Isotope effect studies carbon

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

A. Carbon-14 and Deuterium Isotope Effect Studies of the Diels-Alder... 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

Isotope effect studies of the mechanism of hydration of alkynes with formic acid as water donor485b,485c leading to ketones (equation 234a) have been undertaken recently4854 by observing the kinetic isotope fractionation of 13C in the course of carbon monoxide... 

B. Carbon-14 Isotope Effect Studies of Elimination Reactions. 510... 

Kinetic and carbon-14 isotope effect study of the base promoted dehydrochlorination of substituted 1-phenyiethyl-2-14C-chlorides (391)... 

Swanson T, Brooks HB, Osterman AL, O Leary MH, Phillips MA (1998) Carbon-13 isotope effect studies of Trypanosoma brucei ornithine decarboxylase. Biochemistry 37 14943-14947... 

Eubanks, J. R. I., Sims, L. B., Fry, A. Carbon isotope effect studies of the mechanism of the Hofmann elimination reaction of para-substituted (2-phenylethyl-1- C)- and (2-phenylethyl-2- C)-trimethylammonium bromides. J. Am. Chem. Soc. 1991, 113, 8821-8829. 

Keefer LK, Streeter AJ, Leung LY, et al. 1987. Pharmacokinetic and deuterium isotope effect studies on the metabolism of formaldehyde and formate to carbon dioxide in rats in vivo. Drug Metab Dispos 15 300-304. 

Current Issues in Enzymatic Hydrogen Transfer from Carbon Tunneling and Coupled Motion from Kinetic Isotope Effect Studies... 

The substrate for the carboxylation is HCO3" rather than CO2 (4, 36). When the carboxylation is conducted with HC Os , the Pj produced contains a single atom of 0 (4, 37), apparently because the reaction occurs by way of a carboxy phosphate intermediate (Scheme IV). Carboxy phosphate is a known compound, but it is unstable, hydrolyzing in aqueous solution within a few seconds (58). Presumably at the active site of the enzyme, carboxy phosphate is protected from hydrolysis. Studies of phosphorus stereochemistry (39) and carbon, hydrogen, and oxygen isotope effects (40, 41) are consistent with this mechanism. Isotope effect studies indicate that the phosphate transfer step is rate determining (40). 

O Leary MH (1981) Carbon isotope fractionation in plants. Phytochemistry 20 553-567 O Leary MH, Rife JE, Slater JD (1981) Kinetic and isotope effect studies of maize phosphoenolpyravate carboxylase. Biochem 20 7308-7314... 

Next to the isotope effects of hydrogen the most studied element has been carbon where the effects range as high as 15% with " C measurements of radioactivity are inherently of lower precision than mass spectrometric measurements making C the preferable isotope. Results from isotope effects on carbon can be most informative as the majority of organic reactions involve carbon bond fission. 

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