Autocatalysis direct

Asymmetric Autocatalysis Triggered Directly by Circularly Polarized Light... 

We have demonstrated the enantioselective synthesis of near-enantiopure compounds by asymmetric photodegradation of racemic pyrimidyl alkanol 2c by circularly polarized light followed by asymmetric autocatalysis. This is the first example of asymmetric autocatalysis triggered directly by a chiral physical factor CPL. 

Fig. 8.3. The approach to, or departure from, stationary-state solutions following small perturbations for simple cubic autocatalysis again showing the instability of the middle branch. The turning points (ignition and extinction) have one-sided stability as perturbations in one direction decay back to the saddle-node point, but those of the opposite sign depart for the other...

In self-replicating systems employing three starting constituents competition between constituents can occur [9.205]. Such processes are on the way to systems displaying information transfer, whereas the two-components ones are non-infor-mational. A shift from parabolic kinetics to exponential growth of the template concentration is required for a selection process to take place [9.197]. The evidence for self-replication on the basis of template-directed autocatalysis as in 184 requires detailed mechanistic investigation on the origin of the catalytic effects observed [9.206]. 

Asymmetric autocatalysis with amplification of enantiopurity involves the catalytic automulti-plication of a chiral molecule along with increase in its enantiopurity. This process is a direct method for synthesizing highly enantio-enriched compounds from the same compound with very low enantiopurity and without any other chiral catalysts. 

Indeed, direct irradiation of racemic autocatalyst 12 by left-handed CPL and the subsequent asymmetric autocatalysis produces highly enantioenriched (S)-alkanol 12 with > 99.5% ee (Scheme 13). On the other hand, irradiation with right-handed (r) CPL instead of /-CPL, formed (R)-12 with > 99.5% ee. The process provides direct correlation of the handedness of CPL with that of the organic compound with high enantiomeric excess [84],... 

Direct Examination of Extraterrestrial Chirality in Meteorites Using Asymmetric Autocatalysis... 

Highly sensitive chiral discrimination of amino acids with low ee was described. Amino acids with low ee act as a chiral initiator of asymmetric autocatalysis. In the presence of amino acids with low ee, pyrimidine-5-carbaldehyde was treated with z-P Zn to produce chiral pyrimidyl alkanol with the absolute configuration correlated with that of the amino acid by the consecutive asymmetric autocatalysis with amplification of ee. In addition, direct examination of extraterrestrial chirality was performed using meteorites by applying the asymmetric autocatalysis as the chiral sensor. The results indicated the presence of some chiral factor in the meteorites other than known organic compounds such as amino acids. 

A reaction process in which the product directly increases the rate of the chemical reaction is called autocatalysis. In the present case, the reaction product plays the role of a catalyst and of a potent chiral auxiliary at the same time. Hence Soai s discovery described the first example of a chirally autocatalytic reaction in organic chemistry in which the chiral product and the chiral catalyst are identical. 

When A is positive, as in the case of Fig. 6c, the coefficient of the cubic termB is also positive, and the velocity 4>i vanishes at three values of 0i in the range of - 1 < 0i < 1. This is possible if a strong quadratic autocatalysis > k 2 exists together with a linear recycling X > 0, or if a linear autocatalysis k and a nonlinear recycling x > 0 coexist. By following the direction indicated by the arrows for positive order parameter ends up at a definite value ... 

It is instructive to contrast direct autocatalysis with reflexive (mutual) autocatalysis in the GARD model [23]. In direct autocatalysis, the diagonal... 

The essential criteria of life are twofold (1) the ability to direct chemical change by catalysis, and (2) the ability to reproduce by autocatalysis. The ability to undergo heritable catalysis changes is general, and is essential where there is competition between different types of living things, as has been the case in the evolution of plants and animals. 

Once the aldehyde concentration builds up to a critical level (and even in the oxidation of secondary alcohols, small quantities of aldehydes are produced by side-reactions), then autocatalysis is observed. It is suggested that the aldehydes bring about chain-branching by reacting directly with oxygen... 

Brown and coworkers reported studies of the addition of aryl iodides to a series of palladium complexes of trialkylphosphines with the general formula Pd[(P(Cy) (Bu-f)3 )]2 (n = 0-3)181. They concluded that the complexes of the smaller phosphines (n = 2, 3) react with Phi through an associative mechanism and undergo oxidative addition directly to the PdL2 complex. They also concluded that oxidative addition of Phi to complexes of the bulkier phosphines (n = 0-1) proceed after dissociation of ligand to generate PdL. In unpublished work, Barrios-Landeros and Hartwig have found that the kinetic behavior of the reactions of these complexes is complex and that these reactions occur with profiles that are characteristic of autocatalysis. 

The advance of a reaction interface, once established, into regions of almost perfect crystal, in preference to the appearance of new centres of decomposition in the bulk material, indicates that the reaction contributes to fiuther decomposition by generating conditions that were not present originally. This is a form of autocatalysis. The crystal structure may, however, influence the direction and rate of advance, e g. along layers in layer-type structures. Distortion of the structure adjacent to the reaction zone may lead to destabilization, or reaction may produce surfaces which act as a catalyst for further reaction, e.g. the particles of metal product formed during decompositions of metal carboxylates [18,59,73 and Chapter 16] provide active surfaces for anion breakdown. If a decomposition takes place in several stages, the stability of an intermediate may determine the overall reaction rate [22,79]. 

But what is it about this system that makes us call it self-replicating How could we show that the system progresses through directed template catalysis rather than through simple chemical autocatalysis After all, (6) bristles with functional groups. The imide, amide, ribose and purine functionalities must all be considered as possible explanations for the autocatalysis observed. For example, imidazole is a well-known catalyst for acylation reactions, and the purine contains such a subunit. Could not this functionality be the cause The potentially catalytic functions of the product molecule had to be individually tested in the structural context of (6) and under the conditions where (6) acts as an autocatalyst. 

Bryce and Greenwood studied the kinetics of formation of the major volatile fraction from potato starch, and its components. They limited their interest to the temperature range from 156 to 337 and to the formation of water, as well as of carbon mon- and di-oxide. The results revealed the following facts. Stability toward pyrolysis within the first 20 minutes of the process falls in the order amylose < starch < amylopectin < cellulose. Autocatalysis is absent, as shown by Puddington. Both carbon mon- and di-oxide are evolved as a consequence of each of two first-order reactions. The initial one is fast, and the second is slow. The reasons are not well understood, but they probably involve some secondary physical effects. The amount of both carbon oxides is a direct function of the quantity of water produced from any polysaccharide, which, furthermore, is independent of the temperature. The activation energy for the production of carbon mon-and di-oxide reaches 161.6 kJ/mol, and is practically independent of the polysaccharide formed. At the limiting rates, the approximate ratios of water carbon dioxide carbon monoxide were found to be 16 4 1 for amylopectin, 13 3 1 for starch, 10 3 1 for amylose, and 16 5 1 for cellulose. 

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