Conformation anti-gauche

Ethanediol, like n-butane, exists as an equilibrium mixture of two distinct conformers anti (OCCO dihedral angle = 180°) and gauche (OCCO dihedral angle 60°). 

The conformational anomeric effects design the contrasteric effects observed in acetals which render the more sterically encumbered gauche/ gauche conformers more stable than their anti/gauche and anti/anti conformers. Such effects were first evidenced by Jungins in 1905 and rediscovered by Edward in 1955 and by Lemieux and Chiu in 1958. They observed the higher stability of alkyl a-D-glucopyranosides in comparison with their (3-anomers (Fig. 5).8... 

Obviously Meng et al. [5] have reached an anti-gauche (AG) conformation that probably just is local but not a global minimum. The hydrogen bonding has previously been detected by NMR relaxahon studies on [C4Cilm] [PFJ and related systems in the liquid state [5,6,75,76]. 

FIGURE 2. Newman projections and energies of the MP2/6-31G calculated geometries of the anti, gauche and ortho conformers of permethyltetrasilane... 

The anti-form thus seems generally to predominate for the molecules in Table 5 if no extra strain caused by substituents has been introduced. If substituents are introduced, for example in butadiene, thegaucfte-conformer may prevail. In other cases anti/gauche-equilibria may occur. 

Research on polysilane oligomers and polymers has been the major driving force for the paradigm shift to a new model of rotational conformations.57 58 Until very recently, the structures of polysilanes were mainly described in terms of the older anti-gauche model. This is now known to be incorrect, and as a result nearly 20 years of research will probably need to be reinterpreted. 

Consider a linear polymer, (MX2) . If the substituents X are small compared to the distance M-M between the atoms in the main polymer chain, then the conventional anti-gauche model is valid. This is the case for hydrocarbons, and probably for other (MH2) polymers. However, if the van der Waals radius of the X group becomes a little larger relative to the M-M distance, then repulsions arise between the X atoms on the M atoms in 1,3 positions along the chain. The critical value for such interference is about RX/RM M = 0.8. Beyond this value the anti conformation is no longer a minimum,... 

In a revealing experiment, Toyoda and Fujiki showed that the UV absorption and emission spectra of (PhMeSi) are virtually identical to those of the copolymer, (PhMeSi)o.95 [PhSiCH2C (H)(CH3)CH2CH3]005 . The latter is known to be helical in solution from its CD spectrum. The implication is that (PhMeSi) , which was for many years described in terms of the anti-gauche model, is actually helical in solution also.69 The evidence which is accumulating now suggests that nearly all polysilanes adopt helical conformations in solution. 

S. S. Bukalov, L. A. Leites, R. West and T. Asuke, Macromolecules, 29, 907 (1996). This publication employed the older anti-gauche formalism, but here the data will be interpreted using the newer conformational model. 

It has been found that cyclopropyl ketone exhibits a conformer mixture The preferred component has been identified as the bisected s-c/s conformer At the present time, however, it is not clear whether the second conformer (less stable) corresponds to the bisected s-trans form or anti-gauche form. On the other hand, on the basis of a theoretical analysis (on the semiempirical INDO-level) and a proton NMR investigation for the cis-trans isomeric 2-methylcyclopropyl-l (methyl) ketones 15 and 16 it was suggested that the trans compound 15 exists as an equilibrium of the bisected s-cis and s-trans conformers. The s-c/s and s-trans conformers are separated by a rotational barrier of 5.7 kcal mol ... 

A number of simple diarsines have been structurally investigated and, although individual molecules are similar to those of the corresponding distibines, the diarsines do not show the extended chain structures and consequent thermochromism often found for the latter. The geometry at the two arsenic atoms is pyramidal, though the angles at arsenic can be asymmetric. The substituents occupy anti (gauche) conformations and in some cases the molecules have imposed Cj symmetry. Parameters for four of the compounds are summarized in Table 2. 

By analogy with 1,2-disubstituted ethanes it is reasonable to assume that 1,2-disubstituted disilanes will also exist as mixtures of two conformers, anti and gauche. If the barrier separating the two conformers is considerably larger than 2.5 kJmol (= kT at room temperature), they will exist as well-defined species on the time scale which is typical for infi-ared and Raman spectroscopy Energy... 

Although it is well known that cyclic ethers are readily oxidized to the corresponding lactols or lactones, their asymmetric desymmetrization was not examined until quite recently. However, desymmetrization of prochiral or meso-cydic ethers is expected to be a useful tool for organic synthesis, since many prochiral or raeso-cyclic ethers are available in bulk. Recently, Miyafuji and Katsuki have reported the desymmetrization of 4-terf-butylcyclotetrahydropyran and meso-tetrahydrofurans with the chiral (salen)manganese(III) complex 13 as catalyst (Scheme 9) [24,25]. The oxidation of the former shows only the modest enantioselectivity, while the reaction of the latter exhibits excellent enantioselectivity. The low enantioselectivity (48% ee) observed in the oxidation of 4-ferf-butyltet-rahydropyran has been attributed to the participation of enantiomeric twist-boat conformers. Although 4-ferf-butyltetrahydropyran exists in an equilibrium mixture of chair and enantiomeric twist-boat conformers and the equilibrium ratio of the latter is very small, the latter is considered to be more reactive than the former for stereo electronic reasons. One of a-C-H bonds in the twist-boat conformer almost eclipses the -orbital while those in the chair conformer are gauche or anti to the -orbital. 

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