Cis- and trans-Decalin

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

Examine the structure of cyclodecane, a molecule which contains the same number of carbons as decalin, but only has one ring (a model of the most stable conformation is provided). Compare it to cis and trans decalin. Make a plastic model of cyclodecane. Is it flexible or locked What conformational properties of cyclodecane can be anticipated from the properties of decalins What properties cannot be anticipated How do you account for this ... 

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. 

Interestingly, the electron-transfer activation of cis- and trans-DBC cycloadditions with chloranil as a sensitizer leads to a mixture of cis- and trans-decalin adducts (equation 72). 

Commercial decalin (mixture of cis- and trans-decalin, volume ratio of cis-form/trans-form 1/1) was used for the catalytic reaction test in the present study. Reaction conditions were optimized by varying charged amounts of decalin (0.5,0.75,1.0,1.25,1.5, and 3.0 mL)... 

Since the theory of strainless ring put forward by Sachse required the existence of two isomeric forms and since all attempts to find them had failed the theory was not given much importance till 1918 when Mohr explained the absence of the two forms because of their rapid interconversion. This rapid interconversion can be beautifully illustrated on models where no considerable effort will be needed. Mohr also pointed out that the union of two cyclohexane rings may result in the formation of cis and trans decalins. 

Among the fused ring systems, the most important compound is decalin. This compound exists as the cis and trans forms and the existence of these two forms on the basis of non-planar structures was predicted by Mohr. He thought that the two decalins were formed by the fusion of two chair forms or two boat forms of cyclohexane and so he assigned, the following structures. 

Decachlorotetracyclodecanone, see Kepone Decachlorotetrahydro-4.7-methanoindeneone, see Kepone cis and trans-Decahydronaphthalene, see Decahydronaphthalene Decalin, see Decahydronaphthalene Decalin solvent, see Decahydronaphthalene Decamine, see 2,4-D Decamine 4T, see 2,4.5-T n-Decane, see Decane Decyl hydride, see Decane Dedelo, see p,/7-DDT Dedevap, see Dichlorvos Ded-weed, see 2,4-D Ded-weed brush killer, see 2.4.5-T Ded-weed LV-69, see 2,4-D... 

Becanse of the high concentration of isomeric molecules (>0.1 mol dm ), this equilibrium is established instantaneously. The IP of trans-AtcaYm is 0.02 eV lower than the IP of cw-decalin (9.24 eV versus 9.26 eV). Therefore, the electron-transfer eqnilibrinm is shifted slightly to the left side. Thns, in terms of charge-transfer kinetics, the two ions behave as a single species. It shonld be worth noting that decalin has only two isomeric forms, cis and trans. On the contrary, n-nonane exists in the multitude of conformations. The rate constant of electron exchange between parent nentral molecules of nonane and its cation-radicals is much lower, namely, 2 orders lower than the diffnsion-controlled limit (Borovkov et al. 2007). 

It is worthwhile to compare the stability differences of the analogous hydrindanes and decalins. In both cases, the Gibbs standard free energy differences favor the trans isomers, by 1.3 kJ mol" in the hydrindanes and by about 10 kJ mol in the decalins (94MI3). This direction of the difference is in contrast with that found for the 1,3-heteroanalogs of cis- and trans-hydrindane. 

Probtem 9.48 Decalin, C,oH,g, has cis and trans isomers that differ in the configurations about the two shared C s as shown below. Draw their conformational structural formulas. 

The nearly equal amount of cis and trans products formed from 1,5-dimethylcyclo-hexene is explained by the almost equal degree of hindrance of the homoallylic methyl group with the catalyst surface in the alternate adsorption modes.63 64 Another interesting example is the platinum-catalyzed hydrogenation of isomeric octalins.65-67 If syn addition to the double bond is assumed, in principle, both cis- and mms-decalin are expected to result from l(9)-octalin, but only the cis isomer from 9(10)-octalin. In contrast with these expectations, the isomers are produced in nearly the same ratio from both compounds. Transformation in the presence of deuterium revealed that most of the products contained three deuterium atoms. This was interpreted to prove that the very slow rate of hydrogenation of 9(10)-octalin [Eq. (11.9)] permits its isomerization to the 1(9) isomer. As a result, most of the products are formed through l(9)-octalin [Eq. (11.10)] ... 

Some heteropolycycloalkanes, e.g. indolizidine, quinolizidine, tropane, and decahydro-quinoline, are parent skeletons of alkaloids. Carbon-13 shifts of these and other parent polycycloalkanes [408] (Tables 4.64 and 4.65) are predominantly determined by heteroatom electronegativity, ring size, stereoisomerism, and dynamic effects. Aza- and thiade-calins are representative examples (Table 4.64). As described for cis- and trans-decalin (Section 4.1.3), carbon nuclei in the cis isomers of aza- and thiadecalins are shielded... 

Hydroxylation Perbenzoic acid (C6H6, 100°) can effect hydroxylation of hydrocarbons with a regioselectivity of 60-90% in favor of formation of tertiary alcohols. This reaction involves radicals since it is inhibited by hydroquinone. The hydroxylation of cis-and trans-decalin is stereoselective thus the former is converted mainly into cw-9-decalol (97 3) and the latter is converted mainly trans-9-decalol (95 5). 

Decalin. Decalin was chosen to study the reactivities and selectivities of and a-(5 disubstituted cyclohexane ring as well as the influence of the adjonction of a second saturated Cf, ring. The cis- and trans-isomers were photoxidized in the same conditions. The cis-isomer was found at least 10 times more reactive with a selectivity of 81% in mild oxidation products and 19% in CO2. The following relative selectivities for the main mild oxidation products (2-decalone, with small amounts of 1-decalone and 2-decalol (decahydro-2-naphtol) were found ... 

At this point, it probably will be helpful to construct models of cis- and trans-decalins to appreciate the following (a) The two compounds cannot interconvert unless C-C or C-H bonds first are broken, (b) traw -Decalin is a relatively rigid system and, unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the substituent is fixed in the chair-chair conformation of trans-decalm, (c) The chair-chair forms of cw-decalin are relatively flexible, and inversion of both rings at once occurs fairly easily (the barrier to inversion is about 14 kcal mole-1). A substituent therefore can interconvert between axial and equatorial conformations (Figure 12-24). 

The organic sorbates used were the purest (>99%) available commercially, as were the non-sorbable solvents and internal standards 1,3,5-tri-iso-propylbenzene, 1,3-di(trifluoromethyl)benzene, mesitylene, tetramethylsilane, and cis- and trans-decalin. 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

It should be noted that mixtures of isomers are involved in many cases of organic solvents e.g., mixed xylene isomers, mixed cis- and trans-decalin, mixed 1,1- and 1,2-dichloroethane, or mixed cresols, without obvious detrimental effects on the particular application attempted. However, in the following it is assumed that definite single substances are to be dealt with. 

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