Rotation, conformational isomers

One of the reasons that the enantioselectivity of 5-pyrimidyl alkanol is higher than that of 3-pyridyl alkanol may be due to the presence of symmetry in the pyrimidine ring. When the bond between the asymmetric carbon and the pyridine ring rotates, conformational isomers of pyridyl alkanol will be formed (Figure 9.2). However, with pyrimidyl alkanol, the rotation does not form conformational isomers. Thus, the reduced number of possible conformational isomers may enable 5-pyrimidyl alkanol to serve as a highly enantioselective asymmetric autocatalyst. 

Conformational Isomers. A molecule in a conformation into which its atoms return spontaneously after small displacements is termed a conformer. Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformational isomers (see Fig. 1.1). A pair of conformational isomers can be but do not have to be mirror images of each other. When they are not mirror images, they are called diastereomers. 

The different arrangements of atoms that result from bond rotation are called conformations, and molecules that have different arrangements are called conformational isomers, or conformers. Unlike constitutional isomers, however, different conformers can t usually be isolated because they interconvert too rapidly. 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Even though we define the atropisomerism as above for present purposes, there remain some ambiguities. sym-Tetrabromoethane was obtained in different modifications according to the method of crystallization at low temperature (13). These were found by spectroscopy to correspond to retainers. Similar situations occur in other alkyl halides and acetates (14,15). Such cases will not be included in the discussion, mainly because crystalline atropisomers are isolated at far lower temperatures than die ambient, and their barriers to rotation have not been determined by equilibration. Also excluded is the isolation of chlorocyclohexane (16). The isolation of the equatorial and axial conformational isomers was possible only by crystallization of the former at - 150°C, although it was possible to observe equilibration between the equatorial and the axial forms at higher temperatures. 

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. 

During transformation from the neutral molecules to the corresponding anion-radicals, the rate of the fragment rotation, relative to one another, decreases. This also results in the nonequivalence of the meta and ortho protons. Thus, 3-acetylpyridine gives two different anion-radicals as conformational isomers (Cottrell and Rieger 1967). In the case of 3-benzoylpyridine anion-radical, the phenyl group rotates freely about the carbonyl center, whereas the rotation of the pyridyl group is limited. The ESR spectrum shows that the spin density in the phenyl ortho positions is half of that... 

Conformational isomers, or conformers, interconvert easily by rotation about single bonds. Configurational isomers interconvert only with difficulty and, if they do, usually require bond breaking. We shall study these in turn. 

Bis(2-arylindene)zirconium dichlorides have been studied for the purpose of synthesizing isotactic-atactic stereoblock polymers [Busico et al., 2001 Lin et al., 2000 Lin and Way-mouth, 2002 Nele et al., 2000], Without the phenyl substituents, bisindenylzirconium dichloride yields atactic polypropene because there is rapid rotation of the r 5-ligands. The 2-phenyl substituents in bis(2-arylindene)zirconium dichloride interfere with each other suf-ficently that rotation is slowed to produce isotactic-atactic stereoblock polypropene. Three conformational isomers (conformers) are possible in this metallocene (Eq. 8-54). There is... 

M. Guerra, G. F. Pedulli, M. Tiecco, and G. Martelli, Conformational isomers and rotational harries in the ketyl radicals of thiophen and thienothiophens. J. Chem. Soc., Perkin Trans. II, 562 (1974). 

Atoms within a molecule move relative to one another hy rotation around single bonds. Such rotation of covalent bonds gives rise to different conformations of a compound. Each structure is called a conformer or conformational isomer. Generally, conformers rapidly interconvert at room temperature. 

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