Chirality propagation

In most cases of diastereoselective nucleophilic addition reactions where achiral organometallic reagents are added to chiral carbonyl compounds, the chirulity inducing asymmetric center is in close vicinity to the newly created center and cannot be removed without the loss of chirality of either the inducing center or the newly formed center. This type of reaction is very useful in propagating chirality in a molecule from one center to an adjacent one, or in immolative processes. 

Radical addition to conjugated systems is an important part of chain propagation reactions. The rate constants for addition of cyclohexyl radical to conjugated amides have been measured, and shown to be faster than addition to styrene. In additions to RCH=C(CN)2 systems, where the R group has a chiral center, the Felkin-Ahn rule (p. 148) is followed and the reaction proceeds with high selectivity. Addition of some radicals, such as (McsSijaSi-, is reversible and this can lead to poor selectivity or isomerization. ... 

In CDAD, a chiral experimental geometry is created about a fixed molecular orientation, and the asymmetry in the electron distribution can be observed in directions mutually perpendicular to the photon propagation direction and the... 

The catalytic site is chiral and analogous to that proposed for the isospedfic heterogeneous catalysts (Section 3.2), but, unlike the heterogeneous model site, interconversion between enantiomeric complexes is assumed to be possible, after each insertion step, when the metal atom is pentacoordinated.147,149 The analysis of the nonbonded interactions at the catalytic site showed that a si chain favors the formation of the A complex, which in turn favors the re coordination and insertion of the successive monomer unit, thus assuring the syndiospedfic propagation of the chain. 

Figure 1.23 Transition states for secondary insertion of styrene into secondary growing chain presenting si chirality (that is, generated secondary insertion of. si-coordinated styrene), (a) Model for unlike (syndiospecific) propagation includes fluxional site of R chirality at metal atom, which imposes re-propene coordination, while (b) model for like (isospecific) propagation includes fluxional sites of S chirality at metal, which imposes. si-propene coordination. Syndiospecific transition state (a) is favored because smallest substituent on C atom of chain, the H atom, can be pointed toward Cp ligand, whereas isospecific transition state (b) is of higher energy because Cp of growing chain is oriented toward Cp ring.

As a result, the energy preference of the local adsorption unit is switched to the opposite distortion/reconstruction and, thus, chiral lateral interactions are switched in the direction of the induction and propagation of the chiral assembly occurring in the mirror image construct, leading to a mirror chiral surface. Therefore, from this work, one may conclude that the overall global or local chirality is determined principally at the nucleation stage. 

Recently the density dependence of the symmetry energy has been computed in chiral perturbation effective field theory, described by pions plus one cutoff parameter, A, to simulate the short distance behavior [23]. The nuclear matter calculations have been performed up to three-loop order the density dependence comes from the replacement of the free nucleon propagator by the in-medium one, specified by the Fermi momentum ItF... 

Recently, the possible formation of diquark condensates in QCD at finite density has been re-investigated in a series of papers following Refs. [1, 2], It has been shown that in chiral quark models with nonperturbatrive 4-point interaction motivated from instantons [3] or nonperturbative gluon propagators [4, 5], the anomalous quark pair amplitudes can be very large - of the order of 100 MeV. The diquark pairs that are formed as a result of the attractive inter-... 

What is the origin of biological chirality and how did it successfully propagate itself to dominance The riddle is at once intriguing, central, and profound. It has been persuasively argued and is now widely accepted that homochirality of biopolymers is essential for the existence of life, and that life would be impossible in its absence. [4] Thus the solution to the riddle of terrestrial homochirality may well be intimately related to how life on Earth originated. 

Amplification by Molecular Propagation from a Chiral Center... 

Once chirality is induced and amplified by some mechanism, the excess must first persist and then propagate in order to survive. A distinctive characteristic of homo-chiral protein and nucleic acid biopolymers is that they function within the enclosed environment of cells, which provide a membranous boundary structure that separates the intracellular components from the external environment. It has accordingly been postulated frequently that analogous but simpler enclosed environments must have been available and operative on the primitive Earth. 

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

In the second hypothesis there exists a mechanism of asymmetric growth in which, in each propagation step, the conditional probability of a sequence is different from that of its antipode poo Pll)- In this case an asymmetric catalyst (or broadly speaking, a chiral environment) is required whose action persists throughout the whole course of the polymerization. 

---