Isotopic labelling studies formation

A variety of six-membered carbocycles and heterocycles were synthesized by Shibata et al.81 using Wilkinson s catalyst (Equation (79)). The proposed catalytic cycle (Scheme 24) rationalizes the exclusive formation of the (Z)-isomer. Additionally, the mechanism is supported by the results of a isotope-labeling study reported by Brummond... 

Dahl, I.M. and Kolboe, S. (1995) On the reaction mechanism for hydrocarbon formation from methanol over SAPO-34 2. Isotopic labeling studies... 

The intermediacy of a platinum vinylidene in Yamamoto s reaction was supported by the results of isotopic labeling studies. D FT calculations were used to further probe the proposed reaction mechanism. In contrast to the prevailing model of alkyne/ vinylidene interconversion for Rh(I)-catalysts, direct Ca Cp 1,2-H-migration is implicated in the formation of vinylidene 130. Direct C—H insertion via a single... 

EtOH isopentane Et20) glasses at 77 K, four products 256, 257, 258 and 259 were obtained in overall >95% yield. The major product 256 was shown by isotopic labeling studies to have been derived from the silirene 260 formed by intramolecular coupling of the bis-carbene 261, and pathways for the formation of the other products were proposed. The photolysis of the bis-diazo compound 255 was studied in detail at 405 nm and it was found that the diazirine 262 was formed this on photolysis at 305 nm also gave 260 on the pathway to 256. The chemistry is shown in Scheme 46. 

Attempts to use molecular oxygen as the oxidant failed except in solvents that undergo efficient autoxidation to the corresponding hydroperoxide (e.g., THF). Mechanistic studies, including isotopic labeling studies, indicate that fBuOOH is the source of the oxygen atom incorporated into the product, and the reaction proceeds via a hydride-shift pathway that avoids formation of an enol intermediate (Scheme 12). 

The remaining results are concerned with redox reactions /t-superoxo-/<-peroxo redox reactions have been reported for rhodium complexes The complex [(NC)sCo( -02)Mo(0)(OH2)(CN)5] decomposes on standing in oxygen to [(>7 02)Mo(0)(CN)4] and isotopic labelling studies show that the dioxygen in this complex does not originate from [(NC)5Co(u-02)Mo(0)(OH2)(CN)5] °). The details of this reaction are not clear but the final product could conceivably arise from the formation of a Mo(IV) complex on breakdown of the rj complex followed by addition of dioxygen. 

Supporting evidence for the above mechanistic patterns in the majority of metal systems was first established in elegant isotopic labeling studies, which showed clearly that no intermediate with the symmetry of a cyclobutadiene was involved.In one system, the reaction of a cobaltacyclopentadiene with MeC>2CC CC02Me, benzene formation does not involve direct complexation of the third alkyne to the metal. It has therefore been suggested that the conventional insertion process has been here replaced by a direct Diels-Alder reaction with the metallacycle, perhaps as a result of electronic factors (Scheme 25). o ... 

The mechanism of the P-M oxidation consists of three distinct steps 1) activation of the DMSO by a protonated dialkyl carbodiimide 2) activation of the alcohol substrate and the formation of the key alkoxysulfonium ylide intermediate and 3) the intramolecular decomposition of the alkoxysulfonium ylide to afford the product ketone or aldehyde and the dialkyl urea by-product (established by isotopic labeling studies). The alkoxysulfonium ylide is a common intermediate in all other oxidations using activated DMSO. 

Photolysis of (244) in the presence of alcohols likewise results in the formation of esters of methyl(phenyl)phosphinic acid. However, O-isotopic labelling studies provide evidence of, at least, the partial involvement of a pentacovalent phosphorus intermediate in this reaction, and so the previous assumption of the intermediacy of (245) may not be the whole story. The phosphole oxide dimers (246) have been shown to undergo regioselective reduction and complexation on treatment with the dimethylsulfide-borane adduct, to form the bicyclic system (247). Treatment of the propadienylphosphine oxide... 

Guengerich, F.P., Persmark, M., and Humphreys, W.G. (1993) Formation of 1, N2- and N2,3-ethenoguanine from 2-halooxiranes isotopic labeling studies and isolation of a hemiaminal derivative of N2-(2-oxoethyl)guanine. Chem. Res. Toxicol., 6, 635-648. 

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