Beta-hydrogen elimination

Hexene Polymerization Polymerization of 1-hexene (and also propylene) by the Kaminsky catalyst [(Cp2ZrCl2/(MeAlO) /toluene] differs fundamentally from that of ethylene in that beta hydrogen elimination is the only detectable chain transfer mechanism. Insertion of 1-hexene into the Zr-C bond in Cp2ZrCH3+ produces Cp2ZrCH2CH(CH3)C4H9+. The electron donating... 

Beta hydrogen elimination is the sole chain transfer pathway and the source of the propagating species Cp2ZrH+. 

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. 

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. 

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. 

The activating effects of the halogen atoms on the beta-hydrogen coupled with the reluctance of fluorine to depart as an anion from saturated carbon, especially in the presence of other alpha-fluorine atoms - , make the intermediacy of carbanions in these eliminations highly probable. Unfortunately the olefinic product adds alcohol too rapidly to be isolated and this necessitated careful considerations of alternative mechanisms. A minor fraction of the decomposition may follow an alpha-elimination, viz. 

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. 

Unlike the three fundamental mechanisms of olefin formation already outlined, the a -ji (or ylid) mechanism is only applicable to elimination from onium salts . The base abstracts an alpha hydrogen from the leaving group to form an ylid which subsequently acts as an internal base and abstracts the beta hydrogen atom, viz. 

The Hofmann eliminations (19-21) give very low isotope effects, excepting the controversial intramolecular isotope effects for the 2-p-nitro-phenylethyl-trimethylammonium ion. The conditions vary widely and comparisons are thus difficult. The enhanced acidity of this substrate, however, should promote extensive proton transfer and the small tritium isotope effects would seem most plausibly interpreted as indicative of highly carbanionic transition states for these eliminations. The alkyl ammonium salts, possessing much more weakly acidic beta hydrogen, probably react via less carbanion-like E2 transition states . 

The results for the eliminations giving 1-phenylcyclohexene are very interesting. In the m-2-phenylcyclohexyltrimethylammonium ion, the extent of C -N bond breaking is slightly greater than that for the 2-phenylethyl series. However, in the trans substrate (10, Table 6), in which the C -H and C -N bonds are unable to attain an a/ih-conformation, little bond breaking at the C -N bond occurs in the transition state. Consequently, if the acidity of the beta hydrogen is the same in both cases, the latter elimination has a transition state with considerably more carbanion character. 

The beta-tritium secondary isotope effect for elimination from the propyl-trimethylammonium ion (8, Table 7) seems most probably explained in terms of reduced acidity of the beta hydrogen caused by the greater inductive effect of the bond to the heavier isotope. A hyperconjugative effect also fits the observed data as in the transition state the developing double bond would be more effectively stabilised by the methyl than the monotritiomethyl group. However, the latter explanation seems less likely as the elimination from the propyl compound is slightly slower than that from the ethyl derivative . 

The Bronsted coefficient, yS, (Table 10) suggests, as expected, that electron-withdrawing substituents in the beta phenyl ring promote increased proton transfer to the base in the transition state for elimination in the 2-phenylethyl series. A concomitant increase in ko Jk r is also observed , and an interpretation has been forwarded that increased beta hydrogen bond breaking is accompanied by increased C -X bond breaking. Consequently the ElcB mechanism is extremely rare and has been considered as a near-paradox - . Two peculiar features of these results warrant further comment. 

HX rather than a > -c/i fl/-elimination of DX from the erythro substrate (100) as in the former conformation the beta hydrogen is only flanked by one large substituent and the base approach is less impeded than in the alternative conformation. On the other hand, -hexene is formed by loss of DX in... 

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... 

Some primary esters and those lacking beta hydrogen atoms eliminate via radical intermediates , which may intrude into many of the product studies for which cyclic mechanisms are usually proposed. In fact, mixed mechanistic... 

The low stability of primary carbonium ions makes their intermediacy highly improbable and any rearrangements have to be accounted for usually in terms of concerted mechanisms. The primary dehydration products of n-butanol are 1-butene (97.3%) and 2-butene in which the cis isomer predominates by a two-fold excess. Whereas 1-butene possibly arises from an fl/i/i-elimination, the 2-butenes are best explained as arising by removal of the y-hydrogen with a concerted migration of a beta hydrogen, viz-... 

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]. 

---