Initiation asymmetric

A rationale of asymmetric polymerization was proposed for the first time by Frisch, Schuerch, and Szwarc in 1953 (38). These authors proposed two extreme mechanisms for the formation of optically active polymers. The first involves asymmetric initiation and stereoregular symmetric growth in other words, a... 

Pathway 11 An asymmetric initial division produces two cells of unequal size. The ball of cells may originate from the continued division of the larger vegetative cell while the smaller generative cell may divide a few times before degenerating. 

From racemic mixtures of monomers containing one chiral center in the heterocycle and an achiral lateral chain an OA polymer is obtained with the asymmetric carbon atom in the main chain by a stereoelective process using asymmetric initiators (see Section... 

In the light of absorption spectrum data, the authors assumed that the asymmetric initiators follow the Scheme LIX and confirmed that the asymmetric-selection of monomers is indeed systematically controlled by the spatial arrangement of the asymmetric centers of the initiator (LIXb). 

Fig. 2.8. Evolution of an asymmetrical initial distribution into equihbrium using the master equation with the parameters N=25,k= 1.5 and d = 0...

In Fig. 2.7 the distribution remains symmetrical. The initial fluctuation enhancement and the subsequent drift dominated development of the bimodal distribution into equilibrium can be observed. In Fig. 2.8 the first stages of the development are the same as in Fig. 2.7. In accordance to the asymmetric initial distribution, however, it comes to a bimodal distribution with peaks of differing significance. The final development into the symmetrical stationary distribution is fluctuation dominated and extremely slow. Inserting the explicit equations (2.121,123,131) into (2.116) the transition time for collective opinion change is obtained as... 

Grandbois and Collins have used the asymmetric initiator (R,R)-[Ru]-XIV to kinetically resolve a racemic sample of the doubly armed, C2-symmetric tetraene ( )-48 (Scheme 9.12) [17]. Interestingly, the enantiomeric excess of the [7]hehcene... 

Scheme 9.12 Use of RRCM and an asymmetric initiator to kinetically resolve the helically chiral substrate ( )-48.

For the asymmetric membranes, progressive yield can cause a loss of production rate due to compaction of the matrix in a prolonged operation. The pressure at which hoUow-fiber compaction is initiated, P, can be approximated ... 

This is also known as the asymmetrical breaking current and tends to become the symmetrical r.m.s. value of the fault current / c after almost four cycles from the instant of fault initiation, as discussed in Section 13.4.1(8). 

Furthermore, the situation becomes even worse for an asymmetric potential like that in (3.18), because at low temperature nearly the entire period p is spent on dwelling in the potential well (see appendix A), so that lim -oo < >ins = 0- In other words, unless the potential is strictly symmetric, the transition state position x tends to the minimum of the initial state It is natural to expect that the centroid approximation will work well when x does not deviate too far from x. To summarize, the centroid method is an instructive way to describe in a unique TST-like manner both the high [T > T ) and fairly low [T < T ) temperature regions, but it does not give a reliable estimate for k. ... 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

The mechanism of the asymmetric alkylation of chiral oxazolines is believed to occur through initial metalation of the oxazoline to afford a rapidly interconverting mixture of 12 and 13 with the methoxy group forming a chelate with the lithium cation." Alkylation of the lithiooxazoline occurs on the less hindered face of the oxazoline 13 (opposite the bulky phenyl substituent) to provide 14 the alkylation may proceed via complexation of the halide to the lithium cation. The fact that decreased enantioselectivity is observed with chiral oxazoline derivatives bearing substituents smaller than the phenyl group of 3 is consistent with this hypothesis. Intermediate 13 is believed to react faster than 12 because the approach of the electrophile is impeded by the alkyl group in 12. 

Since Evans s initial report, several chiral Lewis acids with copper as the central metal have been reported. Davies et al. and Ghosh et al. independently developed a bis(oxazoline) ligand prepared from aminoindanol, and applied the copper complex of this ligand to the asymmetric Diels-Alder reaction. Davies varied the link between the two oxazolines and found that cyclopropyl is the best connector (see catalyst 26), giving the cycloadduct of acryloyloxazolidinone and cyclopentadiene in high optical purity (98.4% ee) [35] (Scheme 1.45). Ghosh et al., on the other hand, obtained the same cycloadduct in 99% ee by the use of unsubstituted ligand (see catalyst 27) [36] (Scheme 1.46, Table 1.19). 

This review outlines developments in zinc-mediated cyclopropanation from the initial reports in the 1950s through to the current state of the art methods. The presentation will rely heavily on how the evolution of mechanistic understanding aided in the rationalization and optimization of each new advance in the asymmetric process. 

Although is it possible to delineate the requirements for a suitable catalytic system, reducing this to practice is a daunting prospect. As is the case in many examples of asymmetric catalysis, empirical survey provides the initial leads which, aided by an understanding of the process, allows for accelerated development. 

These initial reports demonstrated that a catalytic asymmetric variant of the Simmons-Smith reaction could be developed. Although good yields and selectivities were obtained, the lack of a clear understanding of the origin of activation, the limited structural information on the active species and the absence of a stereochemical model made rational improvements difficult at best. The next... 

Isoprene can be polymerized using free radical initiators, but a random polymer is obtained. As with butadiene, polymerization of isoprene can produce a mixture of isomers. However, because the isoprene molecule is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions. Six tactic forms are possible from both 1,2- and 3,4- addition and two geometrical isomers from 1,4- addition (cis and trans) ... 

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