Cis- and frans-Decalin

Figure 4.17 Representations of cis- and frans-decalin. The red hydrogen atoms at the bridgehead carbons are on the same face of the rings in the cis isomer but on opposite faces in the trans isomer.

The nitration of chiral alkanes producing racemic nitro compounds, and that of cis-and frans-decalin, both leading to the formation of trans-9-nitrodecalin,170 support the involvement of alkyl radicals. 

Among the most important of the bicyclic hydrocarbons are the two stereoisomeric bicyclo[4.4.0]decanes, called cis- and frans-decalin. The hydrogen atoms at the ring junctions are on the same side in c/s-decalin and on opposite sides in mms-decalin. Both rings adopt the chair conformation in each stereoisomer. 

Reduction of arenes.1 Raney nickel (Mozingo type) in combination with 2-propanol (reflux) effects reduction of aromatic rings in 2-18 hours. Naphthalene is reduced in 18 hours to tetralin (90% yield) and cis- and frans-decalin (10% yield). Anisole is reduced in 110 hours to cyclohexyl methyl ether (90% yield). Nitrobenzene is reduced quickly to aniline and then further to cyclohexylamine and cyclohexylisopropylamine. 

The opposite behavior as sketched before was detected for solutions of PS in DOP [112], Again, the critical temperature (an UCST at Tc = 12 °C in the quiescent state) turned out to be a function of the shear rate to which the solution is subjected. But, in contrast to solutions of PS and PB in DOP, here enhancements of the UCST as large as 28 °C were recorded at a shear rate of 220 s-1. Similar results have been found for PS solutions in di(2-ethyl hexyl)phthalate or in a mixture of cis- and frans-decalin [113], The solutions demixed in a converging flow from a reservoir into a capillary tube. It has been observed that an increase in the deformation rate raised the UCST or reduced the region of miscibility. In both of these studies an increase of the cloud point temperature of the polymer solutions was used as an indication of phase separation. 

We saw in Problem 4.20 that cis-decalin is less stable than frons-decalin. Assume that the 1,3-diaxial interactions in cis-decalin are similar to those in axial methylcyclohexane [that is, one CH2 H interaction costs 3.8 kJ/mol (0.9 kcal/mol)], and calculate the magnitude of the energy difference between cis- and frans-decalin. 

Oxidative Insertion. MTO has been found to catalyze oxidative insertion of remote C-H bonds in the presence of a large excess of hydrogen peroxide. This intriguing observation has been applied to a number of different hydrocarbons. Moreover, the reactions are stereospecific, as exemplified in the case of cis- and frans-decalin (eqs 32 and 33). 

This dry ozonation procedure is a general method for hydrox-ylation of tertiary carbon atoms in saturated compounds (Table 1). The substitution reaction occurs with predominant retention of configuration. Thus cis-decalin gives the cis-l-decalol, whereas cis- and frans-l,4-dimethylcyclohexane afford cis- and trans-1,4-dimethylcyclohexanol, respectively. The amount of epimeric alcohol formed in these ozonation reactions is usually less than 1%. The tertiary alcohols may be further oxidized to diols by repeating the ozonation however, the yields in these reactions are poorer. For instance, 1-adamantanol is oxidized to 1,3-adamantane-diol in 43% yield. Secondary alcohols are converted to the corresponding ketone. This method has been employed for the hydroxylation of tertiary positions in saturated acetates and bromides. 

Smith and Burwell [61) showed that the reaction of deuterium with j9.io-octalin, which is catalyzed by reduced platinum oxide, yields a mixture of cis- and /mws-decalins containing an average of nearly three deuterium atoms per molecule. Under the same conditions J -octalin gave somewhat less of the cis isomer but the saturated products contained an average of two deuterium atoms per molecule. From a detailed analysis of the distribution of deuterium in the products they concluded that most of the cis- and all of the frans-decalin produced from... 

Exercise 12-27 The equatorial form of methylcyclohexane is 1.5 kcal mole-1 more stable than the axial form because the axial form has steric hindrance between the methyl and two hydrogens, one in the 3- and the other in the 5-position. Knowing that c/s-decalin is about 2 kcal mole-1 less stable than frans-decalin, what would you estimate for the relative stabilities of cis- and frans-9-methyldecalin (numbering as in Figure 12-22) ... 

Decalin (Section 29.8A) Two fused six-membered rings. ci.v-Decalin has the hydrogen atoms at the ring fusion on the same side of the rings, and frans-decalin has the hydrogen atoms at the ring fusion on opposite sides of the rings. 

Cis- and frans-3-f-butyl (or 3-p-chlorophenyl) thietane 1-oxides are isomerized on treatment with hydrogen chloride in dioxane. Mixtures of varying isomer composition yield equilibrium mixtures consisting of 85-100% of the cis isomer. A similar result is obtained by heating the sulfoxides at 170-175° in decalin. Likewise, 3-methyl-3-phenylthietane 1-oxide may be isomerized to an equilibrium mixture consisting mainly (74%) of the equatorial, c/s-3-phenyl isomer. ... 

The other alkanes were found to be much less reactive. Cyclohexane gave a cyclohexanol-one mixture, and adamantane was preferentially hydroxylated at the tertiary positions cis-decalin gave a mixture of cis- and frans-9-decanol (together with 1- and 2-decalones), suggesting the transient formation of radical carbon intermediates, which were also revealed by trapping experiments with CCI4. 

Fig. 4-3 [40] shows the separation of cis- and frans-4-methylheptene-3. Separation of naphthalene, tetralin and decalin (Fig. 4-4 [40]) indicates the predominant influence of geometric structure and the possible orientation of an adsorbed molecule on their retention times. The column with M0S2 yields two peaks in exact correlation with the structures of cis- and frans-deceilin. Tetralin is more strongly retained then decalin owing to its more planar structure. The most planar naphthalene... 

Once regioselective and stereoselective controls have been exerted, the cis-decalins must be isomerised to rrans-decalins, the configuration present in the target molecules. Since frans-decalins are thermodynamically more stable than the corresponding cis-decalins, it is possible to isomerise the latter through enolisation, a process that can be favored by the presence of a carbonyl group near to the centre to be inverted. 

Sterically induced CH bond polarizations are also reflected by the shieldings of carbon atoms in crs-decalin relative to its trans isomer [230]. In the 9-methyl derivatives, steric repulsion between the methyl- and the syn-axial 2,4-hydrogens shields the attached carbon atoms relative to the parent frans-decalin. In the cis isomer, shielding of the bridgehead carbons relative to those in the trans compound clearly reflects steric interaction of hydrogen atoms in the 2,4- und 6,8-positions. 

This sequence serves to exemplify the formation and aspects of reactivity of toluene-p-sulphonate esters in monosaccharide systems, and further to illustrate the selective protection afforded to hydroxyl groups by the formation of cyclic acetals by reaction with carbonyl compounds. Thus reaction of methyl a-D-glucopyranoside (26) with benzaldehyde in the presence of zinc chloride gives the 4,6-acetal (27) (Expt 5.118), wherein two fused six-membered rings of the frans-decalin type are present. As a cognate preparation the reaction of benzaldehyde with methyl a-D-galactopyranoside results in a similar conversion to a 4,6-acetal, but in this case the product is the conformationally flexible system of the cis-decalin type, the most likely conformation being that shown below. 

When a cyclohexane ring inverts, the substituents that were equatorial become axial and vice versa. This is fine for ris-decalin, which has an axial-equatorial junction, but it means that ring inversion is not possible for trans-decalin. For frans-decalin to invert, the junction would have to become axial-axial, and it s not possible to link the axial positions to form a six-membered ring. Cis-decalin, on the other hand, ring inverts just as fast as cyclohexane, ring inversion of cis-decatin... 

Usually o.v-dccalin is formed more selectively from tetralin than from naphthalene. Baker and Schuetz obtained a mixture of 77% cis- and 23% frans-decalin in the hydrogenation of naphthalene over Adams platinum oxide in acetic acid-ether at 25°C and 12.8 MPa H2, while cA-decalin was obtained exclusively in the hydrogenation of tetralin in acetic acid under similar conditions.23... 

Now, the story of this ring expansion is a little more complex than we led you to believe, because the starting material has three stereogenic centres ( ) and hence can exist as four diastereoisomers two frans-decalins and two ds-decalins. What is more, the product has a double bond in a ten-membered ring will it be cis or transl (Both are possible— see Chapter 31.)... 

The hydrogenation of substituted cyclic alkenes is anomalous in many cases in that substantial amounts of rran -addition products are formed, particularly with palladium catalysts. For example, the alkene 19 on hydrogenation over palladium in acetic acid gives mainly frans-decalin (7.12), and the alkene 1,2-dimethylcyclohexene 20 gives variable mixtures of cis- and trans-, 2-dimethylcyclohexane depending on the conditions (7.13). Similarly, in the hydrogenation of the isomeric dimethylbenzenes (xylenes) over platinum oxide, the cis-dimethylcyclohexanes are the main products, but some trans isomer is always produced. 

Predict whether cis- (56) or frans-decalin (57) should be more stable, and estimate the energy difference between them. 

The designations cis and trans are also used to describe the stereochemistry of fused saturated rings at the bridgehead positions. Notice the difference between frans-decalin and m-decaUn with regards to the overall... 

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