Diastereotopic atoms/groups

Stereoheterotopic A term that includes both enantiotopic and diastereotopic atoms, groups and faces. Equivalent atoms, groups and faces would be homotopic. 

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

Zheng et al. [1] postulated that the driving force for placing Zr and B on the same carbon might stem from interactions between the zirconium and oxygen or boron and chlorine atoms. However, an X-ray analysis of 22 revealed that there are no intra- or intermo-lecular interactions between any of these atoms [35]. Compound 22 was also unambiguously characterized by 1H-1H double quantum filtered COSY [36] and 13C-1H heteronuc-lear chemical shift correlation NMR spectroscopy [37,38]. Considerable differences in the chemical shifts of the diastereotopic Cp groups were found in both the XH and 13C NMR spectra. The NMR study unequivocally showed that the methine proton was at-... 

Asymmetric bond disconnection is less frequently employed than asymmetric bond formation for the synthesis of chiral, nonracemic compounds. The substrates for these transformations contain either enantiotopic (diastereotopic) hydrogen atoms or enantiotopic (diastereotopic) functional groups. In some cases the classification of a given transformation of such a substrate as asymmetric bond disconnection or bond formation is somewhat arbitrary. Thus, enantiotopic and diastereotopic group differentiation is also described at appropriate places in various sections but more specifically in part B of this volume. 

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

Fig. 7. Bridges containing diastereotopic atoms in Me2CH or PhCH2 groups in compounds of the type Os3H(/j-X)(CO)10.

We can begin with either a carbon or a proton resonance and obtain equivalent results. We will use the carbon axis as our starting point because we usually have less overlap there. For example, a line drawn parallel to the proton axis at about 68 ppm on the carbon axis (the carbinol carbon) intersects five cross peaks none of the five correlations corresponds to the attached proton (VCH) at 3.8 ppm. Four of the cross peaks correspond to the two pairs of diastereotopic methylene groups (2.48, 2.22,1.45, and 1.28 ppm) and these represent, 2iCH, or two-bond couplings. The fifth interaction (3/CH) correlates this carbon atom (68 ppm) to the isopropyl methine proton (1.82 ppm), which is bonded to a carbon atom in the /3-position. The other carbon atom in a /3-position has no attached protons so we do not have a correlation to it from the carbinol carbon atom. Thus, we have indirect carbon connectivities to two a-carbons and to one of two /3-carbons. 

The limited configurational stability of a-alkylthio organolithiums does not extend to those with benzylic lithium-bearing carbon atoms.112 A Hoffmann test reaction of 233 with 6, for example, gives a 40 60 ratio of stereoisomers 234 whether enantiomerically pure or racemic 6 is used.6 Dynamic NMR experiments2 quantified the barrier to racemisation in 233 as 9.95 kcal mol-1 at 213 K, the temperature at which the diastereotopic CH2 group coalesces. 

Bonded to a group that withdraws part of the electron density from around the nucleus. The absorptions of deshielded nuclei are moved downfield, resulting in larger chemical shifts, (p. 568) Nuclei that occupy diastereomeric positions. The replacement test for diastereotopic atoms gives diastereomers. Diastereotopic nuclei can be distinguished by NMR, and they can split each other unless they are accidentally equivalent, (p. 592)... 

Some of the compounds shown below contain enantiotopic or diastereotopic atoms or groups. Which possess this characteristic For those that do, indicate the atoms or groups that are diastereotopic and assign the groups as pro-R and pro-S. 

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