Induced optical activity

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

The observed induced circular dichroism (ICD) is classified as follows  

It is well known that optical activity is induced for achiral species in the presence of chiral species, giving rise to ICD for the absorption band(s) of the achiral species. In the introduction Section, a historical summary is described in brief. The theory of ICD has been developed by Tinoco, Jr.46), Mason 47), and Schipper 48). 

let us consider a system where an achiral molecule (A) and a chiral molecule (C) have a fixed mutual orientation. An electronic transition of the achiral molecule from the ground state z(0 to the excited state Aa, higher in energy by E0a, has a zero-order (non-perturbed) electric dipole moment po0 and an orthogonal magnetic dipole moment ma0. These moments are increased in the molecular pair (A -C) by first-order dynamic coupling as  

The zero- and first-order moments give the first-order rotational strength and the second-order rotational strength which are induced for the given transition Aa - A0 of the achiral molecule  

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Polyisocyanates have attracted much attention owing to their liquid crystalline properties, stiff-chain solution characteristics, and induced optical activities associated with helical chain conformation (Scheme 8). Pattern and Novak [39]... 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

Chiral alcohols have also been used to induce optical activity in sulfoxides during halogenation of sulfides. Thus, Johnson (58) found... 

In the last two decades optically active sulfur compounds have found wide application in asymmetric synthesis. This is mainly because organic sulfur compounds are quite readily available in optically active form. Moreover, the chiral sulfur groupings that induce optical activity can be removed from the molecule easily, under fairly mild conditions, thus presenting an additional advantage in the asymmetric synthesis of chiral compounds. This section deals with reactions in which asymmetric induction in transfer of chirality from sulfur to other centers was observed. This subject has been treated only in a cursory manner in recent reviews on asymmetric synthesis (290-292). 

A very convenient asymmetric synthesis of cyclopropane or epoxide systems developed by Johnson (184) is based on the use of chiral sulfur ylides as the agents that induce optical activity. Generally, this method consists of the asymmetric addition of a chiral sulfur ylide to the C=C or C=0 bond and subsequent cyclization of the addition product to form a chiral cyclopropane or epoxide system together with chiral sulfinamide. A wide range of chiral... 

In recent years three other examples of asymmetric induction have been described in the literature in which the chiral sulfur reagent that induces optical activity is converted into another chiral sulfur compound. The first reaction of this type is the chlorination of 2,2-diphenylaziridine (265) by means of the optically active A -chloro-phenylmethylsulfoximide (266), affording optically active A -chloro-2, 2-diphenylaziridine (267) and the unsubstituted sulfoximide 149 (197). In this case asymmetric induction is observed on the nitrogen atom. 

The signs of optical activity or of the Faraday effect (magnetically induced optical activity see Appendix) used by physicists are frequently opposite to the chemically defined ones. Furthermore, the handness in liquid crystals, such as cholestric or chiral smectic ones, often has been defined erroneously and thus confused. 

Referred to as the induced optical activity, the resultant CD spectrum has measurable peaks only when the transition is either electrically or magnetically allowed. Thus, this IC-D method is useful for discriminating the two types of transition 162). 

The helical directors in a cholesteric phase can induce optical activity for an added dye intercalated into a cholesteric phase. 

The three acetylpyridines have been reduced in the presence of catalytic concentrations of different alkaloids in attempts to induce optical activity in the products 412 The reduction of 3-acetylpyridine gave optically inactive alcohols under all conditions employed, whereas optically active pyridyl-ethanols are produced from 2- and 4-acetylpyridine at 0°C, in a 1 1 aqueous-ethanolic acetate buffer with strychnine (5 x 10-4 M) as chiral catalyst. Under these conditions protonated, adsorbed strychnine is probably acting as a chiral acid. The pinacols obtained as side products were all optically inactive. 

Supramolecular chiral induction into dye chromophores can also be achieved in totally synthetic systems. Interpenetrated ion pairs of cyanine dyes and chiral borate anions show that the twisting of the dye is important for the observation of the induced optical activity by CD spectroscopy [225]. The incorporation of merocyanine dyes into synthetic self-assembled systems provides important information about their stability and chirality [226]. 

The binding of a symmetrical molecule of PLP to the active site of tryptophanase generates so-called induced optical activity ( extrinsic Cotton effect), which was detected... 

Reorientations of the coenzyme associated with the enzymatic catalytic cycle were first proposed by Ivanov and Karpeisky64 for aspartate transaminase on the basis of general topochemical considerations and studies of induced optical activity. Torchinsky and Koreneva65,66 found that induced optical activity of PLP bound to the transaminase is... 

Characteristic of this process is the low concentration of chiral cation necessary to induce optical activity (10% of the substrate concentration is sufficient) and that the chiral salt is recovered mostly unchanged (except for no. 3). Quaternary ammonium ions work as well as substituted ammonium ions, thereby making less probable the mechanism first suggested (Gourley et al., 1967, 1970), namely, hydrogen atom transfer from an ammonium radical R3NH to the substrate. Today it seems reasonably well established that adsorption of the chiral cation, possibly as a complex with an intermediate is connected with the asymmetric induction observed (Horner et al., 1972 Kariv et al., 1973a). 

Since the CD of oriented molecules are extremely sensitive to the interaction of chromophores, such data could be very useful in the investigation of exciton phenomena. The technique also allows one to determine the degree of order and orientation of chromophores in a macromolecule. Thus, CD studies on oriented visual pigments, and possibly on some selected visual pigment analogs, could be very informative in clarifying the role played by induced optical activity of the chromo-phore and/or exciton coupling. 

In the 1980 s there was a great increase in the development and use of enzymatic procedures by synthetic chemists.6 Previously regarded more as scientific curiosities of limited scope than of practical utility, biological-chemical transformations are now used regularly by synthetic chemists. The ability to induce optical activity in molecules where none existed before is the most useful property of these chiral catalysts. Hydrolase enzymes are generally preferred over other kinds of enzymes for transformations of this nature because they are more easily handled and do not require cofactors for activity. In cases where enantiotopic differentiation between ester functions is desired, prochiral meso diesters are more efficient substrates than racemic esters. In the former case it is possible for all starting material to be converted into a single enantiomer, whereas in the latter example only enzymatic resolution is possible. 

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