Cyclooctene chirality

Assemble a model of trans -cyclooctene. Observe the twisting of the r-bond system. Would you expect the cis stereoisomer to be more stable than tranj-cyclooctene Is m-cyclooctene chiral Is trani -cyclooctene chiral ... 

Achiral ds-cyclooctene upon singlet sensitization yields chiral frcms-cyclooctene. While achiral sensitizers yield a racemic tra s-cyclooctene, chiral sensitizers yield enantiomerically enriched trans-cyclooctene (Scheme 23). ° The highest ee reported (73%) thus far is with optically active (-) tetramen-thyl-l,2,4,5-benezene tetracarboxylate at -110°C in diethyl ether. The extent of ee depends on temperature, pressure, and solvent. 

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

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... 

Based on this, asymmetric hydroamination was developed using [Ir(C2H4)4Cl] or lr(coe)2Cl]2 (coe = cyclooctene) with chiral diphosphines to give complexes (57)-(61) (Scheme 40). While (57) afforded only a low yield and poor enantiomeric excess (51% 2S) of exo-2-(phenylamino)nor-bornane, addition of up to one equivalent of fluoride ion gave a six-fold increase in chemical yield (from 12% to 81%) and a reversal of enantioselectivity. In the case of (60), addition of four equivalents of fluoride led to an ee of 95 % The role of fluoride in these reactions has still not been explained satisfactorily.175... 

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

Optical resolution employing a chiral Pt(II) complex enabled them to obtain (—)-l,2-cyclononadiene (the enantiomer of 116), [a] —71° (neat) which was estimated to have an optical purity of 44%. In their second approach, (—)-(R)-( )-cyclooctene (6) was treated with dibromocarbene to yield the ( + )-adduct 115, which was further treated with methyllithium to give (+)-116, [[Pg.19]

Moscowitz, A., Mislow, K. J. Am. Chem. Soc. 84, 4605 (1962). Although they follwed the axial instead of planar chirality convention in their R,S-specification for chiral (rans-cyclooctene, both procedures give the same R,S-notation in this case. 

Fig. 1. Plane of Chirality Enantio-morphous figures and corresponding torsional angles A—X—Y—Z 7). Molecular examples ([n]paracyclophane, [2.2]meta-cyclophane, bridged[10]anulene and ( )-cyclooctene) with descriptors (/ , S)...

On the basis of this definition (Fig. 1) the following classes of chiral compounds will be treated in this article cyclophanes, bridged anulenes and [8]anulenes, and ( )-cyclooctene and related structures. As mentioned above, metallocenes will be excluded. 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

The prochiral aziridine 1 is easily prepared from cyclooctene. Paul Muller of the University of Geneva has shown (Helv. Chim. Acta 2004,87,227) that metalation of 1 in the presence of the chiral amine sparteine leads to the bicyclic amine 3 in 15% , by way of intramolecular C-H insertion by the intermediate chiral carbene 2. The sparteine can be recovered and recycled. 

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.

The molecule that has been investigated more than any other olefin is the trans -cyclooctene. This molecule, which is a twisted olefin, served as a model compound for the olefinic chromophore, the main reason being that chromophores in general can be divided into two types those which are asymmetric (inherently asymmetric) by nature and those which become asymmetric as a result of chiral centers in their vicinity. 

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... 

F-Cyclooctene is chiral, and it was resolved into enantiomers by Cope and coworkers100 by separation of diasteromeric platinum complexes containing 20 and (+)-phenyl-2-aminopropane as ligands. Thermal racemization occurred around 150 °C with a rate... 

Denmark et al. employed the chiral phosphoramide 74 (Scheme 13.37) as nucleophilic activator [75]. As summarized in Scheme 13.37, the best enantiomeric excess was observed for cis-stilbene oxide (87%). The study revealed that enantioselectivity was highly dependent on the ring size (cyclohexene oxide cyclopentene oxide > cyclooctene oxide). 

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