Optically active state

Fluctuations are inherent to any experimental chemical system. Even if these fluctuations are infinitesimally small, they are sufficient to drive the system away from an unstable state. The optically active state is characterized by two equivalent options starting from an unstable racemic situation, the system can evolve into either an R configuration or into an S one. However, each individual experiment remains unpredictable as to which of the optically active states the system will move towards. For a large number of experiments an equal and random distribution between R and S dominance is expected if the initial conditions do not involve any preferences. Due to this unpredictability of the chiral configuration, the phenomenon of mirror-symmetry breaking introduces another element of stochastic behavior into chemical reactions different from that occurring in clock reactions [38,39]. 

During classical asymmetric synthesis, the amplitude of these fluctuations are expected to decrease during the course of the reaction because more and more chiral molecules are formed and eeeXp declines. However, in the presence of chiral autocatalysis, the small ee caused by such fluctuations can be amplified. In such cases, the system is likely to be most sensitive in the initial stage of reaction when the concentration of chiral molecules is still small. If the autocatalytic species are concentrated they can be either in a racemic or optically active state but if they are highly diluted, as at the beginning of the reaction, statistical fluctuations can become significant so that the state... 

The simplest situation occurs if X is highly stereoselective regarding the formation of R and if it shows an overwhelming preference to form the XR over the XS dimer. In this case, the competition between k 6 and k ]0 alone gives rise to the enantioselectivity reversal. Here, R is inhibited by the process X + R-o-XR but promoted by A + Z + X R + X. In this particular case, a stepwise transition between the two optically active states can be obtained by simulation within a certain range of the involved rate parameters when the concentration of the additive is gradually increased. A more general case where all the realistic processes are operative is shown in Fig. 7. 

This function exactly vanishes for uncorrelated states, i.e. states that can be defined as products of local molecular states (as in the mf and EM assumption). Positive (negative) f, indicates instead an increased (decreased) probability of finding I nearby zwitterionic molecules with respect to the uncorrelated state at the same average polarity. Fig. 11 shows the Z dependence of f, calculated for the most optically active state (left panel) and for the gs (right panel) of the same cluster as in Fig. 9. For the active excited state, a sizeable weight is found of wave functions with several (say 2 to 6) nearby fully zwitterionic molecules (we will call these wave functions T-droplets ). This demonstrates that the phenomenon of photoinduced multielectron transfer corresponds to a concerted motion of electrons on several nearby molecular sites [93]. 

The simple kinetic model usually employed to relate spectral characteristics of emission to rates of IVR is as follows. As in the previous discussion, one assumes that the species is excited to some optically active state u b). Subsequently, an irreversible IVR process with rate constant fc1VR occurs such that the state of the species eventually evolves to ab >. With this model, application of kinetics gives... 

Figure 36. General level schematic pertaining to IVR and portraying the possibility of mixing between optically active states outside the laser bandwidth (A

The hole-burnt state (State II) has a higher energy than the unbumt-state (State I) and it relaxes across the activation barrier into the unbumed state as shown in Figure 11-8. The optically activated state (State II in Figure 11-8) relaxes across the activation barrier, V, into the unbumt state (State I), the rate of which is given (Zhang, 1992) ... 

They described a number of new reactions, and classes of compound never prepared before in an optically active state (Scheme 9), including diastereomeric... 

Furthermore, he showed that these considerations apply to catalytic asymmetric syntheses and decompositions alike, though in the former the stability of the optically active state (measured by the ratio of the time required for racemate formation to the time required for attainment of maximum optical activity) will be considerably greater than in the latter. 

As has already been shown, several quite different mechanisnts may operate in optically selective biosynthesis and, given the initial presence of an optically active substance, there is no difficulty in accounting for the perpetuation of asymmetry through a subsequent chain of biosyntheses. But we have still to face the thorny problem of the production of the initial directing agent in an optically active state, and we may conclude this survey by considering the relative merits of the two rival mechanisms which have been proposed to account for this. 

Certain state highway authorities are studyiag the use of ftber-reiaforced polymers, typically thermosets such as epoxy or unsaturated polyester, for bridge constmction. On an even more futuristic scale, fiber optics that employ polymeric jacketing and, ia some cases, optically active polymeric cores, may someday be employed ia place of wines for home security systems, climate control, etc (10,91). 

The dynamic stereochemishy of biaryls is conceptually similar. The energy barrier for racemization of optically active 1,1 -binaphthyl (Scheme 2.2, enhy 3, p. 83) is 21-23 kcal/mol. The two rings are not coplanar in the ground state, and the racemization takes place by rotation about the l,l -bond. 

It has frequently been stated that dipentene has a higher boiling-point than its optically active components, but this is not so, any observation in this direction being undoubtedly due to the presence of minute traces of impurities. 

Excellent stereoselectivity is attained with the optically active boron enolate 7, prepared from the corresponding A-acyloxazolidinone (see Appendix) and diethylboron trifluoromethanesul-fonate177. The reaction presumably proceeds via a transition state similar to 5. 

Mechanisms in the racemization of optically active coordination complexes in the solid state. P. O Brien, Polyhedron, 1983, 2,233-243 (54). 

Treatment of (—)-(S)-276 with allyl Grignard reagents gives optically active allylic sulphoxides 288. This reaction, however, involves an allylic rearrangement via transition state 289 as evidenced by Mislow and his collaborators362 (equation 160). 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

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