Chirality amplification mechanism

As outlined in a later section, dimer formation is believed to play an essential role in the chiral amplification mechanism. Since the thermodynamic stabilities as well as the kinetics of formation of the dimers depend on their structural properties, variations in the amplification capacity by taking different aldehyde substrates may appear rational from this point of view. 

The macromolecular helicity induced in poly(phenylacetylene)s 28-30 (Fig. 14) upon complexation with chiral amines is dynamic in nature, and therefore, the ICD due to the helical chirality immediately disappears when exposed to a stronger acid such as trifluoroacetic acid. However, during the intensive exploration of the helicity induction and chirality amplification mechanism of the poly(phenylacetylene)s, such an induced helical chirality of 28-30 by an optically active amine such as (R)-39 has been found to be maintained, namely memorized , when the chiral amine is completely removed and replaced by various achiral amines, for example, 71 and 72 for 28 and diamines such as ethylenediamine for 29 and 30 in... 

LC phase, while the later systems are based on chirality amplification mechanism. There are many types of chiral LC phases such as cholesteric (N ), chiral smectic (Sm ), blue phase (BP), and twist grain boundary (TGB) phase. Among them, N and SmC phases are the most studied due to their potential technological applications. BPs are also receiving increasing attention due to their interesting 3D cubic structures and potential applications as 3D photonic crystals. 

Self-assembled structures are supramolecular assemblies of covalent backbones structured through intra- and interchain noncovalent interactions. These secondary structures arise from steric constraints and a network of weak interactions (i.e., hydrogen or Van der Waals bonding, dipole-dipole or amphiphilic interactions). Helical morphologies are stiU rarely represented in these artificial species but the control of the heHx sense, and a better knowledge of the chiral amplification mechanism, is highly desirable due to their potential use in many applications. For example, helically chiral polymers can be used as chiral stationary phases for HPLC or for catalysis. 

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. 

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... 

In summary, the origin of the chiral amplification is basically the difference in stability of the homochiral and heterochiral dinuclear Zn complexes. These complexes act as catalyst precursors, but differences in their kinetic behavior also affect the degree of the nonlinear effect. This investigation is probably the first example of elucidation of a molecular mechanism of catalytic chiral amplification (41) and may provide a chemical model of one means of propagation of chirality in nature. 

Chirality amplification, 11,255 catalytic reactions, 289 DAIB complexes, 273 mechanism, 275 organozinc chemistry, 273 Chirality multiplication, 2... 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The presence of N-atoms in the aromatic part of the aldehyde appears essential for chiral amplification. With only one nitrogen, such as in the case of 3-pyridine carbaldehyde, autocatalytic kinetics but no chiral amplification effect has been observed [24,25]. In the case of 3-quinoline carbaldehyde, i.e., in the presence of two nucleophilic centers, autocatalysis as well as moderate chiral amplification were reported [26,27]. Highest amplification capacity is observed in the presence of two N-atoms in the aromatic part of the aldehyde, where for the substituent at the 2-position the amplification capacity increases H < CH3 < f- Bu - C=C -, i.e., with the size and rigidity of this group. So far, detailed studies that could relate the given observations to the possible mechanism of chiral amplification in Soai s reaction are still to be carried out. 

It was soon recognized that in specific cases of asymmetric synthesis the relation between the ee of a chiral auxiliary and the ee of the product can deviate from linearity [17,18,72 - 74]. These so-called nonlinear effects (NLE) in asymmetric synthesis, in which the achievable eeprod becomes higher than the eeaux> represent chiral amplification while the opposite case represents chiral depletion. A variety of NLE have been found in asymmetric syntheses involving the interaction between organometallic compounds and chiral ligands to form enantioselective catalysts [74]. NLE reflect the complexity of the reaction mechanism involved and are usually caused by the association between chiral molecules during the course of the reaction. This leads to the formation of diastereoisomeric species (e.g., homochiral and heterochiral dimers) with possibly different relative quantities due to distinct kinetics of formation and thermodynamic stabilities, and also because of different catalytic activities. 

According to Soai and coworkers [31], the strong chiral amplification sometimes observed in their system cannot be explained by so-called NLE alone since amplification effects in these cases remain at a more moderate level. Hence a more complex reaction mechanism than expressed by the reservoir or the ML model is indicated for the Soai reaction. 

The occurrence of chiral amplification in this case is explained by the mechanism of Figure 10.40. When chiral amplification is observed one always finds—as is the case here—that two molecules of the chiral ligand become linked to each other, albeit indirectly (i.e., via another component of the reaction mixture). In other words, a derivative of the dimer of the chiral ligand is formed. When chiral amplification occurs, this derivative of the dimer exists... 

The cpl-induced asymmetry in photoreactions as described in Sec. B. of this chapter is not very pronounced. In order to obtain ees in excess of a few percent, photodestruction must be chosen and most of the reactant material must be sacrificed. Therefore amplification mechanisms for all types of cpl-induced asymmetric photoreactions would be highly desirable. Autocatalysis, i.e., an asymmetric synthesis where a chiral product acts as a catalyst for its own production [128], and autoinduction, i.e., the stimulation of a chiral catalyst by a chiral product [44,129], are options. Autocatalytic systems that will tilt to one enantiomeric side were introduced by Frank [130] and Seelig [131]. 

A phenomenon that may be related to the origin of biological homochirality was recently reported by Cooks and coworkers 39 Serine sublimes with spontaneous chiral amplification. Sublimation of near racemic sample of serine 55 (Figure 9) yields a sublimate that is enriched in the major enantiomer. The chiral purity maximizes at 190-210 °C, and then falls as thermolysis becomes favorable. This simple one-step sublimation may represent a possible mechanism for the chiral amplification step to explain the origin of biological homochirality. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

The above results allow the definition of one important feature of the assembly of discotic molecules in isotropic solution. There seem to be conditions (controlled by temperature, concentration, and solvent type) in which contact forces are weak and loose binding of the unimers produces short columns with low DP and little or no chiral amplification. Cooperative growth ensues even though a detailed mechanism is often unclear. The critical nucleus size is not readily identified from theory (cf Section n.B.3) but might be associated with the number of disks included in the pitch of the highly correlated helix forming when contact forces increase at low temperature. 

The energy difference between the two enantiomeric forms of alanine has been calculated to be of order 10 kJ/mol (Laerdahl et al. 2000) and this is most probably too small to be a decisive factor in the natural predominance of one enantiomer over another, unless coupled to some (unknown) amplification mechanism. Still, it is of interest to be able to demonstrate these P-odd interactions for a chiral molecule, and the... 

---