Stereochemistry conformational isomers

Since atropisomers are conformational isomers, their stereochemistry should be designated by IUPAC nomenclature rule E (23). For the convenience of the reader, the rule will be outlined here. A sequence number is given to substituents connected to the rotational axis according to the Sequence Rule (24), as follows. 

Next relative configurations (R,S) are possible for 1,2- or 1,3-disubstituted isomers. (The 1,4 isomer has a plane of symmetry.) The relative stereochemistry can be denoted as cis or trans, depending on whedter the substituents point toward the same side or opposite sides of die ring. Finally, die cyclohexane ring can undergo chair-chair interconversion leading to different conformational isomers. These possibilities are shown for metiiylcyclohexanol in (6.6). 

Key point. The spatial arrangement of atoms determines the stereochemistry, or shape, of organic molecules. When different shapes of the same molecule are interconvertible on rotating a bond, they are known as conformational isomers. In contrast, configurational isomers cannot be interconverted without breaking a bond, and examples include alkenes and optical isomers, which rotate plane-polarised light. 

Determination of molecular conformation (structural isomers) and stereochemistry (geometric isomers). 

The structural isomers that differ only with respect to a stereochemistry point of view are named as conformation isomers. 

Sometimes, the difference in stereochemistry results in a difference in the magnetic properties of a complex. Square planar complexes of Ni(II) are diamagnetic, whereas the tetrahedral ones are paramagnetic. Hence, some of the conformation isomers are also spin isomers. 

It was necessary to determine the absolute stereochemistry of vibsanins B (1) and C (2) before discussing the structures of the newly isolated vibsane-type diterpenoids. The NMR of 1 showed two kinds of broad signals at room temperature, but a pair of the sharp signals was observed at 0 °C. This phenomenon indicated that vibsanin B (1) is present in solution as two conformational isomers [11]. 

The study of optical isomers has shown a similar development. First it was shown that the reduction potentials of several meso and racemic isomers were different (Elving et al., 1965 Feokstistov, 1968 Zavada et al., 1963) and later, studies have been made of the ratio of dljmeso compound isolated from electrolyses which form products capable of showing optical activity. Thus the conformation of the products from the pinacolization of ketones, the reduction of double bonds, the reduction of onium ions and the oxidation of carboxylic acids have been reported by several workers (reviewed by Feokstistov, 1968). Unfortunately, in many of these studies the electrolysis conditions were not controlled and it is therefore too early to draw definite conclusions about the stereochemistry of electrode processes and the possibilities for asymmetric syntheses. 

If the carbanion has even a short lifetime, 6 and 7 will assume the most favorable conformation before the attack of W. This is of course the same for both, and when W attacks, the same product will result from each. This will be one of two possible diastereomers, so the reaction will be stereoselective but since the cis and trans isomers do not give rise to different isomers, it will not be stereospecific. Unfortunately, this prediction has not been tested on open-chain alkenes. Except for Michael-type substrates, the stereochemistry of nucleophilic addition to double bonds has been studied only in cyclic systems, where only the cis isomer exists. In these cases, the reaction has been shown to be stereoselective with syn addition reported in some cases and anti addition in others." When the reaction is performed on a Michael-type substrate, C=C—Z, the hydrogen does not arrive at the carbon directly but only through a tautomeric equilibrium. The product naturally assumes the most thermodynamically stable configuration, without relation to the direction of original attack of Y. In one such case (the addition of EtOD and of Me3CSD to tra -MeCH=CHCOOEt) predominant anti addition was found there is evidence that the stereoselectivity here results from the final protonation of the enolate, and not from the initial attack. For obvious reasons, additions to triple bonds cannot be stereospecific. As with electrophilic additions, nucleophilic additions to triple bonds are usually stereoselective and anti, though syn addition and nonstereoselective addition have also been reported. 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

The first element of stereocontrol in aldol addition reactions of ketone enolates is the enolate structure. Most enolates can exist as two stereoisomers. In Section 1.1.2, we discussed the factors that influence enolate composition. The enolate formed from 2,2-dimethyl-3-pentanone under kinetically controlled conditions is the Z-isomer.5 When it reacts with benzaldehyde only the syn aldol is formed.4 The product stereochemistry is correctly predicted if the TS has a conformation with the phenyl substituent in an equatorial position. 

Stereochemistry can be interpreted in terms of conformation effects in the 1,4-biradical intermediates.199 Vinyl enol ethers and enamides add to aromatic ketones to give 3-substituted oxetanes, usually with the cis isomer preferred.200... 

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