Dynamic processes stereochemistry

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Arsines (AsR3) and stibines (SbR3) are generally found to be more labile (sometimes an advantage) than phosphines, in part due to their reduced o-donor properties. Furthermore, arsenic and antimony lack conveniently measurable spin-active nuclei for NMR studies, in contrast to phosphines (31P / = Vi, 100% abundance), for which an enormous amount of informative data is available. Because of the direct bond between phosphorus and the metal centre, information is available about the electronic nature and stereochemistry of the metal centre, in addition to information about dynamic processes. This is further enhanced in the case of complexes involving metals with NMR-active nuclei, e.g. W, Rh, V, Pt, Hg and Ag, where /(M-31P) couplings may be observed. 

High resolution nuclear magnetic resonance is the most effective structural technique for studies in solution. Although in some ideal cases, such studies can define structure and stereochemistry, it is not possible to obtain structural parameters by this method. Nuclear magnetic resonance techniques have proved to be an excellent method for studying dynamic processes in clusters. 

Since the bowl shape of 39 does not undergo a dynamic process on the NMR timescale, each half of the dimer imparts chirality. This property leads to the interesting possibility of multiple diasteromeric reduced species, depending on the position of the connection as shown in Fig. 13.4. Although it was not possible to fully assign the stereochemistry of the dimers, it is apparent from the Jh3, h3 coupling constant (10.0 and 10.5 respectively, for 39a and 39c) that they adopt anti conformation. 

Biooxidative deracemization of racemic sec-alcohols to single enantiomers [47,48] is complementary to combined metal-assisted lipase-mediated strategies [49,50]. In general, deracemization can be realized by either an enantioconvergent, a dynamic kinetic resolution, or a stereoinversion process. The latter concept is particularly appealing, as only half of the substrate needs to be converted, as the remaining half already represents the product with correct stereochemistry. 

Racemization, which results in the loss of optical activity of a chiral compound, is considered to be one of the fundamental processes in dynamic stereochemistry. Quite generally, racemization can be caused by supplying adequate energy by heating or irradiation, or it may be effected by chemical reactions. 

Organosulfur chemistry is presently a particularly dynamic subject area. The stereochemical aspects of this field are surveyed by M. Mikojajczyk and J. Drabowicz. in the fifth chapter, entitled Qural Organosulfur Compounds. The synthesis, resolution, and application of a wide range of chiral sulfur compounds are described as are the determination of absolute configuration and of enantiomeric purity of these substances. A discussion of the dynamic stereochemistry of chiral sulfur compounds including racemization processes follows. Finally, nucleophilic substitution on and reaction of such compounds with electrophiles, their use in asymmetric synthesis, and asymmetric induction in the transfer of chirality from sulfur to other centers is discussed in a chapter that should be of interest to chemists in several disciplines, in particular synthetic and natural product chemistry. 

Such conventional kinetic resolution reported above often provide an effective route to access to the enantiomerically pure/enriched compounds. However, the limitation of such process is that the resolution of two enantiomers will provide a maximum 50% yield of the enantiomerically pure materials. Such limitation can be overcome in several ways. Among these ways are the use of meso compounds or prochiral substrates,33 inversion of the stereochemistry (stereoinversion) of the unwanted enantiomer (the remaining unreacted substrate),34 racemization and recycling of the unwanted enantiomer and dynamic kinetic resolution (DKR).21... 

One of the practical applications of the optical polarization of molecular angular momenta is the investigation of the stereochemical forces in the process of molecule-atom collisions. The most complete information on the dependence of atom-molecule interaction potential on the orientation of the molecule with respect to the relative collision velocity can be obtained by the method of molecular beams, and often in conjunction with inhomogeneous magnetic and electric fields which orient the molecules. Such investigations are undoubtedly very complex, and their realization rather costly. To convince oneself of it one might just peruse the Proceedings of the First Workshop on Dynamic Stereochemistry in Jerusalem in 1986 [66] see also [67, 341],... 

Since we are dealing with heteroaromatics, the dynamic stereochemistry will essentially concern the study of the processes associated with sp2-sp3-bond and sp2-sp2-bond systems, with the sp2-atom X being a carbon or a nitrogen atom belonging to the heteroaromatic system. (Scheme 47). 

The dynamic stereochemistry of spiroarsoranes containing five- and six-membered ring systems has also been studied by Dale and Froyen (44). The variable-temperature NMR results have been interpreted in terms of pseudorotation processes. It has been concluded that the observed spectra do not allow deduction as to whether trigonal-bipyra-midal, rectangular-pyramidal, or any other intermediate structure is the most stable configuration in solution (44, 45). 

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

It is conceivable that the difference in the Doering and Cooke results can be attributed to the isopropyl group in a-thujene which would make the non-allylic radical center tertiary in the proposed intermediate. In the Cooke and Andrews work the corresponding radical center would be secondary. Possibly the difference is enough to make the biradical longer lived and better able to achieve geometrical equilibrium in the experiments of Doering and coworkers. Whether the stereochemistry observed by Cooke and Andrews is the result of involvement of one or more pericyclic processes or whether it reflects dynamic phenomena in the biradical (see next section) is an open question. 

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