Double bond formation 3-hydrogen elimination

The mechanism proposed involves hydrogenation of the C2 C3 double bond, formation of 2-vinylthiophenol by an E2 elimination, and hydrocarbon elimination by homolysis of the S—Caryi bond. This pathway rationalizes the primary formation of (104) observed in some HDS reactions of (102) over Co/Mo/S catalysts, as well as the kinetic evidence that the rate-determining step on real catalysts is the removal of surface sulfur.158-160... 

Allyl-type monomers do not yield high polymers. The substituent on the carbon in the / position with respect to the double bond is easily eliminated (especially hydrogen, halogenides, etc.). The generated radical is resonance-stabilized. It reacts much more readily with growing radicals than with the monomer. The low probability of long chain formation is a consequence of these terminating and transfer reactions. 

Distinct functional groups with acidic hydrogens can also promote these transformations. For instance, benzoylnitromethane (208) or ethyl bromopyruvate (206) react with isoquinoline (6) and acetylenedicarboxylates via the same dipolar mechanism to generate a pyrrolo[2,l-a]isoquinoline scaffold. However, in these cases, after closure of the 5-membered ring, a double-bond formation via dehydrogenation or nitrous acid elimination yields the fully aromatic ring systems 207 and 209 (Scheme 28) [82, 183]. 

In the final 1,4-elimination, the trivalency of a carbon atom does not lead to a double bond formation but to a ring formation sterically facilitated by the fact that carbon atoms participating in double bonds form planar structures characterized by bond angles of 120° (plane trigonal orbitals). After the 1,4-elimination, ejection of a hydrogen ion completes the process. 

The dehydrochlorination reaction continues to be studied - and a variety of mechanisms has been proposed to account for the process. These include a radical-chain mechanism in which the termination step depends upon the stage of the reaction, a monomolecular mechanism involving an activated complex, and a three-step mechanism, beginning with the random formation of a single c s-carbon—carbon double bond, followed by elimination of hydrogen chloride via a six-membered transition state and isomerization of the polyene so formed. ... 

With cyclic substrates, the formation of the new double bond depends on the availability of a c i -/3-hydrogen, which is required for the yw-elimination... 

In the case of the cyclohexane derivative 7 however, that bears an equatorial acetate group, two axial cis-/3-hydrogens are available, and elimination in both directions is possible. The pyrolysis of 7 yields the two elimination products 8 and 6. Formation of product 8 is strongly favored, because the new double bond is in conjugation to the ester carbonyl group. ... 

Braun and Schurek [9] assumed that during polymerization a reaction can occur between the polymer and free radicals that leads to the elimination of hydrogen chloride and formation of a double bond. The formation of HCl during the polymerization of vinyl chloride has been observed [10],... 

These compounds are much more toxic than chlordan (Table III), and yet are stable toward alkaline reagents (15), being unable to eliminate hydrogen chloride without the formation of a double bond at a bridgehead carbon atom. Thus in this type of compound the conclusion must again be reached that dehydrochlorination with alkali and insecticidal activity have no systematic relationship. 

Reactions leading to the formation of the catalytically active nickel hydride species from organonickel precursors (Section III) can be regarded as model reactions for olefin oligomerization reactions. The reactions described by Eq. (8) and Scheme 3 (Section III) show that RNiX compounds (R = methyl orallyl, X = halide or acetylacetonate) activated by Lewis acids add to double bonds under mild reaction conditions (-40° or 0°C). It follows further from these reactions that under conditions leading to olefin dimerization a rapid nickel hydride /3-hydrogen elimination reaction occurs. The fact that products resulting from olefin insertion into the nickel-carbon bond are only observed when /3-hydride... 

Another approach is based on the palladium-catalyzed intramolecular carbocyclization of the allylic acetate moiety with the alkene moiety (Scheme 96). After the formation of a 7t-allylpalladium complex, with the first double bond the intramolecular carbometallation of the second double bond occurs to form a new C-C bond. The fate of the resulting alkylpalladium complex 393 depends on the possiblity of /3-elimination. If /3-elimination is possible, it generates a metallated hydride and furnishes the cycloadduct 394. This cyclization could be viewed as a pallada-ene reaction, in which palladium replaces the hydrogen atom of the allylic moiety.231... 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

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