Homotopic substituents

Homotopic substituents are interconverted by rotational axes. Substitution of either substituent leads to the same compound, so it follows that homotopic substituents are not subject to distinction. 

Z)-l,2-disubstituted alkenes proved to be the most difficult class. In fact, they are not osmylated efficiently with the all purpose ligands 1F/2 F. Further studies, however, led to the discovery of the indolinyl ligands 11/21 that allowed cis dihydroxylation of these alkenes in up to 80% eel0. It should be kept in mind, however, that in the case of 1,1-disubstituted alkenes and of (Z)-l,2-disubstituted alkenes, a lowering of difference in steric requirement between the two vicinal substituents inevitably means a drop in the 7t-face discrimination since the two enantiotopic alkene 7t-faces lend to become quasi-homotopic . 

A. Compound 60 is of the same substitution pattern as dichloromethane, and the hydrogens Ha and Hb are homotopic. Compound 61 has the same pattern as ethanol here Ha and Hb are enantiotopic. In compound 62, Ha and Hb are homotopic here a in other homotopic examples, H and Hb lie astride a symmetry plane. In compound 63, Ha and Hb are diastereotopic. Separate replacement of Ha and Hb by D in 63 gives a pair of diastereoiso-mers (that is stereoisomers that are not enantiomers). In compound 64, Ha and Hb are diastereo topic this can be confirmed by separate replacement of Ha and Hb by D, to give a pair of diastereoisomers. Note that previously (Sections 6.3 and 6.5.2) we have shown that cis and trcms 1,3-disubstituted cyclobutanes and cis and tram 1,4-disubstituted cyclohexanes constitute a pair of achiral diastereoisomers. In compound 65, Ha and Hb are enantiotopic. In fused-ring molecules, substituents are often drawn where Ha/Hb are located in 65 however, anti-clockwise rotation by 120° around an axis through the bridgehead carbons puts Ha and Hb in a position where their enantiotopicity can be seen more easily. 

Abstract The control elements that did not find mention in the earlier chapters are dealt with here. The prominent among these elements are spiroconjugation, peris-electivity in pericyclic reactions, torquoselectivity in conrotatory-ring openings, ambident nucleophiles and electrophiles, a-effect in nucleophilicity, carbene addition to 1,3-dienes, Hammett s substituent constants, Hammond postulate, Curtin-Hammett principle, and diastereotopic, homotopic, and enantiotopic substituents. 

Keywords Spiroconjugation Periselectivity Carbenes Ketenes Torquoselectivity Ambident nucleophiles and electrophiles a-effect Hammett s substituent constants Hammond postulate Curtin-Hammett principle Diastereotopic Homotopic Enantiotopic substituents... 

Unlike the above, the two hydrogen atoms labeled Ha and Hb in propane 184 are homotopic because a C2 operation converts one into the other, so that they are considered to be equivalent in all possible ways. Even if one of these hydrogen atoms is replaced by a substituent other than methyl and hydrogen, resultant molecule is not chiral. Homotopic groups remain indistinguishable under chiral influence, i.e., in the presence of chiral ligands. 

The emphasis to this point has been on the stereochemical relationship of one structure to another. However, the principles developed so far are also relevant to the consideration of s)unmetry relationships within a single molecule. Many of the labels for these relationships are based on the suffix -topic, from the Greek for "place." Terms incorporating this suffix apply both to atoms and to spaces in a molecule, although we usually think of them in terms of atoms. Identical atoms that occupy equivalent environments (both in terms of chemical properties and local or molecular S5munetry) are said to be homotopic (i.e., to have the same place). Identical atoms in nonequivalent environments are said to be heterotopic (for different place). Heterotopic substituents can be either constitutionally heterotopic or stereoheterotopic. Stereoheterotopic substituents can be either enantiotopic or diastereotopic. ... 

Consider the examples in Figures 2.36-2.39. Any proton on either CH3 group of propane (89) is homotopic with any other methyl proton, since the product of replacing any one proton by another substituent (shown as an X in 90) is the same as that produced by replacing any other methyl proton (Figure 2.36). If necessary, the products can be seen to be identical by rotating the entire molecule, by rotation of one part of the molecule about an internal bond, or both. The two methylene protons on C2 of propane are also homotopic. 

Scheme 4.49. Use of its homolog without such substituents afforded the mixture of homotopic M4L4 square-planar metallomacrocycle and octahedral MgL 4 cage complex. The caging ligand 552 is also able to encapsulate a cofacial stacked por-phine dimer forming a 1 2 cage complex by Scheme 4.50 its extended analog 553 is described in [50] to give a heteroguest 1 2 1 compound with triple-decker encapsulated species containing two cofacial porphine molecules that sandwich one tris-pyridyl aromatic syntone 241 (Scheme 4.50).

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