Catenate chiral

Other supramolecular structures such as catenanes and rotaxanes can be formed using zinc as a template ion for example, a benzylic imine catenate formed by Leigh et a/.288 The reversible five-component assembly of a [2]catenane from a chiral metallomacrocycle and a dinaphtho-crown ether has been achieved. Zinc is used as the metal component and drives assembly via the coordination to a bipyridyl unit 7r interactions between the aromatic components are also... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

Out of many exciting recently obtained topologically distinct structures combined rotaxanes 41,400 [23], pretzel-molecule 35 [24a] and bis(pretzelane) 401 [24b], a topologically chiral [2]catenane [25] and an interestiig catenated... 

Resolution using chiral supports in HPLC turned out to be successful for the separation of chiral catenates [135, 136], Unfortunately, this technique seemed to be inappropriate to the resolution of the knot. 

Finally, one may note that catenated and knotted structures display topological chirality [8.280, 8.282c, 9.76, 9.178]. 

Hori, A., Kataoka, H., Akasaka, A., Okano, T. and Fujita, M. (2003) Reversible catenation of coordination rings with Pd(II) and/or Pt(II) metal centers chirality induction through catenation, J. 

To our knowledge, topologically chiral molecules have not yet been resolved into enantiomers. However, we may anticipate that their energy barrier to racemization will be extremely high, compared to Euclidean chiral molecules. Therefore they are expected to be useful in enantioselective interactions or reactions. For example, it has been shown that tetrahedral copper(I) bis-2,9-diphenyl-l,10-phenanthroline complexes (which form the catenate subunits) are good reductants in the excited state [97] therefore the chiral Cu(I) catenates could be used for enantioselective electron-transfer reactions. Alternatively, the resolution of topologically chiral molecules would allow to answer fundamental questions, such as what are the chiroptical properties of molecular trefoil knots ... 

The Schlafli Symbols of these 6-connected nets are 48.66.8 (rob) for L = pyrazine, and 44.610.8 for L = l,4-bis(4-pyridyl)butadiene, respectively, so that the requirements for self-catenation are fulfilled, since the catenated 6-rings are the shortest rings [50], Perhaps one of the most fascinating examples is represented by [Ni(tpt)(N03)2] (tpt = tri-4-pyridyl-l,3,5-triazine) [51]. This contains a remarkable 3-connected chiral uniform net of 123 topology (twt, VS=124.12y. 127) (see Figure... 

The charge-assisted tt- tt stacking methodology has proved to be enormously versatile and has led to an enormous variety of catenated species. Of particular interest is the chiral bis([2]catenane)... 

Synthetically, Sauvage s group have been able to extend this work as far as three metal centres, giving rise to the (unconditionally chiral) doubly interlocked [2]catenate 10.137. ° The synthetic procedure is similar to those employed for the trefoil knots 10.135 and singly interlocked... 

For the simple case with the symmetrical phen-containing threads, such as shown in Figure 20(a), the resultant [2]catenane is nonchiral. However, in the case where nonsymmetrically substituted phens are utilized (such as in Figure 20b), the resultant catenane is chiral, and in this case the catenate formed (shown on the left of Figure 20c) was shown to be chiral using H NMR in the presence of a chiral reagent. 

Chiral catenanes can be obtained by the use of an unsynunetrically substituted 1,10-phenanthroline derivative such as LI 123 in the Cu+-template method. Thus the catenate [Cu(L1124)2] , generated in 12% yield as shown in Scheme 4-26 [247J, is topologically chiral. 

Addition polymer, 395 Alcohol, 370 Aldehyde, 370 Aliphatic, 365 Alkyl group, 367 Amide. 370 Amine, 370 Amino acid, 370 Aromatic. 365 Carbocation, 387 Carboxylic acid, 370 Catenation, 365 Chiral, 383... 

Despite the numerous molecular topologies reported to date, it is clear that very few of the intrinsic properties of these assemblies such as the topologically unconditional chirality of trefoil knots and Solomon links or the incredible kinetic inertness of Cu(I)-based [2]catenates have been exploited. A step forward for a better understanding and use of these supramolecular assemblies resides in the availability of researchers to exploit such properties. 

Limitations on the application of hydrogen bonded capsular assemblies come from the poor stability of these assemblies in polar media compared to those held by covalent bonds. In an attempt to overcome these Umitations, Bohmer described the synthesis of a mechanically locked chiral bis-catenane based on calixarene capsules [55]. Inspired by this pioneering work, our group produced a mechanically locked hybrid tetraurea calix[4]arene-calix[4]pyrrole capsule 30 (Fig. 32.14) [56]. This catenated capsule displayed reversible encapsulation (i.e. reversible assembly of the urea belt). The two hemispheres of the capsule never dissociate completely due to the bis-catenated topology. 

Addition polymer, 420 Alcohol, 390 Aldehyde, 390 Aliphatic, 389 AUcyl group, 390 Amide, 390 Amine, 390 Amino acid, 395 Aromatic, 389 Carbocation, 411 Carboxylic acid, 390 Catenation, 389 Chiral, 408... 

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