Kinetic isotope effects measurement

TABLE 16. The hydrogen-deuterium kinetic isotope effects measured for the oxidation of mandelic acid" by Pb(OAc)4 in benzene and in benzene-pyridine... 

Reduction of aldonolactones and their derivatives with isotopically modified reducing agents leads to sugars labeled at the anomeric center. Glycosides substituted with deuterium or labeled with tritium are widely employed for kinetic isotope-effect measurements, mechanistic studies, isotope-tracing experiments, and so on. 

A potential factor affecting the catalytic power of enzyme-catalyzed reactions in which the enzyme forces the attacking moiety into the valence shell of the entity under attack. Compression is thought to be manifested in the transition state complex. The results of kinetic isotope effect measurements have suggested that compression events factor in the catechol methyltransferase re-actionh... 

Kinetic isotope effect measurements support this interpretation. A small kinetic isotope effect (kR/kD = 0.71) is observed for p-methoxyphenol in agreement with a ratedetermining nucleophilic attack, while a large kinetic isotope effect (kR/kR = 5.27), observed for p-trifluoromethylphenol, strongly supports a mechanism in which a phenolic H (or D) is transferred to 39 in the rate-determining step. Unfortunately, 39 is a symmetric disilene so that diastereoselectivity could not be determined. It will be interesting to examine whether the diastereoselectivity will be effected by the change in the addition mechanism from electrophilic to nucleophilic. 

FIGURE 9.1 Schematic of an apparatus used to determine competitive isotope effects. The reaction chamber (1) is closed off from the atmosphere in the configuration used for kinetic isotope effect measurements and left open in the configuration used for equilibrium isotope effect measurements. 

HO radicals [26]. These results were also consistent with intramolecular kinetic isotope effect measurements on typical Gif oxygenations of l,3-d2-adamantane to... 

Kinetic Isotope Effects Measured at Natural Abundance 153... 

Theory and practice of kinetic isotope effect measurements 153... 

KINETIC ISOTOPE EFFECTS MEASURED AT NATURAL ABUNDANCE... 

J. K. Lee, A. D. Bain, and P. J. Berti, Probing the transitions states of four glucoside hydrolyses with 13C kinetic isotope effects measured at natural abundance by NMR spectroscopy, /. Am. Chem. Soc., 126 (2004) 3769-3776. 

Our recent studies on phenol oxidation by oxoiron(IV) and oxomanganese(IV) porphyrins in aqueous solution (pH 7.6) have shown that the initial step in these reactions is H atom abstraction to generate a phenoxyl radical (Scheme 3). e kinetic isotope effect measured in the present study indicates that H atom abstraction also occurs in the rate determining step of the oxidation of azonaphthol dyes by 1. However, although the structures of azonaphthol dyes are normally shown as azo compounds, in aqueous solution they are in a rapid dynamic equilibrium with their hydrazone tautomers the latter isomer being the dominant species (Scheme 4). This complicates kinetic studies on the dyes, since the substrate is in effect a mixture of two compounds. Consequently one or both the tautomers may be the active form of the substrate providing the H atom for the oxidant. It is important to note that, irrespective of which tautomer is the reactive substrate, H atom abstraction leads to a common azonaphthoxyl radical intermediate and subsequent reactions of this species should be independent of the initial tautomerism (Scheme 5). 

Nuclear tunneling is potentially a significant consideration in outer-sphere radical electron transfer reactions. The case of reduction of NO2 to NO2 is notable in that nuclear tunneling is predicted to increase the self-exchange rate constant by a factor of 79 relative to the classical value.75 Kinetic isotope effect measurements could provide experimental evidence for nuclear tunneling. 180/160 KIE measurements have indeed provided evidence for nuclear tunneling in reactions involving the O2/O2 redox couple.76... 

The mechanism of Brudvig and co-workers, shown in Figure 28, also involves a Mn=0 species.S-state advancement from Sq Si S2 involves PCET path A. The large increases in activation energy and the deuterium kinetic isotope effect measured for the S2 —> S3 transition are taken to mean that there is a switch from path A to path B in this step, triggered Iw the increased net positive charge on the Mn4 cluster in S2. This leads to the formation of a Mn =0 species in S4. Nucleophilic attack on the Mn =0 by a Ca-bound H2O is prompted by... 

Indole protonation is a functional step in the cleavage of tryptophan by tryptophan indole-lyase, based on kinetic isotope effect measurements [26]. 

Scheme 5 Mechanistic possibilities consistent with secondary deuterium kinetic isotope effect measurements...

In their seminal paper on the rhodium(ll)-catalyzed C—H insertion with PhI=NNs, the group of Muller reached the conclusion that the reaction proceeds through the concerted asynchronous insertion of a rhodium-bound nitrene species.This hypothesis was supported by a Hammett analysis (/9= —0.90 vs. tr+), the absence of ring-opened products in reactions involving cyclopropyl radical clocks, and the stereospecific C(sp )—H ami-nation of (Ji)-2-phenylbutane that occurs with complete retention of configuration. However, the very low yields obtained for these test reactions as well as the kinetic isotope effect measured for the reaction from (1,3-D2)-adamantane (KIE = 3.5 0.2) put this conclusion into question as these did not rule out the possible involvement of radicals that could undergo fast recombination. Nevertheless, this initial study already highlighted the discrepancies that could be observed between the carbene and nitrene chemistries in terms of mechanism. The electronic structure of nitrenes, contrary... 

Ph, CH2Ph) undergo one-pot, four component condensation reactions to give 4-aminoimidazolin-2-ylidene complexes 62. 3 jhe activation parameters and kinetic isotope effect measurements for the reaction of a substituted imidazole with an unsaturated Fischer carbene complex [(OC)5M=C(OEt)CsCPh] clearly show that the reaction proceeds via a two-step process with a... 

M. J. Stern, A. Persky, and F. S. Klein, Force field and tunneling effects in the H-H-Cl reaction system. Determination from kinetic isotope effect measurements, J. Chem. Phys. 58 5697 (1973). 

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