Elimination reactions beta-hydrogen

The yield of this reaction for R=R =C2H is 73% (15). Reagents are less toxic than the corresponding sulfates and less volatile than the orthoformates. Esters of phosphoms oxo-acids having beta hydrogens undergo olefin elimination upon pyrolysis, usually beginning at 160—200°C. 

Beta-elimination reactions have been observed in a number of proteins. This reaction occurs primarily at alkaline pH conditions. Abstraction of the hydrogen atom from the alpha-carbon of a cysteine, serine, threonine, phenylalanine, or lysine residue leads to racemization or loss of part of the side chain and the formation of dehydroalanine (26). 

All beta hydrogen elimination occurs from the 14-electron species Cp2ZrR+ where, by definition, R is branched. In contrast, beta hydrogen elimination from Cp2ZrR,+ is slow (because the alkyl group, n-hexyl, is linear) and the rate of this reaction is taken to be zero. 

In order to fit the data, it is not necessary to assume that beta hydrogen elimination from the monolefin complexes Cp2ZrR 0 or Cp2ZrRO+ occurs. The model therefore ignores these reactions. 

These examples of the carbanion mechanism based on substrates possessing alpha sulphone groups would be more certain if founded on kinetic evidence. In all the cases quoted above, the enhanced reactivity of the sulphones relative to the other model compounds may be explicable in terms of beta hydrogen activation and a normal concerted elimination reaction. 

If planar carbonium ions were the intermediates in El reactions in the cyclohexyl series, menthyl and neomenthyl compounds should give the same product ratios. However, the olefin distribution is quite different in the two El processes and the stereospecificity is less marked than in the E2 reactions of these substrates (Table 15). Whereas the concerted eliminations always show anti stereospecificity, the unimolecular eliminations only exhibit this preference when a tertiary beta hydrogen is trans to the ionising group (e.g. neomenthyl series). Possibly in this case the tertiary hydrogen aids ionisation by forming a type of non-classical bridged intermediate, viz. 

It would be unreasonable to expect a nucleophilic species not to interact with both an electrophilic alpha carbon and beta hydrogen at some stage during the reaction. However, whether this interaction is representative of the rate-determining step is a matter of conjecture. Most probably this dual interaction occurs early in the reaction profile and is followed by partitioning to give the two different transition states commonly accepted for bimolecular elimination and substitution. The closer the dual interaction and the transition states are on the reaction profile, then the more closely the elimination rate will respond to carbon nucleophilicity. 

The principal factors affecting orientation in acetate decompositions have been adequately summarised by DePuy and King Essentially three influences were recognised, these being termed statistical, steric and thermodynamic effects. Statistical control is observed in pyrolysis of simple aliphatic esters which under the elevated reaction temperatures experience little resistance to conformational rotation and the number of beta hydrogen atoms in each branch determines the direction of elimination (147)= 37o distortion in statistical control is imposed by the steric influence of a t-butyl substituent (148), and is also illustrated by the predominance of trans- over m-olefin formation (148, 149) due to eclipsing effects . The latter example, however, may also arise from thermodynamic influences which are more certainly demonstrated by preferential elimination towards a phenyl rather than an alkyl substituent (150) . The influence of substituents on olefin stability rather than beta hydrogen acidity seems more critical as elimination occurs more often towards a p-methoxyphenyl rather than a phenyl substituent (151... 

Elimination reactions which give rise to multiple bonds between carbon and a heteroatom occur with particular facility and show many of the characteristics of olefin-forming processes. Most often one of the eliminating fragments is a hydrogen atom, which is easily removed from a heteroatom and the choice of mechanism, between an anion or a concerted process, will depend on the lability of the beta carbon-X bond, e.g. 

Earland and Raven [65] have examined the reaction of A-(mercap-tomethyl) polyhexamethyleneadipamide disulfide (XV) with alkali. Under alkaline conditions that produce lanthionyl residues in wool, no thioether is formed from this polymeric disulfide however, cyanide readily produces thioether from either (XV) or wool fiber. Therefore, the mechanism for thioether formation must be different in these two reactions. Because this polymeric disulfide (XV) contains no beta-hydrogen atoms (beta to the disulfide group), a likely mechanism for formation of lanthionyl residues in keratins, under alkaline conditions, is the beta-elimination scheme [64] (the reaction depicted by Equation F). Other mechanisms that have been suggested for this reaction have been summarized by Danehy and Kreuz [66]. 

Hydride elimination reactions are characterized by hydrogen atom transfer from a ligand to a metal. The most common type of hydride elimination is /3 elimination, with a proton in a j8 position on an aUcyl ligand transferred to the metal by way of an intermediate in which the metal, the a and /3 carbons, and the hydride are coplanar. An example of j8 elimination—the reverse of 1,2-insertion—is in Figure 14.15. Beta eliminations are important in many catalytic processes. 

The carbocation may rearrange, eliminate a proton to produce an olefin, or crack at a beta position to yield an olefin and a new carbocation. Under an atmosphere of hydrogen and in the presence of a catalyst with hydrogenation-dehydrogenation activity, the olefins are hydrogenated to paraffinic compounds. This reaction sequence could be represented as follows ... 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

Olefinic compounds will often insert into carbon-transition metal bonds as CO does, and this reaction is an important step in many catalytic syntheses. When this step is combined with an oxidative addition of an organic halide to a palladium(O) complex in the presence of a base, a very useful, catalytic olefinic substitution reaction results (26-29). The oxidative addition produces an organopalladium(II) halide, which then adds 1,2 to the olefinic reactant (insertion reaction). The adduct is unstable if there are hydrogens beta to the palladium group and elimination of a hydridopalladium salt occurs, forming a substituted olefinic product. The hydridopalladium salt then reforms the... 

By considering the H-migration origin/destination, one may distinguish I, II and III/IV. On this basis, experiments (i) and (ii) with a type A catalyst as shown in Scheme 12.9 eliminated mechanisms I and II from consideration this left III and IV which were both fully consistent with the results. The outcome for (i) is obvious the allylic hydrogens (see Hb in mechanism I, Scheme 12.8) are not involved in the reaction. The outcome for (ii) is more subtle and relates to the stereochemistry attending fceta-carbopalladation and beta-hydride elimination which are both known to proceed with syn stereochemistry. Thus, mechanism II which does not involve a beta-hydride elimination would not affect the alkene stereochemistry (see Hc in II, Scheme 12.8), as was revealed by D-labelling, Scheme 12.9. In contrast, mechanisms III and IV should reverse the stereochemistry (see Hc in III and IV, Scheme 12.8), as was observed. 

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