Optical activity, definition

An interesting aspect of the benzofuran cationic polymerization was uncovered by Natta, Farina, Peraldo and Bressan who reported in 196160,61 that an asymmetric synthesis of an optically active poly(benzofuran) could be achieved by using AlCl2Et coupled with (-)j3-phenylalanine, (+)camphorsulphonic acid or with (-)brucine. The optical activity was definitely due to the asymmetric carbon atoms in the polymer chain, indicating that at least some of the polymer s macromolecules possessed a di-isotactic structure, v/ z.62 ... 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

There are two possible structures for simple alkyl radicals. They might have sp bonding, in which case the structure would be planar, with the odd electron in ap orbital, or the bonding might be sp, which would make the structure pyramidal and place the odd electron in an sp orbital. The ESR spectra of CHs and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon. In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near planar. The IR spectra of CH3 trapped in solid argon led to a similar conclusion. " °... 

To date, direct asymmetric synthesis of optically active chiral-at-metal complexes, which by definition leads to a mixture of enantiomers in unequal amounts thanks to an external chiral auxiUary, has never been achieved. The most studied strategy is currently indirect asymmetric synthesis, which involves (i) the stereoselective formation of the chiral-at-metal complex thanks to a chiral inductor located either on the ligand or on the counterion and then (ii) removal of this internal chiral auxiliary (Fig. 4). Indeed, when the isomerization of the stereogenic metal center is possible in solution, in-... 

The question that emerges at the climax of this survey relates to the possibility of using crystalline inclusion phenomena for optical resolutions of molecular species. Can this be done effectively with suitably designed host compounds The definitely positive answer to this question has elegantly been demonstrated by Toda 20) as well as by other investigators (see Ch. 2 of Vol. 140). An optically active host compound will always form a chiral lattice. Therefore, when an inclusion type structure is induced, one enantiomer of the guest moiety should be included selectively within the asymmetric environment. 

The fundamental scattering mechanism responsible for ROA was discovered by Atkins and Barron (1969), who showed that interference between the waves scattered via the polarizability and optical activity tensors of the molecule yields a dependence of the scattered intensity on the degree of circular polarization of the incident light and to a circular component in the scattered light. Barron and Buckingham (1971) subsequently developed a more definitive version of the theory and introduced a definition of the dimensionless circular intensity difference (CID),... 

Let us now apply the technique in some specific cases. The existence of optical activity of several compounds in solution has been explained due to the presence of several active forms of the compound in equilibrium with each other and various assumptions about the forms were also put forward. The equilibrium between the different forms depended on external conditions. But a definite explanation was put forward in 1930 about tartaric acid and it was said that the molecule exists in the following three conformations and each of which makes a certain contribution to the rotation observed. 

Basie definitions of terms relating to polymerization reactions [1,2] and stereochemical definitions and notations relating to polymers [3] have been published, but no reference was made explieitly to reaetions involving the asymmetric synthesis of polymers. It is the aim of the present doeument to recommend classification and definitions relating to asymmetrie polymerizations that may produce optically active polymers. 

The optical activity of the hexitols is of a low order. It may be enhanced by the addition of various complex-forming salts, borax and ammonium molybdate being the ones most used. Since the amount of enhancement is a function of the relative proportions of polyol and booster, it is regrettable that definite amounts of borax or molybdate have not always been reported in the past. 

The third approach is the main topic of this volume. According to the definition given above it involves enantiomerically pure starting materials which at some point must be provided by resolution or ex-chiral-pool synthesis. It is more or less equivalent to the term asymmetric synthesis defined by Marckwald in 19047 as follows Asymmetric syntheses are those reactions which produce optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . In today s language, this would mean that asymmetric syntheses are those reactions, or sequences of reactions, which produce chiral nonracemic substances from achiral compounds with the intermediate use of chiral nonracemic materials, but excluding a separation operation. 

The first concerns mixtures with a composition not equal to 50 50. The term nonracemic appears particularly apt as it is short and does not require an explicit definition. In view of the availability of this self-explanatory term, it is hard to understand why the vocabulary of stereochemistry should be burdened with new terms such as aracemic or scalemic, these terms being the subject of an extensive discussion in letters to the editor of Chemical and Engineering News during the period September 1990 to August 1991. Although probably not necessary these days, it should be pointed out that the traditional term optically active is not appropriate here. 

Asymmetric synthesis is a term first used in 1894 by E. Fischer and defined4 in 1904 by W. Markwald as a reaction which produces optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . A modem definition was proposed 5) by Morrison and Mosher An asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereosiomeric products (enantiomeric or diastereomeric) are formed in unequal amounts. This is to say, an asymmetric synthesis is a process which converts a prochiral6) unit into a chiral unit so that unequal amounts of stereoisomeric products result . When a prochiral molecule... 

The importance of the theory was further demonstrated by the discovery of the existence of optically active inorganic compounds, and the isolation of the exact number of optical isomers theoretically possible for the spatial arrangement of the atoms.1 Friend 2 and others criticised the theory on the grounds that in simple compounds, such as sodium chloride or cobaltous chloride, the chlorine is ionised and yet is attached to sodium or cobalt atom directly, whereas in the ammino-coinpounds the acid capable of ionisation is that which is not directly attached to metal. For instance, in chloro-pentammino-cobaltic chloride, [CoCI(NH3)5]C12, it is the chlorine outside the first zone which is ionised in solution. Also, the dissociable acidic groups are not attached to any point within the complex, but simply hover round the central complex in an indefinite manner. Thus a definite valency for ionisable... 

Olefins can add to double bonds in a reaction different from those discussed in 5-15, which, however, is still formally the addition of RH to a double bond. This reaction is called the ene synthesis44,1 and bears a certain similarity to the Diels-Alder reaction (5-47). For the reaction to proceed without a catalyst, one of the components must be a reactive dienophile (see 5-47 for a definition of this word) such as maleic anhydride, but the other (which supplies the hydrogen) may be a simple alkene such as propene. There has been much discussion of the mechanism of this reaction, and both concerted pericyclic (as shown above) and stepwise mechanisms have been suggested. The reaction between maleic anhydride and optically active PhCHMeCH=CH2 gave an optically active product,441 which is strong evi-... 

B As we have. seen. Hie formal definition of optical activity is bused upon the absence of an improper axis of rotation. The Iwo definitions arc equivalent. 

The Kyoto group hoped to obtain more definitive evidence for the existence of carbene species bound covalently to copper. If such a species did exist, the use of an optically active copper catalyst should show some asymmetric induction in the cyclopropanation reaction. Indeed,... 

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