Chiral alkenes planar

Scheme 7.8. Asymmetric carbonyl olefinations to give planar chiral alkenes.

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

Prochirality Planar molecules possessing a double bond such as alkenes, imines, and ketones, which do not contain a chiral carbon in one of the side chains, are not chiral. When these molecules coordinate to a metal a chiral complex is formed, unless the alkene etc. has C2V symmetry. In other words, even a simple alkene such as propene will form a chiral complex with a transition metal. So will trans-2-butene, but cis-2-butene won t. If a bare metal atom coordinates to cis-2-butene the complex has a mirror plane, and hence the complex is not chiral. It is useful to give some thought to this and find out whether or not alkenes and hetero-alkenes form chiral complexes. One can also formulate it as follows complexation of a metal to the one face of the alkene gives rise to a certain enantiomer, and complexation to the other face gives rise to the other enantiomer. 

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 sterically overcrowded alkenes shown in Scheme 6 have been exploited in our group since, from the perspective of molecular switches design, they combine a number of attractive structural features. Steric interactions between the groups attached to the central olefmic bond force these molecules to adapt a non-planar helical shape. The chirality of these so-called inherently dissymmetric alkenes 3, is therefore the result of distortion of the entire molecular structure. Beside the heli-cene-like geometry, both a cis- and a trans-stilbene chromophore are present in the same molecule. 

Trisubstituted cyclic alkenes have been kinetically resolved via a chiral dioxirane (4), generated in situ from the ketone and Oxone. A sequential desymmetrization and kinetic resolution of cyclohexa-1,4-dienes has also been achieved. The observed stereochemical results have been rationalized on the basis of a spiro-planar transition state model.93... 

One highly attractive feature of ketone-catalyzed epoxidation via chiral dioxir-anes is that reliable models can be developed to rationalize the observed enantio-selectivities. For the reaction of a dioxirane with an alkene, two extreme transition states can be envisaged the so-called spiro and planar modes (Fig. 12.3). 

An intriguing approach to an enantioselective cyclopropanation has recently been reported (Scheme 27). Enantiomer-ically pure planar chiral -chromium benzaldehydes (71) were prepared in the manner described earlier (Scheme 19) and treated with alkenes at low temperature. For some cases... 

If other elements of chirality are present in the molecule, the descriptors should be arranged in the order central > carbocationic > axial > planar > torsional (note that Sokolov [6] uses a different order for the descriptors). The effects caused by such other types of chirality are discussed elsewhere in some detail [41], e.g., S-cis/S-trans-isomerism in ferrocenyl ketones and alkenes, as well as the chemistry of biferrocenyl (containing a direct bond between two ferrocenes). 

Davis, F. A., Harakal, M. E., Awad, S. B. Chemistry of oxaziridines. 4. Asymmetric epoxidation of unfunctionalized alkenes using chiral 2-sulfonyloxaziridines evidence for a planar transition state geometry. J. Am. Chem. Soc. 1983, 105, 3123-3126. 

With BF3, the syn product 156 predominates by 87 13 but with MgBr2, the anti 157 is even more dominant (92 8). Both products have the same stereochemistry at C-5, determined by the Me3Si group as in 154. The mechanism 158 forces the electrophile to approach the alkene from the top face - opposite the Me3Si group - but a detailed transition state such as 159 is needed to explain the new chiral centre at C-6. Panek suggests that 159 is the anti-peri-planar transition state and that MgBr2 adopts a synclinal transition state. 

Figure 83. Examples of molecules prepared in enantiomerically enriched form using Sharpless KR procedure, (a) Compounds having alternative sites of oxidation acetylene [38], furan [39], and amine [40], (b) Compounds bearing axial chirality [38]. (c) An alkene with planar chirality [41],...

Two enantiomers are chemically identical because they are mirror images of one another. Other types of stereoisomers may be chemically (and physically) quite different. These two alkenes, for example, are geometrical isomers (or cis-trans isomers). Their physical chemical properties are different, as you would expect, since they are quite different in shape. Neither is chiral of course as they are planar molecules. 

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