Stereochemical Phenomena

As mentioned earlier, an electrode reaction implies some orientation effect because a substrate molecule must be fairly close to the electrode surface. The following pairs of cis and trans isomers were reported to exhibit identical reduction potentials l,2-dimethyl-l,2-diphenylethylenes (Weinberg and Wienberg 1968) l,2-bis(4-cyanophenyl)-l,2-bis(4-methoxyphenyl) ethylenes (Leigh and Arnold 1981) and l,2-bis(4-acetylphenyl)-l,2-diphenylethylenes (Wolf et al. 1996). 

Particularly, the trans isomer of l,2-dimethyl-l,2-diphenylethylene is coplanar and the cis isomer is noncoplanar. However, both isomers are oriented in an identical manner within the electrode space and electric field (Homer and Roder 1969). The energy needed for such an orientation is not markedly reflected in the value of a potential. For oxidation potentials, there are also data that these potentials are not sensitive to diastereoisomerism (Fukui et al. 2007). 

Analogously, l,2-dicyano-l,2-diphenylethylene, which is free from steric strains, and its strained isomer l,l-dicyano-2,2-diphenylethylene are reduced at practically the same potentials (Ioffe et al. 1971, Todres and Bespalov 1972). In DMF, with the support of Et4NI, the reversible two-step one-electron reductions are characterized by the following potentials (mercury pool as a reference electrode) -0.48 and -0.98 V for 1,2-dicyanoethylene and -0.50 and -1.07 V for 1,1-dicyano isomer. Thus, electrochemical reduction does not fix the difference in isomer structures. 

In summary, the space strain is indicative of the stability of electron-transfer products. Electrode reactions fail to reveal such an effect. In liquid-phase processes, this effect, however, plays a decisive role. As Baizer and Lund s book (1983, p. 907) underlines 

There are no passing into the solntion volnme, with the following electron being exchanged there with nonoxidized molecnles of stilbene. 

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We hope that this review of chiral sulfur compounds will be useful to chemists interested in various aspects of chemistry and stereochemistry. The facts and problems discussed provide numerous possibilities for the study of additional stereochemical phenomena at sulfur. As a consequence of the extent of recent research on the application of oiganosulfur compounds in synthesis, further developments in the field of sulfur stereochemistry and especially in the area of asymmetric synthesis may be expected. Looking to the future, it may be said that the static and dynamic stereochemistry of tetra- and pentacoordinate trigonal-bipyramidal sulfur compounds will be and should be the subject of further studies. Similarly, more investigations will be needed to clarify the complex nature of nucleophilic substitution at tri- and tetracoordinate sulfur. Finally, we note that this chapter was intended to be illustrative, not exhaustive therefore, we apologize to the authors whose important work could not be included. 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

The trend of structural selectivity can be summarized as degenerate metathesis of terminal alkenes (exchange of methylene groups) > cross-metathesis of terminal and internal alkenes > metathesis of internal alkenes > productive metathesis of terminal alkenes (formation of internal alkene and ethylene).87 Since different catalyst systems exhibit different selectivities, a simple general picture accounting for all stereochemical phenomena of metathesis is not feasible. 

It was possible to explain the stereochemical phenomena of Scheme 18 by assuming the intermediacy of partially bridged cations 3 [89]. A detailed analysis of the transition states, which explains the simultaneous formation of rearranged products, will be developed in Section III.B.2. 

Rare examples of unique topologies in such systems are known. However, it was recently realized that when the analysis includes cofactors and prosthetic groups such as seen in quinoproteins or iron-sulfur cluster proteins, interesting topologies including knots and catenanes are in fact more common than previously realized. As always, in considering stereochemical phenomena, our definition of connectivity is crucial. Earlier studies had counted only the amino acids as contributing to the connectivity of the system. When cofactors are included, more complex connectivities result. 

In summary, therefore, Staudinger s efforts in setting the stage for studies of the properties of polymers are demonstrated across the spectmm of polymer classes in stereochemical phenomena associated with chirality studied at the Herman F. Mark Polymer Research Institute at the Polytechnic Institute of Brooklyn [12-17],... 

Paul Pfeiffer discovered a very interesting stereochemical phenomenon, which now bears his name — the Pfeiffer effect this has received a good deal of attention.30 When an optically active substance which is stable in solution is added to a solution of a labile chiral substance, the optical rotation of the solution changes, reaching a new level in some hours. Several theories have been advanced to explain the phenomenon, the most satisfactory based on the supposition that the optically active ion or molecule forms an association with one isomer of the racemic pair of the labile substance and thus shifts the dextro—levo equilibrium. In general it is not possible to use this as a means of resolution, for when the added optically active substance is removed from the labile material, the latter immediately racemizes. 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

The ability of certain metal complexes to exist in stereoisomeric forms, and particularly to interconvert, adds another dimension to the study of the mechanisms of their reactions. There are two aspects from which the phenomenon of stereochemical change may be regarded. 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

When a racemic substance is hydrogenated or when the reduction leads to the production of centers of asymmetry, the phytochemical reduction will take at first a completely or partially asymmetric course. Examples of such asymmetric reactions are the conversions of pure racemic valeraldehyde, acetaldol, furoin and furil, diacetyl and acetyl-methylcarbinol to optically active alcohols. Occasionally meso forms also arise, as for example in the case off glycols (p. 84). The reasons for the stereochemical specificity of these reactions have not been clarified. This type of phenomenon has frequently been observed in the related intramolecular dismutation of keto aldehydes, especially if enzyme materials of differing origins are used. 

These reactions are characterized by the phenomenon that the frontier orbitals of the reactants maintain a defined stereochemical orientation throughout the w hole reaction. Most noteworthy in this respect, is the principle of orbital symmetry conservation ( Woodward-Hoffmann rules la), but the phenomenon is much more general, as shown by the following examples of Self-Immolative Stereoconversion or Chirality Transfer . This term describes processes by which a stereocenter in the starting material is sacrificed to generate a stereocenter in the product in an unambiguous fashion. This is, of course, the case in classical SN2-displacements. 

Normally, additions depicted by model C lead to the highest asymmetric induction. The antiperiplanar effect of OR substituents can be very efficient in the Houk model B ( , , , , ) however it plays no role in model C. Furthermore, the Houk model B must be considered in all cycloaddition-like reactions. The Felkin-Anh model A is operative for nucleophilic additions other than cuprate additions ( ). The epoxidation reactions are unique as they demonstrate the activation of one diastereoface by a hydroxy group which forms a hydrogen bridge to the reagent ( Henbest phenomenon ). The stereochemical outcome may thus be interpreted in terms of the reactive conformations 1 and 2 where the hydroxy function is perpendicular to the olefinic plane and has an optimal activating effect. 

In this section we mention the paramagnetic monocationic complexes [Mo(CO)2(dppe)2]+ and [Mo(CO)2(bipy)2]+, both of which are produced by oxidation of the parent Mo° complexes. In the former case, the oxidants used include [MeCftKUNJfBFj,46 NO[PF6], I2 and AgIla-47 and a variety of analogues [Mo(CO)2L2]+ has been obtained (L2 - dppm, dmpe, diars, etc.).1 47 Electrochemical oxidation has also been used, with other measurements, to show that a rapid cis — trans isomerization follows oxidation and an explanation for this phenomenon has been proposed on the basis of extended Huckel molecular orbital calculations, the stereochemical change being dependent on the number of valence electrons and the nature of the coligand jr-donor or -acceptor capacity. Similar studies have been made upon the compounds [Mo(CO)2L2]+ (L = bipy or phen).47... 

Probably the most important effect contributed by metal coordination to ligands is stereochemical in nature. Because of the rather strict coordination geometry imposed by metal ions, ligands can be held in suitable juxtaposition for reactions to take place between them. This phenomenon is the hallmark of metal template reactions and is also a crucial feature of metal enzyme reactions, where high specificity occurs. 

In their extensive studies of metal nitrosyl chemistry, Enemark and Feltham showed that the mode of NO bonding can be altered by the simple addition of another ligand (168, 204, 205). An example of this phenomenon is illustrated by reaction (83b), and has been described by these investigators as stereochemical control of valence (204). 

The conclusions reached by Isbell8 and other workers3-4 5 thus rendered clear the possible influence of neighboring-group participation on the stereochemical path of reaction. But the really profound effect that neighboring-group participation may have on the rate of reaction was not recognized in carbohydrate chemistry until after the discovery of this phenomenon by Winstein and coworkers. For example, a study by these... 

An important approach to stereochemical problems is to make use of the concept of chirality. Chirality (7), namely, the phenomenon that a chiral object and its mirror image cannot be superimposed, has been classified according to different elements of chirality. Chiral molecules may contain chiral centers, axes, and/or planes (2, 3). 

Many reactions mediated by Sml2 require the presence of a proton donor. The primary role of the proton donor is to quench alkoxides and carbanions produced as intermediates upon reduction or reductive coupling. The most commonly utilised proton donors are alcohols, glycols and water. It is now very clear, however, that proton donors can have a considerable impact on the efficiency of Sml2-mediated reactions and their regiochemical and stereochemical outcome. Often, even a modest change in the proton donor or its concentration can have a profound impact on product distributions. Two important examples of this phenomenon are discussed below. 

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