Isotope labeling studies

At least three distinct mechanisms can be written for these reactions. Write down some possible mechanisms, and suggest isotopic labeling studies that could distinguish among the possibilities you have proposed. 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

The mechanism shown in Figure 21.7 is supported by isotope-labeling studies. When ethyl propanoate labeled with lsO in the ether-like oxygen is hydrolyzed in aqueous NaOH, the l80 label shows up exclusively in the ethanol product. None of the label remains with the propanoic acid, indicating that saponification occurs by cleavage of the C-OR bond rather than the CO—R bond. 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

The characterization of the species was accomplished with the aid of isotopic labeling studies and ab initio calculations. The Be-CO stretching frequency of 2,189.5 cm-1, above that of CO itself in solid argon (2,138.4cm-1) indicates the considerable strength of BeO as a Lewis acid.65... 

A variety of six-membered carbocycles and heterocycles were synthesized by Shibata et al.81 using Wilkinson s catalyst (Equation (79)). The proposed catalytic cycle (Scheme 24) rationalizes the exclusive formation of the (Z)-isomer. Additionally, the mechanism is supported by the results of a isotope-labeling study reported by Brummond... 

ESI represents a powerful method by which to transfer organometallic ions from catalytically active solutions into the gas phase. ESI-MS systems allow the characterization of the gas-phase ions using CID, reactivity, and isotope-labeling studies. The application of ESI-tandem-MS systems allows gas-phase preparations and isolation of desired organometallic ions in the first ESI-octopole-quad-rupole, followed by characterization or reactivity studies in the second octopole-quadrupole. 

Isotopic labeling studies, 21 137-139 Isotopic mixtures, hydrogen, 13 765 Isotrate ER, molecular formula and structure, 5 110t Isotretinoin, 25 789... 

Isotopic labeling studies have shown that H202 formed in irradiated suspensions of ZnO contained oxygen atoms derived exclusively from 02. 

Dahl, I.M. and Kolboe, S. (1995) On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 2. Isotopic labeling studies... 

Lastly, the positive charge is neutralized via loss of a proton, giving the alkene lanosterol. There is no obvious energy advantage in such tertiary-to-tertiary cation changes, but it must be appreciated that this is an enzyme-catalysed reaction, and the enzyme plays a crucial role in the reactions that occur. These hydride and methyl migrations definitely do occur, as demonstrated by isotopic labelling studies. 

In addition to characterization of intermediates, in situ spectroscopic techniques can be applied in kinetic studies, providing additional mechanistic insight. Also, isotopic labeling studies have proven very useful, especially vhen studying the individual steps of a catalytic cycle [5, 6]... 

In this review, we will discuss the use of in situ spectroscopic techniques, in combination with kinetic and isotopic labeling studies, to obtain a detailed mechanistic insight of the rhodium catalyzed hydroformylation. 

The intermediacy of a platinum vinylidene in Yamamoto s reaction was supported by the results of isotopic labeling studies. D FT calculations were used to further probe the proposed reaction mechanism. In contrast to the prevailing model of alkyne/ vinylidene interconversion for Rh(I)-catalysts, direct Ca Cp 1,2-H-migration is implicated in the formation of vinylidene 130. Direct C—H insertion via a single... 

Intramolecular nucleophilic attack on the a-position of the vinylidene complex 148 affords a zwitterionic species (149). Next, a formal 1,3-H-shift must occur before release of the observed product (146). Based on the results of an isotope labeling study, this process is believed to occur via two discrete 1,2-H-migration events (149—>150—>147). Interestingly, no back-migration of heteroatom substituents is observed [50]. 

The condensation of a 2-aminobenzoxazole 141 with a-bromo ketones 142 gives 2(3/f)-oxazolones 148 on heating in ethanol. Isotope labeling studies with have shown that the additional oxygen that is incorporated into the ring is derived from the solvent, ethanol (Fig. 5.36). ... 

Weeg-Aerssens, E., Tiedje, J. M., and Averill, B. A. (1988). Evidence from isotope labeling studies for a sequential mechanism for dissimilatory nitrite reduction. J. Am. Chem. Soc. 110, 6851-6856. 

I-Ph, or LNiIH-0-NiIUL) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

It is known through isotope labeling studies (see Section 3.4.1) that in the hydrogenation of CO the primary source of methanol is CO2. It is also known, however,... 

Isotope labeling studies furnished a convincing body of evidence to indicate that even in the presence of carbon monoxide, only carbon dioxide is converted to methanol [Eq. (3.2)].394-398 In a review article371 Chinchen considers this as the dominant route, with CO merely serving as a source of C02 via the water-gas shift reaction. 

Van den Berg et al. suggested an intramolecular Stevens rearrangement of the oxo-nium ylide to ethyl methyl ether to interpret carbon-carbon bond formation463 [Eq. (3.55)]. Olah and coworkers, however, provided evidence (based on isotopic labeling studies) that the oxonium ylide undergoes intermolecular methylation to ethyl dimethyloxonium ion [Eq. (3.56)] instead of Stevens rearrangement 447... 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of... 

---