Labeling experiments, isotopic

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

Mechanism I was ruled out by an isotopic labeling experiment. The mixed anhydride of salicylic acid and acetic acid is an intermediate if nucleophilic catalysis occurs by mechanism 1. This molecule is known to hydrolyze in water with about 25% incorporation of solvent water into the salicylic acid. 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

The mechanism outlined above is supported by experimental findings. An intermediate 5 has been isolated, " and has been identified by and N-nuclear magnetic resonance spectroscopy. Side-products have been isolated, which are likely to be formed from intermediate 4. N-isotope labeling experiments have shown that only the nitrogen remote from the phenyl group is eliminated as ammonia. 

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

Evidence in support of the mechanism shown in figure 21.4 comes from isotope-labeling experiments. When 180-labeled methanol reacts with benzoic acid, the methyl benzoate produced is found to be l80-labeled but the water produced is uniabeled. Thus, it is the C-OH bond of the carboxylic acid that is broken during the reaction rather than the CO—H bond and the RO-H bond of the alcohol that is broken rather than the R-OH bond. 

Problem 29.5 Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope-labeling experiments. If acetate labeled with 13C in the methyl group ( CFtyCC H) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the, 3C label to appear ... 

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

The literature contains ample evidence that methanol is synthesized from CO2 according to (19) and not from CO as in (18). Isotopic labeling experiments have demonstrated that CO2 is the main source of carbon in methanol formed from synthesis gas [G.C. Chinchen, P.J. Denny, D.G. Parker, M.S. Spencer, K.C. Waugh and D.A. Whan, Appl. Catal. 30 (1987) 333], In addition, it has been shown that a Cu(lOO) single crystal readily produces methanol from a mixture of GO2 and H2 at 2 bar [P.B. Rasmussen, M. Kazuta and I. GhorkendorfF, Suif. Sci. 318 (1994) 267]... 

Further evidence for different reaction pathways is obtained in isotope labelling experiments, illustrated in fig.6. Here we present a comparison of the effects of preadsorbing various amounts of deuterium with methyl groups cm the two metal surfaces. 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

After approximately 3 min on stream in the isotopic mixture of CH4/02/C 02/He, a steady-state value in the rate of formation is obtained (Fig. 3). This value is used to estimate the relative contribution of the CO2-reforming route to the overall production of CO (direct + indirect routes). Proper analysis of this result, taking into consideration scrambling of isotopic oxygen Figure 3. Transient isotopic labelling experiment atoms between the CO2 and O2 molecules... 

Results of isotopic labeling experiments conducted over the Ru/Ti02 catalyst. 

A reaction that has been much investigated is the hydrolysis of esters, e.g. (164), by aqueous base, i.e. eOH. It is found to be kinetically second order, and lsO isotopic labelling experiments on (164) have... 

Wentrup and co-workers have carried out systematic flash vacuum thermolysis studies with a series of fused tetrazoles. Investigations of the isomeric tetrazolo[l,5- ]pyrazine 17 and tetrazolo[l,5-f]pyrimidine 20 showed that, in both cases, ring contraction takes place to afford imidazoles in high yields, but isotope labeling experiments revealed that the mechanisms of the openings of the two ring systems are different <2002JOC8538>. 

Several experiments have been done to probe the mechanisms of these unusual addition reactions,21 although certain aspects of the mechanisms are still not fully understood. Based on isotopic labeling experiments, a mechanism has been proposed involving formation of a triprotonated species (Scheme 2). Like the... 

W. Wiechert and A. A. de Graaf, Bidirectional reaction steps in metabolic networks. Part I Modeling and simulation of carbon isotope labelling experiments. Biotechnol. Bioeng. 55,101 117 (1997). 

The rearrangements of b-(acyloxy), (i-(phosphatoxy)alkyl, and related systems have been reviewed [51,52] and representative kinetic data are given in Table 2 above. As revealed by isotopic labeling experiments, the acyloxy... 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

None of the methods described in this chapter utilize a pre-formed metal vinylidene as an active catalyst precursor. The occurrence of metal vinylidene intermediates is instead inferred on the basis of product structure, isotopic labeling experiments, and computational studies. 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

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