Stereogenic atoms substituent

Scheme 2.27 Preparation of a diphosphine borane containing a P-stereogenic atom with a menthyl substituent. Only the Rp diastereomer is represented.

Atropisomerism is significant because it introduces an element of chirality in the absence of stereogenic atoms. Axial chirality is observed with stereoisomers (or atropisomers) that result from hindered rotation about a single C—C or C—N bond. The barrier of rotation between atropisomers must be high enough to allow for their isolation. A minimum of three or/ho-substituents are generally required for an axially chiral biphenyl to have substantial stability toward racemization at room temperature. For general definitions and descriptions, see references [3-5]. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

There are a number of important kinds of stereogenic centers besides asymmetric carbon atoms. One example is furnished by sulfoxides with nonidentical substituents on sulfur. Sulfoxides are pyramidal and maintain dieir configuration at room temperature. Unsymmetrical sulfoxides are therefore chiral and exist as enantiomers. Sulfonium salts with three nonidentical ligands are also chiral as a result of their pyramidal shape. Some examples of chiral derivatives of sulfur are given in Scheme 2.1. 

The reason that the third stereoisomer is achiral is that the substituents on the two asymmetric carbons are located with respect to each other in such a way that a molecular plane of symmetry exists. Compounds that incorporate asymmetric atoms but are nevertheless achiral are called meso forms. This situation occurs whenever pairs of stereogenic centers are disposed in the molecule in such a way as to create a plane of symmetry. A... 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

The addition of dibutylcupratc to the a-substituted /1-formyl esters 1 preferentially affords, via chelation control, the cw-disubstituted y-lactone 241. These results are in agreement with those found with a-unsubstituted /1-esters39-41 (vide supra), assuming a seven-membered chelate as transition state of the addition reaction. The diastercosclectivity is somewhat lower with esters 1 as the stereogenic center is one carbon atom further removed from the reaction center and therefore the steric influence of the substituent R1 is less pronounced. 

The effect of stereogenic centers along the chain, connecting the 7t-nueleophile and the nitrogen atom, has been studied. For substituents adjacent to the nitrogen atom, an axial orientation is preferred in order to avoid A 1-31 strain between the substituent and the carbonyl function of the A-acyl group146. 

It is the purpose of this chapter to summarize what is currently known about the stereochemistry and conformation of organogermanium, tin and lead compounds. Coverage is selective rather than exhaustive. The first section deals with compounds in which substitution by four different groups causes the metal atom to be stereogenic. We have limited our discussion to those cases in which at least three of the four substituents are alkyl or aryl. In this section we also briefly discuss pentacoordinated triorgano halostannanes. 

The development of the next major class of ligands occurred during the 1990s, with Burks DuPhos (42) family of phospholane ligands [222, 223]. (An individual member of the family is named after the substituent R in Me-DuPhos, R=Me.) This structure could be considered an improvement on the DIOP-derived ligands, where the stereogenic centers are now closer to phosphorus. In addition to the aromatic spacer of DuPhos, there is also the related BPE (43) family, where the spacer between the two phosphorus atoms is less rigid. In both series the phosphorus is... 

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