Heterotopic faces

Re and Si Labels used in stereochemical descriptions of heterotopic faces. If the CIP priority of the three ligands a, b, and c is assigned as a > b > c, the face that is oriented clockwise toward the viewer is called Re, while the face with a counterclockwise orientation of a — b — c is called Si, as shown in Figure 1-34 ... 

Figure 13. Two synthetic strategies leading to chiral crown compounds with heterotopic faces starting from asymmetric precursors. Note that, although the product obtained in (b) has Cj symmetry, the symmetry element does not exchange the environment of the two faces.

Returning to the above-mentioned unsaturated compounds, the reader might also note that the faces in four of the five are homotopic, the exception being trans-1,2-dichloroethylene which has heterotopic faces.6... 

The addition criterion tends to be confusing when applied to a molecule like ethylene where addition occurs at both ends of the double bond. The reader is advised, in such cases, either to use the symmetry criterion or to choose epoxidation as the test reaction for the addition criterion. For additional examples involving the heterotopic faces of not only olefins and carbonyl compounds... 

These mirror image representations cannot be made to coincide as long as they are maintained in the plane of the paper. As we have already seen (Sect. 3), such two-dimensionally chiral representations may be taken to depict heterotopic faces. If the sequence is A > B > C, triangle I represents the Re face and triangle II the Si face. 

The stereoselective complexation see Stereochemistry) of the heterotopic faces of Cp ligands can be realized. For example, starting from the isodicyclopentadienyl anion and MCLi or () -L)MCl3, the resnlts clearly indicate the temperatnre dependence in the stereo differentiation of the two heteropic sides of the anion. ... 

RelSi Descriptors for heterotopic faces in a prochiral molecule. See Eliel and Wilen, p. 484 for a description. 

An asymmetric annelation sequence analogous to the foregoing achiral version has been reported (Scheme 29) (55a). Although only moderate yields were achieved, the diastereoselectivity and discrimination between heterotopic faces achieved in the process are both excellent. 

Asymmetric synthesis is a reaction or reaction sequence that selectively creates one configuration of one or more new stereogenic elements by the action of a chiral reagent or auxiliary, acting on heterotopic faces, atoms, or groups of a substrate. The stereoselectivity is primarily influenced by the chiral catalyst, reagent, or auxiliary, despite any stereogenic elements that may be present in the substrate. 

Figure 1.4. Two types of reactions that distinguish heterotopic faces.

Homofacial, heterofacial The relative configuration of stereocenters (in different molecules) having three identical ligands and one different is homofacial if the fourth ligand is on the same heterotopic face in both, and heterofacial if on opposite faces, as shown below. [91,92,101]. See also relative configuration. 

Prochiral Tetrahedral atoms having heterotopic ligands, or heterotopic faces of trigonal atoms, may be described as being prochiral. Note that it is inappropriate to describe an entire molecule as being prochiral [105]. Heterotopic faces are described using Re, Si if reflection variant, and re, si if reflection invariant [91]. If the CIP priority of the three ligands is clockwise, the face... 

Interligand asymmetric induction. Group-selective reactions are ones in which heterotopic ligands (as opposed to heterotopic faces) are distinguished. Recall from the discussion at the beginning of this chapter that secondary amines form complexes with lithium enolates (pp 76-77) and that lithium amides form complexes with carbonyl compounds (Section 3.1.1). So if the ligands on a carbonyl are enantiotopic, they become diastereotopic on complexation with chiral lithium amides. Thus, deprotonation of certain ketones can be rendered enantioselective by using a chiral lithium amide base [122], as shown in Scheme 3.23 for the deprotonation of cyclohexanones [123-128]. 2,6-Dimethyl cyclohexanone (Scheme 3.23a) is meso, whereas 4-tertbutylcyclohexanone (Scheme 3.23b) has no stereocenters. Nevertheless, the enolates of these ketones are chiral. Alkylation of the enolates affords nonracemic products and O-silylation affords a chiral enol ether which can... 

If the electrophile is an aldehyde or an unsymmetrical ketone, does the organometallic add selectively to one of the heterotopic faces (Scheme 3.27c) ... 

Figure 6.6. Inset Heterotopic faces for determining relative topicity (note inversion at the stereo-genic RLi). (a,b) Syn product is formed by two combinations of ul topicity. (c,d) Anti product is formed by two combinations of Ik topicity. In transition states a-d, the metal is omitted. When R is an alkyl group, it would be bridged to the C-0 bond, antiperiplanar to the allyl group (cf. Figure 6.4a, b). If R is a carbonyl, the metal will be attached to the enolate oxygen (cf. Figure 6.5c).

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