Chirality observation

It is interesting to stress that the spin chirality observed in the gadolinium radical chains differs from the more usual one that characterizes anisotropic materials and is solely due to the significant strength of NNN interactions between lanthanide ions that are very far apart. The mechanism responsible for this interaction remains unclear and the complexity of the system has, up to now, hampered an ab initio investigation of the phenomenon. 

The chirality observed in this kind of substituted allene is a consequence of dissymmetry resulting from restricted rotation about the double bonds, not because of a tetrahedral atom carrying four different groups. Restricted rotation occurs in many other kinds of compounds and a few examples are shown in Table 13-3, which includes trans-cycloalkenes (Section 12-7), cycloalkyli-denes, spiranes, and ort/zo-substituted biphenyl compounds. To have enantiomers, the structure must not have a plane or center of symmetry (Section 5-5). 

This dichotomy reflects itself in the different possible choices for decompositions of the thermal density operator Dp into pure states, viz. (i) A decomposition of the thermal density operator Dp into (symmetry-adapted) eigenstates of the Hamiltonian. If superpositions of these eigenstates are not considered, one obtains a classical energy obsen able. (ii) A decomposition of the thermal density operator Dp into pure handed states. If superpositions of these handed states are not considered, one gets a classical chirality observable, (iii) A decomposition fi of the thermal density operator Dp into pure states such that the average dispersion... 

Most linear optical phenomena such as refraction, absorption and Rayleigh scattering are described by the first term in Eq. (1) where is the linear susceptibility tensor. The higher order terms and susceptibilities are responsible for nonlinear optical effects. The second-order susceptibility tensor T underlies SFG, whereas and BioCARS arises within As we are concerned with optical effects of randomly oriented molecules in fluids, we need to consider unweighted orientational averages of the susceptibility tensors in Eq. (1). We will show that the symmetries of the corresponding isotropic components and correspond to time-even pseudoscalars the hallmark of chiral observables [2]. 

Parity, or space inversion, is the symmetry operation that interconverts a chiral molecule into its mirror image. All coordinates (x, v, <) are replaced everywhere by (—X, — y, —z) under space inversion [2]. A chirality specific response in liquids and gases requires that the isotropic component of the susceptibility is odd under parity. Further, since the isotropic part of any tensor is necessarily a scalar, it follows that pseudoscalars - independent of the choice of coordinate axes and of opposite sign for enantiomers - underlie chiral observables in fluids. The isotropic medium may... 

These boron enolates can be considered as chiral nucleophiles wherein chirality observed in the products of the aldol reactions arises from the chiral auxiliary mandelic acid. An alternative approach to the diastereo- and enantioselective carbon-carbon bond forming reaction is to react an achiral anion precursor with an electrophilic equivalent containing a chiral auxiliary derived from mandelic acid. 

Calculations indicate that the position of oxidation of substituted phenols by PIDA and phenyliodonium bis(trifluoroacetate) is in accord with the intervention of phen-oxenium ions as intermediates. The lack of induction of chirality observed in the reaction, whether using a preformed chiral iodonium reagent or a homochiral alcohol as the medium, also supports this hypothesis. ... 

Methods are required to detect chirality. A so-called chirality observation answers the question whether the system is chiral or not with yes or no. Then, a chirality measurement yields a value and a sign for a quantity that gives information about the chirality of the molecules or the phases but, in general, gives no measure for chirality itself. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40]. 

In other approaches, chirahty descriptors were developed with the intention not of measuring chirality but of describing chirality in a way that correlations could be established with observable properties. These descriptors have different values for opposite enantiomers, in order that chirality-dependent properties can be predicted from them. They are usually multidimensional. 

Chirality codes are used to represent molecular chirality by a fixed number of de-.scriptors. Thc.se descriptors can then be correlated with molecular properties by way of statistical methods or artificial neural networks, for example. The importance of using descriptors that take different values for opposite enantiomers resides in the fact that observable properties are often different for opposite enantiomers. 

Most importantly, enantioselectivity benefits considerably from the use of water. This effect could be a result of water exerting a favourable influence on the cisoid - transoid equilibrium. Unfortunately, little is known of the factors that affect this equilibrium. Alternatively, and more likely, water enhances the efficiency of the arene - arene interactions. There is support for this observation"" . Since arene-arene interactions are held responsible for the enantioselectivify in many reactions involving chiral catalysts, we suggest that the enhancement of enantioselectivity by water might well be a general phenomenon. 

Molecular chirality is most often observed experimentally through its optical activity, which is the elfect on polarized light. The spectroscopic techniques for measuring optical activity are optical rotary dispersion (ORD), circular di-chroism (CD), and vibrational circular dichroism (VCD). 

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

Enamines derived from ketones are allylated[79]. The intramolecular asymmetric allylation (chirality transfer) of cyclohexanone via its 5-proline ally ester enamine 120 proceeds to give o-allylcyclohexanone (121) with 98% ee[80,8l]. Low ee was observed in intermolecular allylation. Similarly, the asymmetric allylation of imines and hydrazones of aldehydes and ketones has been carried out[82]. 

Complete chirality transfer has been observed in the intramolecular allyla-tion of an alcohol with the activated allylic ester of 2,6-dichlorobenzoic acid 338 to give the 2-substituted tetrahydrofuran 339[208]. 

Asymmetric hydrogenolysis of allylic esters with formic acid with satisfactory ee was observed[387], Geranyl methyl carbonate (594) was reduced to 570 with formic acid using l,8-bis(dimethylamino)naphthalene as a base and MOP-Phen as the best chiral ligand, achieving 85% ee. 

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