Activation parameters intramolecular electron transfer

Table I. Intramolecular Electron Transfer Rates and Activation Parameters ...

Amjad and McAuley (1974) have investigated the oxidation of malic (2-hydroxy-butane-1,4-dicarboxylic) acid in perchloric add. The reaction occurs at stopped-flow lifetimes and is unaffected by either Ce(III) or nitrate. The Michaelis-Menten plot is linear with finite positive intercept indicating precursor complex formation. The resolved values for the formation constants are comparable to those reported for the hydroxy and keto monocarboxylates, suggesting that the second carboxylate group is not bound in the activated complex. The intramolecular electron transfer rate parameters are less than those reported by Hanna and Fenton for the cyclic a-hydroxycarboxylic acids. 

The stopped-flow technique has been used to study the anaerobic reduction of fungal laccase B by hydroquinone, ascorbate, and ferrocyanide, for which a model is formulated, and the reduction of caeruloplasmin by the hydrated electron has been followed by pulse radiolysis. The latter technique has also been used to study the inhibition of another copper-containing enzyme, superoxide dismutase, by cyanide ions. Comparison of the rates and activation parameters for the reduction by chromous ion of blue copper in laccase, stellacyanin, and spinach and French-bean plasto-cyanins indicates that reduction of the Cu(614) site in laccase may occur by intramolecular electron transfer from one of the Cu(330) sites. The value of 17.4 kcal mol for the activation enthalpy associated with the reduction of cytochrome c by the same species is consistent with a mechanism in which major conformational changes in the protein accompany electron transfer. 

The rate and activation parameters for the intramolecular electron transfers from Os(II)(NH3)5 to Ru(III)(NH3)5 across oligoproline peptide bridges have been determined. The rate constants decrease from 3.1 x 10 s to 50 s as the number of proline units is increase from 1 to 4, and the metal-metal separation distance increases from 12 to 21 A. A time-correlated single photon counting method was employed to investigate the kinetics of the forward [ Ru(II)- Rh(III)] and reverse [Ru(III)<-Rh(II)] intramolecular electron transfer reactions within covalently linked Ru/Rh polypyridine complexes in aqueous, acetonitrile, and methanol solvents. The rate constants for the outer-sphere electron transfer reactions of [Ru(edta)pyz] and... 

Stellacyanin from R. vernicifera has been modified by coordination of [Ru(NH3)5] " to surface histidines at positions 32 and 100. The rate constant and activation parameters for the intramolecular electron transfer from Ru(II) to the stellacyanin Cu(II) have been determined, and were found to be in good agreement with Marcus theory. The replacement of methionine by lysine at position 44 in P. aeruginosa azurin by site-directed mutagenesis results in a decrease in the Cu(I)/Cu(II) electron self-exchange rate constant (36 °C) from 1.3xl0 M... 

Forward and reverse rate constants and activation parameters have been determined for the intramolecular electron transfer (13.0 A) in several ruthenium-modified (histidine 39) zinc cytochrome c complexes.These rate constants are 3 times as large as the values for the Ru(His-33) analogs (13.3 A), with slightly greater electronic coupling between the metal centers. The temperature dependence of the rate constant for electron transfer in the Ru(His-33) modified cytochrome c complex has been measured over the 10-200 K range.The rate is independent of temperature (3.6 s" ) between 10 and 100 K, above which two regions of Arrhenius-like behavior (transition at about 150 K) are observed. 

Photosensitised [2+2] cycloreversions have been reported in the aryl cage compounds (115), yielding the dienes (116). Analogous cycloreversions have also been observed in cis- and trans-bicyclo[5.2.0]non-8-enes.Intramolecular photosensitised pyrimidine dimer splitting in the indole-pyrimidine (117) is remarkably solvent dependent and is thought to proceed via electron transfer from the excited indole moiety. The relative values of the activation parameters in photosensitised pyrimidine dimer splitting have been determined. ... 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

Overall, these early studies of carboxylate-assisted intramolecular C-H activation established the key features of these AML A/CMD processes, where an electron-deficient metal center works in concert with a pendant carboxylate base to promote C-H activation. This is most evident when an agostic intermediate is involved and such species also rationalize how these systems can also perform C(sp )-H bond activation. Whether C-H activation is achieved as a one- or two-step process appears rather system dependent. Alternative mechanisms, for example, proton transfer onto a halide ligand, oxidative addition, or AMLA-4 processes involving proton transfer onto the inner (Pd-bound) oxygen of the carboxylate were all ruled out. Likewise, no evidence for S Ar processes had been reported. Subsequent work was set against this background and considered the various other parameters that may affect the C-H bond activation process. 

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