Stereoisomeric characterization

As we have seen previously (Section 7.1), within two decades of Schraube and Schmidt s discovery (1894) of isomeric diazoates, (Z)-and (ii)-isomers were found for all major stable addition products of arenediazonium ions with nucleophiles with the exception of triazenes. However, in the 1970s Wiberg and Pracht (1972), and also Fanghanel et al. (1975 a, 1975 b), discovered examples of stereoisomeric triazenes. They showed that 3,3-di-(trimethylsilyl)-l-phenyltriazenes (13.1, 13.2) and l-aryl-3-[3 -methylbenzothiazolinylidene(2 )]triazenes (13.3, 13.4) exist in two isomeric forms that can be separated and characterized on the basis of their chemical and UV spectral properties as (Z)- and ( -isomers. 

A unique pair of stereoisomeric dimolybdenum amidinate complexes has been prepared and structurally characterized. The reaction of Li[PhC(NSiMe3)2] with dimolybdenum tetraacetate afforded trans- and ds-Mo2(02CMe)2[PhC (NSiMe3)2]2- While the acetates coordinate to the M02 core via a bridging mode in both compounds, the benzamidinates are bridging in the trans complex and... 

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

The new terms, which ought to be used for characterization of steric elements only and not of compounds, permit several generalizations. Any chiral molecule for which all elements of stereoisomerism have been determined must contain at least one that is pherochiral. A compound must be chiral if the total number... 

A third type of configurational interdependence exists if two elements are so interrelated that a change in the configuration of one automatically alters that of the other. This characterization applies to the two centers of 1,4-cyclohexanediol of the type Cg+g hi (5,51). Consequently only two isomers exist and a single pair of descriptors suffices for their distinction. We can remove the mutual dependence of the two elements by waiving the requirement that a line of stereoisomerism be occupied by bonds. The H and OH ligands have different distributions in the isomers about the line between C(l) and C(4), and the usual terms cis and trans express this relationship. Undoubtedly this is the most convenient description and the only one now available, but should we go further and say that the proper element of stereoisomerism in this case is this achiral line of torsion, and that its further factorization into two graphochiral centers is unwarranted ... 

The stereoisomerism in some asymmetric tetrahedral complexes of the type M(AA,)2 has been characterized by nmr. - Tetrahedral complexes of the type M(AA,)2 are chiral, but their lability requires nmr methods for measurement of the rates of interconversion of the enantiomers. The pioneering work of Holm and his collaborators has been consolidated by Minkin and his Russian group and Zn, Cd and, more recently, Ni chelates of the type M(AA[)2 have been extensively studied. 

Enantiomers are characterized as nonsuperimposable mirror images. Enantiomers are said to be chiral (note that some diastereomers may be chiral as well). In the context of the same bonding pattern or connectivity, which atoms are bonded to which, enantiomers have handedness and are related to each other as the right hand is related to the left hand. In the specific example we saw earlier, the carbon atom is linked to four different atoms. Such molecules have non-superimposable mirror images. Stereoisomerism occurs in some molecules that do not have such a carbon atom but these cases are more exotic than we need to worry about here. Stereoisomers frequently have different, and sometimes strikingly different, biological properties, exemplified by the thalidomide case. 

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

Lucchini, Pasquato, and co-workers have obtained stereoisomeric 2,3-disubstituted 1-methylthiiranium ions, reported characterization (1H and 13C NMR, X-ray crystallography), and studied their anionotropic rearrangements. The synthesis was performed by reacting methylbis(methylthio)sulfonium salts with alkenes184,185 (Scheme 4.7). 

Oxidation of dialkenyl disulfides gave the stereoisomeric 2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane 5-oxides 36 and 37, which are found in extracts of onion. Further conversion and oxidations afforded a series of various 2,3-dimethyl-5,6-dithiabicyclo[2.1.1]hexane derivatives, which were characterized on the basis of the comparative X-ray structural and NMR and IR spectroscopic data. The 111 NMR peak assignments for 36 and 37 were facilitated by LAOCOON III analyses of these 10 spin systems and by examination of the shifts induced by Eu(fod)3 and d6-benzene (fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadione) <1996JA2790>. 

Another Fischer achievement was the synthesis of small peptides via the condensation of amino acids. Fischer suggested that there was a common linkage that held pairs of amino acids together in all proteins—the peptide bond. He understood that proteins were tremendously complex, owing to the large number of constituents and the fact of stereoisomerism. By 1916 Fischer had synthesized and characterized 100 peptides, but knew they represented a tiny fraction of what was possible, see also Amino Acid Carbohydrates Isomerism van t Hoff, Jacobus. 

---