Curtin-Hammett relation

It is well known that we usually deal with a mixture of rotamers in chemical reactions. If they are interconverted rapidly at a given temperature (eq. [6]), then the Curtin-Hammett relation (171) (eq. [7]) will explain the product dis-... 

Applied to the use of reactions to locate the position of a probe in a middle, the Curtin-Hammett principle suggests that not only the site of the probe but its reactivity at various sites must be considered. For example, if the reactivity of a probe at various sites along a detergent chain were independent of site, reactivity would reflect probe position, i.e., the extent of product formation would related to the concentration of probe at each site. If on the other hand, if the reactivity of the probe were much greater near the head of the detergent chain or near the tail... 

It is noted that a general discussion of the Curtin-Hammett principle (product composition is related to the relative concentrations of the conformers) may be of value in any discussion of this equilibrium reaction. For this process to work, some estimation of F° (hereafter called H°) must be made. In connection with the... 

If conformations resembling the half-chair conformer 58 ( H4) and the twist conformers 59 and 60 were also responsible for the reactivity of the related oxocarbenium ion, nucleophilic addition to the oxocarbenium ion should be a-selective. Nucleophilic attacks from the p-face to conformers analogous to 56-58 should be disfavored over the corresponding attacks from the a-faces, because they would lead to transition states with 1, 3-diaxial interactions (Fig. 4.24). Alternatively, if any of these modes of nucleophilic attack were slow relative to stereoelectronically controlled addition to the higher energy half-chair conformer 60, a-selectivity would still be predicted based upon the Curtin-Hammett principle [82, 83]. This interpretation was supported by the nucleophilic substitution reaction of acetate 66, which yielded the a-product 67 (Figs. 4.25 and 4.26). 

These examples serve to illustrate some of the ways in which stereochemistry can affect reactivity. In later chapters and in Part B, other examples will be given as specific reactions are discussed. One other important point relating conformation to reactivity will be raised in Chapter 4. This is the Curtin-Hammett principle, which states an important limitation on the validity of arguments that attribute reactivity differences to conformational effects. 

The first scenario is relatively simple and relates to the previous example of group transfer. If interconversion of the diastereomeric olefin complexes I and T is slow, relative to the rates of conversion of the olefin complexes to product, the enantioselectivity-determining step is binding the prochiral olefin faces to the metal (Figure 14.12A). In contrast, if interconversion of the diastereomeric olefin complexes is significantly faster than their reaction to form product, the enantioselectivity-deternrining step will be the reaction to form product (Figure 14.12B). This latter scenario is stated to meet "Curtin-Hammett conditions."... 

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