Stereoisomerization interconversion

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

In principle, cri-2-butene and fran5-2-butene may be interconveited by rotation about the C-2=C-3 double bond. However, unlike rotation about the C-2—C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal circumstances. It is sometimes said that rotation about a car bon-carbon double bond is restricted, but this is an understatement. Conventional laboratory sources of heat do not provide enough energy for rotation about the double bond in alkenes. As shown in Figure 5.2, rotation about a double bond requues the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment—in effect, the tt component of the double bond must be broken at the transition state. 

The 1,3-allylic rearrangement has also been observed with several cyclic sulfoxides. For example, the interesting thermal interconversion of the bicyclic stereoisomeric pair 141... 

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

The stereoisomerism in some asymmetric tetrahedral complexes of the type M(AA,)2 has been characterized by nmr. - Tetrahedral complexes of the type M(AA,)2 are chiral, but their lability requires nmr methods for measurement of the rates of interconversion of the enantiomers. The pioneering work of Holm and his collaborators has been consolidated by Minkin and his Russian group and Zn, Cd and, more recently, Ni chelates of the type M(AA[)2 have been extensively studied. 

The rearrangement is of interest because the corresponding enzymatic interconversion of aldoses and ketoses is an important part of the biosynthetic, photosynthetic, and metabolic pathways, as we shall see in Section 20-9. Although the biochemical rearrangement also may proceed by way of enediol intermediates, it is highly stereospecific and yields only one of two possible stereoisomeric aldoses. For example, glucose, but not mannose, can be enzymatically interconverted with fructose as the 6-phosphate ester derivative ... 

Electrophilic attack on //-vinylidene complexes can occur either on the methylene carbon, or at the metal-metal bond. With the manganese complexes (45, R = H or Me), protonation affords the//-carbyne complexes (46), which in the case of R = Me, exist in the stereoisomeric forms shown (57). Interconversion of the two forms is slow at room temperature ... 

The two types of addition were proposed to occur through different surface absorbed forms240,243 (Scheme 6). The 24 -adsorbed intermediate is suggested to result in 1,2 addition (Type A metals). The 25 and 26 jr-allyl species, in contrast, ensure 1,4 addition. The selectivity of the formation of stereoisomeric 2-butenes, in turn, depends on the interconversion of the possible half hydrogenated syn and anti surface jr-allyl complexes... 

The interconversion of isomers in the case of optical or geometric pairs, if structurally feasible, may only take place by the breaking of a- or 7c-bonds. However, there is a further area of stereoisomerism wherein the isomers are interconvertible by rotation about a single (conformational isomers or conformers. 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

In the examples of Schemes 9.13 and 9.14, formation of the equilibrating intermediate has been rate limiting. Any reversibility of intermediate formation would provide a pathway for interconversion of the stereoisomeric reactants. 

It is generally accepted that the oxidative power of charcoal is due to the chemisorbed layer of oxygen on its surface.1 This oxygen layer appears to be important also in other ways T. W. J. Taylor s studies on the interconversion of stereoisomeric oximes and sulphoxides show that oxygen is necessary for this reaction. The mechanism appears to be an exchange of the active, adsorbed oxygens on the surface of the carbon, with the molecules of the substances undergoing isomerization in solution. The nature of the layers adsorbed on charcoal influences, naturally, its behaviour as an electrode, and the kind of ions which it sends into solution ... 

If one assumes a sufficiently low relative interconversion rate for the stereoisomeric diazoazides. Scheme 7, a variation of Scheme 5, becomes an attractive mechanistic interpretation based on the two-phase experiment (64AHC(3)373,63T1801). 

According to the above analysis, the isomerism of the 1,3-0-benzylidene-glyceritols is not to be regarded as an example of isomerism at the acetal carbon atom, as the phenyl group will be equatorial in each isomer. Indeed, true isomerism at the acetal carbon atom in simple, six-membered rings (other than the rare ST-rings) is most improbable. It has been claimed that 1,3 5,7-di-O-benzylidene-D- and-L-perseitol are obtainable in stereoisomeric forms, " but the two forms had identical rotations, and it is difficult to see how the reported interconversion of the isomers could be effected in the basic solvent used. The two substances are probably dimorphic modifica-... 

Alternatively, interconversion between the stereoisomeric allyl cations can take place by capture of a nucleophile at either end, followed by rotation about the more or less normal single bond, and then regeneration of the cation by ionisation. Interconversion between the corresponding anions can take place similarly by cr coordination (771) to a metal at one end or the other. Because of the availability of these pathways, experimental measurements of the barrier to rotation have confirmed that it is less than the very approximate theoretical value of 116 kJ mol-1 (28 kcal mol ). Furthermore, measurements have generally been made on significantly more substituted systems. Such substitution can stabilise the filled, half-filled or empty p orbital, or the double bond, even when these components are no longer conjugated, and so appropriate substituents lower the barrier to rotation. 

This type of isomerization still seems to be of limited importance. For example, a mixture of unseparable valence-bond isomers 75 and 76 was obtained by thermolysis of an appropriate cyclobutene precursor (92TL883). Another 2//-pyran intermediate postulated in the Z-E interconversion of stereoisomeric retinals could be neither isolated nor identified (92JOC1110). 

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