Propylene, reaction with deuterium

It is also clear that accumulated chemical evidence on warmed products may in some instances permit reasonable, although not optimum, deductions about low-temperature reactions. For example, it seems clear from experiments with deuterium that addition of hydrogen atoms to some olefins followed by abstraction does occur at 77°K, while with some other olefins this abstraction is not apparent [ ]. From the propylene reaction with D rather than with H atoms, it was established that virtually all of the propane was formed by... 

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. 

Fig. 8. Partial results of the course of reaction of propylene with deuterium over palladium-alumina at —20° (31).

With Thompson and Irsa the study of the Fischer Tropsch synthesis using deuterium gas,(89,90) with Lewis Friedman on reduction of acetone,(91,92) with G. C. Bond, the reaction of propylene(93) and cyclopropane(94) with deuterium. Using the ethylene-deuterium reaction as an example,(98) the mass spectroscopic approach permitted analysis of each of the five ethylenes and seven ethanes at the same time and showed, among other interesting data, that the first product of the reaction between deuterium and ethylene was an ethane containing no deuterium. 

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. ... 

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. 

Orchin and Roos (108) examined the isomerization of allylbenzene by HCo(CO)4 and DCo(CO)4 at ambient temperature and pressure. Both HCo(CO)4 and DCo(CO)4 catalyzed isomerization to propenylbenzene at the same rate, and when DCo(CO)4 was used as catalyst 5% of the propenylbenzene produced was found to contain a deuterium atom. Hydroformylation of propylene with residual DCo(CO)4, after an isomerization of allylbenzene, yielded RCDO with no detectable RCHO. The authors chose to reject a mechanism involving addition of D—Co to the olefinic double bond, on the grounds that the lack of an isotope effect indicated breaking of D—Co, or H—Co, was not the rate-determining step, and that only a relatively minor amount of deuterium was incorporated into the isomerized reaction product. Instead, the authors favored a mechanism expressed as... 

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... 

Katzer (28) observed that counterdiffusion of benzene and cumene within the pores of H-mordenite does not occur at low temperatures. However, H-mordenite shows activity for the alkylation of benzene with propylene to form cumene under the liquid phase conditions used for the diffusion studies, and he has suggested that reaction must occur on the external crystallite surface, or just within the pore mouth. In earlier studies on the isomerization of 2,3-dimethylbutene-l at 0°-20°C over a deuterated Y-type faujasite (62), we observed that the extent of isomerization (2,3-dimethylbutene-2) was far greater than the extent of deutera-tion only a fraction of the total deuterium on catalyst OD groups was exchanged. One possible explanation—assuming a protonic isomerization mechanism—is that, because of lowered intracrystalline diffusion rates... 

The calculated values are those obtained by assuming the ratedetermining step to be abstraction of an allylic hydrogen with an isotope effect equal to the discrimination effect obtained from the retention of deuterium in the products. The observed kinetic isotope effects show clearly that this first abstraction is the slow step. The good agreement with the discrimination values gives excellent confirmation for the stepwise mechanistic scheme proposed. In the tracer experiments using deuterium it was found that appreciable propylene isomerization occurred unless ammonia was present. The oxidation rate was unaffected by the ammonia. This was taken to indicate that the initial oxidative abstraction reaction had no carbonium ion characteristic. However, this conclusion does not apply in the case of cuprous oxide, where ammonia very severely inhibited the oxidation reaction. 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

This mechanistic tour de force was concluded with one final, stereochemical probe on degenerate propylene metathesis (Scheme 10.15). A mixture of (Z)-propylene-l-(ij and propylene-d5 was subjected to the MoOj catalyst. Propylene-dg was formed with 80% initial retention of stereochemical information (where retention here means that the deuterium of the methylidene group ended up on the same side as the methyl group, as in (Z)-propylene-l-dj). While the stereochemical purity of this product eventually degraded due to secondary metathesis, it is mechanistically significant that the stereochemistry of the methylidene moiety was initially preserved. Again, this reaction could proceed through either a metal-methylidene or ethylidene intermediate. 

Methane" Figure 2 indicates that methane-di is produced in largest concentrations of all methanes produced when deuterium is mixed with the propylene, and methane-do is the second largest. The methyl radical, produced by hydrogenolysis of propylene or by reaction (e), is converted either to methane-di by reaction (f ) or to methane-do by reaction (g)... 

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