Optical isomerism racemates

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

Significant progress has been made in enantioselective C—C bond formation in ir-allylpalladium chemistry, where the source of the chirality is the phosphine ligand on the palladium. A number of potential difficulties must be addressed in order for this approach to result in synthetically useful optical yields. Racemization can occur by a variety of mechanisms. Intermediates not bearing substituents at both allyl termini can undergo syn-anti isomerization, which accomplishes racemization (equation 347). 

Before 1940 optically active compounds could only be obtained in stereo-isomerically pure form by isolation from natural sources, by resolution of racemic mixtures, or by a few laboratory controlled enzymic reactions. Many of the chemical reactions described in this book lead to products which contain chiral centres, axes, or planes, but in which the isolated material is the optically inactive (racemic) form. This is a direct consequence of the fact that the reactants, reagents, or solvents are achiral or are themselves racemic. The following selection of reactions drawn from the text illustrate this statement they may be cross-referenced to the relevant discussion sections, namely (a) Section 5.4.1, p. 519, (b) Section 5.4.3, p.542, (c) Section 5.11.7, p.687, (d) Section 8.1.3, p. 1133, e) Section 5.2.4, p. 504 and (/) Section 5.4.2, p. 531. 

Optical isomerism occurs in molecules with an asymmetric carbon atom - that is one that is attached to four different atoms or functional groups. The two different optical isomers are called optical isomers or enantiomers and rotate polarized light in opposite directions. The cZ-isomer rotates it clockwise and is called dextrorotatory the Z-isomer rotates it anticlockwise and is called Zaevorotatory. Despite their structural similarity, optical isomers exhibit significantly different physiological properties. In the case of essential oils their smells can be very dissimilar. When a compound is made up of equal amounts of the d- and Z-isomers it will be optically inactive and is called a racemic mixture, or racemate. 

Geometrical isomerism is potentially possible among many of the lanthanide chelates. The 1,3-diketone chelates, the species derived from the aminepolycarboxylic acids, the tropolonates, and many other complex derivatives are asymmetric and, thus, potentially capable of exhibiting optical isomerism. That the only resolutions reported have been limited to some tris (diketone) compounds (35) may refiect more the tendency of these ionically-bonded species to racemize rapidly in polar environments than in the absence of asymmetry. The existence of more than a single modification of each of several tetrakis(di-keto)europium(III) compounds may refiect asymmetry also (1). 

In addition to the stereochemical changes that accompany chemical reaction, these systems can be rearranged without chemical change. Several such examples of geometrical and of optical rearrangements are known for metal complexes. A brief account is given of some examples of geometrical (or cis-trans) isomerization in this section, and the next section contains a few examples of optical isomerization (or racemization). 

Thus from a study of the crystalline sodium-ammonium salt of racemic acid and of dextro tartaric acid Pasteur showed, conclusively, the relationship of these two acids to each other and also discovered the existence of a third isomer optically active but of opposite direction to the ordinary tartaric acid already known. Racemic acid, therefore, is optically inactive because it consists of equal molecules of the ordinary dextro tartaric acid and the newly discovered levo tartaric acid. Also racemic acid can be resolved into its optically isomeric components by mechanically separating the two forms of crystals of the sodium-ammonium salt. The two active forms of tartaric acid, when mixed in equal molecular amounts, yield the inactive or racemic acid. Later, Pasteur prepared the fourth variety of tartaric acid, viz., meso-tartaric acid, by heating the cinchonine salt of dextro tartaric acid. This new acid proved to be inactive like racemic acid, but, unlike it, was unable to be resolved into optically active components. Its relation to the other three forms of tartaric acid was unexplained by Pasteur. 

An optically active and electroconductive polymeric sorbent was prepared by coating the PS-L-proline-Cu(II) beads with conducting polymer polypyrrole. Applying a potential difference of 1.5 V to the column, racemic lysine and aspartic acid were separated according to their charge characteristics and were simultaneously resolved with respect to their optical isomerism. 

Although both geometrical and optical isomerism are in principle possible in seven-coordinate complexes, no e.xamples are known. Note, for example, that the pyjtren complexes must be optically active (see Fig. 12.35b), subject of course to kinetic stability with respect to racemization. 

Rule 12. Designation of optical isomerism. Where optical isomerism can occur, the optically active compound is designated by (+) or (—) depending upon the sign of rotation alternatively, d- or /- may be used. The racemic mixture is designated by (db) or dl- and the inactive form by meso. 

A method introduced by Velluz (1957) involves seeding of a supersaturated solution of a racemate (an equimolar mixture of d- and 1-isomers obtained in the last step of synthesis of a compound capable of showing optical isomerism) with crystals of the optically active isomer sought. Resolution of dl-chloramphenicol... 

The absorption spectra of optically active (d or /), racemic and inactive isomers are generally found to be similar. Optical, isomerism has no effect on Aauuc or However, when a compound has more than one asymmetric... 

A detailed investigation of the stereochemistry of cis- and tran5-[RuCl2(L-L)2] (l L = o-QH< (EMePh)2 E = As, P) has been undertaken. The optically active, racemic, meso, syn, and anti forms of the trans compound were isolated for both ligands each of these subsequently isomerized to the corresponding cis complex by reaction with AlEts and the various diastereoisomers of the latter separated and characterized. Whereas the trans isomers are relatively inert, the cis complexes readily undergo stereospecific halogen substitution by 1 and A minor product obtained... 

Racemate. A pharmaceutical which is a mixture of optical isomers. Many pharmaceuticals exhibit optical isomerism and most of these are marketed as racemates. Since effects and side-effects are not usually the same for different optical isomers of the same molecule, an optically pure form could show a higher therapeutic ratio. Difficulties in obtaining such forms, however, usually prohibit this from being a practical possibility. 

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