And stereogenic atoms

The most common, although not the only, cause of chirality in an organic molecule is the presence of a carbon atom bonded to four different groups—for example, the central carbon atom in lactic acid. Such carbons are now referred to as chirality centers, although other terms such as stereocenter asymmetric center, and stereogenic center have also been used formerly. Note that chirality is a property of the entire molecule, whereas a chirality center is the cause of chirality. 

Ueno and coworkers49 have developed a procedure for the synthesis of chiral sulfinic acids. Treatment of (R)-( + )-23 with disulfide 24 and tributylphosphine in THF gave (S)-( — )-25. Compound 25 was oxidized with potassium permanganate to the sulfone, which was then reduced to the sulfinic acid, (S)-( — )-26, by treatment with sodium borohydride. Conversion of 26 or an analog to an ester would lead to diastereomers. If these epimers could be separated, then they would offer a path to homochiral sulfoxides with stereogenic carbon and sulfur atoms. 

Polymerization of optically active isonitiiles, 72, also leads to the formation of helical polymers with a preferential screw sense (219-222). Various factors distinguish this system from the preceding ones In the isonitrile case no new stereogenic atoms are formed during polymerization the helices are rigid and there is no indication of conformational equilibrium in the system the formation of a preferential screw sense is very probably a kinetically controlled process. 

For an alkene to exhibit cis-trans isomerism, the two groups on one end of the double bond must be different and the two groups on the other end of the double bond must be different. That is, in terms of the following structure, A must be different from B, and D must be different from E. When this is the case, both of the carbons of the double bond are said to be stereocenters. A stereocenter or stereogenic atom is defined as an atom at which the interchange of two groups produces a stereoisomer. 

The examples shown in Scheme 16, which are representative for all other sulfinates studied, demonstrate a high level of stereoselectivity (around 80-90% of the original enantiomeric excess is retained after switching the stereogenic atom from sulfur to carbon). Despite these encouraging results, the somewhat tedious and stereochemically unreliable preparation of the starting sulfinates [47] may be the reason for the reluctance of the chemical community to use this chemistry as an entry to enantiomerically pure 2-alkenyl sulfones. 

Studies of structurally related phosphonoamidates possessing P- and C-stereogenic centers indicated that the alkylation of diastereoisomer (436) is mostly influenced by the chirality of the asymmetric phosphorus atom, while the alkylation of I diastereoisomer (437) depends on a combination of both the chirality of phosphorus and carbon atoms (Scheme 103). ... 

Compounds with Tervalent Stereogenic Atoms. Atoms with pyramidal bond-ing might be expected to give rise to optical activity if the atom is connected to three different groups, since the unshared pair of electrons is analogous to a fourth group, necessarily different from the others. For example, a secondary or tertiary amine where X, Y, and Z are different would be expected to be chiral and thus resolvable. Many attempts have been made to resolve such compounds, but until 1968 aU of them failed because of pyramidal inversion, which is a rapid oscillation of the unshared pair from... 

---