Measurements of Insertion Kinetics

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

Pirrang, Liu, and Morehead [22] have elegandy demonstrated the application of saturation kinetics (Michaehs-Menten) to the rhodium(II)-mediated insertion reactions of a-diazo /9-keto esters and a-diazo /9-diketones. Their method used the Eadie-Hofstee plot of reaction velocity (v) versus v/[S] to give and K, the equilibrium constants for the catalytic process. However, they were unable to measure the Michaelis constant (fC ) for the insertion reactions of a-diazo esters because they proved to be too rapid. 

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

The mechanism of oxidation of alkanes with dimethyldioxirane has been examined by measurement of the primary kinetic isotope effect for the oxidation of cyclohexane and methylcyclohexane in solution and in the gas phase. These experiments indicated that the major products (cyclohexanol and methylcyclohexanol) are probably formed via an electrophilic oxygen-insertion reaction while minor by-products may arise from radical reactions.90... 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

A concerted insertion mechanism with highly ordered transition state, close to three-centered, was corroborated by examination of the kinetic isotope effect, which was measured by competitive GeC insertion reactions into the C—Cl bonds of labeled 14CCl4 and 12CC14. The value obtained, ku/k 2 = 1.01 0.01, is very close to that calculated from the stretching frequencies and the ratio of the masses and moments of inertia of the isotopic molecules for a synchronous reaction (0.993) and differs significantly from the calculated value for a dissociative mechanism (0.900)52. 

FIGURE 8.3 Comparative reactivation kinetics of soman-inhibited human butyrylcholinesterase single mutant G117H ( ) and double mutant G117H/E197Q ( ). Note that the recovery rate of the double mutant is very fast (with reaction rates of 77,000 and 128,000/min for the PsCs and PsCr isomers of soman, respectively), whereas the single mutant does not recover measurably. The insert shows that reactivation of the double mutant... 

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