Stereochemical freedom

The structural chemistry associated with polysulfide molecules and materials reflects the stereochemical freedom of the sulfur atom as mentioned above, disulfide ligands can coordinate in a number of ways. Similarly, larger rings may adopt a variety of geometries (from X-ray studies) and the evidence that is available suggests that many of the rings are fluxional in solution. Muller has classified and discussed some aspects of this problem for molecular species, but the rationalization of the structures adopted in solid-state materials is not yet complete. ... 

In summary, when using a ligand catalyst ratio of 1.75 1 at pH 5-6 the enantioselectivity of the Diels-Alder reaction between 3.8c and 3.9 is dictated by the activated complexes involving ligand, copper(ir) ion, dienophile and diene. Considering that four different products are formed in this reaction (see Scheme 3.5), at least four different activated complexes are involved However, each of these complexes hus two degrees of freedom that determine the stereochemical outcome of the... 

Notice that "stereoselectivity" means "order" and order means "negative entropy" which, in turn, means "rigidity", "lack of flexibility" or "absence of degrees of freedom". It is always easier to exert stereochemical control in rigid monocyclic or polycyclic systems than in the more flexible open-chain compounds (remember the limitations imposed by Corey s rule number 6 see 7.2.1). 

As opposed to fundamental stereochemists, biochemists cannot choose or design the molecules they plan to investigate from a stereochemical viewpoint. These molecules tend to be characterized by their structural complexity, among other aspects a large number of degrees of conformational freedom, and several elements of stereoisomerism. 

Diastereofacial selection in nucleophilic additions to acyclic aldehydes and ketones is of major importance in synthetic organic chemistry." The greater conformational freedom of acyclic systems makes predictions as to the stereochemical outcome of such reactions more difficult than when the carbonyl group is contained within a cyclic framework. 

As noted earlier, the factors that affect crystal solubility differ substantially from those that affect other solubility boundaries. Foremost among these are the shape of molecules (symmetry and stereochemical structure) and conformational freedom. The thermodjmamic stability of the crystal phase is strongly influenced by the ability of molecules to pack densely together in this phase. Structural features which inhibit this packing may dramatically alter the temperature range of the crystal solubility. An historic example is the effect of the cis double bond in oleates, which lowers the Krafft eutectic significantly relative to that of stearates. The trans double bond has a smaller effect, which parallels the influence... 

VOA presents advantages over the other chiroptical methods, but also it possesses some drawbacks. VCD and ROA can be virtually measured for any molecule, in solid, liquid, and gas phases without the requirement of either UV-Vis chromophores for ECD, derivatizations, or simulations of excited-state wavefunctions. In addition, as these two spectroscopic methods represent global molecular properties, all degrees of freedom (3N-6, where N is the number of atoms) of a molecule can be investigated, at least in theory. Nevertheless, at the same time, it becomes more difficult to extract stereochemical information from VOA spectra, which increases the dependence on the accuracy of the calculations. 

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