Stability configurational

Hawkins J M, Nambu M and Meyer A 1994 Resolution and configurational stability of the chiral fullerenes C-g, C g, and Cg. A limit for the activation energy of the Stone-Wales transformation J. Am. Chem. Soc. 116 7642-5... 

S.08.2.3.1 NMR investigations on oxaziridines and diaziridines, configurational stability at nitrogen... 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

One major task is the selection of the optimal cation" for a given carbanion . It determines to a great extent the mechanism of the reaction and the positional and configurational stability of the allyl moiety, and thus, the regioselectivity, EjZ selectivity, and diastereoselectivity. Some reviews cover these topics in general1 " 5. 

A simple experimental test of the potential configurational stability of carbanions was recently published26 48 it is based on double stereodifferentiation3 and can be carried out with the racemates. 

If R1 differs from R2. the preparation may lead to both regioisomers. In these cases, a synthetic route which does not rely on allyl anion substitution is often the most advantageous one. Thus, the best results are recorded for allylboronates and -silanes which also possess the required constitutional and configurational stability. 

Investigation of the configurational stability of allylmetals 2a, prepared by means of Rieke metals from geometrically pure l-chloro-2-decenes 1 a in THF, showed that the (Z)- and (ZT)-lithium, -sodium, and -potassium derivatives preserve the configuration of the precursors to a preparatively useful extent below — 90°C, — 50°C, and >0°C, respectively. For the pairs of 3,7-dimethyl-2,6-octadienyl derivatives (Z)- and ( )-2b, which differ less in their thermodynamic stability, the respective temperatures are — 60 °C, — 40 °C and >0°C124. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

Simple diastereoselection in the reactions of 2-butenylboron compounds and aldehydes is critically dependent on the configurational stability of the reagentslb. As a general rule, most 2-bulenylorganometallics arc sensitive to sequential 1,3-metal shifts (1,3-metallotropic rearrangements) that result in E- to Z-olefin isomerization via the l-methyl-2-propenylmetal isomer. 

In contrast to the 2-butenylboranes, 2-butcnylboronates have found widespread application in acyclic diastereoselective synthesis owing to their ease of preparation (Section 1.3.3.3.3.1.1.), configurational stability and highly stereoselective reactions with aldehydes3 4. The results of reactions of substituted allylboronates and representative achiral aldehydes are summarized in Table 1. 

The numerous examples of optically active sulfoxides reflect their configurational stability. Optically active sulfoxides resist thermal racemization by pyramidal inversion, so... 

Further evidence has been adduced for the configurational stability of phosphoranyl radicals. Thus photolysis of iodobenzene in the presence of (11) gave a 95% yield of (12). Reaction of the phosphonium salt (13) with lithium alkyls produces the phosphoranyl radical (14). ... 

Nucleophilic substitutions of 0-activated 2-hydroxy carboxylic acids and esters, respectively, are well established, but little is known about the analogous reactions of activated cyanohydrins. Chiral 2-sulfonyloxynitriles, accessible from non-racemic cyanohydrins, have a relatively high configurational stability. They react with nucleophiles under very mild conditions under inversion of configuration (Scheme 8). ° ... 

The inductive and electrostatic effects, steric constraints and conjugative interactions are the major factors that determine the configurational stability of a-sulfonyl carbanions. These are thought to be pyramidal with appreciable electrostatic inhibition to racemization by way of inversion. LCAO-MO-SCF calculations have indicated the conformer 195 in which the lone pair is directed along the bisector of the OSO angle to be the most stable in acyclic sulfones. ... 

When configurational stability is truly low and results in half-lives of racemization or epimerization, which are on the order of minutes or hours, the phenomenon has pharmacological significance. On the other hand, when the half-lives are on the order of months or years, the phenomenon has pharmaceutical significance and may shorten the shelf life of drugs [47],... 

The methods which have been used to determine the configurational stability of organotin compounds and those which have successfully been applied to synthesize such optically active molecules are reviewed, including the chromatographic resolution of racemic mixtures. 

The Use of Diastereotopic Groups as a Probe for the Determination of the Configurational Stability at the Tin Atom... 

It is therefore possible to determine the configurational stability at tin by looking at the NMR spectrum of a racemic mixture of suitably substituted organotin compounds of type M (see figure 1) bearing diastereotopie groups. Some of the compounds examined by this elegant method are listed in Table 1. 

Table 2. Molecules of type N, i.e. U(CH3)A(CH3)BSn—V—CH(CH3)Z used to determine the configurational stability at tin (st. stands for configurationally stable within the NMR time-scale (Avab) u. for configurationally unstable RT stands for room temperature)... 

If two asymmetric atoms are present in the molecule (see 4.1), NMR can still be used to determine the configurational stability but intermolecularly diastereo-topic groups are then used instead of intramolecularly diastereotopic ones. 

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