Chiral Tetrahedra

The three classes of molecular tetrahedra with different stoichiometries M4L6, M4L4 and M6L4 (reprinted from reference ). 

The Saalfrank method to produce the first molecular tetrahedron. 

X-ray molecular structure of cobalt cage 5.28 (reprinted with permission from reference ). 

Enantiomerization of AAAA)-5.21 to AAAA)-5.21 (courtesy of Professor Rolf W. Saalfrank, University Erlangen-Niirnberg, Germany, from reference ). 

More recently Saalfrank and coworkers extended this strategy to the preparation of enantiomerically pure Cu(II) cubanes using chiral bis-1,3-diketones as tetradentate ligands. 

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These counterintuitive properties of racemization paths of three-dimensional labeled and unlabeled chiral tetrahedra, noted by Mislow, are referred to as Mislow s label paradox. More recently, it has been shown that Mislow s label paradox is general for n-chiral simplices in all finite dimensions n, and sufficient and necessary partial vertex labeling conditions have been given for chirality-preserving interconversion paths of mirror images of chiral -dimensional simplexes. 

In substitution reactions on chiral tetrahedra that involve trigonal-bipyramids as intermediates, only isomerizations with the leaving group as a pivotal (or unique) ligand affect the stereochemical course of the overall substitution reaction. 

A map that describes associative substitution of chiral tetrahedra to give isomerizable square-pyramids has been described 4). A diamond lattice map with an internal network has also been developed 4) that traces the Berry pseudorotations of chiral trigonal-bipyramids. This earlier paper provided the same generalizations as did the more detailed map of Fig. 7. 

Supramolecular architectures with T symmetry are chiral because they have no improper symmetry operations, but possess a combination of two- and three-fold rotation axes. They represent the most investigated metal-based chiral assemblies.There are three types of chiral tetrahedra with different metal-to-ligand stoichiometry, M4L6, M6L4 and M4L4 (see Figure 5.7). 

L. Pu, Recent developments in asymmetric catalysis using synthetic polymers with main chain chirality, Tetrahedron Asymm. 1998, 9, 1457-1477. 

Fig.24 Overview of the synthesis and resolution of solution stable, chiral tetrahedron 6 [119]. a Formation of racemic, homochiral (AAAA)-6 and (AAAA)-6 b Chiral resolution and separation upon addition of s-nic ions (38) by formation of diastereomeric ion-pair (AAAA)-6-38 c Ion-exchange of chiral auxiliary 38 by achiral counterions such as NMe4+ and NEt4+ maintains the chirality of the resolved tetrahedra (AA AA)-6...

Baudoux J, Judeinstein P, Cahard D et al (2005) Design and synthesis of novel ionic liquid/ liquid crystals (IL2Cs) with axial chirality. Tetrahedron Lett 46 1137-1140... 

New terminology is helpful in such cases, and the members of the cycle, the chiromers , participate in a monoligostatic stereochemical cycle if a single ligand at a chiral tetrahedron is common to all chiromers. ... 

As shown in Fig. 15, inversion of a chiral tetrahedron, with the base ABC in a fixed 2D plane, changes the 3D chirality but appears as a simple rotation in 2D. We anticipate similar discrepancies between 4D symmetry and its projection into 3D as responsible for the irregular variation of optical activity as a function of 3D chirality. 

We contend that molecular chirality appears as a four-dimensional symmetry which is incompletely interpreted in three dimensions. The type of anticipated error is demonstrated by the way in which three-dimensional chirality is projected into two dimensions, as in Fig. 6. The two-dimensional chiral system is defined here in the plane that supports the triangular base of a three-dimensional chiral tetrahedron. The symmetry element, shown as a solid vertical line, represents an inversion (I) in three dimensions and a twofold rotation (R) in two. The horizontal broken line represents a twofold rotation in three dimensions, but a reflection (M) in two dimensions. To complete the argument, the three-dimensional reflection that operates diagonally also appears as a two-dimensional reflection. Two-dimensional inversion is equivalent to rotation. In summary,... 

Noyori R. Organometallic ways for the multiplication of chirality. Tetrahedron 1994 50(15) 4259-4292. 

Piva O, Pete JP. Highly enantioselective protonation of pho-todienols an unusal suhtituent effect of the induced chirality. Tetrahedron Lett. 1990 31 5157-5160. 

The H4 system is the prototype for many four-elecbon reactions [34]. The basic tetrahedral sfructure of the conical intersection is preserved in all four-electron systems. It arises from the fact that the four electrons are contributed by four different atoms. Obviously, the tefrahedron is in general not a perfect one. This result was found computationally for many systems (see, e.g., [37]). Robb and co-workers [38] showed that the structure shown (a tetraradicaloid conical intersection) was found for many different photochemical transformations. Having the form of a tetrahedron, the conical intersection can exist in two enantiomeric structures. However, this feature is important only when chiral reactions are discussed. 

Preparation and Properties of Chiral Ruoroorgamc Compounds Bravo, P, Resnati G Tetrahedron Asvmmetry / 661-692 300 ... 

Vogt P. E., Miller M. J. Development and Applications of Amino Acid-Derived Chiral Acylnitroso Hetero Diels-Alder Reactions Tetrahedron 1998 54 1317 1348 Keywords nitroso and acylnitroso dienophlles, befero-Dlels-Alder reaction... 

Hultin P. G., Earle M. A. and Sudharshan M. Synthetic Studies with Carbohydrate-Derived Chiral Auxiliaries Tetrahedron 1997 53 14823-14870 Keywords sugar-linked dienes, sugar-linked dienophiles... 

Aversa M. C., Barattucci A., Bonaccorsi P. and Giannetto P. Chiral Sulfinile-1,3-dienes-Synthesis and Use in Asymmetric Reactions Tetrahedron Asymmetry 1997 9 1339-1367... 

Compounds With Other Quadrivalent Chiral Atoms. Any molecule containing an atom that has four bonds pointing to the comers of a tetrahedron will be optically active if the four groups are different. Among atoms in this... 

Suitably Substituted Adamantanes. Adamantanes bearing four different substituents at the bridgehead positions are chiral and optically active, and 14, for example, has been resolved. This type of molecule is a kind of expanded tetrahedron and has the same symmetry properties as any other tetrahedron. 

For a review of chirality in Mobius-strip molecules catenanes, and knots, see Walba, D.M. Tetrahedron, 1985, 41, 3161. 

In theory, the chiral center can be anywhere in the molecule, but in practice, reasonable diastereoselectivity is most often achieved when it is in the a position. For examples of high diastereoselectivity when the chiral center is further away, especially in reduction of P-hydroxy ketones, see Narasaka, K. Pai, F. Tetrahedron, 1984, 40, 2233 Hassine, B.B. Gorsane, M. Pecher, J. Martin, R.H. Bull. Soc. Chim. Belg., 1985, 94, 597 Bloch, R. Gilbert, L. Girard, C. Tetrahedron Lett., 1988, 53, 1021 Evans, D.A. Chapman, K.T. Carreira, E.M. J. Am. Chem. Soc., 1988, 110, 3560. 

Wallbaum, S. Martens. J. Tetrahedron Asymmetry, 1993, 4, 637 With a chiral oxazoline additive, 60% ee... 

Cohen F, Overman LE (1998) Planar-chiral cyclopalladated ferrocenyl amines and imines as enantioselective catalysts for allylic imidate rearrangements. Tetrahedron Asymmetry 9 3213-3222... 

Doucet, C. Vergely, I. Reboud-Ravaux, M. Guilhem, J. Kobaiter, R. Joyeau, R. Wakselman, M. Inhibition of human leucocyte elastase by functionalized V-aryl azeti-din-2-ones. Stereospecific synthesis and chiral recognition of disymmetrically C3-substi-tuted P-lactams. Tetrahedron Asymmetry 1997, 8, 739-751. 

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