Ethylene 1,2 hydrogen shift

Let us finally consider two Z-matrices for optimization to transition structures, the Diels-Alder reaction of butadiene and ethylene, and the [l,5]-hydrogen shift in Z-1,3-pentadiene. To enforce the symmetries of the TSs (Cj in both cases) it is again advantageous to use dummy atoms. 

Figure 12 shows the spectrum for the deformation region in an ethylene-helium stream and an ethylene-hydrogen stream. In the deformation region the two bands at 1451 and 1438 cm-1 due to ethylene alone appear to weaken and shift slightly and a new band (or perhaps two) appears at about 1415 cm-1. Figure 12 also shows that under reaction conditions the ZnH band is shifted from 1709 to 1655 cm 1 the corresponding shift in the OH band is from 3490 to 3510 cm-1. 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

When the NMR spectrum of a 30% (w./v.) solution of peroxide in toluene was recorded at 34°C., absorption was observed between 8 2.74 and 5.46. There were seven main resonances, all multiplets, which were interpreted in terms of aliphatic hydrogen shifted by oxygen. Resonance from ethylenic hydrogen amounted to only a fraction of a proton. However, the sample darkened while in the instrument and probably decomposed extensively. When the spectrum of a solution of peroxide prepared by oxidation to 10.4 mole % was recorded using a cold probe at —35°C. a different picture was obtained. There was complex absorption from both ethylenic and saturated hydrogen which was interpreted as arising from a mixture of 1,2 and 1,4 oxygen copolymers in an approximate jatio of 1 to 2. In this sample the residual chloroprene amounted to 0.15% of the monomer units in the peroxide and dimers of chloroprene to 0.6% of the peroxide. 

The oxidation of olefins to carbonyl compounds by palladium (II) ion can be regarded as an addition of a palladium hydroxide group to the olefin followed by a hydrogen shift. Kinetic evidence suggests the following mechanism for the oxidation of ethylene by palladium chloride in aqueous solution containing excess chloride ion 21, 49, 99). 

Alternatively, if the reaction involved trivalent w-butyl cation 468 (from ethylation of ethylene) the ion would inevitably rearrange via 1,2-hydrogen shift to sec-butyl cation 19, which in turn would isomerize into the ferf-butyl cation (1) and thus give isobutane (461) [Eq. (3.125)]. 

They proposed that 1-butene is obtained from ethylene by intermediate (I) in which breaking one bond and hydrogen shift occurs. 2-Butene is produced by double-bond isomerization, and propylene is formed from ethylene and 2-butene by breaking ring bonds Cj —C2 and C3 C4 of structure (II). 

The reaction with (E) or (Z)-l,2-bis(phenylsulfonyl)ethylene leads to the exclusive formation of the fraras-cycloadduct, the (Z)-dipolarophile rearranging into its thermodynamically more stable (T)-isomer before cycloaddition reaction takes place.407,408 The reaction with DMAD leads to the corresponding pyrrol derivative with good yield, a 1,5-hydrogen shift taking place from the adduct.407,408... 

The compounds (256) and (257) show in their mass spectra a considerable peak at M-28, indicating that the elimination of ethylene is still an important degradation process. The loss of ethylene may be explained by assuming a hydrogen shift in the molecular ion, [261]->[262]. The intermediate [262] can now lose another ethylene... 

Figure 7.1. Geometry of the S,-S conical intersection of ethylene calculated at the CASSCF level, indicating the possibility of cis-trans isomerization and [1,2] hydrogen shift a) side view, b) Newman projection with localized nonbonding orbitals (Freund and Klessinger, 1995).

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