Electronic isokinetic temperature

For the aqua metal ions having pKa around 7, this mechanism is virtually unperturbed by the nature of the metal ion and steric and electronic factors of its ligand envelope as evident from a linear isokinetic plot (A H = AG(f + (10AS, (10 is the isokinetic temperature)... 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

The stereoselectivity in the electron transfer reactions between reduced spinach [2Fe-2S]-ferredoxin and optical isomers of [Co(alamp)py], [Co(promp)H20] and [Co(promp)py] (alamp = iV,W -[(pyridine-2,6-diyl)bis(methylene)]bis[(5)- or (iJ)-alaninate] and promp = iV,W -[(pyridine-2,6-diyl)bis(methylene)]bis[(S )- or (R)-proline]) have been investigated. The stereoselectivities are observed to be temperature dependent, with the large differences in the enthalpies of activation compensated by the entropies in an isokinetic relationship. 

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