Conformational processes

Molecules that are chiral as a result of barriers to conformational interconversion can be racemized if the enantiomeric conformers are interconverted. The rate of racemization will depend upon the conformational barrier. For example, -cyclooctene is chiral. E-Cycloalkenes can be racemized by a conformational process involving reorienting of the... 

Interestingly, the sulfur-linked bis-crown ligand [8] shows an unprecedented cathodic potential shift upon addition of K+ cations to the electrochemical solution (Table 3). It is believed to be a conformational process that causes the anomalous shift of the ferrocene/ferrocenium redox couple and not a through-space or through-bond interaction, as these effects would produce the expected anodic potential shift of the ferrocene redox couple. The origin of the effect may be a redirection of the lone pairs of the sulfur donor atoms towards the iron centre upon complexation. This would increase the electron density... 

One should expect the activation entropy (AS ) to C=C rotation in Case 1 push-pull ethylenes to be negative, since the increase in polarity in the transition state should increase the order in the solvated structure. The effect should increase with increasing difference in polarity between ground and transition states, and also with increasing solvent polarity. These expectations have been completely borne out by experiments (78,140,143), as Table 22 shows. Contrary to what is generally found for conformational processes (144), AS values -20 e.u. are frequently found for C=C rotation in push-pull systems. 

Fortunately, the partial decoupling of the ET and conformational processes afforded by the absence of synchronous events in principle and in practice allows for the identification of an observed decay rate constant. For example, if one constructs a series of systems in which the ET energetics (or electronic coupling) is modified without change in the conformational equilibrium, thus leaving the conformational rates unchanged, then the observed rate constants will be unchanged if the reaction is controlled by a conformational rate, but will vary if this is not so. 

Give the number of different ehemical environments for the magnetic nuclei H and C in the following compounds. Assume that any conformational processes are fast on the NMR timescale unless otherwise indicated. 

Another important conformational process, ring inversion , is best typified by cyclohexane. This molecule undergoes motion in which axial and equatorial ring positions interconvert. 

Special cases of these involving transition states for rotation about single bonds, inversion of pyramidal nitrogen and phosphorus centers and ring inversion in cyclohexane, have been discussed in the previous chapter. The only difference is that these conformational processes are typically well described in terms of a simple motion, e.g., rotation about a single bond, whereas the motion involved in a chemical reaction is likely to be more complex. 

The lowest energy conformational process in 2,2-dimethyl-l,3-dioxocane is the conversion of (433) to (434) via the boat-boat intermediate (BB-1,3) which has C2 symmetry. The methyl groups (A and B) in (433) and (434) have exchanged sites. This process is actually similar to the high energy conformational process in 1,3-dioxocane itself. This crossover of mechanisms is shown diagrammatically in Figure 1. 

Figure 3 Conformational processes in 5-substituted 5,6,11,12-tetrahydrodibenz[6,/]azocines (456c-e). The boat to chair process takes place via the transition state (462), while the boat-ring inversion has the transition state (463)...

So far we have not been able to treat chains with bond correlations in more than one dimension. The introduction of more detailed or realistic models of local conformational processes, such as those of Reneker34 or of Schatzki,35 has, therefore, not been feasible. We may remark that the theory of dielectric relaxation by Work and Fujita,36 which applies Glauber s methods25 to delayed (dynamic) correlations between chain dipoles, is also in essence a one-dimensional affair. 

Electroswitching of structure takes place when a redox change induces a reversible structural or conformational process in a molecule, such as an electrochemically activated intramolecular rearrangement [8.259]. On the supramolecular level it consists of the electroinduced interconversion between two states of different superstructure. A case in point is the reversible interconversion of a double-helical dinuclear Cu(l) complex M2L22+ [8.260] and of a mononuclear Cu(ll) complex ML2+ in a sequential electrochemical-chemical process [8.261] ... 

An enzyme can deactivate irreversibly for two kinds of reasons (i) conformational processes, such as aggregation (intermolecular), or incorrect structure formation (intramolecular), such as scrambled disulfide bond formation between wrong side chains, and (ii) covalent processes, such as reduction and thus destruction of disulfide bonds, deamidation of asparagine (Asn) or glutamine (Gin) side chains, or hydrolysis of (usually) labile asp-X bonds in the protein sequence. 

Deactivation by conformational processes, such as aggregation, an intermolecular process, or through incorrect structure formation, an intramolecular process. The latter can happen if the wrong cysteine side chains form disulfide bonds, which renders the protein structure inactive. 

When the reactivity of the c -unsaturated carbonyls is considered, their stereochemistry is very important. However, literature data concerning this problem are very rare because of the high rate of isomerization and conformational processes in enones. Some examples of such an influence are discussed in this monograph as well. 

Other phenomena besides conformational processes can also lead to multizoning effects with polypeptides and proteins when they interact with adsorptive HPLC sorbents. The so-called split peak effect is probably the easiest of these phenomena to be identified and steps taken to remedy. The split peak effect is very often seen in HP-BAC, RP-HPLC, and HP-HIC and to a lesser extent in the HP-IEX of proteins.325-327 This effect is manifested by the presence of a weakly retained (or occasionally as a nonretained peak) and a more strongly retained peak with the bound-to-free ratio between the weakly retained to strongly retained species dependent on the diffusion and adsorption kinetics. An extreme case of the split peak effect involves the weakly interacting component elution in or near to the column breakthrough volume. In this case, the amount of protein in the breakthrough zone is influenced by the nominal pore diameter and ligand density of the sorbent, the flow rate, and the injection volume. This effect can be circumvented by... 

After a short qualitative introduction to the principles of dynamic nuclear magnetic resonance spectroscopy , the proposed interconversion processes for cycloheptanes and cycloheptenes are explained in detail. According to calculations, the most favourable conformation for cycloheptanes seems to be the twist chair cycloheptenes prefer the chair form. Possible conformational processes for chair and boat forms are discussed and illustrated. 

Ken has continued to explore and influence new areas of chemistry. For example, he has recently made an important discovery in molecular recognition. His finding that a conformational process ( gating ) is the rate-determining step in complex formation and dissociation in Cram s hemicarceplexes has produced a new element in host design. Ken s investigations of the stabilities and mechanisms of formation of Stoddart s catenanes and rotaxanes have already led to discovery of gating phenomena in and electrostatic stabilization of these complexes. 

Figure 5 Schematic representations of the conformational processes allowing the interconversion between four limit conformers of calix[4]arenes.

---