Since compounds with alkyl equatorial substituents are generally more stable, trans-1,2 compounds, which can adopt the ee conformation, are thermodynamically more stable than their cis-1,2 isomers, which must exist in the ae conformation. For the 1,2-dimethylcyclohexanes, the difference in stability is about 2kcal moP (8 kJ mol" ). Similarly, trans-1,4 and cis-1,3 compounds are more stable than their stereoisomers. 

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