Configurational isomer

As we have now seen, conformational isomers interconvert easily by rotation about single bonds. Configurational isomers, on the other hand, are isomers that interconvert only with difficulty, and it usually requires bond breaking if they do interconvert. 

The spatial arrangement of atoms or groups in molecules is known as configuration. Compounds with the same molecular formula and bonds, which cannot be interconverted without breaking a bond, have different configurations (and are known as configurational isomers). 

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The next sections describe briefly the nomenclature of configurational isomers, and how this stereochemistry can be handled by computer. 

Isomer separation beyond physical fractional crystallization has been accompHshed by derivatization using methyl formate to make /V-formyl derivatives and acetic anhydride to prepare the corresponding acetamides (1). Alkaline hydrolysis regenerates the analytically pure amine configurational isomers. 

PerfluoroaUcenes are converted to vicinal diols when no fluonne atom is present at the double bond Configurational isomers of perfluoroalkenes [29] (equations 20 and 21) are oxidized stereospecifically Perfluorbicyclo[4 3 0]non-1(6) ene gives the corresponding 1,6 diol m a 24% yield upon oxidation with potassium permanganate at 18 °C for 1 h [29]... 

NH2OH can exist as 2 configurational isomers (cis and trans) and in numerous intermediate gauche conformations as shown in Fig. 11.7. In the crystalline form, H bonding appears to favour packing in the trans conformation. The N-O distance is 147 pm consistent with its formulation as a single bond. Above room temperature the compound decomposes (sometimes explosively) by internal oxidation-reduction reactions into a complex mixture of N2, NH3, N2O and H2O. Aqueous solutions are much more stable, particularly acid solutions in which the compound... 

Although the limited examples of AE reactions on 2,3Z-substituted allyl alcohols appear to give product epoxides in good enantioselectivity, the highly substituted nature of these olefins can have a deleterious effect on the reactivity. For example, Aiai has shown that the 2,3E-substituted allyl alcohol 30 can be epoxidized with either (-)-DET or (+)-DET in good yields and enantioselectivity. However, the configurational isomer 32 is completely unreactive using (-)-DET, even after a 34 h reaction time. 

Reductive alkylation with chiral substrates may afford new chiral centers. The reaction has been of interest for the preparation of optically active amino acids where the chirality of the amine function is induced in the prochiral carbonyl moiety 34,35). The degree of induced asymmetry is influenced by substrate, solvent, and temperature 26,27,28,29,48,51,65). Asymmetry also has been obtained by reduction of prochiral imines, using a chiral catalyst 44). Prediction of the major configurational isomer arising from a reductive alkylation can be made usually by the assumption that amine formation comes via an imine, not the hydroxyamino addition compound, and that the catalyst approaches the least hindered side (57). 

Three configurational isomers are possible for a diazoanhydride with the same (substituted) arene residues on both sides the (Z),(Z), the (Z (E), and the (E),(E), as shown in Scheme 5-15, formulas 5.4, 5.5, and 5.6. Kauffmann et al. think that in their product the residues attached to both azo groups are in the (Z)-configuration. However, their arguments are not clearly convincing. 

Since diazoates can be considered to be derived from oximes by substitution of nitrogen for the methine group, Hantzsch (1894) put forward the hypothesis that configurational isomerism was also occurring here. He therefore represented the isomeric diazoates by the structures 7.1 and 7.2, assigning the syn structure (7.1) to the labile diazoate and the anti (7.2) to the stable isomer. Nowadays the description recommended by IUPAC (1979) for such configurational isomers, namely (Z) instead of syn and (E) instead of anti, should be used. 

Furlei and coworkers44 studied the negative ion mass spectra of several cyclic sulfones (82-98) upon dissociative electron capture and concluded that the negative molecular ions were notably stabilized by the introduction of electron-withdrawing substituents and/or unsaturation. Some difference was found in the negative ion mass spectra of configurational isomers (85 vs. 86 and 87 vs. 88) in contrast to the situation in their positive ion spectra. A strong S02 ion (m/z 64) was observed also for all the compounds studied. 

This crisscross or von Halban-White-type cyclization product is formed from the (E)-configured intermediate 87, which cannot undergo the 67r-electrocy-clization like the (Z)-configured isomer 88, to yield the benzannelation product 86 [78,79]. While the diastereoselectivity of the alkyne insertion must have been controlled by the electronic and not the steric factors of the substituents on the alkyne, the anti-configuration of the tricyclic system 85 was confirmed by an X-ray structure analysis [77]. 

Phenylserine derivative (132), precursor to the enantiomer of the antibiotic thiamphenicol, has been prepared with 92% de and >99% ee using a recombinant D-ThrA from Alcaligenes xylosoxidans, whereas the opposite L-configurated isomer was obtained by i-ThrA catalysis with only low diastereoselectivity [196]. 

Compared to the parent a-amino acids, /9-amino acids are characterized by a much greater chemical diversity (five substitution positions versus three for a-ami-no acids) and conformational versatility. With the extra carbon atom in the backbone, the number of possible configurational isomers for / -amino acids increases dramatically (eight possible configurations versus two for a-amino acids) (Fig. 2.5). 

In the case of the smaller macrocycle [13]aneN4, six configurational isomers exist for [Ni([13]aneN4)]2+ due to the orientation of the N—H protons above or below the plane of the macrocycle. X-ray crystallography and NMR spectroscopy, respectively, revealed that the R,iS,7 ,iS -[Ni([13]aneN4)]2+ form (trans- ) is the preferred one both in aqueous solution and in the solid state.1 43... 

A survey of transition-metal polyazacycloalkane complexes in general, with data also for Cd and Hg species in particular, has been published.180 Structural, 3H and 13C NMR, UV/vis, and conductivity data of several transition-metal complexes, including Cd and Hg complexes, with derivatives of the 16-membered ligand l,9-dithia-5,13-diazacyclohexadecane have been compiled and compared. In particular, conversion between configurational isomers and exchange processes in solution have been discussed.181... 

Similar studies have been conducted on poly(vinyl chloride) (PVC) to assign different IR signatures obtained from different stereo-configurational isomers. The sensitivity of the vC-Cl bond on the stereochemical environment has been utilized using IR spectroscopy. The characteristic vibrations of the vC-Cl bonds are inherently tied in to the configuration as well as the conformation of the... 

Anhydro Sugars as Intermediates in the Interconversion of Configurational Isomers... 

Water-white to light straw colored liquid that is odorless. There are two configurational isomers of this agent that have been studied. 

Colorless to brown oily liquid that is odorless when pure. Crude material may have a violet to purple color. Impurities give it an odor similar to geranium that is detectable at approximately 0.9 ppm. There are two configurational isomers of this agent that have been studied. 

Water-white liquid that becomes pale yellow on contact with air. It has a tarry, extremely irritating and suffocating odor that is detectable at 0.13 ppm. It exists as two configurational isomers. This material is hazardous through inhalation and ingestion, and produces local skin/eye impacts. 

As exemplified in Eq. 8.38, thermal [2 + 2] cycloadditions of 4-vinylidene-2-oxazoli-dinone 287 and alkynes such as phenylacetylene result in the formation of 3-phenyl-substituted methylenecyclobutene 288 [149]. The authors confirmed by NMR analysis that only the Z-configuration isomer was formed. It is worth noting that the [2 + 2] cycloaddition of allenes 287 is not restricted to alkynes even olefins such as acrylic esters or silyl enol ethers furnish the corresponding methylenecyclobutanes... 

The first two classes of selectivity distinguish between constitutional isomers the last one between stereoisomers (configurational isomers and, eventually, conformational isomers). 

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