Conformation cyclooctene

Levin and Hoffmann14 carried out their calculations on two trans-cyclooctene conformations, 1 and 2. To introduce the two conformations here and also on a number of simpler prototypes, one of them was twisted ethylene, using 29°, 20° and 10° torsion angles. They have suggested three possible sources for the optical activity of trans -cyclooctene the chirally disposed allylic substituents, the transannular portion of the methylene chain... 

Racemic (1 / A,3RiS )-3-methyl-1-phenyl-3,4,5,6-tetrahydro-l//-2,5-benzoxazocine-5-carbonitrile (35) crystallized as a racemic compound in an achiral space group.1 X-ray structure analysis showed that one of the enantiomers in the crystal was bent into the (retro,S)-TBC conformation (for torsion angles of the X-ray structure, see Fig. 18).1 TBC is a relatively rare d.v-cyclooctene conformation, and is the fourth and highest energy conformation for d.v-cyclooctene within an energy window of 33.4 kJ from the ground state.1 Selected FI and 13C solution-state and 13C cp/mas... 

Adam, W., Stegmann, V. R. Unusual Temperature Dependence in the cis/trans-Oxetane Formation Discloses Competitive Syn versus Anti Attack for the Patemo-Buchi Reaction of Triplet-Excited Ketones with cis- and trans-Cyclooctenes. Conformational Control of Diastereoselectivity in the Cyclization and Cleavage of Preoxetane Diradicals. J. Am. Chem. Soc. 2002,124, 3600-3607. 

Adam, W. and Stegmann, V. R., Unusual temperature dependence in the cisltrans-oxtXd.nt formation discloses competitive syn versus anti attack for the Paternh-Buchi reaction of triplet excited ketones with cis- and trans-cyclooctenes conformational control of diastereoselectivity in the cyclization and cleavage of preoxetane biradicals, /. Am. Chem. Soc., 124, 3600, 2002. 

Dibenzo[a,d]cyclooctene, 5,6,7,12-tetrahydro-Dibenzo[a,d]cyclooctene, 5,6,7,12-tetrahydro-conformation, 7, 706... 

Molecules that are chiral as a result of barriers to conformational interconversion can be racemized if the enantiomeric conformers are interconverted. The rate of racemization will depend upon the conformational barrier. For example, -cyclooctene is chiral. E-Cycloalkenes can be racemized by a conformational process involving reorienting of the... 

The course of alkylation is also influenced by the steric arrangement of the enamine. 1-Pyrrolidino-l-cycloheptene gave approximately equal quantities of the C- and N-alkylated products in dioxane, while 1-pyrroli-dino-l-cyclooctene, and 1-pyrrolidino-l-cyclononene afforded N-alkylated products exclusively under similar eonditions (29). The reason for N alkylation in the eight- and nine-membered ring compounds is to be found in the conformation of these rings, which prevents full interaction of the unshared electrons on nitrogen with the n eleetrons of the double bond. 

Fig. 12. trans-Cyclooctene a calculated (19) and observed (78) (in parentheses) torsion angles (deg) of the crown conformation b calculated torsion angles of the distorted chair conformation... 

FIGURE 4. Ground state conformation of 2-cycloocten-l-ol and 2-cyclononen-l-ol... 

One of the most interesting developments in the stereochemistry of organic compounds in recent years has been the demonstration that trans-cyclooctene (but not the cis isomer) can be resolved into stable chiral isomers (enantiomers, Section 5-IB). In general, a Wa/w-cycloalkene would not be expected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However, when the chain connecting the ends of the double bond is short, as in trans-cyclooctene, steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. 

Figure 12-20 Representation of (a) achiral and (b) chiral conformations of frans-cycloalkenes, using frans-cyclooctene as a specific example. For frans-cyclooctene, the achiral state is highly strained because of interference between the inside alkenic hydrogen and the CH2 groups on the other side of the ring. Consequently the mirror-image forms are quite stable. With frans-cyclononene, the planar state is much less strained and, as a result, the optical isomers are much less stable. With frans-cyclodecene, it has not been possible to isolate mirror-image forms because the two forms corresponding to (b) are interconverted through achiral planar conformations corresponding to (a) about 1016 times faster than with frans-cyclooctene.

Conformational enantiomerism. trans-Cyclooctene is strained, unable to achieve a symmetric planar conformation. It is locked into one of these two enantiomeric conformations. Either pure enantiomer is optically active, with [a] = 430°. 

Even a simple strained molecule can show conformational enantiomerism. trans-Cyclooctene is the smallest stable trans-cycloalkene, and it is strained. If trans-cyclooctene existed as a planar ring, even for an instant, it could not be chiral. Make a molecular model of trans-cyclooctene, however, and you will see that it cannot exist as a planar ring. Its ring is folded into the three-dimensional structure pictured in Figure 5-18. The mirror image of this structure is different, and trans-cyclooctene is a chiral molecule. In fact, the enantiomers of iran.v-cyclooclcnc have been separated and characterized, and they are optically active. 

The dramatic switching of product chirality by temperature is entropic i origin, for which the different degrees of conformational changes induced by thj rotational relaxation to the enantiomeric twisted cyclooctene singlets (/ )- am (S)-lp within the exciplex are thought to be responsible [30,31]. This idea w supported experimentally by using chiral pyromellitate sensitizers 45g-j. C ing chiral auxiliaries with an electron-rich aromatic substituent as a donor moiel... 

The photoisomerization of (Z)-cyclooctene (30) (Scheme 12) to the (E)-isomer (31) was sensitized by enantiopure alkyl benzenecarboxylates immobilized in zeolite to give modest ees. The use of an antipodal sensitizer pair of (R)-and (S)-1 -methyIheptyl benzoates, 32d and 32e, yielded enantiomeric 31 in — 5% and +5% ee, respectively, while the same sensitizers gave practically racemic 31 upon irradiation in homogeneous solutions. This small, but apparent, enhancement of the product ee observed upon irradiation in modified zeolite supercages is likely to arise from the decreased conformational freedom of the adsorbed sensitizer, the hindered approach of 30 to the sensitizer, and/or the different exciplex structure in confined media. In this context, it is interesting to examine the effect of temperature on the supramolecular photochirogenesis in modified zeolites and to compare the results with those obtained in the homogeneous phase. Such an examination will reveal the distinctly different role of entropy in confined media, which should be clarified in a future study. 

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