Isotopical scrambling selectivity

Selectivities to various isomers are more difficult to predict when metal oxides are used as catalysts. ZnO preferentially produced 79% 1-butene and several percent of i7j -2-butene [624-64-6] (75). CdO catalyst produced 55% 1-butene and 45% i7j -2-butene. It was also reported that while interconversion between 1-butene and i7j -2-butene was quite facile on CdO, cis—trans isomeri2ation was slow. This was attributed to the presence of a TT-aHyl anion intermediate (76). High i7j -2-butene selectivities were obtained with molybdenum carbonyl encapsulated in 2eohtes (77). On the other hand, deuteration using H1O2 catalyst produced predominantly the 1,4-addition product, trans-2-huX.en.e-d2 with no isotope scrambling (78). 

The unreacted ethanol and the diethylether product retained >98% of from the starting 0-ethanol, indicating that no isotope scrambling occurred. Data in Table 4 demonstrate that was retained in the mixed ether and ethanol attack of the acid-activated 2-pentanol via an axial S 2 rear-attack was the predominant synthesis pathway. Evidently, the shape selectivity induced by the 2 M-5 zeolite channel structure (Figure 2) plays an important role in achieving the remarkably higher configuration inversion... 

The selective oxidation of propylene to acrolein was suggested to occur via a Mars-van Krevelen mechanism (i.e. reaction of the hydrocarbon with lattice oxygen), where in the first step bismuth dehydrogenates propene to form an allylic species. This has mainly been concluded from isotopic scrambling studies. The oxidation step itself is believed to occur via the mechanism shown in Figure 7.11. 

From this finding, the authors conclude that the imine formation may be selectivity determining. The imine that was added in a crossover-competition experiment was not reduced under the reaction conditions, which indicates that the imine is formed and consumed in the coordination sphere of the iridium catalyst. The monohydridic nature of the active catalyst was determined by the absence of isotope scrambling in an alcohol that had been labeled with deuterium in the a-position. 

An even more sophisticated approach comprises protein synthesis by cell-free expression as the amino acids added in a cell-free system are less exposed to bacterial metabolism, efficiency and selectivity of incorporation of specific isotope-labeled amino acids are only marginally hampered by isotope scrambling often observed in in vivo expression systems [19]. Even non-canonical amino acids can be incorporated allowing for site-specific labeling of proteins [20, 21]. 

Some of the problems associated with selective vibrational photochemistry can be avoided in electronic photochemistry, and dye lasers are available to provide tunable radiation. To trap the selectively excited species, reactions involving molecules as distinct from radicals are preferred, since then the opportunities for isotopic scrambling are minimized. This approach to isotope... 

Elements.—The use of a tunable narrow-frequency laser to produce an isotopically selected chemical reaction has been described by Leone and Moore 1 this approach should permit efficient isotope separations to be performed. In the example reported, natural Br2 (79Br/81Br = 1) is photo-predissociated by selective excitation into the 3n0+ state. Bromine atoms, enriched in one isotope, react with HI before scrambling occurs, to produce 80—85% enriched H81Br. 

Scheme 74 shows selective deprotonation of one of two methyl groups that depends upon the presence of an ortho carboxyl,4603 and Scheme 75 shows a more complex but similar example.461 Related C—C bond-forming reactions are known.462 Scrambling of isotope into the methyl group of 5-methyl-2-furoic acid when it is decarboxylated at about 240 C in deuterium oxide463 probably involves a similar carbon deprotonation. The behavior... 

Figure 8.S Avoiding H/D scrambling by adjustment of the declustering conditions in the electrospray ion source. The ETD mass spectra display the c -lon (left panel) and charge-reduced precursor Ion (right panel) obtained from ETD experiments on (a) unlabeled and (b-c) selectively labeled peptide PI. The Isotopic labeling of the model peptide Is shown by the schematic. The ETD spectrum in (b) was recorded at defined mild declustering ion source conditions for minimal collisional activation, while the ETD spectrum in (c) was recorded with excessive collisional activation. The centroids are indicated by a dotted red line. Extensive scrambling in (c) is evident by the mass increase (2.1 Da) due to the presence of deuterium in the c-ion (compare (b) with (c)). Note the deuterium content of the c -ion corresponds to complete (100%) H/D scrambling. Reproduced with permission from [47] 2009 American Chemical Society. (See insert for color representation of the figure.)...

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