Isomers, rotational

Figure 2-81. The permutation matrices oftwo structures that differ through rotation by 120 T The permutation matrix of the rotated Isomer can be brought Into correspondence with the permutation matrix of the reference isomer by two Interchanges of two ligands (transpositions),...

The steric bulk of the three iodine atoms in the 2,4,6-triiodoben2ene system and the amide nature of the 1,3,5-substituents yield rotational isomers of the 5-A/-acyl-substituted 2,4,6-triiodoisophthalamides. Rotational motion in the bonds connecting the side chains and the aromatic ring is restricted. These compounds also exhibit stereoisomerism when chiral carbon atoms are present on side chains. (R,5)-3-Amino-l,2-propanediol is incorporated in the synthesis of iohexol (11) and ioversol (12) and an (3)-2-hydroxypropanoyl group is used in the synthesis of iopamidol (10). Consequendy, the resulting products contain a mixture of stereoisomers, ie, meso-isomers, or an optical isomer. 

More recendy the cis and trans isomers of the mosquito repellent CIC-4, a mixture of citroneUa isomers, have been separated by preparative hplc and bioassayed for effectiveness (23). Chiral-phase capillary gas chromatography and mosquito repellent activity of some oxazoUdine derivatives of (+)-and ( —)-citroneUal have been studied to find stmcture—activity relationships (24). Several 2-aLkyl- -acetyloxahdines have been synthesized and tested against mosquitoes, with further efforts using nmr to determine the rotational isomers of the more active N-acetyl-2,2-dimethyloxazohdine (25). 

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

Variable temperature NMR studies of [IrCl2(PMe2Ph)4]+ (XXV L = PMe2Ph) show a broad line in the 31P spectrum at room temperature but on cooling to 80°C a mass of sharp lines are observed, owing to three to four rotational isomers (Figure 2.89) the predominant ABCD pattern is... 

Cross Polarization — Magic Angle Sample Spinning NMR Studies. With Respect to the Rotational Isomeric States of Saturated Chain Molecules of Aliphatic Chain Rotational Isomers... 

Oki, M.The Chemistry of Rotational Isomers, Springer-Verlag, Berlin, 1993. 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

Because a double bond between two carbons prevents the carbons from rotating, isomers involving the atoms bonded to the carbons are possible, as shown above with dichloroethylene. Such isomers are called geometrical isomers, in contrast to the structural isomers discussed previously. When the substiuent groups are on the same side of the molecule, the compound is designated the cis- isomer. When the substituent groups are on the opposite side, the compound is the trans- isomer. Like all isomers, cis- and trans-isomers have the same molecular formula, but differ in certain physical and chemical properties. For example, cw-l,2-dichloroethylene boils at 60°C whereas 1,2-dichloroethylene boils at 48°C. 

The occurrence of more than two v(CO) bands in the IR spectrum of Cl2MeSiFe(CO)2Cp has led to the suggestion that conformational or rotational isomers are present, and the simple NMR spectrum is explained by assuming rapid interconversion of the two species 148). Assignments of particular v(CO) bands to the corresponding conformers have been made on the basis of v CO) studies 93, 93a). A similar situation has been observed in the analogous Sn and Ge compounds, and, in the latter, two of the four CO bands disappear at —40° C in agreement with the postulate 193). 

These data can be understood in terms of two rotational isomers of BMA (1 and 2). 

The application of the one-pot Ugi four-component reaction by stirring a mixture of the aldehyde 486, benzyl amine 487, isocyanide 479, and acrylic acid 385 in methanol at room temperature for 36 h afforded the triene 488 as a 74 26 mixture of amine rotational isomers in 80% combined yield. The triene 488 on heating in DMSO at 120 °C for 12 h underwent cycloaddition to give the tricyclic compound 489 as a single diastereomer in 98% yield (Scheme 111). 

An interesting series of high Ts HTMs based on novel indolo[3,2-b]carbazoles has been discovered by the Xerox group [85]. These compounds not only showed the desired hole transport properties and high Ts of 164°C but also display an unusual atropisomerism with two discrete trans- and U.v-rotational isomers (Scheme 3.14), which greatly improves their tendency to form stable amorphous glasses. 

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

In the above instance, the field effect shifts the C = 0 frequency in the rotational isomer (k) and not in the isomer (1). As both isomers are usually found to be present together, therefore, two C = 0 str. absorptions are observed in the spectrum of this compound. 

The concept of atropisomerism developed to a considerable extent following other developments in chemistry, especially those in spectroscopy. Early work by Kohlrausch (4) and Mizushima (3), based on Raman spectra and dipole moment studies, established that rotational isomers—rotamers—must exist in 1,2-dichloroethane. Pitzer established that there are three energy minima when ethane is rotated about its C—C axis (6). Rotamers about single bonds have been found in a wide variety of organic compounds since then, mainly as a result of the application of vibrational spectroscopy to organic molecules (7). 

Compounds of type 111 should, in principle, be resolvable into stable enantiomers if the barrier is high enough. However, these compounds do not carry a functional group convenient for resolution. Oki and associates (162) modified one of the methyls in the isopropyl group to make the rotational isomers dia-stereomeric. They prepared 9-(2-methoxy-l-methylethyl)triptycenes (114) and... 

In the first of four chapters in this volume of Topics in Stereochemistry, Michinori Oki presents a comprehensive review of atropisomerism with special reference to the literature of the past two decades. The review summarizes restricted rotation about sp2-sp2, sp2-sp, and sp3-sp3 bonds and it concludes with an analysis of reactions of isolated rotational isomers. It places particular emphasis on the magnitude of rotation barriers as a function of structure (incidentally identifying some of the largest barriers yet measured to conformer interconversion) and on the isolation of stable single-bond rotational diastereomers. 

Owens, P.K., Svensson, L.A., and Vessman, J., Direct separation of captoprU diastereoisomers including their rotational isomers by RP-LC using a teicoplanin column, 7. Pharm. Biomed. Anal, 25, 453, 2001. 

N. Blechta et al. [63] used LC-NMR experiments with H- Si indirect detection to analyze mixtures of siloxan polymers. Other studies take advantage of the unique ability of NMR to study dynamic processes like isomerization, for example, the interconversion of rotational isomers or enol-keto tautomers [64,65]. 

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