Propylene deuterium labeled

The spectra of C3H6 and C3D6 show that chemisorption of propylene is dissociative, but they fail to identify which carbon-hydrogen bond is broken on adsorption. To this end the spectra of a number of deuterium-labeled propylenes were studied and compared. These results are summarized in abbreviated form in Table VI, which specifies the hydrogen fragment formed on adsorption the fragment was identified as an OH if a band appeared near 3593 cm-1 or as an OD if a band appeared near 2653 cm-1. In those cases where the spectrum changed with time the summary... 

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. 

The use of isotopic tracers has demonstrated that the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Probably the most convincing evidence is presented by the isotopic tracer studies utilizing, 4C-labeled propylene and deuterated propylene. Adams and Jennings 14, 15) studied the oxidation of propylene at 450°C over bismuth molybdate and cuprous oxide catalysts. The reactant propylene was labeled with deuterium in various positions. They analyzed their results in terms of a kinetic isotope effect, which is defined by the probability of a deuterium atom being abstracted relative to that of a hydrogen atom. Letting z = kD/kH represent this relative discrimination probability, the reaction paths shown in Fig. 1 were found to be applicable to the oxidation of 1—C3He—3d and 1—QH —1 d. 

Studies with specifically deuterium labeled 2-propanols also indicated that this was a contributing pathway. In situ generation of propylene necessarily must result in loss of one [I deuterium from the isopropyl precursor. Under the conditions employed for these labeling studies, the hydride would be derived from HI (as opposed to DI), so that such an overall reaction would be expected to result in loss of one fi deuterium in going from isopropyl... 

The most well accepted feature of the mechanism is the formation of an allylic intermediate via a-hydrogen abstraction from propylene in the ratedetermining step. The structure of this intermediate and its subsequent steps involved in its conversion to selective products are much less well understood. It has been suggested from deuterium-labeling studies (77) that this intermediate undergoes a second hydrogen abstraction followed by O (oxidation) or N insertion (ammoxidation) (Scheme 3). The formation of an... 

Groves and co-workers have reported that the epoxidation of franj-l-deuterio-propylene by a reconstituted system with purified P-450lm2 proceeded with significant loss of the deuterium labels (778). Further, the epoxidation of propylene by this enzyme system in D2O afforded predominantly rran5-l-deuteriopropy-lene oxide. When NADPH/O2 was replaced with PhIO in the enzyme system, no incorporation of deuterium from D2O in recovered propylene was observed. 

Nucleophilic attack of stabilized carbon nucleophiles on coordinated olefins is also known. Hegedus developed the alkylation of olefins shown in Equation 11.31. The (olefin)palladium(II) chloride complexes did not react with malonate nucleophiles, but the triethylamine adduct does react with this carbon nucleophile to provide the alkylation product. This reaction has recently been incorporated into a catalytic alkylation of olefins by Widenhoefer. - Intramolecular reaction of the 1,3-dicarbonyl compounds with pendant olefins in the presence of (GHjCNl PdCl occurs to generate cyclic products containing a new C-C bond (Equation 11.32). Some intermolecular reactions with ethylene and propylene have also been developed by this group. Deuterium labeling studies (Equation 11.32) have shown that the addition occurs by external attack on the coordinated olefin. ... 

Alternatively, if the propylene interacts with a metal-methylidene intermediate (for illustrative purposes, the deuterium label has been moved to the vinyl terminus in Scheme 10.1b), again two intermediate metallacycles are possible. The formation of a P-substituted metallacycle exchanges the termini of the olefin and degeneratively regenerates a methylidene. If, instead, an a-substituted metallacycle is formed, an alkylidene intermediate is generated upon cycloreversion, thereby productively moving the catalyst species back to the first set of pathways. 

Low monomer concentration can also decrease the isotacticity of polypropylene. The three-coordinate metallocene alkyl species that exists in the absence of a n-coordinated monomer is capable of racemizing the methyl group of the last-inserted monomer unit. Evidence for the occurrence of chain-end epimerization has come from studies using deuterium-labeled propylene " the relevant mechanistic steps are believed to be a series of P-hydride eliminations and subsequent isomerizations with a tertiary alkyl species as an intermediate (Scheme 1.8). 

At low propylene concentrations, isolated m stereoerrors were also observed for 35a-c/MAO and found to be more numerous than mm stereoerrors. The isolated m stereoerrors were found to have a strong dependence on propylene concentration. This gave further support for site epimerization as the major stereoerror-forming mechanism for 35a-c/MAO. However, since chain epimerization occurring with simultaneous site epimerization can also lead to isolated m stereoerrors, the authors undertook a deuterium labeling study to probe the importance of chain epimerization as a potential mechanism for forming both single m and double mm stereoerrors. 

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. 

Confirmation and extension of this mechanism was obtained by Adams and Jennings 97,109) using propylene labeled with deuterium in various positions. Ammonia was also added to the feed to produce acrylonitrile. After reasonable corrections for small effects of propylene isomerization and deuterium exchange, the results were in quantitative agreement with a model in which allylic hydrogen abstraction occurs to form an allylic intermediate followed by hydrogen abstraction from either end. The model is illustrated by the following scheme for l-propene-3d. 

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