Reverse-Labeling Schemes

By considering the H-migration origin/destination, one may distinguish I, II and III/IV. On this basis, experiments (i) and (ii) with a type A catalyst as shown in Scheme 12.9 eliminated mechanisms I and II from consideration this left III and IV which were both fully consistent with the results. The outcome for (i) is obvious the allylic hydrogens (see Hb in mechanism I, Scheme 12.8) are not involved in the reaction. The outcome for (ii) is more subtle and relates to the stereochemistry attending fceta-carbopalladation and beta-hydride elimination which are both known to proceed with syn stereochemistry. Thus, mechanism II which does not involve a beta-hydride elimination would not affect the alkene stereochemistry (see Hc in II, Scheme 12.8), as was revealed by D-labelling, Scheme 12.9. In contrast, mechanisms III and IV should reverse the stereochemistry (see Hc in III and IV, Scheme 12.8), as was observed. 

Mechanistic studies on the intramolecular hydroacylation by using deuterium-labeling experiments have been reported by several groups [95-100]. The results of their studies showed that the addition of the Rh-H bond to a carbon-carbon double bond takes place in syn fashion [95,96]. They also demonstrated that C-H bond cleavage, hydrid transfer to the double bond, and carbonyl deinsertion are all fast and reversible steps (Scheme 3) [99]. 

A very elegant experiment to show a similar effect was conceived and executed by Russell, Wool and co-workers (Russell et al. 1993). They studied interdiffusion at an interface between two isotopie triblock copolymers of identical length. On one side of the initial interface the diblock had hydrogenated ends and a deuterated centre, whereas the polymer on the other side of the interface had the labelling scheme reversed, with deuterated end blocks and... 

For the qualitative test one has to carry out dediazoniations with a diazonium ion labeled with 15N in either the a- or P-position or both, in a solution containing the highest possible concentration of unlabeled molecular nitrogen (14N2). If the reaction is stopped before completion, the 15N content of the residual diazonium ion should be smaller due to the reverse reaction of (a) in Scheme 8-8. The result of this test was indeed positive (Bergstrom et al., 1974, 1976). 

Elgiire 11.3. A flow-model scheme intended to represent relevant nitrogen flows, especially with regard to which flows are reversihle. The labeled reactions 1, 11, 111 IV are all potentially iso-topically fractionating. Because reaction 11 is not reversible, subsequent fractionations in the excretory pathway should not influence the isotopic composition of the body protein pool. 

Wiberg et have performed the reaction in the presence of C-labelled cyanide ion and find no incorporation of activity into product ferrocyanide. Evidently the reversible ligand displacement proposed by the Czech workers does not take place and the electron-transfer scheme of Swinehart is preferable. Recent spectroscopic studies indicate that a complex [Fe(CN)5(CNS03)] functions as an intermediate in this reaction. 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

Scheme 22.5 Deuterium-labeling studies suggest reversible hydrometallation for keto-enone substrates.

Another example of hypermonomer-based dendrimer synthesis was reported by Gilat et al. for the rapid preparation of a new family of laser dye-labeled dendrimers that were critical to our study of energy transfer in light harvesting dendrimers (Scheme 6) [29]. In order to avoid undesirable through-bond energy transfer in the poly(benzyl ether dendrimers, the reversed monomer unit,... 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

The results of deuterium labeling are consistent with intervention of a symmetric iridium 7i-allyl intermediate or rapid interconversion of a-allyl haptomers through the agency of a symmetric 7i-allyl (Scheme 20) [280], Competition experiments akin to those previously described (see Scheme 12) again demonstrate rapid and reversible dehydrogenation of the carbonyl partner in advance of C-C coupling. 

II,D,l,a) that in the 3-amino compound obtained, the label is nearly exclusively present on the nitrogen of the amino group (Cl 96% Br 93%), proving that the aminodechlorination and aminodebromination have taken place according to the Sn(ANRORC) mechanism. Exploring the amino-lysis of 3-iodo-4-phenyl-l,2,4-triazine (109, X = I) in a reverse manner, which means reaction of unlabeled 109 X =1) with N-labeled potassium amide/liquid ammonia, gave as result that the 3-iodo compound reacts 63% according to the Sn(ANRORC) mechanism. The conclusion from all these experiments is that the formation of the 3-amino compound 110 from 109 (X = Cl, Br, I) can be explained as described in Scheme 11.49 for the aminodemethylthiolation. 

Competitive with -deprotonation, a-deprotonation furnishes the carbenoid-type oxiranyl anion species 10. In selected cases anion formation has been established to be a reversible process by deuterium-labeling experiments. As opposed to -deprotonation which gives only allylic alcohols, a-deprotonation can give rise to a variety of products as summarized in Scheme 4. This behavior will be further discussed in Section V. Some... 

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