Cis- and trans- 1,2-Dimethylcyclopropane

Exclusive of compounds with double bonds four hydrocarbons are constitutional isomers of cis and trans 1 2 dimethylcyclopropane Identify these compounds... 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospeciflcally with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper 2S,S7,60 85), palladium 86), and rhodium catalysts S9,87>. However, cyclopropanation of c -D2-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization88). Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and trans-1,2-dimethylcyclopropanes 89). 

The reactions of alkylcyclopropanes with bromine and with hydrogen bromide have been investigated in detail. Methylcyclopropane (4, R = Me) scarcely underwent addition of bromine in the cold when light and catalysts were excluded. Similar observations were made with other alkyl-substituted cyclopropanes 4 such as ethylcyclopropane, butylcyclopropane, cis-and trans-1,2-dimethylcyclopropane. ... 

FIGURE 3.20 The enthalpy difference between cis- and trans-1,2-dimethylcyclopropane can be determined from their heats of combustion. Van der Waals strain between methyl groups on the same side of the ring makes the cis isomer less stable than the trans. 

We can measure the energy difference between cis- and trans- 1,2-dimethylcyclo-propane by comparing their heats of combustion. As illustrated in Figure 3.20, the two compounds are isomers, and so the difference in their heats of combustion is a direct measure of the difference in their energies. Because the heat of combustion of trans-1,2-dimethylcyclopropane is 5 kJ/mol (1.2 kcal/mol) less than that of its cis stereoisomer, it follows that trans-1,2-dimethylcyclopropane is 5 kJ/mol (1.2 kcal/mol) more stable than cis-1,2-dimethylcyclopropane. 

For example, photolysis of diazomethane in liquid cw-2-butene gives only c/j-l,2-dimethylcyclopropane, and in liquid tra j-2-butene gives only transA,2-dimcthylcyclopropane. Addition here is stereospecific and syn. Photolysis of diazomethane in gaseous 2-butene—either cis or trans—gives both cis- and trans-1,2-dimcthylcyclopropanes. Addition here is non-stereospecific. 

Unlike the case of cis- and irons-1,2-dimethylcyclopropane, the cia-trans-isomerization of the dimethylcyclobutanes is considerably slower than their decomposition reactions. The Arrhenius equations for all these processes have been measured and are given below ... 

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

Electrophilic addition of bromine at — 60°C to cis- and trans-1,2-dimethyl-cyclopropanes has been shown to open the C-1—C-3 bond preferentially in a non-stereospecific manner. This rules out predominant reaction through a 1,3-bromonium ion. The principal difference between the cis- and transisomers is that the former gives 33 % of a 1,2-dibromide by 1,2-hydride shift within an intermediate carbonium ion. It has been reported that oxymercura-tion of optically active trans-1,2-dimethylcyclopropane occurs with at least 85 % inversion of configuration at the site of nucleophilic attack in a reaction which also cleaves the less substituted cyclopropane bond. Under the influ-... 

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

Butene-2. The experimental results for the reaction of methylene with cis and trans butene-2 are summarized in Table III. In the liquid phase and in the gas phase at high pressures (>200 mm.) the major products are 1,2-dimethylcyclopropane (addition to double bond), pentane-2, and 2-methylbutene-2 (insertion products), and the steric configuration of the butene-2 is predominantly retained in the products... 

If you worked Problem 2.29 (p. 85), you saw that there are two isomers of ra r-l,2-dimethylcyclopropane. We will see in this chapter why two trans-1,2-dimethylcyclopropanes exist and continue with a detailed discussion of stereochemistry, the stmctural and chemical consequences of the arrangement of atoms in space. We have already seen several examples of stereoisomeric molecules, compounds that differ only in the spatial arrangement of their constituent parts. The compounds cis-and ra r-l,2-dimethylcyclopropane are stereoisomers (as are all cis/trans pairs). Substituents on both ring compounds and alkenes can be attached in two ways cis and trans (Fig. 4.1). We will soon see that the two rflw -l,2-dimethylcyclopropanes are stereoisomers of a different kind. 

The cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have then- atoms bonded in the sane order—that is, they have the sane constitution, but they differ in the anangernent of atoms in space. Stereoisomers of the cis-trans type are sometimes refened to as geometric isomers. You learned in Section 2.18 that constitutional isomers could differ in stability. What about stereoisomers ... 

In the presence of propene, photolysis of diazoethane yields cis and trans isomers of 1,2-dimethylcyclopropane ... 

In agreement with the results of experiments on pyrolysis of l,l-bis(trimethyl-silyl)cyclopropane, additional (2/2)CASPT2/6-31G calculations predict that a rapid 1,2-silyl shift will occur in 12a, forming 13a. However, if cis- and trans-l,l-bis(trimethylsilyl)-2,3-dimethylcyclopropanes (11b and 11c) were pyrolyzed, then, as shown in Figure 22.10, the stereochemistry of ring opening could, presumably, still be inferred from the stereochemistry of the double bonds in the expected rearrangement products (13b and 13c). 

Having documented that we are perplexed by the differences we found for the enthalpies of formation of simple cyclopropyl and phenyl derivatives, it is premature and perhaps foolish to go from singly to doubly substituted species and expect any better understanding. Steric repulsion between vicinal groups on a cyclopropane may be expected, and hence the trans-isomQv of 1,2-dimethylcyclopropane is expected to be more stable than the cis. This is correct the enthalpies of formation of trans-1,2- and cis-1,2-dimethylcyclopropane (20c and 20b, X = Me) respectively, are (-30.7 0.8) and (-26.3 0.6) kJ moF for the liquids, and-3.8 and 1.7 kJmoF for the gases. The 1,1-dimethyl isomer (20a, X = Me) is more stable yet (-33.3 0.7) and (-8.2 1.2) kJ moF for the liquid and gas, respectively. What comparisons with benzene derivatives can we make With the 1,1-disubstituted cyclopropane isomer, seemingly none. With the cw-l,2-isomer, let s try the o-disubstituted benzene, o-xylene (47a, X = Me). With the trans-l,l-isomcr, let s try the -xylene isomer (47b,... 

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