MislowSiegel - referred to as stereogenic if ligand...

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L. D. Shirtcliff – CHEM 3053 Stereogenicity & Chirotopicity Reference: K. Mislow and J. Siegel, J. Am. Chem. Soc. 1984 , 106 , 3319. Mislow and Siegel showed that all segments of a chiral molecule are chiral and that segments of an achiral molecule may be chiral or achiral. A new stereochemical terminology was introduced. (Statements below are direct quotes or are paraphrased from the referenced paper). Chirotopicity: chirotopic – any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in a chiral environment (i.e. no S n axis passes through it) achirotopic – any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in an achiral environment (i.e. must have S n axis pass through it) This property, chirotopicity , is a purely geometric attribute based on symmetry considerations rather than bonding considerations. Stereogenicity: stereogenic – any mono- or polyatomic permutation center or skeleton in a molecule may be
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Unformatted text preview: referred to as stereogenic if ligand permutation (interchange of two ligands (groups)) produces stereoisomers. For example, switch a and b in tetrahedral C abxy and the enantiomer of the original molecule is produced. (The abbreviated term stereocenter is often used to denote the carbon in such a molecule, and * is often used to denote it.) Switch a and b in alkene ab C=C xy and a diastereomer of the original is produced. nonstereogenic ligand permutation about the center/skeleton doesnt lead to stereoisomers (Note that the regular tetrahedron is the only skeleton in which every transposition of ligands is equivalent to a reversal in the sense of chirality of the ligated assembly. Thus, for this geometry there is unique link between chirotopicity and stereogenicity.) C C a b x y C C b a x y a/b interchange C b y x a a/b interchange C a y x b C b y x a...
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