Chirality conformation

We now consider the alternative strategy with which to create chiral molecules using Gay-Berne molecules as a basis. In this two rod-like molecules are linked together resulting in a wide range of chiral conformations. One of... 

Because the time scale of the Raman scattering event ( 3.3 x 10-14s for a vibration with wavenumber shift 1000 cm-1 excited in the visible) is much shorter than that of the fastest conformational fluctuations in biomolecules, the ROA spectrum is a superposition of snapshot spectra from all the distinct chiral conformers present in the sample. Together with the dependence of ROA on chirality, this leads to an enhanced sensitivity to the dynamic aspects of biomolecular structure. The two-group model provides a qualitative explanation since it predicts ROA intensities that depend on absolute chirality in the form of a sin x dependence... 

McGrath et al. have also thoroughly studied the chiroptical properties of dendrimers such as 40. They compared the optical activities of the series of 1st-, 2nd- and 3rd-generation compounds of type 40, considering the molar rotation per chiral unit ([ ]D/n) [75]. A big difference of the values was found between the generations which could possibly indicate chiral conformations inside the dendrimers, that enhance the optical rotation values per unit when... 

Comparison of the optical activity showed that the dendrimer 63 with branches of (S)-configuration has a specific rotation and a molecular ellipticity which clearly deviate from the expected values [88,90]. All other 2nd-generation dendrimers (even those with additional spacers between the branches and the core) have specific rotations that are comparable to those expected by simple addition of appropriate values for their building blocks. The deviation may therefore signal the presence of chiral conformational substructures in the 2nd-generation dendrimer 63. 

The dynamic NMR (DNMR) spectroscopy has been used in studies of stereomutations of non-symmetrical di-Schiff bases [18].39 It was shown that the hindered Schiff bases exist in DMSO in two chiral conformations. The presence of a pair of conformers being in equilibrium was explained by the existence of two stereogenic axes a g (aligned to Cl—N8 bond) and a 7 (aligned to C6—N7 bond) due to restricted rotation around two Ar—N bonds. The trans to cis interconversion as well as enantio- or diastereoisomerisation barriers for the compounds studied have been established using line shape analysis. 

A similar mechanism could operate also for bis(imino)pyridyl Fe(II)-based catalysts which have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138139 In fact, preliminary calculations indicate that the chirality of the growing chain would determine a chiral conformation of the bis-pyridyl ligand, which in turn would discriminate between the two monomer enantiofaces. 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

If both alcohol monomers forming a dimer are on average achiral, one may still have chirality synchronization events, where the two monomers match their transient chiral conformations when they bind to each other. A particularly simple example is that of ethanol dimer, where the lowest-energy conformer involves two gauche monomers of the same helicity [80,91]. However, the energy difference to other conformers is so small that efficient isomerizing collisions in a supersonic jet expansion are required to favor the lowest-energy form over the others. A more... 

We have found that the novel chiral self-dimerizahon of V complexes produces an active unsaturated structure with a new chiral conformation on a Si02 surface, and provides highly enanhoselective achve sites for the asymmetric oxidative coupling of 2-naphthol [35, 36]. Table 10.3 summarizes catalyhc activities, selectivities and enantiomeric excess (e.e.) of homogeneous and heterogeneous V catalysts for the oxidative coupling of 2-naphthol. The homogeneous reactions on a representative L-leucine based V monomer did not exhibit the catalyhc activity. Only in the presence of chlorotrimethylsilane as an acidic promoter did the reachon proceed, but conversion was as low as 15% at 293 K, selectivity to BINOL and e.e. to (P)-BINOL were quite low at 73% and 8%, respectively (Table 10.3). Furthermore, loss of activity occurred and no reachon proceeded after 9 days. 

Notably, the enantioselectivity of the coupUng reaction is determined by the chiral conformation on the V center rather than the chirality of the Schiff-base ligands. The chirality of the ligands affects stericaUy the chiral self-dimerization... 

Scheme 14 Yamada s chiral conformation-switch catalyst applied to the KR of aryl alkyl sec-alcohols [136, 144]...

Recker J, Tomcik DJ, Parquette JR. Folding dendrons the development of solvent-, temperature-, and generation-dependent chiral conformational order in intramolecularly hydrogen-bonded dendrons. J Am Chem Soc 2000 122 10298-10307. 

The relative population of chiral conformers changes with temperature. 

An increase in the optical rotation caused by an achiral contaminant may occur as the result of the induction of a preferential chiral conformation in an achiral contaminant33. Thus, careful purification of both sample and solvent is mandatory in polarimetry, Utmost care has to be exercised, however, to avoid accidental de- or enrichments due to fractionation during incomplete recoveries (see also Section 3.1.3.). 

In the case of flexible molecules all chiral conformers contribute to the observed CD spectrum. This usually leads to substantial reduction of the magnitude of the exciton Cotton effect. Nevertheless, if the conformation of the main contributor is established independently (e.g.. by molecular mechanics or NMR spectroscopy), its absolute configuration can be deduced from the exciton Cotton effect. Thus, for 3-(l-naphthalenyl)phthalide 5, the preferred conformation is 5 a. The negative couplet is in accordance with the left-handed screw between the phthalide 1 Ld and the naphthalene lBb transition dipole moment vectors, when the absolute configuration is R121. 

An X-ray analysis 34a) of ( ),( ),( )-l,5,9-cyclododecatriene (38) suggested that this molecule is most comfortable in a twisted chiral conformation of D3 symmetry, and a dynamic NMR study and a force-field calculation Mb) estimated AG = 8.6 kcal/mol for the racemization process between two enantiomeric D3 conformers. A pseudochair conformation of similar D3 symmetry was also suggested35) for 1,5,9-cyclodo-decatriyne (39), following an ab initio STO-3G calculation and photoelectron spectroscopy. 

Chiral Conformity During Complexing in Heterocyclic Structures D. J. Cram et al., Usp. Khim. Geterotsikl., 1976, 287-306. 

Figure 12-20 Representation of (a) achiral and (b) chiral conformations of frans-cycloalkenes, using frans-cyclooctene as a specific example. For frans-cyclooctene, the achiral state is highly strained because of interference between the inside alkenic hydrogen and the CH2 groups on the other side of the ring. Consequently the mirror-image forms are quite stable. With frans-cyclononene, the planar state is much less strained and, as a result, the optical isomers are much less stable. With frans-cyclodecene, it has not been possible to isolate mirror-image forms because the two forms corresponding to (b) are interconverted through achiral planar conformations corresponding to (a) about 1016 times faster than with frans-cyclooctene.

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