13C isotope effect

Corey et al. investigated the kinetic isotopic effect (KIE) in asymmetric dihydroxylation. 12C/13C KIE was measured for the dihydroxylation of styrene, p-nitrostyrene, and 4-methoxy-benzoate (Figure 7).197 The observed similar 12C/13C isotopic effect of two olefinic carbons... 

As an example of this technique we consider 12C/13C isotope effects in the Diels-Alder condensation of isoprene and maleic anhydride (Equation 7.32). The terminal carbons of isoprene carbons are numbered 1 and 4, the methyl substituted carbon is number 2. The reaction proceeds via a concerted and slightly asynchronous mechanism. 

The 13C NMR spectra of oxetanes and oxetanones are also very useful in structure determination. The chemical shift values for the a-carbon atoms tend to be higher than for other cyclic ethers. Some representative 13C chemical shift values are given in Table 3. The 13C isotope effect on 13C chemical shifts has been determined as +0.27 and +0.23 Hz for C-2 and C-3 in oxetane <780MR(11)157). 

The predictive capabilities of results of theoretical calculations of isotope effects have again been questioned,94 following an experimental and theoretical study of the decarboxylation of 3-carboxybenzisoxazole at room temperature (Kemp s reaction). The experimentally determined 15N isotope effect in acetone is 1.0312 0.0006 and the 13C isotope effect (1.0448, 1.0445, 1.0472, and 1.0418 in 1,4-dioxane, acetonitrile, DMF, and water, respectively) is independent of solvent polarity even though the reaction rate is markedly solvent dependent. Theoretical models at die semiempirical (AMI, PM3, SAMI) and ab initio (up to B3LYP/6-31+ + G ) levels were all unable to predict die experimental results quantitatively. 

The deoxygenation of styrene oxide with dichlorocarbene exhibits a normal 13C isotope effect of 1.016 for the -carbon of the styrene oxide and an inverse isotope... 

The substantial differences of isotope effects in diastereoisomers 15 and 16 indicate differences in mechanisms or rate-determining steps in the formation of these products. Significant 13C isotope effect at a and (3 carbon atoms and no effect at carbonyl atom suggest rate limiting of the Ca-C(3 bond in fra s-(3-lactone 15. The formation of cz s-(3-lactone 16 is accompanied by substantial isotope effects at all carbons, what can be associated with concerted cycloaddition or13C KIE at the... 

Bromination of alkenes belongs to the most important and comprehensively studied reaction in organic synthesis. 2H and 13C isotope effects were measured for bromination of 1-pentene 17 in CCI4 at 25°C.72 Unreacted 1-pentane was recovered from reactions taken to 88-95% and then brominated. The resulting 1,2-dibromopentane 18 was analysed by 2H and 13C NMR in comparison to 1,2-dibromopentane obtained from original 1-pentene. Experimental isotope effects are summarized in Figure 7. 

Procedure of 13C NMR analysis was identical like in the case of 1-pentene bromination.72 Calculated 2H and 13C isotope effects are summarized in Figure 8. 

The mean values of 13C isotope effects determined in three independent experiments for unreacted substrate (81-92% completion) and product (10% completion) are summarized in Figure 12. 

C isotope effect at C3 (1.025) corresponds to the bonding change at this atom in the rate-determining step. Smaller but significant 13C KIE at Ca atom is indicative for the butyl group transfer in the same step. Such results are consistent with reductive elimination process in the rate-determining step. 

The resulting isotope effects determined relative to the para carbon are summarized in Figure 13. Independently of used catalyst similar 13C isotope effects were observed for olefinic carbons. This fact suggests that in the presence of the bulky chiral catalyst as Rh2(S-DOSP)4 the geometry of the cyclopropana-tion transition state was not change. 

The intermolecular and intramolecular 13C isotope effects were used in studies of the mechanism of the ene reaction of singlet (1Ag) oxygen with alkenes.89 The intermolecular 13C isotope effect was determined for reaction of 3-isopropyl-2,4-dimethylpent-2-ene 40 with 102 in methanol/2-propanol (1 1) at 10°C using a sunlamp and Rose Bengal as sensitizer. 

The intramolecular 13C isotope effects on the ene reaction with 102 oxygen were determined for 2,3-dimethyl-l-buten-3-ol 43 prepared from 2,3-dimethyl-but-2-ene 89... 

The analysis of 13C integrations in context of different mechanistic models (concerted, biradical or per epoxide) led authors to the conclusion that 13C integration pattern is consistent with the perepoxide model and supports mechanism proposed earlier from 2H KIEs.90 The following intramolecular 13C isotope effects for formation of 43 were calculated from changes of 13C integrations (Figure 18). 

So far, three computational studies of isotope effects related to the ODCase mechanism have been published Singleton, Beak and Lee used 13C isotope effects to elucidate the mechanism by which the uncatalyzed decarboxylation of orotic acid takes place.46 Phillips and Lee calculated 15N isotope effects and compared them to known experimental values to show that oxygen-protonation mechanisms are viable for the enzyme-catalyzed process.47 Kollman and coworkers focused on the 15N isotope effect associated with C5-protonation.27 Each study is described further below. 

The authors also compared their values to a previously measured 13C isotope effect of 1.043 0.003 for the carboxylate carbon in the E. coli ODCase-catalyzed decarboxylation of OMP.65 This value differs substantially from the experimental value of 1.013 measured by Singleton for the decarboxylation of orotic acid in sulfolane, implying that the uncatalyzed and catalyzed reactions are quite different. 

The presence of a significant 13C isotope effect at both ends of the diene system (but not for the central carbon atoms) in 16 in the Diels-Alder... 

This is probably the most important part of your poster, so be sure it is short, pithv, and attractive to the eye. Two sentences is the recommended maximum. Ideally, the conclusion will be the answer to the question posed at the outset. For example, The observed 13C-isotope effect is 2.5%. Decarboxylation is thus 50% rate-limiting, and product release is also 50% rate-limiting, both steps occurring at equal rates. The conclusion should not be a simple summary. 

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