Fluxional conformers

Dynamic Processes Fluxional, Conformational, and Exchange Equilibria... 

UB3LYP theory predicts four minima of Sg which possess Cj, Cjhy O2 d C2 symmetries. At the UMP2 level of theory, no stationary point corresponding to the C2 minimum can be located and two new local minima with and D2 symmetries appear. The Sg conformers are found to be very prone to pseudorotation and are predicted to interconvert readily. For this reason, Cioslowski et al. refer to Sg as a fluxional species [93]. Interestingly, they found that the structures corresponding to local minima are not directly interconvertible. 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

Besides the phosphite ligands based on BINOL, phosphite ligands based on bisphenol are also used in rhodium-catalyzed hydrogenation. These ligands are shown in Scheme 28.7 and consist of a bisphenol with different substituents on the 3,3, 5,5, and 6,6 -positions. The ligands without substituents on the 6,6 -positions are only fluxionally chiral. The use of readily available chiral alcohols (21 aa-21 aj) such as menthol in combination with bisphenol was thought to induce one of the bisphenol conformations in preponderant amounts [49]. The... 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

The principle aim of the reported studies was to model structures, conformational equilibria, and fluxionality. Parameters for the model involving interactionless dummy atoms were fitted to infrared spectra and allowed for the structures of metallocenes (M = Fe(H), Ru(II), Os(II), V(U), Cr(II), Cofll), Co(ni), Fe(III), Ni(II)) and analogues with substituted cyclopentadienyl rings (Fig. 13.3) to be accurately reproduced 981. The preferred conformation and the calculated barrier for cyclopentadienyl ring rotation in ferrocene were also found to agree well with the experimentally determined data (Table 13.1). This is not surprising since the relevant experimental data were used in the parameterization procedure. However, the parameters were shown to be self-consistent and transferable (except for the torsional parameters which are dependent on the metal center). An important conclusion was that the preference for an eclipsed conformation of metallocenes is the result of electronic effects. Van der Waals and electrostatic terms were similar for the eclipsed and staggered conformation and the van der Waals interactions were attractive 981. It is important to note, however, that these conclusions are to some extent dependent on the parameterization scheme, and particularly on the parameters used for the nonbonded interactions. 

The unsymmetrical diene ligands in 85b and 85c may form two dia-stereomers with the less substituted double bond in the cis or trans configuration with respect to the Mo—C a bond. For 85b, only the isomer with the less substituted double bond cis to this bond is found. Compound 85c forms both isomers, but the cis isomer is distinctly preferred. In contrast with the fluxional >j2-diene complexes 84a-84c, compounds 85a-85c do not show temperature-dependent H-NMR spectra for the //4-diene ligands. However, this does not disprove a hindered rotation of the f/4-diene ligands when one of the two conformers (o or u) is energetically more favored than the other. 

H-NMR spectroscopy is particularly useful for studying DNA because of the well-resolved base and H(l ) regions of the spectrum. Additional information can be gathered from studies with nuclei such as 31P, 13C, 15N, and 195Pt. Because DNA is conformationally fluxional, often the isolated crystallized form is A-form DNA, whereas a variety of physical methods show that DNA is normally B-form in aqueous solution. Thus, solution studies are particularly important for DNA. 

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