Auto-catalysis

Another aspect of NLE is asymmetric autocatalysis as an event following symmetry breaking in nature. On the origin of chirality in nature, two major mechanisms have been proposed [28]. (1) Chance mechanism To generate an optically 

The non-equivalence of enantiomers through the spontaneous breaking of mirror-symmetry in nature is amplified by asymmetric autocatalytic reaction [34], e.g. Frank s spontaneous asymmetric synthesis [35, 36] (Fig. 7-8). Alberts and Wyn-berg have reported in enantioselective autoinduction that chiral lithium alkoxide products may be involved in the reaction to increase the enantioselectivity (Eq. (7.9)) [37]. The product % ee however does not exceed the level of catalyst % ee. In asymmetric hydrocyanation catalyzed by cyclic dipeptides, the (Si-cyanohydrin product complexes with the cyclic peptide to increase the enantioselectivity in the (S)-cyanohydrin product, the reaction going up to 95.8% ee (Eq. (7.10)) [38]. In the presence of achiral amine, (/ )-l-phenylpropan-l-ol catalyzed carbonyl-addition reaction of diethylzinc has been reported to show lower % ee than that of the catalyst employed [39]. 

Soai has reported the remarkable example of asymmetric autocatalysis in carbonyl-addition reactions of diisopropylzinc [40- 3, 45]. Usually, zinc alkoxide forms an inactive tetramer. However, the use of pyridyl aldehyde as a substrate to give pyridyl alcohol product can loop the catalytic cycle without formation of the inac- 

a little non-equivalence of enantiomers caused by symmetry breaking can be amplified through asymmetric autocatalysis to a large enantiomeric non-equivalence in molecules as found in nature. 

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Entropy, Entropy Production, Auto Catalysis and Oscillating Reactions 69... 

Mossel, E. and Steel, M. (2005). Random biochemical networks the probability of self-sustaining auto-catalysis. J. Theor. Biol., 233, 327-336... 

It must be realized that the basic reason for bifurcation is that the function F is multiple-valued and therefore non-linear. Other sources of non-linearity, like auto-catalysis have been explored systematically and have proven to be the starting point of geochemical catastrophes (e.g., Ortoleva, 1994). 

Fig. 1.3. The variation of reactant concentration, a, and reaction rate, — da/dt, with time for a system obeying quadratic auto catalysis initial reactant concentration a0 = 0.1 mol dm-3 ...

These reactions are catalysed by acids such as Lewis acids, phenols, and alcohols. The hydroxyl groups formed by the amine epoxide addition are active catalysts, so that the curing reaction usually shows an accelerating rate in its early stages, typical of auto catalysis. In some cases when the amine is present in less than stoichiometric concentrations, reaction of epoxide and hydroxyl may occur to produce an ether group ... 

We also want to scale the reaction rate, although we cannot guarantee that it will be less than 1 if auto-catalysis is present. 

Auto-catalysis by the stable end products of the chemical reaction under consideration. The concentration of these catalyzing products is related by the stoichiometric equation to the decrease in the concentration of the original substance. Thus, for a reaction equation,... 

In both cases of auto-catalysis the initial concentration of the catalyzing product is taken equal to zero. 

They have the induction period required for active site accumulation in the system in amounts that cause a noticeable transformation of the initial substance. In both cases (for analysis, refer to the next chapter) the initial reaction is supplementary, just defining the induction period. Hence, the main oxidation reaction proceeds after the induction period, and its stoichiometric equation does not take into account the consumption of initial reagents (as well as consumption of initiators of the induction and initiation period of chain reactions). Thus at the initial stage of oxidation, initiation and auto-catalysis play the role of reaction launchers . 

As shown in Scheme 9, various organic compounds can act as a chiral initiator of asymmetric auto catalysis. 2-Methylpyrimidine-5-carbaldehyde 9 was subjected to the addition of z-Pr2Zn in the presence of chiral butan-2-ol, methyl mandelate and a carboxylic acid [74], When the chiral alcohol, (S)-butan-2-ol with ca. 0.1% ee was used as a chiral initiator of asymmetric autocatalysis, (S)-pyrimidyl alkanol 10 with 73% ee was obtained. In contrast, (,R)-butan-2-ol with 0.1% ee induced the production of (A)-10 with 76% ee. In the same manner, methyl mandelate (ca. 0.05% ee) and a chiral carboxylic acid (ca. 0.1% ee) can act as a chiral initiator of asymmetric autocatalysis, therefore the S- and IC enantiomers of methyl mandelate and carboxylic acid induce the formation of (R)- and (S)-alkanol 10, respectively. Chiral propylene oxide (2% ee) and styrene oxide (2% ee) also induce the imbalance of ee in initially forming the zinc alkoxide of the pyrimidyl alkanol in the addition reaction of z-Pr2Zn to pyrimidine-5-carbaldehyde 11 [75]. Further consecutive reactions enable the amplification of ee to produce the highly enantiomerically enriched alkanol 12 (up to 96% ee) with the corresponding... 

The chirality in the organic compound (even though with small ee) can be converted into almost enantiomerically pure pyrimidyl alkanol by asymmetric auto catalysis with amplification of chirality. 

Indeed, in the presence of L-leucine with only 2% ee as a chiral initiator, the reaction of 2-methylpyrimidine-5-carbaldehyde 9 with i-Pr2Zn produced (iC-pyrimidyl alkanol 10 with an enhanced ee of 21% (Scheme 11) [74,81]. In contrast, when D-leucine with 2% ee was used as a chiral initiator, (S)-10 with an increased ee of 26% was obtained. As described in the preceding section, the ee of the obtained pyrimidyl alkanol can be amplified significantly by consecutive asymmetric auto catalysis to achieve homochirality. 

Hexahelicene is a chiral hydrocarbon with a helical structure. We found that (P)-hexahelicene with 0.13% ee, a lower ee than that induced by CPL [3,80], acts as a chiral initiator for asymmetric autocatalysis (Scheme 11). The reaction between pyrimidine-5-carbaldehyde 11 and i-Pr2Zn gave (S)-pyrimidyl alkanol 12 with 56% ee [83]. On the other hand, when (M)-hexahelicene with 0.54% ee was used instead of (P)-hexahelicene, (P)-12 with 62% ee was formed. As already described, these ee can be enhanced by further asymmetric autocatalysis. Thus, the chirality of CPL has been correlated with that of alkanol 12 with high ee by using hexahelicene as the chiral source of asymmetric auto catalysis. 

We found that inorganic helical structures such as helical silica serve as chiral triggers for asymmetric autocatalysis (Scheme 23). In the presence of helical silica, the enantioselective addition of z-P Zn to 2-alkynylpyrimidine-5-carbaldehyde 11 was examined. In the presence of right-handed helical silica, (S)-5-pyrimidyl alkanol 12 was formed [123]. In contrast, in the presence of left-handed helical silica, (S)-5-pyrimidyl alkanol 12 with high ee was obtained. These results clearly show that asymmetric auto catalysis can discriminate the helical structure in artificially tuned inorganic silica. 

As described, asymmetric auto catalysis is closely related to the origin of the homo chirality of organic compounds. 

The conformational distinction between homo- and heterochiral dimers indicates why a bulky dialkylzinc may be important in limiting the scope of amplifying auto catalysis the Soai prescription remains unique. Since it is the product of reaction that is also the catalyst, a further question needs to be addressed. In the conventional Oguni-Noyori reaction discussed earlier [60-71] the zinc alkoxide product normally plays no further part in the proceedings because it forms a stable cubic tetramer [81-87]. There are scattered exceptions in zinc-mediated catalysis, arising when the product structure is conducive to its further involvement [88,89]. 

This book illustrates the recent aspects of amplification of chirality by asymmetric auto catalysis and by forming helical structures. The first four chapters summarize experimental asymmetric autocatalysis with amplification of enan-tiopurity, the mechanism of asymmetric autocatalysis examined by NMR and calculation, the computer simulation models of the reaction mechanism of asymmetric auto catalysis, and the theoretical models of amplification of chirality. The last chapter deals with the amplification of chirality by the formation of helical structures. However, the amplification of enantiopurity in non-auto catalytic asymmetric reaction and the amplification by enantiomer separation involving crystallization or sublimation are beyond the scope of this book. 

One of the main features of asymmetric autocatalysis and the formation of the helix is that the initial extremely low enantioenrichment is amplified significantly to near enantiopure. These processes of amplification of chirality have become powerful tools to elucidate the origin of chirality of organic compounds. For example, by using asymmetric auto catalysis, spontaneous absolute asymmetric synthesis without the intervention of any chiral factor has been realized. 

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