Localized Chirality Transfer

Finally, we feel it is worthwhile to stress one more time the importance of the kinetic inertia in the (reversible) chiral transfer and memory processes of our porphyrin systems. Inertia provides evidence that the system is trapped in an energy minimum. In the above examples the minimum is local the real minimum is that reached from the achiral system whose formation involves the same enthalpic contribution of the chiral one but a more favourable entropic contribution. In particular, the network of electrostatic interactions ensures a quite deep local energy minimum (that is a high value of EA). 

Photosensitized enantiodifferentiating reactions are synthetically attractive and mechanistically interesting photochemical processes. The chiral information of the sensitizer is transferred to the substrate by short-Hved interactions in the excited state (i.e., during the lifetime of an exciplex of a reaction intermediate and the chiral sensitizer that is involved in the reaction mechanism) hence, the chirahty is multiplied, and only catalytic amounts of the optically active sensitizer are required. The stabilization energy of an exciplex compared to the locally excited state and its lifetime are often found to be strongly dependent on both electronic and steric properties of its components. Chiral induction can be achieved by different stabilization energies or lifetimes of the exciplex between the sensitizer and the intermediates that lead to the enantiomeric photoproducts. The absence of other reaction pathways without intimate contact to the sensitizer and of racemization processes in the further course of the reaction mechanism is an additional requirement to ensure effective chirality transfer. 

Figure 8 Microenvironmental polarity control upon enantiodifferentiating polar photi addition of alcohol (ROH) to aromatic olefin (D) sensitized by naphthalenedicarboxylai with saccharide auxiliaries (A ) the local polarity is enhanced around the saccharic moieties, facilitating electron transfer from exited sensitizer (A ) to substrate olefin (I to produce a radical cation (D +). The radical cation produced cannot escape from tl high polarity region around the saccharide to the low-polarity bulk solution and is accor< ingly attacked by ROH in the chiral environment of saccharide to produce the adduct i high ee.

Kaczmarski et al. used a similar model for the calculation of the band profiles of the enantiomers of 1-indanol on a chiral phase in HPLC [29,57]. These authors ignored the external mass transfer and assumed that local equilibrium takes place for each component between the pore surface and the stagnant fluid phase in the macropores (infinite fast kinetics of adsorption-desorption). They also assumed that surface diffusion contribution is much faster than pore diffusion and neglected pore diffusion entirely. Instead of the single file Maxwell-Stefan diffusion, these authors used the generalized Maxwell-Stefan diffusion (see Chapter 5).The calculation (see below) requires first the selection of equations to calculate the surface molecular flux [29,57,58],... 

In the case of achiral, C2v-symmetrical malonate addends, three constitutionally isomeric bis(methano)fullerenes are formed [54,131 ] An achiral one ( 2 -symmetrical 45), and two chiral ones (C2-symmetrical ( )-46 and ( )-47) which are obtained as pairs of enantiomers with an inherently chiral addition pattern [54] (Fig. 7). The tree constitutional isomers are formed in the approximate ratio 2.8 6.2 1 which differs substantially from the statistical 1 2 2 ratio and shows a marked preference for the second addition taking place at bonds C(56)-C(57)/C(59)-C(60) as compared to C(67)-C(68)/C(69)-C(70) (for the numbering of C70, cf. Fig. 3). Electroiuc factors such as the coefficients of the LUMO of the mono-adduct, to which the electron density of the incoming nucleophile is transferred [95], may explain the observed selectivity among distant bonds of (nearly) identical local curvature. 

To conclude this section on chiral conductors, it appears that the introduction of low symmetry counter ions systematically favor the presence of multiple crystallo-graphically independent molecules, with then the possibility of different degrees of charge transfer, and competition with charge localization [37]. 

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