Conjugate addition activation parameters

An extensive spectroscopic and mechanistic study on the enantioselective Cu/ferrocenyl bisphosphine-catalyzed conjugate addition has been performed. Several parameters such as solvent, nature of the halide present in the Grignard reagent and Cu(I) source, and additives (i.e. dioxane and crown ethers) were identified. These factors directly affect the formation and nature of the intermediate active species, and therefore the selectivity, rate and overall outcome of the reaction. Importantly, the presence of and Br ions in the reaction are essential in order to achieve high selectivity and efficiency. 

The ratio of the reactivities of ions and ion pairs (kp+/kp ) are also included in Table 13. They were determined from kinetic studies of the apparent rate constants at either different acid concentrations which vary the extent of dissociation into free ions, or in the presence of tetrabutylam-monium salts with common counteranions such as perchlorates and triflates. This results in ratios of the reactivities of ions and ion pairs of approximately 6 to 24. However, addition of an equimolar amount of salt to triflic acid may lead conjugation of acid with anions [215], with complete deactivation of the system. Therefore, the lower rate constants of propagation for ion pairs may be partially due to removal of the acid from the system. Thus, the values reported in Table 13 can be considered the upper limit of kp+/kp. The true ratio might be lower, with very similar reactivities for ions and ion pairs as in model systems [4]. Miscalculations of the ratio of reactivities of ions and ion pairs has led to unrealistic values of activation parameters calculated for propagation by ions (Ep = 51 kJmol-, ASP = +54 Jmol- K-1) and ion pairs (Ep = 21 kJmol- ASP = -84-mol-,-K l) [17] the latter values are similar to the overall activation parameters for ionic propagation and are quite reasonable. Extrapolation of Kunitake s data to - 80° C shows ion pairs being 30 times more reactive than ions [17], which contradicts the available experimental data [213]. 

The reactivity of a range of alkenes in addition reactions of peroxyl radicals has been reported. Parameters that described the relationship between the activation energy and enthalpy were calculated. An activation energy of 82 kJ moP was determined for the addition of alkylperoxy radicals to isolated C=C bonds, rising by 8.5kJmor when the alkene was conjugated with an aromatic substituent. 

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