Deuterium labelled olefins

The equilibrium constant of this reaction depends on the temperature. The tantalacycle is exclusively trans-, / -disubstituted. This decomposes to give 2,3-dimethyl-1-butene. In the presence of an excess of propylene the dimer forms catalytically. Detailed studies with deuterium labelled olefins show that the dimer is not directly formed by reductive elimination from an alkenyl hydride intermediate. The most satisfactory explanation is that the tantalum hydride adds back to the alkenyl double bond to give a tantalacyclobutane which then rearranges to one of two possible olefins. This unexpected ring contraction supposes a rapid and irreversible decomposition of the less favored ring strained species. 

Proper deuterium-labeling experiments involving (CD3)4Sn and [1,1,1,-10,10,10-D6]-2,8-decadiene confirmed that propylene is indeed the first-formed olefin, and its structure indicated that the methylidene and ethyl-idene moieties originated from Me4Sn and 2,8-decadiene, respectively. 

Such stability is only relative, however, given the possibility of the acid-catalyzed 1,2-shift of a proton observed in some olefin epoxides of general structure 10.10 (Fig. 10.3) [12], Such a reaction occurs in the in vivo metabolism of styrene to phenylacetic acid the first metabolite formed is styrene oxide (10.10, R = Ph, Fig. 10.3, also 10.6), whose isomerization to phenyl-acetaldehyde (10.11, R = Ph, Fig. 10.3) and further dehydrogenation to phenylacetic acid has been demonstrated by deuterium-labeling studies. A com-... 

Kinetic analyses and deuterium-labeling experiments have demonstrated that, remarkably, the reductive elimination of TEA and the formation of intermediate C is the rate-determining step in the (de)hydrogenation cycle. Accordingly, hydrogenation of the acceptor appears to be slower than dehydrogenation of the alkane substrate. This contrasts with the fact that catalytic olefin hydrogenation is well-established in transition-metal-mediated chemistry [10]. 

According to a deuterium-labeling experiment, Miyuara s hydroboration is actually a 1,1-addition process with concomitant 1,2-H-shift, rather than a true transaddition. The olefin geometry of the product boronate is presumably determined during an insertion of boron or hydrogen into the a-position of rhodium vinylidene intermediate 43 (Table 9.8). 

The related system 336 with an olefinic a,p-bond cyclized in a similar way to give the 3//-2-benazepines (337) (203). Deuterium labeling studies showed that the cyclization step is irreversible. 

In the hydroformylation of optically active alkenes, all aldehydes were shown to be formed with predominant retention of configuration.53,54 This was interpreted to prove, in accordance with the transformation of deuterium labeled alkenes,55 56 that stereoselective hydrogen shifts of the coordinated olefins occur without dissociation of the isomeric alkenes.30... 

Evidence for the Fe2+/Fe3+ redox cycle was provided later by ESR measurements [205], while recent experiments with deuterium-labelled butene indicate that C—H cleavage is involved in the rate-controlling step [138]. In agreement with the views of Schuit [281], chemisorption of the olefin on an anion vacancy is assumed, but O- is postulated as the active oxygen species rather than O2-. An argument in favour of O" is that otherwise much more, and rather complicated, elementary reaction steps are required to account for the transfer of charge. 

The results of deuterium labeling experiments shown in Scheme 37 clearly show the operation of a monohydride mechanism in the BINAP-RuOI) catalyzed hydrogenation of unsaturated carboxylic acids. However, with many olefinic substrates with a neutral, rather than anionic, secondary binding site, the products exhibit a similar degree of isotope incorporation at the two hydrogenated centers (Scheme 39). The out-... 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

The stereospecific deuterium labeling and subsequent photo-oxidation of the olefin 5 and similar olefins 6 and 7 revealed the hidden regioselectivity of the... 

One significant aspect of the cyclohexane-1,4-diyl radical cation has been a point of contention the question whether it undergoes cleavage to the hexa-1,5-diene radical cation, i.e. whether it completes the Cope rearrangement. Results obtained in the laboratory of Miyashi, particularly the exchange of a deuterium label between the terminal olefinic and allylic positions, seem to suggest such a... 

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