Chiral template effect

In this work a new approach is desribed, which can help to understand ED over heterogeneous catalysts We also hope that this approach can be used to find new modifiers for enantioselective heterogeneous catalytic reactions. The basis for this approach is the steric shielding known in organic chemistry [7,8]. A chiral template molecule can induce shielding effect (SE) in such a way that it preferentially interacts with one of the prochiral sites of the substrate. If a substrate is preferentially shielded its further reaction can take place only fi"om its unshielded site resulting in ED. 

The rhodium(II) catalysts and the chelated copper catalysts are considered to coordinate only to the carbenoid, while copper triflate and tetrafluoioborate coordinate to both the carbenoid and alkene and thus enhance cyclopropanation reactions through a template effect.14 Palladium-based catalysts, such as palladium(II) acetate and bis(benzonitrile)palladium(II) chloride,l6e are also believed to be able to coordinate with the alkene. Some chiral complexes based on cobalt have also been developed,21 but these have not been extensively used. 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

On the basis of this discussion a particularly promising scheme for synthesizing chiral compounds with great isomer purity and high overall yield would be to start with a productive asymmetric synthesis (to assure high yield) and follow by a destructive one (to assure unlimited isomer purity of the desired product). Such a procedure has the essential advantage that under suitable conditions it leads to pure chiral compounds without cumbersome separation of stereoisomers. With proper choice of the reactants the procedure will also permit an effective recovery of the auxiliary chiral materials needed as chiral templates in the asymmetric synthesis. 

While technically simpler than the covalent approach, the self-assembly approach creates more heterogeneous sites and also requires templates with specific functional groups.8 Since sol-gel chemistry is aqueous based, H-bonding interactions are significantly weaker compared to the conventional organic polymerization methods. Often, hydrophobic effects and ionic interactions are utilized. A number of other examples of the noncovalent approach to imprinting in sol-gel-derived materials are provided in recent reviews.5 17 In the sections below, the focus will be on some of the newer aspects of small molecule imprinting in silica that involve the use of chiral templates... 

Chiral templates can be considered a subclass of chiral auxiliaries. Unlike auxiliaries that have the potential for recycle, the stereogenic center of a template is destroyed during its removal. Although this usually results in the formation of simple by-products that are simple to remove, the cost of the template s stereogenic center is transferred to the product molecule. Under certain circumstances, chiral templates can provide a cost-effective route to a chiral compound (Chapter 25). Usually, the development of a template is the first step in understanding a specific transformation and the knowledge gained is used to develop an auxiliary or catalyst system. 

Fig. 25 Enantioselective non covalent synthesis of double rosettes exploiting the chiral memory effect. Pyridyl residues on the dimelamine units bind chiral dicarboxylic acids leading mainly to one diastereoisomer. Once the chiral templating carboxylic acid is removed by precipitation, the enantioenriched double rosette persists for several hours...

Our hypothesis is that J-aggregates are inherently chiral and exist in aqueous solution as racemate. We anticipate that their enantiomorphic distribution can be altered by vortex action the enantiomer favoured by stirring is deposited on the cuvette wall, the other remains in solution. This is possible because stirring induces a thermodynamic unbalancing in the racemate solution. The situation is complicated because, being a weak thermodynamic force, vortex action competes with other forces, such as, for example, the (stronger) thermodynamic effect exerted by the presence of high concentrations of chiral templates. 

The next experiment is very useful to compare the stirring effect (which is a weak thermodynamic effect) with a stronger thermodynamic effect, such as that exerted by the presence of high concentrations of chiral templates. The next system, in our design, has to behave differently to the previous ones and helps in delineating a model. 

In this paper, the chirodiastaltic energy between the inorganic host and the complex guest is calculated using the molecular mechanics. The results imply that there is chiral recognition effect between the inorganic chiral motifs and chiral complex templates. The absolute configuration of the chiral templates determines that of the chiral motifs in the networks. 

The stereochemical outcome of the addition of carbanions to ketones yielding tertiary alcohols (or secondary alcohols in the case of aldehydes) is variable and depends on the substrate, the counterion and the solvent. Numerous applications of this strategy to natural product synthesis from carbohydrates can be found in the literature and this approach was fruitful in pioneering syntheses of polyketide-type products. Here again, the template effect of the sugar plays a tremendous role in the stereochemical outcome of the reaction. Chelation controlled nucleophilic addition can also be used to form chiral centers in a highly predictable way. 

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