Properties stereochemical

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

A molecule editor can draw a chemical structure and save it, for example as a Molfile. Although it is possible to include stereochemical properties in the drawing as wedges and hashed bonds, or even to assign a stereocenter/stereogroup with its identifiers R/S or E/Z), the connection table of the Molfile only represents the constitution (topology) of the molecule. 

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. 

Chemical and stereochemical properties of compounds with silicon- or germanium-transition metal bonds. E. Colomer and R. J. P. Corriu, Top. Curr. Chem., 1981, 96, 79-107 (68). 

Km and values) and stereochemical properties. Ideally, screening should he carried out under reaction conditions that mimic the final process as closely as possible. This step is often one of the most time-consuming phases of process development, and improvements here can have significant impacts. 

Colomer, E., and Corriu, R. J. P. Chemical and Stereochemical Properties of Compounds with Silicon or Germanium-Transition Metal Bonds, 96, 79-110 (1981). 

Another hypothesis was provided by Mikio Shimitso (1982) on the basis of studies of steric effects in molecular models. It had been noted years previously that the fourth nucleotide at the 3 end of the tRNA molecules (referred to as the discrimination base) might have a recognition function. In the case of certain amino acids (i.e., their tRNA-amino acid complexes) this base pair, in combination with the anticodon of the tRNA molecule, can select the amino acid corresponding to the tRNA species in question this is done on the basis of the stereochemical properties of the molecule. Since the anticodon of a tRNA molecule and the fourth nucleotide of the acceptor stem are far apart in space, two tRNA molecules must complex in a head-to-tail manner. The pocket thus formed can then fit specifically to the corresponding amino acid. 

Synthetic models of myoglobin and hemoglobin are complex molecules that mimic the stereochemical properties of the protein active center [24] and have oxygen affinities similar to those measured for the protein [25-27]. The first heme model that reversibly binds oxygen (i.e. the picket-fence-oxygen complex Fe(TpivPP)(l,2-Melm)(02), shown in Fig. 3.3) was obtained in the early nine-teen-seventies by Collman and coworkers (TpivPP = tetrapivalami-nophenyl porphyrin 2-meIm = 2-methylimidazole) [18]. Research on synthetic models of the protein has led to a deeper understand-... 

Bohle and co-workers (133) have demonstrated that varying the electronic and stereochemical properties of porphyrin substituents can strongly influence the rates of NO labilization (Eq. (11)). For example, the displacement of NO from Fe(TPP)(NO) by pyridine is many orders of magnitude slower than from Fe(OBTPP)(NO) (OBTPP = octabromo-tetraphenylporphyrin). An analysis of the kinetics of the latter reaction indicated saturation in [L], and the mechanism was suggested to involve reversible formation of Fe(OBTPP)(L)(NO) followed by NO dissociation (Eq. (50)). Clearly changes in porphyrin properties can lead to enhanced reactivity toward NO loss. 

We now proceed to discuss in detail two other approaches which are directly relevant to the ideas presented in this book. Thus, in a recent paper, Kollman405) has questioned the role of nonbonded attraction in determining the stereochemical properties of difluoroethylenes and related molecules. He has suggested instead that the attraction is due to changes in the nature of the C-X bonding orbital as X becomes more electronegative. 

This chapter covers only the chiral compounds that are cited in the literature by virtue of their optical activity. To keep the chapter to an acceptable length, a discussion of the stereochemical properties of sulfenamides showing axial chirality is omitted (17). Similarly, to limit the scope of the review, the chemistry of penicillin, cephalosporin sulfoxides and related compounds (14,18,19), steroidal sulfoxides (15,16), and other naturally occurring chiral sulfur compounds (4) is not discussed. For the same reason, only selected results are discussed and in some cases only references are given to recent papers and review articles on special topics. 

Further information concerning the stereochemical properties of the rearrangement were evaluated by submitting rigid cyclohexane derivatives 254/255 to the reaction conditions. In 1975, House described the allylation of a cyclohexyl cyanide 248 [53]. The initial deprotonation with LDA led to a ketene imine anion 249, which was then treated with allyl bromide. Two potential paths rationalized the outcome an AT-allylation generated the intermediate ketene imines 250/251, which underwent aza-Claisen rearrangement to deliver the nitriles 252/253 alternatively, the direct C-allylation of249 produced the nitriles. 

Chemical and Stereochemical Properties of Compounds with Silicon or Germanium-Transition Metal B(nids... 

If several molecules of the chiral auxiliary are involved in autoassociation or in any type of molecular species, then the passage from enantiopure auxiliary to nonenantiopure auxiliary may produce new diastereomeric entities. Consequently, the enantioimpure system may have new stereochemical properties. 

A stereochemical property of compounds arising from the ability of an enzyme s active site to distinguish between two chemically identical substituents covalently bound to a tetrahedral center (usually carbon and, in some cases, phosphorus). Prochirality is also termed prostereoisomerism. The classical example is citrate with its two carboxymethyl group substituents. Likewise, the Cl carbon atom of ethanol has two prochiral hydrogens. See Chirality Chirality Probes... 

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