Deuterium Labelling Experiments

A similar reaction the with rran.v-isomer 3b gave c -3,5-dimethylcyclohexene (4) with very high diastereoselectivity. Accordingly, the stereochemistry of this substitution is anti. Deuterium labeling experiments using the 1-deuterio or 3-deuterio derivative of 3 a showed that the ratio of SN2 /SN2 with lithium dimethylcuprate was about 50 50, while the ratio with lithium cyano(methyl)cupratc was >96 4. 

A catalytic mechanism, which is supported by deuterium-labeling experiments in the corresponding Ru-catalyzed procedure [146], is shown in Scheme 47. Accordingly, the reactive Fe-hydride species is formed in situ by the reaction of the iron precatalyst with hydrosilane. Hydrosilylation of the carboxyl group affords the 0-silyl-A,0-acetal a, which is converted into the iminium intermediate b. Reduction of b by a second Fe-hydride species finally generates the corresponding amine and disiloxane. 

Scheme 10 Deuterium-labeling experiment providing evidence for a metallacyclic reaction mechanism [17]...

As a mechanistic hypothesis, the authors assumed a reduction of the Fe(+2) by magnesium and subsequent coordination of the substrates, followed by oxidative coupling to form alkyl allyl complex 112a. A ti—c rearrangement, followed by a syn p-hydride elimination and reductive elimination, yields the linear product 114 with the 1,2-disubstituted ( )-double bond (Scheme 29). This hypothesis has been supported by deuterium labeling experiments, whereas the influence of the ligand on the regioselectivity still remains unclear. 

The oxidation state of the metal is of the utmost importance since no conversion is observed with complexes in a different oxidation state. CatalyticaUy active species are electron-rich d or d complexes. A general catalytic cycle has been proposed on the basis of deuterium-labeling experiments (Scheme 4-14) [280]. It is beUeved to occur for aU the catalysts used. 

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. 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

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]. 

In contrast to thioethers 63 and 64, the corresponding sulfoxides (e.g., 66 and 69, respectively) cyclize in the course of an extremely mild thermolysis in DMF at 110°C [84H(22)467], Thereby, 2-(chloroallylsulfinyl)tropone 66 is transformed to thiopyranotropone 67 and its sulfoxide 68. Crossover deuterium labeling experiments confirm the reaction to be a radical, non-concerted, intermolecular process. 

Deuterium labeling experiments show that oxidation of 2-phenylethyl iodide proceeds through the bridged carbonium ion 11 [21]. Both iodocyclobutane and... 

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... 

Entry 6 is particularly interesting. Indeed, it has been shown by deuterium labeling experiments that the presence in the reaction mixture of a strong donor solvent such as HMPA induces a change in the stereoselectivity of the -elimination, from syn to anti (see above, Section Ll.C.). Therefore, both the increase in rate and the enantioselectivity... 

It is interesting to note that no reaction is observed with oxirane 72a if the hydroxyl group is protected. Moreover, whereas deuterium labeling experiments indicate a clean /3-deprotonation process for both oxiranes 69 and 72a, the same enantiomer of base 71 furnishes the corresponding allylic alcohols 70 and 73a with the opposite absolute configurations (Scheme 30 vs. 31). The same studies on vicinal disubstituted analogues 72b,c showed that both the sense and the level of enantioselectivity are unchanged, which... 

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