Amplification chirality

There are a number of examples of enantioselective catalysis with a zinc catalyst. The use of zinc as a catalyst in the Diels-Alder reaction of (R,R)-4,6-dibenzofurandiyl-2,2 -bis(4-phenylox-azoline) with cyclopentadiene shows high enantioselectivity. Chiral amplification is observed in the... 

In a related study involving structurally similar chiral methylzinc anisyl fencholates, both chiral amplification and depletion were observed in the catalytic alkylations of benzaldehyde.209 Thus, methylzinc anisyl fencholates, bearing sterically small substituents in the ortho-position of the anisyl group, crystallized preferentially as homochiral dimers, as shown for the methyl-substituted anisyl group in Scheme 91. Because of the greater stability of the homochiral dimers, scalemic mixtures of both enantiomers of the ligand showed a chiral depletion of the benzyl alcohol. 

By constrast, the methylzinc complexes bearing the sterically more encumbered anisyl fencholates with tert-butyl and trimethylsilyl substituents dimerized preferentially in the heterochiral form, resulting in the more common chiral amplification effects observed for classical dimethylaminoisoborneol systems. 

CHIRAL AMPLIFICATION, CHIRAL AUTOCATALYSIS, AND THE ORIGIN OF NATURAL CHIRALITY... 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

A recent interesting example of the chiral amplification of a small initial e.e. has been reported by Soai et al. [113,114] involving the induction of a chiral center in an achiral aldehyde using diisopropylzinc as an alkylating reductant and a very small... 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

These schemes expressly induded the idea that clays and porous minerals adsorbed, absorbed, and ultimately concentrated any extant organics from a dilute oceanic broth on the early Earth. This idea is critical because it is difficult to imagine that the kind of polymerization and chiral amplification processes discussed above could or would have occurred in a water solution not much more than millimolar in organics. [131]... 

The metal-catalyzed amplification of e.e. in small molecules, demonstrated by Soai and coworkers, along with the chiral enrichment of amino arid polymers by sequential polymerization/depolymerization steps, have shown that small enantiomeric excesses in nearly racemic mixtures can be reactively amplified to produce chiral dominance. These real chemical systems, which include plausible prebiotic reactions, experimentally demonstrate the principle of the chiral amplification of a spontaneously broken chiral symmetry in a dynamic and authentic chemical milieu. Therefore amplification to dominance of a small chiral excess of both small and polymeric molecules can be credibly incorporated into an origin-of-life model. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

The enantioselective incorporation of these amino acids into the serine octamers represents an example of chiral transmission to elementary biomolecules and a possible way of chirality amplification on primitive earth. 

Dramatic chiral amplification is observed in alkylations catalyzed by partially resolved DAIB7 Reaction of benzaldehyde and diethylzinc in toluene containing 8 mol % of (2S)-DAIB of 15% ee leads to (S)-1-phenyl-1-propanol in 95% ee, a value close to the 98% ee obtained with enantiomerically pure (2S)-DAIB. 

Figure 11.32 (Top) Helical conformational equilibria of dendron branching units and (bottom) chiral amplification in dendrons.

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