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.

Entry 6 shows a stereoselective formation of a highly substituted frans-decalin system. The reaction in Entry 7 establishes a taxanelike structure. The stereochemistry is consistent with a TS in which both the carbonyl oxygen and the methoxy group are coordinated to aluminum. 

Fig. 6. Specific viscosity, r sp, as a function of the product c- [q] for narrowly distributed polystyrene in toluene (good solvent) (A) and frans-decalin (poor solvent) ( ) at 25 °C. Experimental data for the polystyrene/toluene system at 30 °C (taken from [65]) are represented by (O). 

A comparison of the solution behaviour of PS in both solvents, toluene and frans-decalin, reveals that the limiting power of the molar mass dependence of r 0 (3.35 and 3.28, respectively) is very close to the value of 3.4 observed in highly concentrated solutions and melts. The concentration dependence of r 0, however, is clearly different in each of the solvents ... 

Table 2. Characterisation data and viscosities, T]0, of polystyrene (molar mass distribution Mw/Mn<1.3) in frans-decalin at 25 °C. Theoretical viscosities, T)0(theor) were calculated from Eq. (15). In the last column A represents the relative deviation of T)0(theor) from r)o(exp)...

Parameters Toluene cis-Decalin frans-Decalin Cyclohexane... 

Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

FIGURE 2.1.1.2.14.4 Logarithm of vapor pressure versus reciprocal temperature for frans-decalin. 

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. 

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... 

Now, frans-decalin forms a rather rigid system, and it transpires that the only conformational mobility possible is ring flip of chairs to very much less favourable boats. Since both bonds of the second ring are equatorial with respect to the first ring, any other type of conformational... 

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. 

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. 

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. 

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) ... 

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. 

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. 

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... 

In most reactions of tram-decalins, the conformational principles of simple six-inembered rings can be used, but you may expect lighter control from the greater rigidity. If you wish to design a molecule where you are quite certain of the conformation, a frarrs-decalin is a better bet than even a t-butyl cyclohexane as frans-decalins cannot flip. 

First a brief rentin 1 of the conformation of c/s-decalins (see Chapter 18). Unlike frans-decalins, which are rigid, th caii flip rapidly between two all-chair conformations. During the flip, all... 

The experiments also provide stereochemical evidence that a carbocation is an intermediate in both reactions. Both starting materials are ns-decalins but the product is a frans-decalin. The carbocation intermediate has no stereochemistry and can react with the nitrile from either face. Axial attack is preferred and it gives the stable frcws-decal in. The formation of the carbocation is shown only by the Beckmann fragmentation formation from the alcohol by the S l mechanism is obvious. 

In the Ritter reaction a rather different kind of evidence for the cation is the fact that families of isomeric alcohols all give the same product. In all these cases, rearrangements of the first formed carbocation (Chapter 37) can easily account for the products. Another example in the decalin series is this Ritter reaction with KCN as the nitrile in acidic solution so that HCN is the reagent. The starting material is a spirocyclic tertiary alcohol but the product is a frans-decalin formed by... 

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... 

The protonation leads specifically to the fran -decalin system, though reduction could apparently give rise to two stereoisomeric products. The guiding principle appears to be that protonation of the intermediate allylic anion 12 takes place axially, orthogonal to the plane of the double bond, and to the most stable conformation of the carbanion which allows the best sp -orbital overlap on the yS-carbon with the. -orbital system of the double bond. 

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