Curtin product distribution

If all the cyclizations and cleavages are rapid relative to H-atom transfer, then the product distribution depends on the equilibrium constant between (49) and (50) and the rate of hydrogen atom transfer to (49) and (50) (Curtin-Hammett kinetics). 

As a consequence of the Curtin-Hammett principle, one of the most useful means of investigating a mechanism is (not altogether unexpectedly) to measure the distribution of product(s). One prerequisite for measuring product distribution is a proper assay. 

Olefin polymerization is not a simple single-step olefin insertion process it involves complex kinetics and multiple equilibria. Insertion is preceded by olefin complexation (Scheme 7.2a) or olefin complexation coupled with counterion displacement (Scheme 7.2b). In view of this complexity, how can the use of barrier differences be justified The use of barrier height differences arises from the Curtin-Hammett principle. The Curtin-Hammett principle is a kinetic analysis that describes product distributions for reactions involving a pair of equilibrating reactants or intermediates, each capable of forming a product. The stereodifferentiation of general isotactic polymerization can be... 

FIGURE 7.8 Potential curves for Curtin-Hammett/Winstein-Holness kinetics limiting cases (a) LI limit where the free energy difference between barriers dictates product distribution and (b) L2 limit where the free energy difference between equilibrating intermediates dictates the product distribution. 

If it is assumed that the Curtin-Hammett principle applies, one need only to compare the energies of the minima on the solid and dashed curves to be able to predict the structure of the major product. These curves also allow a direct comparison of Cram s, Cornforth s, Karabatsos s and Felkin s model for 1,2 asymmetric induction. Both Figures show the Felkin transition states lying close to the minima. The Corn-forth transition states (Fig. 3) are more than 4 kcal/mol higher and should contribute little to the formation of the final products assuming a Boltzmann distribution for the transition states, less than one molecule, out of a thousand, goes through them. Similarly, Fig. 4 shows the Cram and Karabatsos transition states to lie more than 2.7 kcal/mol above the Felkin transition states, which means that they account for less than 1% of the total yield. 

According to the Curtin-Hammett principle, the ratio of the distribution between two products depends only on their rates offormation (Figure 9.1). If the two products are conformers, the energy barrier between the two states tends to be small and the ratio between the two products is an indicator of the free enthalpies of the transition states. The Curtin-Hammett principle can be applied when two conformations, tautomers, or isomers of a starting material are in rapid equilibrium compared to the rate of a reaction. In this case, the product ratio provides no information about which conformation, tautomer, or isomer was present in the starting material. 

This interpretation could be complicated by the conformational equilibrium Hs/ H4 which gives to 3,4,6-tri-O-acetyl-D-glucal 10, for instance, a 40% contribution of the (d) conformer with all its substituents axial. A vinylogous anomeric effect could explain this unusual distribution [28]. However, assuming a lower energy barrier for this conformer equiUbrium in comparison to the subsequent reactions, the Curtin-Hammett principle can be apphed and the product ratio will be governed by the activation energy of the electrophilic addition, rather than by the actual concentration of conformers. 

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