Kinetic advantages

In studies on Pt dotted silicon electrodes, PMC measurements revealed that tiny Pt dots increased the interfacial charge transfer compared with bare silicon surfaces in contact with aqueous electrolytes. However, during an aging effect, the thickness of the oxide layer between the silicon and the platinum dots gradually increased so that the kinetic advantage again decreased with time.11... 

Sumegi, B., Porpaczy, Z., Alkonyi, 1. (1991). Kinetic advantage of the interaction between the fatty acid P-oxidation enzymes and the complexes of the respiratory chain. Biochim. Biophys. Acta 1081, 121-128. 

Either reaction b or reaction d would preclude subsequent formation of (PhH)2Cr. After the initial period of chemical and thermal excitation, the CO would enjoy a strong kinetic advantage over the ring, both because of its small size and because of its favorable 7r-electron distribution, so that reaction d would be preferred over c, and f would be preferred over e. 

Tne kinetic advantages of five-membered rings over other izes are well illustrated by the radical reaction leading to cyclic amine (3) used on p T 247. Chlori-... 

Wang Y, Li L, Hu L, Zhuang L, Lu J, Xu B. 2003. A feasibility analysis for alkaline membrane direct methanol fuel cell Thermodynamic disadvantages versus kinetic advantages. Electrochem Commun 5 662. 

Su YO, Kuwana T, Chen SM. 1990. Electrocatalysis of oxygen reduction by water-soluble iron porphyrins. Thermodynamic and kinetic advantage studies. J Electroanal Chem 288 177. 

The kinetic advantages of IMDA additions can be exploited by installing temporary links (tethers) between the diene and dienophile components.132 After the addition reaction, the tether can be broken. Siloxy derivatives have been used in this way, since silicon-oxygen bonds can be readily cleaved by solvolysis or by fluoride ion.133 The silyl group can also be used to introduce a hydroxy function by oxidation. 

The differences in the rate constant for the water reaction and the catalyzed reactions reside in the mole fraction of substrate present as near attack conformers (NACs).171 These results and knowledge of the importance of transition-state stabilization in other cases support a proposal that enzymes utilize both NAC and transition-state stabilization in the mix required for the most efficient catalysis. Using a combined QM/MM Monte Carlo/free-energy perturbation (MC/FEP) method, 82%, 57%, and 1% of chorismate conformers were found to be NAC structures (NACs) in water, methanol, and the gas phase, respectively.172 The fact that the reaction occurred faster in water than in methanol was attributed to greater stabilization of the TS in water by specific interactions with first-shell solvent molecules. The Claisen rearrangements of chorismate in water and at the active site of E. coli chorismate mutase have been compared.173 It follows that the efficiency of formation of NAC (7.8 kcal/mol) at the active site provides approximately 90% of the kinetic advantage of the enzymatic reaction as compared with the water reaction. 

Detailed accounts of fibers and carbon-carbon composites can be found in several recently published books [1-5]. Here, details of novel carbon fibers and their composites are reported. The manufacture and applications of adsorbent carbon fibers are discussed in Chapter 3. Active carbon fibers are an attractive adsorbent because their small diameters (typically 6-20 pm) offer a kinetic advantage over granular activated carbons whose dimensions are typically 1-5 mm. Moreover, active carbon fibers contain a large volume of mesopores and micropores. Current and emerging applications of active carbon fibers are discussed. The manufacture, structure and properties of high performance fibers are reviewed in Chapter 4, whereas the manufacture and properties of vapor grown fibers and their composites are reported in Chapter 5. Low density (porous) carbon fiber composites have novel properties that make them uniquely suited for certain applications. The properties and applications of novel low density composites developed at Oak Ridge National Laboratory are reported in Chapter 6. 

An example of how cyclic voltammetry can be used to apply the kinetic advantage method is summarized in Figure 3.29. The goal is to investigate the reaction of low-valent iron porphyrins, Fe1 and Fe°, with an alkyl halide. 

Use of the kinetic advantage method thus points clearly to the occurrence of chemical catalysis with the low-valent metalloporphyrins. This is confirmed by repeating, with iron(I) octaethylporphyrin and cobalt (I) etioporphyrin, the stereochemical experiments carried out earlier with the anion radical of 1,4-diacetylbenzene. Complete stereospecificity is observed in both cases The meso isomer of 4,5-dibromooctane is converted totally into the c/.v-olcfin the d,l isomer is converted totally into the trans-olefin. The reaction again exhibits a clear antiperiplanar preference. 

The kinetic advantage to the inner-sphere pathway is nicely illustrated in Figure 3 in which the kinetic data are treated as one would treat outer-sphere electron transfer reactions. Thus, for reactants designated with a and b subscripts,... 

The use of outer-sphere parameters predicts a much larger activation barrier (or smaller rate constant) than is actually observed. It appears that much of this kinetic advantage derives from differences in the equilibrium constants for outer-... 

Among comparative kinetic studies, the kinetic advantage method has been used systematically in several cases. It has been developed for the first time for investigating the ET versus 8 2 problem in the reaction of iron(i) and cobalt(ii) porphyrins with primary butyl halides (Lexa et al., 1981), yielding the corresponding a-butyl-iron(iii) and cobalt(iii) complexes according to the overall reaction (142). 

The kinetic advantage method, again using aromatic anion radicals as reference ET reagents, combined with the investigation of steric constraints at the electrophilic carbon centre, has also been used for examining the reaction of alkyl halides with several purely organic nucleophiles. The... 

Azido aldehydes and a-protected amino aldehydes have been used to incorporate the nitrogen in the aldolase-catalyzed reaction (for an extensive review on aldolase-mediated synthesis of iminocyclitols, see [28] and references therein). The steri-cally unhindered azido and N-formylamino aldehydes display a marked kinetic advantage over analogs having larger and/or poor water-soluble N-protecting groups [29]. 

In contrast, 1,1-difluorohexa-l,5-diene (3) is ca. 5 kcal - mof1 less stable than its 3.3-difluoro isomer 4, a thermodynamic driving force that also translates into a kinetic advantage.3... 

Kinetic studies by Doering and his collaborators at Harvard150-154 based on five sets of chiral 1,2-disubstituted cyclopropanes, with 1-cyano, 2-(phenyl or propen-2 -yl or -(E)-propenyl or phenylethynyl) (3) and 1 -phenyl-2-(propen-2 -yl) (4) substitution, established the ralative rotational propensities of these substituents and tested the proposition that they might be related to substituent moments of inertia. In all of these cases, the balance between one-center and two-center epimerizations from a trans isomer, reflected in (kt + k2) kl2, was fairly constant, ranging from 1.4 1 to 2.1 1. The kinetic advantages for one-center epimerizations at cyano-substituted carbons for the four cases studied were modest and not especially system-dependent the k, k2 ratios were 2.5,2.2,2.4 and 1.8, thus establishing that rotational propensities are not dictated by some simple function of the moments of inertia of substituents. 

Unusual non-hydrolyzing homonucleoside analogs 95 have been synthesized from a suitably adenine substituted lactaldehyde precursor 96 [308], Due to its thermodynamic (and possible kinetic) advantage, the D-fructose related diastereomer was formed predominantly (9 1). The L-sorho-configurated isomer 95 resembling adenosine was obtained selectively, however, when the enantiomerically pure (S)-aldehyde served as the substrate. 

It was initially presumed that one would observe a polarity-driven kinetic advantage in 5-hexenyl radical cyclizations if one constructed such radicals with either a hydrocarbon radical site adding to a fluorinated alkene segment or, vice-versa, with a fluorinated radical site adding to a hydrocarbon alkene segment. As it turns out, only the latter combination led to a significant cycliza-tion rate enhancement. 

Likewise, conversions where a reactant is unstable should be linked with the previous step to keep the yield as high as possible, and conversions where the product is unstable should be linked with the following step to keep the yield as high as possible. In addition, reactions requiring deracemization to take advantage of the unreacted enantiomer will also benefit from one-pot operation to improve the yield. In each of these cases it is an increase in the thermodynamic yield of product on substrate rather than any kinetic advantage that is gained. 

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