Curtin Hammett equilibrium constant

The efficient kinetic resolution of chiral aminoalkenes has been achieved utilizing the binaphtholate complexes (R) 38 Ln (Table 11.3) [52, 124]. Various chiral amino pentenes were kinetically resolved with resolution factors/(defined as f= x fefast/fesiow, where is the Curtin Hammett equilibrium constant between the two diastereomeric substrate/catalyst complexes and kfast/fesiow being the ratio between the faster and the slower reaction rate constant) as high as 19 and enantiomeric excess for recovered starting material reaching >80% ee at conversions dose to 50%. The... 

Quantitative data for the difference in complexation of ethene and CO to hydrocarbylpalladium(dppp)+ were reported by Brookhart and co-workers [15,33], The equilibrium between CO and ethene coordination amounts to about 104 at 25 °C. Multiplied by the concentrations of the two gases and the two individual rate constants for the insertion they calculated that the ratio of CO insertion versus ethene insertion is about 105 in an alkyl-palladium intermediate under Curtin-Hammett conditions, that is to say fast exchange of coordinated CO and ethene ligands compared to insertion reactions. Figure 12.9 summarises this. 

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). 

The synthesis of amino acid esters can be carried out enantioselectively when optically active EBTHI zirconaaziridines are used. After diastereomeric zir-conaaziridines are generated and allowed to equilibrate (recall Scheme 3), the stereochemistry of the chiral carbon center in the insertion product is determined by competition between the rate constants kSSR and ksss for the epimerization of zirconaaziridine diastereomers and the rate constants [EC] and ks[EC] for ethylene carbonate (EC) insertion (Eq. 31) [43]. When kR[EC] and ks[EC] are much greater than kSSR and ksss> the product ratio reflects the equilibrium ratio as shown in Eq. 32. However, the opposite limit, where epimerization is much faster than insertion, is a Curtin-Hammett kinetic situation [65] where the product ratio is given by Eq. 33. 

The Curtin-Hammett principle (see below) applies to this scheme when the rate constants for product formation K and 2 are small relative to rate constants for interconversion over the reactants and intermediates k, k k- and k-2 (in other words, when I j and 1 2 ate in rapid equilibrium). A kinetic analysis of such a process shows that the selectivity depends only upon the free-energy difference between the transition states [ZMSfl and [ -f f]. The ratio of the diastereoi-someric primary products is given by the same relationship as before ... 

A quantitative description of this behavior is given in Figure 4. Whereas the competition constant observed in presence of excess BCI3 can be derived from the known addition rate constants of the two carbenium ions (AAG " AGi - AG2 ), a Curtin Hammett situation is encountered in presence of catalytic amounts of Lewis acid. Now, the ratio of the two individual rate constants has to be multiplied with the equilibrium constant for the ionization (AAG AGj " - AG2 " + AAG ). 

---