Stereochemistry of Dynamic Processes

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 the topological features of such processes. The cases that are of most importance in organic chemistry are chemical reactions, conformational processes, 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. 

1 (Z)-3-Decenoic acid (the sex pheromone of the furniture carpet beetle) 

2 Methyl (2f,6 ,10Z)-10,11-epoxy-3,7,11-trimethyltridecadienoate (the juvenile hormone of the tobacco hornworm) 

In describing the stereochemical features of chemical reactions, we can distinguish between two types stereospecific reactions and stereoselective reactions. A stereospecific reaction is one in which stereoisomeric starting materials afford stereoisomerically different products under the same reaction conditions. A stereoselective reaction is one in which a single reactant has the capacity of forming two or more stereoisomeric products in a particular reaction but one is formed preferentially. A stereospecific reaction is a special more restrictive case of a stereoselective reaction. 

The stereochemistry of the most familiar reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds that are made or broken are on the same or opposite faces of the plane of the double bond. 

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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... 

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). 

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. 

The expansion of coordination at silicon. Pentacoordinated species seem to be quite intimately involved in many processes taking place at silicon. Expansion of coordination is the fundamental step not only in the nucleophilic induced racemization reviewed some years ago (13), but also in nucleophilic substitution activated by nucleophiles. A part of this review is devoted to the stereochemical and mechanistic aspects of nucleophilic activation. Furthermore, in connection with a possible isomerization of trigonal bipyramidal silicon by Berry pseudorotation, the dynamic stereochemistry of pentacoordinated silicon compounds is discussed. 

The NMR has provided an invaluable tool in the elucidation of both the static and dynamic stereochemistry of metal complexes. Applications to both diamagnetic and paramagnetic species have been rewarding. In particular, the unique role of NMR as a mechanistic probe for the elucidation of fluxional processes in polynuclear metal carbonyl complexes should be noted. 

The dynamic stereochemistry of biaryls is similar. The energy barrier for racemization of optically active 1,1-binaphthyl (see Scheme 2.1, entry 3, p. 46) is 21-23 kcal/mol. The two rings are not coplanar in the ground state, and the process by which racemization takes place is rotation about the l,l -bond. 

The first investigation on the stereoselectivity of the stepwise thermal denitrogena-tion of the diethoxy-substituted azoalkane has been reported. The double inversion product was the major isomer of the ring closure product. In photochemical denitro-genation, the inverted azoalkane was also the major isomer at 70 °C, although the stereoselectivity of the reaction was temperature dependent. An inversion process was proposed as the mechanism that determines the stereochemistry of the denitrogenation reaction products. The inversion product may alternatively be formed as a result of dynamic effects. 

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. 

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